RASA telescope design technology

The Rowe-Ackermann Schmidt Astrograph isn't your typical telescope, it’s a high-speed astrophotography machine that you can't actually "look through" - you have to use a camera to see the fruits of it's optics. Think of it as the “mothership” of aperture and speed. I’ve owned Fastar-enabled Celestron SCTs before, using that unusual-looking assembly on the corrector plate, but only in the 9.25" size. This time, I finally took the plunge and picked up the 11" RASA along with the CGX-L mount. Now that I own one - and have it permanently installed in my observatory as a primary instrument, I thought it would be fun to explore the technology behind the RASA design, from its invention and optical magic to practical uses, quirks, and why it’s coveted by newcomers and veteran astrophotographers alike. We’ll cover all the sizes (6″, 8″, 11″, and the giant 14″ RASA 36) including what’s special about the 11″ RASA V2 that now sits in my observatory.

What is a RASA telescope (Rowe-Ackermann Schmidt Astrograph)?

The Rowe-Ackermann Schmidt Astrograph, or RASA for short, is a specialized telescope designed purely for imaging. Unlike a regular telescope you might use with an eyepiece to look at planets or galaxies, an astrograph like the RASA is all about the camera. In fact, if you try to peek through a RASA, you’ll just see the back of a camera or a blank window – there is no way to use an eyepiece on it. The RASA’s job is to collect as much light as possible and focus it onto a flat camera sensor, capturing wide swaths of the sky in a very short time.

The RASA design has its roots in a famous concept from the 1930s: the Schmidt camera. Bernhard Schmidt invented a telescope that used a combination of a spherical primary mirror and a thin aspheric corrector lens to take wide-field photographs of the sky. Schmidt’s original camera could take very “fast” photos (meaning short exposures) by using a large mirror at around f/1.7 focal ratio – extremely fast for its time (and very problematic). However, Schmidt cameras had a curved film holder at the focal plane, which made them impractical for modern digital sensors that are flat. Fast forward to today: the RASA is a modern reinvention of that idea, built for digital astrophotography.

“Rowe-Ackermann” honors the two optical engineers, Dave Rowe and Mark Ackermann, who partnered with Celestron to create this astrograph. They essentially tackled the challenge of making a super-fast telescope that produces a sharp image on a flat sensor. The RASA uses a mix of mirrors and lenses (that’s what “catadioptric” means) to correct aberrations across a wide field. In simple terms, it’s a Schmidt telescope with extra optics: it has a Schmidt corrector plate at the front (like a typical Celestron Schmidt-Cassegrain telescope), a large primary mirror at the back, and in place of a secondary mirror it has a specially designed 4-element lens group near the front. This lens group brings the light to focus on your camera sensor and corrects any distortions, so stars stay pinpoint even toward the edges of the picture. The entire design is optimized to give a flat, wide image with very tiny star spots, suitable for modern CMOS and CCD sensors.

Here’s another way to picture it: imagine an 11-inch wide “light bucket” that funnels starlight directly onto a camera chip. The camera actually sits at the front of the telescope, facing down into the tube. This is the ingenious twist of the RASA (and earlier Fastar/HyperStar systems) – instead of having light go all the way to an eyepiece at the back, the light comes to focus just behind the corrector plate, where you attach your camera. By doing this, the focal length stays very short relative to the aperture, which yields an incredibly fast focal ratio (f/2.2 in the case of the 11″ RASA). In photography terms, that’s like having a giant 279 mm aperture lens at f/2.2. For comparison, most high-end telephoto camera lenses are f/2.8 or f/4, and many telescopes for imaging are f/5 to f/7. The RASA is faster than almost all of them, meaning it can expose images in a fraction of the time.

The Quest for Speed: Why Focal Ratio Matters In RASA telescope design technology

Astrophotographers often talk about telescopes being “fast” or “slow,” and this has nothing to do with slewing speed – it’s about the focal ratio (f-stop) of the optics. The focal ratio (f/number) is defined as the focal length divided by the aperture. A lower f-number means a faster optical system, which collects the same amount of light per unit area on the sensor in less time. The RASA’s fast optics are one of its biggest selling points. For instance, the 8″ RASA has a focal ratio of f/2.0, and the 11″ RASA is f/2.2. These numbers are dramatically lower (faster) than traditional telescopes.

To put this in perspective, let’s compare a RASA to a typical beginner telescope for imaging, say an 8″ Newtonian astrograph at f/5 (like some Sky-Watcher or Orion models). If you photograph a nebula with the f/5 scope, and it requires a 5-minute exposure to reach a certain brightness, the exact same nebula framed similarly with an f/2.2 RASA would only need roughly (5/2.2)² = about (2.27)² = 5.2 times less exposure time. In other words, around just 1 minute might be enough for the RASA to gather the equivalent amount of light! If you compared to an f/4 astrograph, the RASA still is about (4/2.2)² ≈ 3.3 times faster – what you capture in 3 minutes on that f/4 Newt might take only 1 minute on the RASA. And compared to a common f/10 Schmidt-Cassegrain used without reducers? The RASA is about 20 times faster – something that might take 20 minutes through an f/10 SCT could pop out in 1 minute with the RASA. This speed advantage is eye-opening, especially for faint deep-sky objects.

What does this really mean for you as an astrophotographer? It means you can gather more signal (photons) in less time. If you live in a place with fickle weather or have only a couple hours under dark skies, a fast system lets you maximize what you capture. It also means you can potentially get away without guiding or with simpler tracking, since each individual exposure can be so short. For example, with my RASA 11 V2, I often take one-minute exposures. Even at one minute, a surprising amount of detail emerges in galaxies and nebulae. With a slower scope I might need five-minute or ten-minute exposures, which absolutely require very accurate guiding and polar alignment. The RASA shortens that learning curve: you still need a decent equatorial mount, but the tolerance for error is higher when you’re taking many short shots instead of a few long ones.

It’s important to note a trade-off: fast focal ratio goes hand-in-hand with shorter focal length (unless you build a truly enormous telescope). The RASA’s design is inherently wide-field. For instance, the 8″ RASA has a focal length of about 400 mm, and the 11″ RASA about 620 mm. This is great for capturing large swaths of sky – huge nebulae, broad star fields, entire galaxies or galaxy groups in one frame – but it also means it doesn’t magnify small targets as much as a longer focal length telescope would. If your goal is to image tiny planetary nebulae or really zoom in on a small galaxy’s core, a RASA might not give you the image scale you want without resorting to techniques like cropping or using smaller-pixel sensors. In essence, the RASA is like a wide-angle lens for the universe, whereas something like an 11″ SCT at f/10 is like a telephoto lens. Each has its place. But for many of the popular deep-sky targets – the Orion Nebula, the Andromeda Galaxy, the Lagoon and Trifid Nebulae side by side – the RASA’s wide field is a perfect fit, and it lets you capture those targets in a breathtakingly short time.

