Fast telescope vs slow telescope: The Space Koala

One debate never seems to end in astrophotography: are faster, low f-ratio telescopes really better than slower, long focal length telescopes? Luca (SpaceKoala), explores this question with both theory and real-world examples, revisiting core concepts like focal length, field of view, and camera sensor characteristics, showing how they work together to shape the final image. This emphasizes that comparing only telescopes,or only cameras doesn’t make much sense, you have to evaluate the full system as a whole. The most important question isn’t which telescope is faster on paper, but how long it takes each setup (scope plus camera) to achieve the same final image with matching resolution and field of view.

Fast telescope vs slow telescope: Image framing + pixels

To make fair comparisons, you need to account for image framing and pixel resolution. Without matching those, it’s like comparing two completely different shots, one may show a wide area of sky, the other may show more fine detail, but they aren’t equivalent. Several important techniques used to align images across setups: binning (combining pixels to improve signal-to-noise), drizzling (reconstructing higher resolution from dithered exposures), cropping (cutting down field of view), and mosaicing (stitching multiple panels together for a wide view). These processing methods let astrophotographers work around the differences between telescopes and sensors.

Binning improves signal but reduces resolution, making it useful only when fine detail isn’t critical. Drizzling, on the other hand, increases resolution but requires careful data collection using dithering. Cropping and mosaicing are more straightforward but come with their own challenges, mosaicing, for example, lowers efficiency because of the need to overlap panels.

A big focus is on understanding the real meaning of focal ratio, or f-number. While it’s often used as shorthand for telescope speed, it can be misleading if you don’t account for central obstructions in reflectors. Luca introduces the idea of an “effective focal ratio” that takes this into account. For instance, a Celestron RASA 8, advertised at f/2, is closer to f/2.2 when you consider the obstruction. The PlaneWave Delta Rho, advertised at f/3, ends up at about f/3.6 in real terms. This more accurate figure helps astrophotographers better judge actual light-gathering ability.

From theory to practice, here are three example comparisons. In one, two telescopes with the same focal length but different apertures, 200 mm f/4 versus 100 mm f/8, show that the faster scope produces a brighter image. To match the brightness using the slower scope, you need four times the exposure time. In another example, two telescopes with the same aperture but different focal lengths (100 mm f/4 versus 100 mm f/8) show that while the faster scope covers a wider field, the slower one delivers more detail. To match that wider field with the slower scope, you need mosaicing and binning; to match the detail with the faster scope, you need cropping and drizzling, and both come with time costs.

In a third case, comparing two telescopes with the same focal ratio (f/4) but different sizes: 100 mm aperture with 400 mm focal length versus 200 mm aperture with 800 mm focal length. Despite having the same f-ratio, the larger scope wins on detail because of its size. To match its performance, the smaller scope needs significantly more exposure time. This example breaks the common assumption that focal ratio alone determines speed or efficiency.

To help navigate these trade-offs, she has built a free telescope comparison calculator on his website, thespacekoala.com. It allows users to input the specs of two telescope-camera systems and compare their light-gathering efficiency under realistic conditions, including the losses caused by panel overlap in mosaics. This tool takes the math off your plate and helps astrophotographers see clearly how two systems stack up.

One of the most surprising comparisons is between the Celestron C14 at f/11 and the RedCat 51 at f/4.9, both using full-frame sensors. Even though the C14 is much slower on paper, its massive aperture allows it to outperform the faster RedCat in many mosaic setups. When compared against the RASA 8 at f/2, the C14 still holds its own in certain scenarios, though the RASA’s combination of speed and aperture lets it lead in most use cases.

Things get more complicated when small-sensor, small-pixel cameras enter the equation. Some astrophotographers try to compensate for a fast scope’s wide field by pairing it with a small planetary camera, like the IMX715, to “zoom in.” But this shows this actually works against the photographer. Tiny pixels divide incoming photons into smaller buckets, lowering the signal per pixel, and a small sensor effectively crops the image circle, wasting much of the telescope’s collected light. In some cases, a slower telescope with a full-frame sensor outperforms a faster astrograph paired with a small sensor, delivering better results across most fields of view.

The overall message is clear: fast astrographs are powerful tools, but they come with trade-offs that need to be understood. They typically support smaller sensors due to their limited illuminated circle, they’re more sensitive to focus, tilt, and collimation, and they require specialized filters. Trying to regain detail by using tiny sensors cancels out many of their advantages, leaving photographers to deal with the system’s weaknesses without benefiting from its speed.

So which telescope is “best”? This reminds viewers that the answer depends entirely on your goals. If you want to shoot wide-field nebulae, a fast astrograph with the right camera is a great choice. If you’re after detailed planetary or galaxy images, a slower, long-focal-length telescope with a large aperture is the way to go. And more often than not, the best telescope is the one you already have, as long as you understand how to use it effectively, with the right camera pairing and processing strategies.