“Petzval refractors offer the best image quality of any optical system”
I tested the incumbent King; William Optics v3 RedCat 51 WIFD and two new kids on the block; Askar SQA55 and Touptek’s Hope D60
WINNER
Askar SQA55
Although this little refractor looks like a camera lens it performance ranks it top of the class. It is simply the best small refractor I have used. Its also one of the few sub £1k telescopes with a full frame flat field (and I’m getting one!).
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Why Petzvals? A good Petzval is sharp, apochromatic, meaning it focuses red, green, and blue light in exactly the same spot just like a Newtonian and crucially, unlike a Newtonian, they naturally provide a flat field—hopefully even across a massive full-frame sensor.
“Hold on biscuit! “, I hear you say, “Ritchey–Chrétien telescopes are also apochromatic and flat field and they are bigger, so they gather more light, so surely they offer an even better optical system.”
Well if you need to be more zoomed in then yes RC’s are good but their central obstruction reduces their contrast and their focal ratio is normally about F8 as compared with these petzvals which are faster than f5 and so deliver a signal that is 3 times stronger. The downside of pezvals is that to make a big one would cost the earth – even televue’s 127mm one costs nearly 8K but a new breed of tidly little ones is now on the market and they all cost less than a grand… so if like me you think widefield shots of the milky way are most interesting then you’ll be wanting to know which one of these Petzvals you’ll need to tell Father Christmas about!
I love how widefield shots of the milky way with these small petzvals shows how different nebula are connected in our galaxy.
The Contenders
William Optics RedCat 51: This is the stalwart of the wide-field astrophotography world. It’s an f/4.9 scope where the actual lens moves internally to achieve focus, keeping the imaging train perfectly static. Being the slowest of our test bunch, you would naturally expect it to perform extremely well optically.
Touptek Hope D60: This beast is the heaviest and the fastest of them all at f/4.6. (Apparently it isn’t strictly a Petzval, but I don’t care!). The danger with ultra-fast telescopes is that the lens elements have to be lined up perfectly; any slight error will introduce tilt or aberrations in the optical dungeon.
Askar SQA 55: The new kid on the block. It is slightly fatter and slightly faster at f/4.6, looking and feeling very much like a high-end camera lens. It even has an adjustable diaphragm inside to change the focal ratio. Could this actually hold a flat field for a full-frame sensor?
Why the RedCat Fell Short
The William Optics RedCat 51 is a genuine apochromat that produces remarkably sharp center stars, but it ultimately fell short when it came to maintaining a perfectly flat field. The simplest way to test for this is by shooting a star field and thoroughly checking the corners. During a test where I shot some galaxies from my mum’s back garden, the corners of the RedCat’s images were not brilliant. This was especially disappointing considering I was only using an APC-sized sensor, rather than a demanding full-frame sensor. When you examine the wide galaxy fields in redcat wide.tiff and compare them to askar wide.tiff, it is clear that the Askar maintains a significantly flatter field than the RedCat.
Why Touptek’s Hope D60 Fell Short
The Hope D60 had a bit of a rocky journey in this shootout. Because it is the fastest optical system in the lineup, the internal lens elements need to be lined up absolutely perfectly. My initial testing in the optical dungeon revealed that the first model had a slight collimation error, meaning it struggled with sharpness and showed some chromatic aberration on bright stars. Touptek did send me an updated, beautifully collimated V2 for the Exmoor shootout, but it still ran into problems under dark skies. While shooting the Veil Nebula the veil was less sharp than the askar and the stars in stwo of the corners were awful. The issue with the corner stars is either the draw tube mechanism it uses for focusing, which could introduce tilt into the optical train (which in theory you can correct for) or there is some deeper underlying issue with the optics. Now I am a wise old biscuit I know that trying to dick around with these telescopes can be a colossal waste of time and a true winner like the Askar will just work!
The Verdict
The Askar SQA 55 is absolutely in the top spot. Because it is apochromatic and a true flat field, I don’t even mind manually focusing it—it doesn’t seem overly affected by temperature shifts anyway.
Just like the Takahashi Baby Q was the legendary scope of the 2010s, the Askar SQA 55 is the 2020’s legendary small refractor as far as I’m concerned. Combined with a modern full-frame sensor, this is serious, serious astrophotography. If you want the ultimate budget Petzval, this is really the business.
My favourite full frame sensor
ZWO asi6200mm Pro
The reason I purchased one of these more expensive full frame cameras is that it gives me the resolution I need to produce an enormous A0 sized print without having to do mosaicing. And there are other reasons too. Not only is this particular camera terrifically sensitive it also has an extremely low read noise.Of course if you need a telescope that has a full frame flat field – like this Askar – to make the most of it.
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