One memorable example for me was capturing the Whirlpool Galaxy (M51) with the 8″ RASA owned by a fellow club member. M51 galaxy is a relatively small object (its spiral arms span only a few arcminutes), so it doesn’t fill the frame of a RASA like a wide nebula would. But even so, the RASA’s speed made up for the small size by delivering a clean, noise-free image quickly. In about 5 minutes of total exposure (stacking several 30-second shots), we could already see the spiral structure of M51 clearly and even the faint tidal bridge between the galaxy and its smaller companion. With a typical 8″ f/5 Newtonian, getting that kind of signal-to-noise on M51 would have needed perhaps half an hour or more of exposure. The fast optics give you a kind of instant gratification – details just pop out sooner.

Evolution of a Design: From Fastar to RASA V2

Celestron’s journey to the RASA involved some stepping stones. Back in the late 1990s, before digital SLRs were common in astrophotography, Celestron introduced something called Fastar for their Schmidt-Cassegrain Telescopes (SCTs). The idea was to remove the secondary mirror from an SCT and replace it with a small lens assembly and a CCD camera, effectively converting an f/10 SCT into an f/2 imaging system. Fastar was revolutionary: suddenly an 8″ or 11″ SCT could take pictures 25 times faster than before. In practice, early Fastar systems were used with tiny CCD sensors of the time and were a bit finicky, so Celestron never fully commercialized the larger Fastar lenses. But a third-party company, Starizona, picked up the baton and created the HyperStar lens, which accomplished the same goal. HyperStar allowed many SCT owners to do F/2 imaging by swapping in that lens assembly. I used to gaze longingly at HyperStar setups, seeing how quickly they could grab, say, the Horsehead Nebula in just a couple of minutes of exposure. The downside was that HyperStar was an add-on; it had to be collimated each time, and it was optimized for specific telescopes but always a compromise because the telescope’s primary mirror and the add-on lens weren’t originally designed as a single system. HyperStar can give great results, but it tends to illuminate only smaller sensors (APS-C or so) well and can show some vignetting or star distortions off-axis if things aren’t perfectly aligned.

The RASA represents the next step: an integrated system purpose-built for this kind of fast, wide imaging. Instead of repurposing an f/10 SCT’s primary mirror, the RASA’s optics from the ground up are designed for f/2.2. This means the lenses, the mirror, the corrector plate – all of it is tuned together. The result is better image quality over a large image circle. For example, the 11″ RASA can accommodate full-frame DSLR sensors (about 43 mm diagonal) with stars that remain tight to the corners and only modest vignetting. HyperStar on an 11″ SCT, by contrast, is typically recommended for sensors no larger than APS-C (~28 mm diagonal) if you want decent stars at the edge. By integrating everything, RASA provides a big, well-corrected field.

The first RASA Celestron released was the 11″ version in 2014. This was followed by a giant 14″ model (sometimes called the RASA 36, because 36 cm aperture) aimed at observatories and institutions, then a more budget-friendly 8″ RASA in 2018 that really made the “speed for everyone” idea come true. Most recently, Celestron even introduced a portable 6″ RASA. Each of these shares the same core concept but scaled for different needs and budgets.

Let’s talk specifically about the 11″ RASA and its Version 2 (V2) update, since that’s what I have sitting next to me. The original RASA 11 (call it V1) was groundbreaking, but like any first-generation product it had some room for improvement. Users loved the optical performance, but one complaint that emerged was with focusing. The RASA (like most Schmidt-Cassegrain derived designs) focuses by moving the primary mirror forward or backward on a set of slides or rails. If that mirror tilts or shifts even a tiny bit during focusing, you see the image wobble or shift – this is called focus shift or mirror flop. On long focal length SCTs, a bit of mirror flop is mostly just an annoyance, but on a super-fast f/2 system, even a tiny shift can throw off collimation or focus sharpness across the field. The RASA 11 V1 used a standard focusing mechanism and some users noted that achieving critical focus without any image shift took a delicate touch.

Celestron addressed this in the RASA 11 V2 by introducing what they call the Ultra-Stable Focus System (USFS). In practical terms, the focusing mechanism in V2 has a precision linear ball bearing supporting the primary mirror as it moves. This constrains the mirror so it cannot wobble side-to-side as you turn the focus knob. The effect is that focusing is smoother and you don’t get that little “image dancing around” problem. It also helps ensure that once you focus and lock it, the mirror stays put even if the telescope slews to a different part of the sky (preventing mirror flop). As an astrophotographer, I find this improvement invaluable – you can imagine how frustrating it would be to chase perfect focus on an f/2 system if the target keeps shifting. With the V2, I can use a Bahtinov mask or electronic focuser and dial in sharp focus much more easily and repeatably. In fact, I equipped my RASA 11 V2 with Celestron’s focus motor, which screws onto the focus knob shaft, and it’s been a dream for making minute adjustments while watching the star sizes on my laptop.

The RASA 11 V2 also incorporated a few other nice tweaks. It has an integrated MagLev cooling fan on the rear cell – this quietly pulls air through the optical tube to help the big primary mirror reach thermal equilibrium with the night air faster. (Anyone who’s used an SCT knows the pain of waiting for a tube to cool down to avoid blurry views; with RASA’s fast optics, thermal currents can really soften an image, so active cooling is a welcome addition.) The fan is paired with a filter to keep dust out, so you’re not sucking pollen or dust onto your optics. The original RASA 11 had a fan too, but the V2’s MagLev fan is a refined version that is quieter and vibration-free.

There’s also the matter of compatibility and ergonomics: the RASA 11 V2, like the V1, comes with adapters for both DSLR/Mirrorless cameras and dedicated astro cameras. It provides the common backfocus distance (55 mm from the camera adapter flange) that matches DSLR standards, but V2’s improved focus stability means it can better support heavier camera bodies. A minor but handy thing on the RASA V2 (and also the RASA 8) is they include threaded mounting points on the front ring for accessories like a carry handle or piggyback mount. And on the big RASA 36, Celestron even added carrying handles and multiple dovetails, a sign that they’ve learned what astrophotographers need in the field or observatory.

In summary, the RASA evolution from the early Fastar concept to today has been all about making a fast system easier to use and higher performance. The 11″ RASA V2’s improvements in focusing and cooling might sound technical, but they directly translate to less fiddling and more consistent results for me when I’m spending precious clear nights capturing photos. Bottom line: the V2 is a more polished experience – it’s the same fundamental f/2.2 light-gathering monster, now tamed and tuned so you can focus precisely and trust it to hold that focus.

Meet the Family: RASA 6, 8, 11, and 14 (36)

One great thing Celestron has done is bring the RASA concept to multiple sizes. Let’s do a quick tour of the RASA family and see how they compare, not just in aperture but in personality:

RASA 6-inch (RASA 6):

This is the baby of the lineup, a 6″ (152 mm) aperture astrograph at f/2.2 with a focal length of only ~335 mm. Don’t let the size fool you – it packs the same technology into a compact package. At around 8.5 lbs (about 3.8 kg) and 24 inches long, the RASA 6 is highly portable. You can even put it on smaller mounts (an AVX class mount can carry it) and take it to a dark site without breaking your back. It’s perfect for ultra-wide shots: think giant swaths of the Milky Way, the North America Nebula, the whole Orion’s Belt region, or multi-object fields. The trade-off is that its image circle is smaller – it’s optimized for sensors up to about 22 mm diagonal (APS-C or smaller). Large full-frame sensors will vignette and see some blurring at the corners. In practice, this means the RASA 6 is best used with dedicated astronomy cameras or mirrorless cameras that have APS-C sensors, or even micro 4/3 sensors. If you’re a beginner, that’s not a huge issue: some of the most popular astro cameras (like those from ZWO, QHY) have APS-C or smaller sensors and work great. The RASA 6 comes with an integrated focus system (moving mirror) similar to its big siblings and a built-in clear filter window that you can swap for a light pollution or narrowband filter using a convenient drawer system. It’s a friendly little powerhouse and probably the fastest telescope of that aperture you can find. A beginner could start with a RASA 6 and do real-time live stacking to see deep-sky objects almost as they appear – it’s that fast.

