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FRA400 with small pixel Full Frame camera?

Askar FRA400 Refractors Buying Advice
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GTom avatar
GTom avatar
I am thinking on adding a grab and go scope, besides the William GT71, the FRA400 is the other candidate.
However, the expectations are high, I need great performance in the corners with small pixel, large sensor cameras also when using the reducer!

How does the FRA400 fare in this league?
Tried to find the glass they used, no info - that didn't make me optimistic.
jmarinotero avatar
jmarinotero avatar
I think the type of glass used is largely irrelevant to this topic. A scope can be a turd even with FPL-53 glass, it depends on how it's manufactured. Askar FRA series are OK, I wouldn't say stellar but very good for the price. 

I own this scope, and have tried it on full frame. Being a Petzval design, at native focal length you just have to put the camera, reach focus, and you are already at optimum distance. The stars in the corners are not round. You can see a couple of examples I took a while ago below; the corners are not great and probably not to your expectations, but I will say that these images would be processed very differently today and with BlurX they might clean up nicely (you can zoom in on these images to full resolution). I should go back and reprocess some of these older images

https://www.astrobin.com/k9qxuh/

https://www.astrobin.com/x2xecr/


I don't think this scope with the reducer will produce the results you want on a full frame sensor with small pixels, and it might be too much even for BlurX. I haven't tried though. Interestingly enough the FRA400 w/reducer + APS-C sensor is roughly equivalent in FOV to the FRA400 + Full Frame sensor (the latter having much better resolution but being a bit slower f/ratio)

Here is an example with the reducer and APS-C sensor. Unfortunately all of these have been taken under very light polluted skies, so I don't have many examples with LRGB filters or OSC camera, which is more of a torture test for optics and CA

https://www.astrobin.com/1vi9bh/B/
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Tralfi avatar
Tralfi avatar
Hi GTom,
here is a exsmple with 6my pixel full-frame: https://www.astrobin.com/y5ehhw/?q=Tralfi

The stars in the corners are elongated but for wide field very usable.

The scope produces also ring reflections from bright stars outside the field of view which needs to be treated in post.

CS
Tralfi
robonrome avatar
robonrome avatar
I have the scope and reducer. I found the amount of chromatic abberation and star shape towards edge of frame objectionable even with an apsc sensor. No way would I consider this suitable for full frame. Even without the reducer edge of frame stars and CA are far from perfect
Denis Peter avatar
Denis Peter avatar
I use my FRA400 with an APS-C sized sensor OSC camera (IMX571, 3.76 μm[b] [/b]pixels), both with and without a reducer.

When used with the reducer, even on an APS-C sized sensor, there is very noticeable chromatic aberration and some slight star shape distortion towards the corners and along far left/right edges. When using just a UV/IR Cut filter, BlurXterminator mostly has no problems fixing the stars, so I would not consider it too much of an issue.

It's different when using a dual-narrowband Ha+OIII filter. In that case I get clear separation of red and cyan signal of individual stars, separated enough that BlurXterminator by default tends to 'correct' red and cyan shapes separately, resulting in double stars. It can be fixed though, it just takes a couple extra steps - splitting RGB channels, running BlurX in Correct Only mode on all channels, star aligning the separated channels and then recombining them. This process mostly corrects chromatic aberration, it just leaves a little residual colour noise around the brighter stars. The distortion of star shapes on the other hand is not an issue, BlurX handles that without problems in either case.

Without the reducer I've found the performance much better, while there's still some chromatic aberration, it is much better then with the reducer.

Considering my experience with an APS-C sensor, I definitely wouldn't recommend this particular scope for use with a reducer and a full frame OSC camera. The chromatic aberration can be worked around as described above, but it would definitely be nicer to not have to deal with it in the first place.
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GTom avatar
GTom avatar
Many thanks for all the feedback! Seem to reflect what I found when browsing processed results here on astrobin: I had that "YMMV" feeling. Surprised to see CA at times! By far not as bad as the early Sharpstar 61EDPH mk though.
verticalmagic avatar
verticalmagic avatar
I use the FRA 400 with IMX571 color and I am satisfied with that, but I did not buy the reducer because I saw many images with degraded performance (elongation, CA) compared to no reducer.