Does telescope size really matter

Who could have imagined that optics the size of a standard finder scope would prove to be so valuable in astrophotography? But the Redcat 51 from William Optics, despite its compact 2-inch aperture, has earned a reputation for delivering impressive results on just about any object you point it at in the night sky. As a refractor telescope with a focal length of just 250mm, it is designed for wide-field imaging, making it ideal for capturing large nebulae, star fields, and other deep-sky objects. Its apochromatic optics ensure sharp, color-accurate images, minimizing chromatic aberration - a common issue in smaller scopes. Paired with a high-quality camera, the Redcat 51 showcases its versatility by producing stunning, highly detailed images of the night sky, proving that even a small, portable telescope can compete with much larger systems when it comes to astrophotography.

Does telescope size really matter?

Astrophotographer and ScopeTrader contributor Kayali Ogetay of NASA APOD recently sat down with us to showcase a unique approach to capturing stunning images, all taken with his compact 2" refractor (Redcat 51) in remote Bortle 1 skies. His work challenges the expectations typically associated with long focal length telescopes, demonstrating that even with a smaller instrument, one can achieve extraordinary results. His images push the boundaries of what’s possible with such equipment, blending artistic framing with technical precision.

The William Optics Redcat 51 specs

4-element Petzval refractor telescope
250mm focal length, f/4.9 focal ratio
Premium quality FPL-53 glass

ScopeTrader: How do small wide-field refractors contribute to capturing unique images, especially when compared to larger telescopes in terms of field of view and detail? And a follow-up question – has technology now come far enough with optical clarity and camera sensitivity that size doesn’t matter?

Ogetay: You take an image and realize it'd be nice if the field of view is a bit larger so you could fit the object better, right? This happened to all of us at some point. Or even fit another nearby object so now you have a cosmic couple! But here's the thing, we all think about this at some point, no matter what our field of view is. A large FOV gives me a good idea about the surrounding objects. We mostly do not realize the faint, overlooked details around the famous targets. Redcat's 250mm focal length gives me that chance. And this saves you a lot of time because you don't have to do a mosaic and double the time needed! We all know time efficiency is the key to astrophotography. One might worry about the details, but I'll tell you this. Due to undersampling, I do 2x and even sometimes 3x drizzling. Folks didn't think it'd make such a difference at first, but when I showed the difference everyone was surprised. So if I ever need to shoot something deeper than this, I'd aim for at least around 1000mm. No reason for me to switch to 500mm unless I'm decreasing my F ratio without sacrificing the quality.

I think we are at a point where technology is bottlenecked by other factors, especially light pollution. Being under dark skies makes much more difference than upgrading your equipment in a light-polluted city. No amount of money will beat the light pollution. Especially if you are chasing really faint stuff, you even start worrying about internal reflections and the coating used on the lens. In that sense, F-ratio (or effective F-ratio for multiple telescope setups) is what matters. I think nowadays, trying to reduce the F-ratio without sacrificing the optical quality is the ultimate goal. Unless I'm trying to shoot really deep targets like distant galaxies or small planetary nebula, I'm not interested in longer focal lengths (and of course solar system objects).

ScopeTrader: What advantages do wide-field refractors offer in terms of image quality, especially when capturing large celestial structures like nebulae or the Milky Way?

Ogetay: We have now more astrophotographers who discover new nebula! As a professional astrophysicist and someone in the astrophoto community for 15 years, this is what I have always dreamed of. Because I know what astrophotographers can achieve. Some of the discovered supernova remnants are many full moon-sized objects. If you only fit 20% of it on your FOV, then you may not even realize it and think of it as some filament. And if you decide to do mosaic... Remember a single frame sometimes needs hundreds of hours to reveal those details. In such cases, fitting the target in one frame is a dream. So small FOVs give you quite a time efficiency. Not to mention, sometimes I can shoot two (or even more) targets at once!

The details you'll get depend on the target. Some targets are diffuse without any distinct details, some are really sharp-edged. Most of the time the concern is that you can not get enough details with wide-field setups. This is not correct unless the target is really really tiny. When undersampled images are drizzled correctly, you achieve similar details that one gets with a much longer focal length. I generally do this for targets that I can reveal more details and works like charm, especially with a mono camera.