RASA 8-inch (RASA 8):

This model really opened the floodgates for amateur astrographs when it launched. An 8″ aperture (203 mm) at f/2.0 is serious light grasp in a still-manageable package. The 8″ RASA weighs about 17 lbs (7.7 kg) – still portable, though you feel the heft more than the 6″. Its focal length is ~400 mm. That means it captures a two-degree field on an APS-C sensor (four times the area of the full moon) – fantastic for big targets like the Andromeda Galaxy (which you can frame with room to spare) or the Lagoon and Trifid Nebula together. The 8″ RASA’s optical design is essentially a scaled-down 11″ with some tweaks. One interesting thing: the RASA 8 has an even faster focal ratio at f/2.0 exactly, and to achieve that speed the camera sensor has to sit very close to the corrector – only about 25 mm of backfocus is available from the included adapter. This is why the RASA 8 does not play nicely with most DSLRs – those cameras have a long flange distance (usually ~45 mm) which far exceeds 25 mm, so they simply can’t reach focus on the RASA 8. Instead, the RASA 8 is aimed at use with dedicated astro cameras or mirrorless cameras with short flange distances (and even then often using a thin adapter). I use mine with a small CMOS color camera; some folks use cooled monochrome cameras with filter drawers. Celestron anticipated the filter need: the RASA 8 has a removable optical window at the front that you can swap out for a 2″ mounted filter. That way, you can do something like add a light pollution filter or a multi-band nebula filter without wrecking the image quality. This model, like its siblings, has the Ultra-Stable Focus System built in, so focusing is surprisingly smooth. And even though it costs a bit more than an 8″ imaging Newtonian, it brings the exotic f/2 world into reach for a lot of amateurs. I often recommend the RASA 8 to folks who want to dive into deep-sky imaging and are most interested in large, diffuse objects and want quick results. With it, you can stack a dozen 20-second shots and start to see live detail in something like the Helix Nebula right on your laptop screen. It almost feels like cheating!

RASA 11-inch (RASA 11):

This is the mid-size heavy hitter and perhaps the most popular among advanced amateurs. The 11″ RASA has a 279 mm aperture and f/2.2 focal ratio, giving ~620 mm focal length. It’s a considerable piece of equipment: about 32 cm (12.5″) in diameter and 84 cm (33″) long, weighing in around 43 lbs (19.5 kg) for the OTA alone. This is a telescope that typically lives on a beefy mount (Celestron CGX, Losmandy G11, SkyWatcher EQ6-R or the like at minimum). You can transport it – I do, carefully – but it’s on the edge of what one person can comfortably set up at night. The RASA 11’s big advantage over the 8″ is that it can fully illuminate bigger sensors. It has a usable image circle of roughly 52 mm with good stars, meaning full-frame (43 mm diagonal) cameras are well-supported. I attach my full-frame DSLR to the RASA 11 V2 easily with the included M48 adapter. Stars out to the corners are sharp enough that I don’t hesitate to use the whole frame; there is some light fall-off at the extreme corners (around 70-80% illumination there), but with flat-field calibration frames that’s no problem. The 11″ also just gathers more total light than the 8″ – roughly (11/8)^2 ≈ 1.9 times as much area – so not only is it fast, it’s larger. That means you can go deeper or get better signal-to-noise in the same time, or alternatively, see similar detail in an even shorter exposure. For example, a well-defined shot of the Whirlpool Galaxy that might take 5 minutes on the 8″ RASA can be done in maybe 2–3 minutes on the 11″ under the same sky conditions, and with finer resolution to boot (because the 11″ has a longer focal length, the galaxy covers more pixels). The RASA 11 V2 I own includes dual Losmandy-style dovetail rails (top and bottom) for mounting and accessories – I often attach a small guide scope or simply a handle on top. It has mirror locks inherited from Celestron’s EdgeHD line, allowing me to lock down the primary mirror after focusing to avoid any slip. In practice, once I lock it, the Ultra-Stable Focus system has done its job and I rarely see any shift even if I slew to a completely different target; focus stays tack sharp for the night unless the temperature changes significantly. Overall, the RASA 11 is something like the “Ferrari” for astrophotography enthusiasts: high performance, requires proper handling, but immensely rewarding. Beginners can and do use them (some jump straight into the deep end!), though I usually advise getting some experience with milder instruments first because the RASA 11 will expose weaknesses in your mount, your collimation know-how, and your cable management due to its fast optics. Speaking of cables: one quirk of all RASAs (and HyperStar systems) is that the camera is up front, so any cables from the camera (USB, power, etc.) also dangle in front and will diffract light. This can cause bright stars to have spikes or slight artifacts. A neat trick is to route the cables with a gentle curve or 45-degree angle rather than straight across the corrector; this spreads out the diffraction into a mild halo instead of a single spike. The larger the camera body and cables, the more obstruction you add – another reason the 11″ benefits from being bigger than the 8″, since a given camera blocks a smaller percentage of an 11″ aperture.

RASA 14-inch (RASA 36):

Now we enter the realm of the serious observatory instrument. The RASA 36 is a 14″ aperture, 355 mm to be exact, operating at f/2.2 for a focal length around 790 mm. This monster weighs about 75 lbs (34 kg) for the optical tube and is nearly a meter long. It’s not something you’d casually set up in the backyard for an hour – it usually lives in a permanent observatory or at least takes two people to mount safely. However, it is arguably the most powerful wide-field astrograph you can buy off the shelf. The 14″ RASA was designed with scientific and industrial applications in mind, not just pretty pictures (though it produces stunning images too). With its huge aperture, it can collect photons at an incredible rate. It’s marketed as an affordable instrument for things like space surveillance and Space Situational Awareness (SSA) – in other words, monitoring satellites and orbital debris – because it can scan large swaths of sky quickly and detect faint fast-moving objects. The RASA 36 can cover a 7.5° x 5° field on a 60 mm diagonal sensor (that’s much larger than full-frame). To put that in perspective, at 790 mm focal length, 5 degrees is about 87 full moons side by side! It can literally image multiple constellations at once if paired with a very large sensor or mosaic setup. This model also has an extended spectral range, focusing light from 400 nm (blue) all the way to 900 nm (near infrared). That’s particularly useful for certain research cameras or detecting satellites that might shine in IR wavelengths, and it means even more photons (including those just outside human vision) are brought to focus. Using a RASA 14 for astrophotography is an absolute joy if you’re an experienced imager – you get the benefit of a big telescope’s light grasp plus the fast optics. For example, with a 14″ RASA, exposures of even 30 seconds under dark skies can reveal incredibly faint galaxies or nebula details that would be unthinkable in such short time on slower systems. I once saw an image taken with a RASA 14 that captured the Integrated Flux Nebula (the extremely faint dust clouds in the high galactic latitude sky) in just a couple hours, whereas traditionally that target needs nights of exposure. The catch: everything around the RASA 14 must be top-notch – the mount needs to be rock solid and accurately aligned, and you have to manage the very short depth of focus carefully. Focusing the big RASA is aided by the same USFS system (it actually debuted on the RASA 14 and then trickled down) and by mirror locks, and owners follow a trick of finishing focus adjustments in the direction against gravity (like always ending turning the focus knob counter-clockwise) so that mirror slop is taken out. The RASA 14 provides about 77 mm of backfocus, which can accommodate large filter wheels or custom camera adapters. Often these units are used with large-format monochrome CCD cameras (the kind used in professional surveys) and multi-filter setups, or even arrays of multiple cameras at once, since the front lens group is so large it can hold heavy payloads (Celestron says up to ~38 lbs of camera equipment can hang off it!). It’s a very special instrument – many of us only dream of owning one. But even if you never plan to get the 14″, it’s comforting to know the same design principles are in the smaller RASAs we use; it’s like having a miniature version of a research telescope at home.

Comparing Performance: It’s tempting to ask, “So what does a 14″ RASA show that an 8″ cannot?” Besides the obvious (fainter magnitude limit and finer resolution due to nearly double the aperture), one way to illustrate it is in exposure time. Suppose with your camera and sky an 8″ RASA can record a certain faint galaxy to a decent signal level in 10 minutes of stacking. The 11″, having ~3 times the light grasp of the 8″, might do it in about 3–4 minutes. The 14″, with about 2.5 times the area of the 11″ (and 7.7 times the 8″’s area), could snare that galaxy in perhaps 1–2 minutes at the same quality. So when it comes to grabbing something fleeting, like a fast-moving comet or an Earth-grazing satellite that doesn’t stay long in your field of view, the larger RASA gives you a better chance of recording it before it moves out or fades. For aesthetic imaging, the larger RASA simply digs deeper; in the same total exposure time, it will pull out more nebulosity, more distant background galaxies, more subtle details. For example, in my own use, the 11″ RASA reveals the outer halo of M51 or the dim outer loops of the Helix Nebula far more clearly in an hour of exposure than an 8″ RASA would in that hour. A friend of mine who had access to a 14″ RASA managed to capture the extremely faint tidal tail of the Pinwheel Galaxy (M101) in just a few hours – something that normally would be a challenging target for amateur scopes at all. These differences aren’t magic; they’re just physics: aperture wins, and aperture + speed wins big.

That said, all RASAs share the fundamental advantage of “speed” over most other designs, and any of them can transform your imaging. Choosing between them often comes down to practical matters: budget, weight, and the size of camera sensor you want to use. The RASA 8 is a superb entry to fast imaging for enthusiasts, the 11″ suits the advanced amateur wanting full-frame coverage and a permanent setup, and the 14″ is targeted at the serious observer or institution aiming to survey the sky rapidly or capture research-grade data. The 6″ carves out a niche for ultra-portability and cost-conscious beginners who still want that f/2.2 experience.

The Optics: How RASA Delivers Wide, Sharp Views

It’s worth digging a bit deeper into the optical tech that makes RASA special. From the outside, a RASA telescope looks a lot like a standard Schmidt-Cassegrain – it has that big corrector plate at the front and a shiny tube. But internally the design is unique. Light enters through the Schmidt corrector plate, which is a thin aspheric lens that corrects spherical aberration. Without it, the spherical primary mirror would produce a very blurry image; the corrector “pre-distorts” incoming light in just the right way that the mirror can then bring it to a focus. After passing the corrector, light reflects off the large primary mirror at the back of the tube. In a normal Schmidt-Cassegrain, that light would then go to a secondary mirror and back through a hole in the primary to an eyepiece. In the RASA, instead of a secondary mirror, there’s a multi-element lens group (often called the field corrector or field lens assembly) mounted in the center of the corrector plate. This lens group takes the converging light from the primary and refines it – it corrects for coma (the stretching of stars into comet-like shapes off-axis), for field curvature (so that the flat sensor can be in focus across its entire area), and for chromatic aberration. Chromatic aberration might sound odd in a mostly-mirror system (mirrors themselves don’t cause it), but once you put lenses in the path, if they’re not designed right, different colors could focus differently. The RASA’s lens group uses special “rare-earth” glass types to minimize any false color, ensuring that stars don’t have blue or red halos. The result is an image that is sharp across a wide field – typically RASA spot diagrams show star images just a few microns across even at the edge of a large sensor.

One challenge with any fast optical system is alignment. At f/2, even a tiny miscollimation can introduce aberrations. Celestron aligns each RASA at the factory, so ideally you should get it perfectly collimated out of the box. There are set screws you can adjust if absolutely needed (on the lens group for tilt, and possibly on the primary via the mirror locks if one were to tweak them), but generally owners don’t have to touch collimation unless something goes seriously out of whack during shipping. I was relieved to find my RASA 11 V2 produced symmetric round stars right away – I did check by defocusing a star and the doughnut looked concentric, indicating good alignment. If adjustment is needed, it’s a bit more involved than with a simpler Newtonian telescope, but it’s doable with patience and a star test or a collimation camera tool.

The RASA optics also incorporate Celestron’s StarBright XLT coatings on all surfaces: that means the corrector plate has a high-transmission multi-coating (it lets through as much light as possible with minimal reflection), and the mirrors have enhanced reflective coatings (aluminum with protective overcoats that boost reflectivity into the 90+% range). The net effect is high light throughput; around 97% of light that hits the corrector makes it through, and the primary reflects something like 95% of that, and then the lens group perhaps transmits another 90+%. It all multiplies, so you end up with something on the order of 80% or more of the starlight actually reaching the sensor. In comparison, older uncoated Schmidt cameras or some simpler scopes might only deliver 50-60% of the light after multiple elements. When you’re fighting for every photon, these coatings matter.

The design is not without its difficulties, though, and it’s worth acknowledging the “problems” or rather the challenges inherent in the RASA technology:

Focusing Tolerance:

The depth of focus (the range over which the image stays sharp) at f/2 is extremely small. This means you must focus very precisely. A tiny turn of the focus knob changes the focus noticeably. It’s a bit like using an f/1.4 aperture on a camera lens – the focus is razor-thin. If your telescope or camera changes temperature by a few degrees, that can slightly change the focus position (because materials expand/contract). So RASA users often refocus throughout the night as things cool. Also, many invest in motorized focusers and even auto-focus software so the computer can quickly adjust focus as needed. The V2’s stable focus mechanism mitigates the shift issue, but you still have to nail the focus itself. I’d say focusing a RASA is no harder than focusing an SCT with an f/6.3 reducer or a fast refractor, but you can’t be sloppy about it or your stars will bloat.