ScopeTrader: In your experience with APOD, have you found that small refractors are more popular among astrophotographers due to their portability and ease of use? If so, how does this affect the diversity of images submitted?

Ogetay: This is a great and interesting question. APOD, on the contrary to astrophotography contests, is a public outreach platform. What matters first is what APOD can tell about the image. During meteor showers, a wide-field landscape image achieves this, or sometimes a longer focal length telescope shooting a target luckily photobombed by a meteor! That tells a story. During comets like today's Comet Tsuchinshan-ATLAS, wide-field telescopes are more practical. For some interesting, rarely imaged deep objects, much longer focal length scopes have to be preferred. APOD does not favor one over the other, but some options such as easier-to-carry short focal length systems offer astrophotographers practicality. I believe this enables many astrophotographers to take more pictures and share them with others. Because for any kind of problem, practicality is a major part of the solution.

ScopeTrader: Are there specific challenges or limitations in processing images taken with small refractors, and how does APOD’s review process account for these when selecting images?

Ogetay: Honestly, with Redcat 51's 250mm focal length and an APS-C-sized sensor (2600MM), I do not experience a particular challenge. The only limitation is really small objects, by really small, I mean a fraction of the image. If it is only 20 pixels wide in your frame, there is nothing much you can do. Just go for longer focal lengths. That being said, I just checked a small galaxy appearing on the background of one of my images which is about 60x70 pixels wide. And with x3 drizzling I got to see some details of the galaxy! Of course, it is nothing like it is at 2000mm focal length, but considering the outcome, it is unbelievable to get this with only 250mm! For APOD, the ultimate result is what matters. If you have a good image with a good story, doesn't matter what you use!

ScopeTrader: How do small refractors perform under different sky conditions (e.g., light pollution, seeing conditions) compared to larger telescopes, and how does this impact their use for APOD-worthy submissions?

Ogetay: My setup is at Starfront Observatory in Texas which has Bortle 1 skies. It is much easier to see the difference in such a dark sky because when there is a full moon out there, you get to see the difference between your images taken throughout the same month.  And that difference... Is quite a lot. In terms of seeing, bigger scopes have much more trouble of course. Every time there is a bad wind or bad seeing, their tracking struggles, meantime small scope owners like me chill at 0.35" --which even 2.5" would be ok! When bigger scope guys post their guiding plots, we can't help ourselves and post ours like there is no wind or bad seeing. Sorry big guys! On the APOD side, we don't generally see this effect. Because either the photographer takes care of those frames or gets fixed in postprocessing. And to be honest, we do not pixel peep because it is not an astrophotography contest.

ScopeTrader: What’s your favorite object to shoot, and why?

Ogetay: I enjoy discovering. Curiosity is what drives me. Although I take pictures of common objects too, I try to find the ones that are overlooked. Always try to find something new. You know even if it is known, it might be still unknown to me! That's why I sometimes browse through my images, zoom in and out to check out every teeny tiny galaxy in the background. Or I think about how I can come up with something novel on a common object. Change the composition, find a different orientation, focus on different parts... This helps me to challenge myself too, and makes it more fun! Anything that fits this purpose is a good target for me. But thanks to the dark skies that Starfront offers, I became more addicted to faint dust. Currently, I spend the New Moon days shooting broadband targets.

ScopeTrader: What’s your favorite/most impactful post-processing tool to use, and why?

Ogetay: I mostly do the preprocessing and initial postprocessing on PixInsight. I quite like it and have been using it for many years now, it is hard to replace it. However, I have to say the user interface and user experience have to be improved. I see folks using APP and getting tempted by it every time I get annoyed by PI. But then I use Photoshop as well. Usually for the final steps. But there is one thing that I always use, which is, as everyone might guess, BlurXTerminator. Nothing replaced this deconvolution tool so far. If you don't want to pay for BlurXTerminator, SetiAstro recently released CosmicClarity tool which I believe doing fairly a great job compared to BlurXT. Especially considering it is free.