Dew and Front Optics:

Since RASAs have a big glass corrector plate right up front, they have the same dew issues as any Schmidt-Cassegrain or refractor. In a humid night, that plate will radiate heat to the sky and cool below ambient temperature, and dew will condense on it if you don’t prevent it. A dew shield (basically an extended tube or collar around the front) is a must-have accessory for most RASA owners in all but the driest climates. I use a flexible dew shield and also a gentle heater strap around the perimeter of the corrector when needed. The RASA 8 and others might come with a stock lens cap that can double as a bit of a shield, but usually you’ll want a longer one. It’s a minor inconvenience, but nothing new to those experienced with SCTs.

Camera Limitations:

As mentioned, not every camera is suitable for every RASA. The 8″ and 6″ are particularly limited in backfocus, so DSLRs are out for those two (unless it’s a very small mirrorless that can be adapted in <25 mm, and even then performance at the corners will suffer if full-frame). The 11″ and 14″ can take DSLRs and big pro cameras just fine, but here the limitation might be the physical size of the camera. If your camera is too large in diameter, it will start to obstruct more of the telescope’s aperture, effectively turning your 11″ into maybe a 9″ or 8″ in light-gathering area. The general guideline is to keep the camera (plus any filter drawer, adapter, etc.) as compact as possible. Mirrorless cameras with low-profile adapters work great; dedicated astro cameras are usually cylindrical and narrow, which is perfect. My full-frame DSLR (Canon body) does block a chunk of the corrector – roughly a 4″ (100 mm) circle in the center is taken by the camera body. That sounds bad (100 mm obstruction in a 279 mm scope is about 36% linear obstruction), but in practice it’s okay because even with that the RASA 11 collects vastly more light than say a 4″ refractor with no obstruction. However, the obstruction does reduce contrast a bit for very faint details. Serious imagers using RASA often prefer cooled astronomy cameras which are smaller than DSLRs and have no shutter mechanisms that cause vibrations.

Filter Use:

Imaging at f/2 changes how filters behave. If you use narrowband filters (like H-alpha or O III filters commonly used for nebulae), the bandpass of the filter can shift at fast light cones. Some standard narrowband filters are designed for f/4 or slower and may lose some efficiency at f/2, meaning they’ll block some of the light you want. The solution is to use high-speed compatible filters that are designed for f/2 systems (these are made by a few manufacturers and labeled as such). Light pollution filters and broadband filters also need to be high quality or you risk aberrations. Celestron addressed this by making their own Light Pollution Imaging Filter that replaces the RASA’s optical window – it’s a large 72 mm (or 68 mm on the 11″, 2″ on the 8″) specially-made filter that doesn’t distort the wavefront. When I shoot in my suburban backyard, I often use that filter on the RASA 11; it cuts the skyglow without introducing weird halos or focus shifts.

No Visual Use:

This is obvious but bears repeating – you cannot really use a RASA for stargazing with your eye. There’s no practical way to attach an eyepiece and even if you did, your head would block the corrector similar to how the camera would. So a RASA is a one-trick pony: astrophotography (or scientific imaging). If you also want to do visual observation, you’ll need a separate telescope. Many hobbyists thus have an observational scope (like a Dobsonian or a small refractor) alongside their RASA imaging rig, to enjoy eyeball views while the camera is busy snapping photos.

Cost and Complexity:

RASAs, especially the larger ones, are not cheap. You’re paying for a lot of precision glass and custom engineering. They’re still often cheaper than equivalent aperture premium refractors or exotic telescopes, but compared to the common “8-inch Dobsonian for $500” a RASA is on another level (the 8″ RASA OTA runs over $2k, the 11″ around $4.5k, and the 14″ is around $15k just for the tube). And because you need a decent mount and a camera to go with it, the total investment is significant. Also, the complexity means more things to manage: cables, power (especially for cooling fans and possibly the camera cooler), computer control, etc. It’s not a casual grab-and-go setup in most cases (except maybe the 6″ if you’re doing a quick EAA session).

Despite these challenges, the RASA design’s advantages usually outweigh the downsides for those of us focused on imaging. The learning curve can be steep, but no steeper than learning astrophotography in general; in fact, one could argue that starting with a fast system might keep a beginner motivated because they actually see results in a single night that might take multiple nights on a slower rig. I’ve seen newbies take amazing photos with an 8″ RASA on an Advanced VX mount and a simple one-shot-color camera, in part because the short exposures forgave some of the mount’s shortcomings and because focusing was made easier with the mask and the stable optics. Their images had that rich depth you get from lots of photons.

Photo above: My own personal RASA 11" inside the Ozark Hills Observatory

Focusing the RASA: Getting Sharp Stars at F/2

Focusing any telescope for imaging is crucial, but at f/2 it becomes something of an art (with a bit of science and tech to help). The RASA’s focusing mechanism, as mentioned, is the primary mirror moving on a threaded rod. On my RASA 11 V2, the focus knob is actually a FeatherTouch 10:1 dual-speed focuser (Celestron partnered with Starlight Instruments for that), which means I have a coarse knob and a fine knob for focusing by hand. The fine knob lets me make very tiny adjustments, which is wonderful because at f/2.2 the critical focus zone (the range in which the image stays acceptably sharp) might be only a few microns wide on the sensor. If you overshoot focus, stars will start getting bigger again.

The Ultra-Stable Focus System ensures that as I turn the knob, the mirror glides smoothly without tilting. I typically perform an initial focus by pointing at a bright star, putting a Bahtinov mask on the front of the RASA, and taking a quick exposure. The Bahtinov mask creates a diffraction pattern – three angled spikes that move as you change focus. When the central spike is perfectly centered between the other two spikes, you’re in focus. Because the RASA is so fast, the diffraction spikes from a Bahtinov mask are extremely pronounced and easy to see even in a 1-second exposure. It’s a great tool to get rough focus quickly. Then I remove the mask and often let my autofocus routine in software fine-tune the focus by analyzing star sizes across the field. The focus motor I use can step in very small increments, and thanks to the design improvements, the first focus I settle on tends to hold well.

One tip I learned (also hinted at in Celestron’s manuals for the bigger RASAs) is to always finish focusing with an upward motion of the primary mirror. This usually means turning the focus knob counter-clockwise for the last move (for Celestron SCT focusers), which pushes the mirror up against gravity, so it’s snug against the mechanism with no slack. This helps ensure the focus doesn’t shift when you reverse direction or when the scope slews around. On the RASA 36, they explicitly say do final focusing CCW to get best star images at the edge. I follow the same practice on the 11″ and 8″ and it indeed helps consistency.

Once focused, I also engage the mirror locks (on the 11″ and 14″ models) gently to secure the mirror. Those locks clamp the mirror in place so it can’t drift. On the RASA 8, there aren’t separate locks, but it doesn’t seem to need them as much; likely the smaller mirror holds better on its own, and the linear bearing does its job keeping it stable.

During a night of imaging, I’ll refocus maybe every hour or if the temperature drops by more than 2–3°C. The RASA’s aluminum tube will contract as it cools, slightly changing the focus position. Some advanced users install temperature compensating focus controllers or even fully automate focusing between exposures. Personally, I keep an eye on the first few sub-exposures of a new sequence – if I see star size creeping up, I pause and refocus. On a good night, especially after the scope is fully cooled, the focus doesn’t shift much at all. The MagLev fan running can also keep the internal air uniform which prevents focus drift due to internal turbulence. Occasionally, if I’m shooting near the zenith and then slew to a low altitude target, I check focus again. The “mirror flop” effect that plagued older SCTs is virtually non-existent in RASA V2, but a big change in orientation can still shake things a touch. With the locks and linear bearing, it’s greatly minimized.

One question people often ask is: can I use a secondary focuser (like a Crayford style) on a RASA, since it has no visual back? The answer is no, not in the usual sense, because the camera is at the front. You can’t attach an external focuser in front of the corrector; focusing must be done by moving the primary mirror. So, all the focus mechanics are internal. This makes it even more important that the internal focuser is solid (which thankfully it is in modern RASAs). It also means an electronic focus motor that turns the knob is the only practical motorized solution, and that’s exactly what Celestron provides as an option.

From a beginner’s perspective, focusing a RASA might be a new experience because you are focusing on a camera screen or a computer, not through an eyepiece. But this is actually great training for astrophotography in general. You learn to trust the feedback from the camera and perhaps the software’s metrics (HFR – half-flux radius of star images, etc.). After a few nights, you can dial in focus quickly and it becomes routine.

Best Cameras and Accessories for RASA

One of the wonderful things about the RASA telescopes is how democratizing they can be in terms of camera choice – as long as your camera fits the form factor constraints, you don’t necessarily need the absolute top-of-the-line model to get good results. Because the scope is so fast, even modest astro cameras will produce low-noise images since you stack many short exposures rather than fewer long ones.

Camera Types: The ideal cameras for RASA are usually dedicated astronomy cameras (CMOS or CCD) that have a small footprint. For example, the popular ZWO ASI series or QHY cameras, which are cylindrical and attach via T-thread or M48, work great. Many of these have APS-C or micro 4/3 sized sensors which match well with the RASA 8 and RASA 6 coverage. If you have a full-frame astro camera (like the ASI6200, 36 mm x 24 mm sensor), that pairs excellently with the RASA 11 or 14.

Mirrorless consumer cameras also work if you get the correct adapter. People have successfully used Sony A7 series, Canon R series, Nikon Z series mirrorless bodies on the RASA 8 and 11. For RASA 8, you need an adapter that basically replaces the camera’s normal lens mount adapter and gives the right 25 mm backfocus – some third-party adapters exist for Canon RF to RASA, etc. For RASA 11, you can use a standard T-ring or M48 ring (the scope comes with both). I occasionally attach my Canon EOS R (full-frame mirrorless) to the RASA 11; it works nicely and focuses at the correct distance, because the RASA 11’s included camera adapter is built to place a DSLR sensor at focus with the proper 55 mm spacing. I just have to make sure to manage the camera’s orientation so that the bulk of the body doesn’t sag or twist (the RASA’s camera adapter lets you lock the rotation).

Sensor Size Considerations: As alluded earlier, sensor size should match the RASA model for best results. On RASA 6, a 4/3 sensor (~21 mm diagonal) is ideal; APS-C (~28 mm diagonal) is pushing to the edges of what the optics fully correct, but still mostly good; full-frame (43 mm diagonal) will show noticeable aberrations in the corners and heavy vignetting – you can still use it for fun, but you’d likely crop or lose quality at the edges. On RASA 8, up to APS-C (approx 30 mm diagonal) is excellent, and some users even use certain full-frame cameras knowing they’ll crop edges or are doing work where corners aren’t critical. RASA 11 loves full-frame – it was practically built with that in mind. I get sharp stars across my full-frame images; if I pixel-peep the extreme corners, I might see a tiny elongation, but nothing that standard processing can’t handle or that detracts from the image. RASA 14 is made for beyond full-frame, handling so-called “large format” sensors (like the 52 mm diagonal CCDs). If you’re lucky enough to use a RASA 14, you probably have a scientific CMOS or CCD with that large chip and you’ll get a gigantic field.

One-Shot Color vs Monochrome: Many beginners start with one-shot color (OSC) cameras or DSLRs, which is perfectly fine on RASA. In fact, because the RASA can’t use a filter wheel easily (it would block too much and also there’s no backfocus for it on 8″ and limited on 11″), OSC is a convenient choice. You capture full-color images in one go. If you do want to do narrowband imaging (for nebulae, for instance) with a RASA, one approach is to use a one-shot color camera with a dual-band or tri-band filter that passes, say, H-alpha and O III wavelengths to different channels. That way, in a single exposure you still collect two narrow bands. This works well with RASA’s fast optics if you get filters designed for fast beams. I’ve used an L-eXtreme filter (dual band) on my color camera + RASA 8 to capture the red hydrogen and blue-green oxygen nebulosity in the North America Nebula in just a couple hours from my light-polluted yard – something that would be tough without a filter.

Monochrome cameras can also be used on RASA, but since you can’t easily put a filter wheel at the front, you have to swap filters manually or build a custom slider. Some RASA imagers have made or bought 3D-printed filter holders that allow them to change filters (like R, G, B, Hα, O III) one at a time during a session. It’s more hassle because you must refocus slightly after filter changes (different wavelengths can focus differently), but it can be done. Monochrome sensors will make the absolute most of the RASA’s photon torrent, since they don’t waste any light via Bayer color filters. If you’re doing scientific work (like searching for supernovae or tracking variable stars), a mono camera on RASA is superb.

Other Accessories: There are a few notable accessories that RASA users often consider:

A quality mount – not an accessory per se, but worth noting that to exploit short exposures you don’t need the world’s best mount, but you do need one that can carry the weight and track decently for at least 30-60 seconds unguided. Many people with RASA 8 use an ZWO AM5, Celestron AVX or SkyWatcher EQ6 class mount and do short unguided subs with great success. The wide field of RASA actually hides minor tracking errors; a small periodic error might smear a star a tiny bit, but if the image scale is like 3 arcseconds per pixel, that might not be noticeable. With a long focal length scope, that same error would ruin a sub. So RASA is relatively forgiving to mounts, but at the same time, once you load cameras, dew shields, etc., make sure your mount is rated for more than the total weight.

Dew shield / dew heater – essential to prevent that corrector plate from fogging. I never run a RASA without at least the shield in place, and I turn on the heater strip preemptively if the night is damp.

Power – you’ll need to power the cooling fan (12V DC). It’s a small draw, but don’t forget to plug it in. If you use a cooled astro camera, that’s another power draw. Cable management at the front can be tricky; I run my camera USB and power, plus the dew heater cable, together along the dovetail or out to the side in a gentle loop to minimize diffraction issues. Velcro and zip ties are your friends to tame the cable spaghetti.

Filter drawer – on RASA 8 and 6, a filter drawer is usually included or available, letting you slot in one 2″ filter at a time in place of the optical window. On RASA 11, since there’s more backfocus, some have used slim filter drawers between the camera and adapter (but any added distance must be carefully accounted for in the 55 mm backfocus – a thin drawer of 5 mm can work if you adjust spacers). The RASA 14 has the luxury of 77.5 mm backfocus and large clear aperture, so you could even use a full frame filter wheel if you custom mount it (some professional setups do). For most of us, if doing filters, a simple drawer to swap a filter at a time is adequate.

Field flatteners / reducers – none needed! Unlike some telescopes that require an extra field flattener lens for imaging, the RASA’s built-in lens group does that job. And forget reducers – you’re already at f/2-ish, you can’t practically reduce that further (and wouldn’t want to as it would mess up focus and correction). Sometimes people ask if they can use a Barlow or extender to make RASA into a longer focal length instrument for planets; technically you might stick something like a 2x in front of the camera sensor and get f/4, but that’s not what RASA is made for. If you want to do planets or small objects, it’s better to use a different scope altogether.

Guiding: Because RASA often does short exposures, many users often skip guiding or do minimal guiding. However, if you plan to stack lots of one or two + minute exposures, having a guide scope can still improve your star sharpness if your mount drifts. Mounting a guide scope on a RASA can be done via the top dovetail rail. Just be mindful that at f/2, even a hint of misfocus or wind can degrade an image, so extra weight on the tube should be balanced well. Also, if doing extremely short exposures (like live stacking of 5-second or 10-second subs for near-real-time viewing), guiding is unnecessary. The RASA sort of enables an intersection of astrophotography and observational astronomy called EAA (Electronically Assisted Astronomy) where you observe via the accumulation of images on the screen. A beginner with a RASA can get into EAA and literally see, say, the spiral arms of a galaxy materialize after a few tens of seconds of stacking – something impossible through an eyepiece on a traditional scope except at large aperture under pristine dark skies.

Beyond Pretty Pictures: What Else is RASA Used For?

We’ve touched on it a bit, but RASA isn’t only for taking beautiful images (though that’s certainly my main use!). Its combination of wide field and fast exposure ability makes it ideal for any application where you need to survey or monitor the sky.

One such application is satellite and space debris tracking. If you’re trying to spot or monitor satellites in orbit, especially in higher orbits, you often need a wide field to catch them and a fast system to see the faint ones quickly. RASA 14s have been deployed in systems that watch for “space junk” – discarded rocket parts, defunct satellites, etc. – that could threaten active spacecraft. They can scan large areas of sky and detect faint moving points of light in short exposures. The fast f/2.2 optics allow detection of objects maybe a magnitude or two fainter in the same time than a slower f/4 system would, which is a big deal if you’re trying to catalog tiny pieces of debris.

RASA scopes are also useful for near-Earth asteroid searches and transient events. The name of the game there is covering lots of sky fast. A cluster of RASA 8s or 11s with sensitive cameras could be set up to patrol the sky looking for things that change – like the sudden appearance of a new supernova in a galaxy, or an asteroid streaking by. Amateur astronomers do contribute to these searches. A RASA’s wide field means you might catch several potential targets in one shot. For instance, an 11″ RASA image might include dozens of distant galaxies; if one of them has a supernova, your data could reveal it without specifically pointing at that galaxy alone.

Another interesting use is comet imaging and discovery. Comets often have large diffuse coma and tails that benefit from a wide field. With a RASA, an amateur can take deep images of comets quickly, monitoring how they develop night by night. If a new comet is in the sky, a RASA can help find a very faint tail or gas cloud around it that slower scopes might miss until the comet brightens.

I also mentioned EAA (Electronically Assisted Astronomy) – while this is not a separate scientific application, it’s a way to use the RASA for outreach or personal observing in quasi real-time. I’ve taken my RASA 8 to public star parties, hooked up a sensitive video astronomy camera, and we were able to show visitors colorful images of the Orion Nebula and even the Whirlpool Galaxy live on a screen, updating every few seconds. It’s a jaw-dropper for the public, who normally would only see a faint smudge through an eyepiece. The RASA essentially turns the night sky into a dynamic canvas you can explore with a screen. It’s like having the power of a big observatory but condensed into near-live feedback.

For scientific photometry and variable star observation, one might not think of using such a fast wide scope, but it can be useful if you want to monitor many stars at once. For example, capturing an entire star cluster with a RASA and then analyzing brightness of numerous variable stars simultaneously is feasible. The short exposures also mean bright stars won't saturate as easily, giving a good range for photometric measurements.

Finally, RASA has been recognized as a tool in professional-looking astrophotography that’s accessible to advanced amateurs. Photographers like Trevor Jones of AstroBackyard (whom the style inspiration mentioned) have used these kinds of fast scopes to great effect, capturing broad mosaic images of the Milky Way’s deep-sky objects that look like they came from a far more expensive setup. It enables creative projects: wide, high-resolution mosaics, time-lapse imaging of deep-sky objects over a single night, etc., that simply wouldn’t be practical with slower gear.

Why RASA for Beginners and Experts Alike?

When considering a telescope, beginners are often told to start with something like an 80 mm refractor or a simple Newtonian – those are indeed more general-purpose and forgiving instruments. But I’d argue a RASA (particularly the 8″ or 6″) can be a fantastic first astrophotography telescope if the user is committed to doing imaging (and doesn’t mind that it can’t be used visually). Why? Because success in astrophotography is all about collecting enough good-quality data. The RASA tilts the odds in your favor by collecting light so quickly. A newbie with a modest mount and basic polar alignment can get away with 30-second exposures and still build a gorgeous image after stacking hundreds of them. Those short exposures mean even if the polar alignment or tracking isn’t perfect, the stars won’t trail much. It means you can see the progress as you go – every subframe shows the object clearly, boosting confidence and excitement. There’s less frustration with “I took a 5-minute shot and still hardly see anything, is it working?” which is a common hurdle in the beginning. Instead, a 30-second shot on a RASA will show you the galaxy or nebula quite obviously. That immediate feedback is motivating and educational.

The RASA also simplifies some choices: you don’t worry about field flatteners, focal reducers, or other optical add-ons – just put the camera on and go. With a decent one-shot color camera, the workflow is straightforward. Yes, you have to manage focusing and a computer from the get-go, but almost all astrophotography requires that anyway.

For an expert, the appeal of RASA is maximizing productivity and pushing boundaries. An experienced imager might have a permanent observatory and want to gather a library of targets quickly – a RASA can allow capturing multiple objects in one night with lots of detail, rather than one object over multiple nights. The wide field means an expert could create large mosaics efficiently. For instance, imagine an advanced astrophotographer wanting to image the entire Cygnus region (which spans tens of degrees) in high resolution. Using a slower scope, they might mosaic dozens of panels, each needing hours. With a RASA, each panel might need only a fraction of the time, or they can use a larger sensor to cover more area per frame.

Experts also appreciate that the RASA’s quality is at a level where it’s not the bottleneck – meaning if their image has issues, it’s likely due to conditions or user error, not the telescope’s inherent aberrations. The design’s been proven to deliver near professional-grade results (in fact, some images taken with RASAs have been published in magazines and even used in research contexts). There’s a reason it made “Hot Product” lists in astronomy publications: it took what used to require a custom or very expensive instrument and made it relatively attainable.

The combination of approachability (point it and shoot, see results fast) and performance (f/2.2 11-inch is nothing to scoff at) truly spans the spectrum of users. I often liken the RASA to a sports car that somehow is as easy to drive as a family sedan – a beginner can start the engine and cruise, but a pro can push it to the limits on the track.

Is There Anything Faster? Pushing Beyond f/2.2

Given how much I’ve sung the praises of f/2, a natural question arises: can we go even faster? Is an f/1 telescope possible for astrophotography? In theory, yes, but in practice there are huge challenges. Historically, a few telescopes have reached around f/1: Schmidt cameras for film (like some Schmidt telescopes in observatories) and special mirrors like the Baker-Nunn cameras (satellite tracking cameras in the 1950s that were f/1). Those systems had extremely curved focal planes or were built for specific purposes. For a flat digital sensor, going much below f/2 requires extremely strong and precise lens corrections and usually results in a very small useful image circle.

In the commercial space, Celestron’s RASA is already among the fastest. The only similarly fast option is the HyperStar on certain SCTs (for example, a HyperStar on a C14 runs at about f/1.9). That shaves a bit off f/2.2, but not dramatically. You might get 20% shorter exposures, at the cost of the complexity and edge performance issues of the add-on. Some fast Newtonians exist (like f/3 astrographs from specialized companies) and some refractor-Petzval designs at f/3.9 or f/2.8 (e.g., Takahashi Epsilon at f/2.8), but none reach f/2.

One interesting avenue is the use of multiple telescopes simultaneously – not a lower f-ratio per scope, but you can effectively double speed by using two identical scopes and cameras in parallel on the same target, then combining images. This is a trick some advanced imagers do: two RASA 8s side-by-side can gather twice the photons in a given time (like having one scope that’s √2 faster in f-ratio terms). Of course, that doubles cost and complexity too, but it’s a clever way to “cheat” the exposure time down. Some research surveys use arrays of multiple fast astrographs to increase coverage and depth.

Could an amateur build an f/1.5 astrograph? Possibly, with a custom optical design and a very small sensor, but it would likely have such a narrow good field that it wouldn’t be broadly useful. The RASA designs are a sweet spot balancing speed with image quality and sensor size.

I find that f/2 is already an aggressive realm: focusing and filtering become exponentially harder as you go faster. For example, the reason Starizona’s HyperStar settles around f/2 is that below that, the angle of incoming light gets so steep that common filters act weird and sensors may not catch the light uniformly (pixels have depth, and very oblique rays can miss the photodiodes). So, f/2.2 is a practical floor given current technology and sensor designs.

One could also consider aperture as another path: an f/3 telescope that’s 1 meter (40 inches) aperture collects way more light than an f/2 8″, simply due to area. But those are one-of-a-kind or extremely expensive scopes. Projects like the Vera Rubin Observatory (for professional astronomy) will have an 8.4 m f/1.2 (!) system – but that’s a $600 million instrument! For us in the backyard, the RASA hits the limits of what’s feasible to mass-produce affordably.

So, practically speaking, if you’re asking “Can I get an even lower f-stop than RASA’s f/2.2?” the answer is: not really, not without caveats. You can attach a camera lens – say a 200 mm f/2 Nikon lens – and that’s technically faster (f/2) and will give you wide images too, but it’s only 200 mm aperture divided by 2, which is 100 mm aperture. That gathers far less total light than an 8″ (200 mm diameter) mirror at f/2, because focal ratio doesn’t tell the whole story for extended objects: you need both aperture and speed. The RASA marries a large aperture with fast speed. A camera lens has fast speed but small aperture, so for extended deep-sky objects a RASA still massively outperforms camera lenses in capturing faint detail.

In summary, RASA owners can rest easy knowing they have about the fastest tool available for deep-sky imaging. Any marginal gain from going “faster” would come with disproportionate headaches. If one’s craving more speed, the answer is usually “go bigger aperture RASA” or “use multiple scopes” rather than trying to push f-ratio much lower. RASA has essentially set the bar in the amateur realm for fast optics.

Astrophotography Made Approachable (and Powerful)

Sitting here under the stars next to my RASA 11″ V2, I sometimes think of how Mark Twain might describe the experience if he were an astrophotographer. Perhaps he’d quip something like: “You spend a lot of nights feeling cold, looking at faint smudges, until one day you catch a fast, bright break and the heavens reward your patience.” The RASA feels like that fast, bright break. It condenses those cold nights into a more manageable bite, giving you more reward for your patience and time. It doesn’t perform magic – you still have to deal with polar alignment, calibration frames, processing the images, and so forth – but it accelerates the learning curve. It’s a bit like having Carl Sagan’s cosmic insight delivered in a plainspoken, common-sense way (to channel the mix of personas the question suggested). There’s nothing mystical about why RASA works so well; it’s just solid optical engineering allowing us to collect gobs of starlight quickly. Yet the result feels magical when you see your first image pop out.

For me as a citizen scientist and astrophotographer, the RASA has been transformative. It made it practical to attempt projects I wouldn’t have before. If I want to capture a faint nebula that covers 5 degrees of sky, I can do it in one frame with a RASA 8 or 6. If I want to see detail in a galaxy cluster, I can gather enough light in one night with the 11″ that I don’t need a week’s worth of imaging. It’s no wonder these scopes are often on backorder or have waiting lists – they opened up a niche that many people didn’t even realize they wanted until it existed.

If you’re a newbie considering a RASA as “a good choice,” the key things to know are: you’ll need to invest in a decent mount and a compatible camera, and you’ll be doing imaging from day one (which means using a laptop or similar). If that excites you more than it intimidates you, a RASA could indeed be a fantastic choice. You’ll be able to produce gallery-worthy images of big, beautiful deep-sky objects without having to spend countless nights gathering data. And the experience of near-instant feedback will teach you a lot about the night sky as you hop from nebula to nebula in a single session.

If you’re an expert or have already gone through the school of hard knocks with slower systems, a RASA feels like the ultimate tool. It’s like upgrading to a top-tier power tool after using hand tools – suddenly your throughput doubles or triples. Many advanced imagers add a RASA to their arsenal not to replace their long focal length scopes, but to complement them. It’s the difference between a wide watercolor and a fine pencil sketch; sometimes you want one, sometimes the other. With a RASA, you gain the ability to paint the Milky Way’s canvas quickly and brilliantly.

In the end, the Rowe-Ackermann Schmidt Astrograph is more than just a piece of hardware. It represents an evolution in how accessible deep-sky astrophotography has become. The universe has countless wonders, and RASA telescopes help us capture those wonders faster – which means we can explore more of them in the limited time we have under clear dark skies. Whether you’re tracking space junk, hunting for supernovae, or just trying to make the prettiest picture of the Andromeda Galaxy on your block, the RASA’s speed and optics give you an edge. And as someone who is deeply passionate about the night sky, I find that anything that gets me from the moment of setting up to the moment of saying “Wow, look at that detail!” in less time is a welcome innovation.

RASA 11 - The Power of Astroimaging at f/2