Askar SQA85 refractor lens telescope astrograph

[Elp]

And here it is, the Askar SQA85:

https://www.sharpstar-optics.com/Products_1/SQA85.html

 

https://m.youtube.com/watch?v=wpZxzlmEHMw

 

[Elp]

Bit disappointed it's not a variable aperture like the sqa55 but I suppose the weight of the 85 is impractical to use as a daytime telephoto anyway.

[Phillyo]

I wish I understood all the different charts and spot diagrams etc. It looks really really nice and I'm super tempted by it, especially with the lack of back focus requirements and a max 70mm back focus distance. Means you can have mono cam, filter wheel, rotator and OAG. From what I can see (with limited knowledge) it looks almost within the FSQ85 territory?

 

https://www.firstlightoptics.com/askar-telescopes/askar-sqa85-85mm-f48-quintuplet-petzval-astrograph.html

[Elp]

59 minutes ago, Phillyo said:

I wish I understood all the different charts and spot diagrams etc

Maybe Mr V can help @vlaiv?

From what I gather the spots you have to consider what measurement scale they've used, the amount of light scatter and where on the sensor the measurement has been taken (the scope will be best corrected in the centre and taper off at the edges, but the spots are very small scale).

Someone else mentioned FSQ85 territory on CN, I don't think it'll be that level of quality, and the price to fit in I'd guess would be around the 1500-2000 mark, otherwise it wouldn't fall within the tiers of Askars/Sharpstars other scopes. We'll have to see on price. The SQA55 is already priced higher than some much faster camera lenses (used prices), though the RRP of the 55 is in line with the WO Redcat, but has the benefit of variable aperture (if one wants to use it for daytime). The 85, maybe is similar to the WO Redcat 71.

Bit disappointed it's all SD glass though, even the 55 has one ED glass and one SD.

[Stuart1971]

Just now, Elp said:

Maybe Mr V can help @vlaiv?

From what I gather the spots you have to consider what measurement scale they've used, the amount of light scatter and where on the sensor the measurement has been taken (the scope will be best corrected in the centre and taper off at the edges, but the spots are very small scale).

Someone else mentioned FSQ85 territory on CN, I don't think it'll be that level of quality, and the price to fit in I'd guess would be around the 1500-2000 mark, otherwise it wouldn't fall within the tiers of Askars/Sharpstars other scopes. We'll have to see on price. The SQA55 is already priced higher than some much faster camera lenses (used prices), though the RRP of the 55 is in line with the WO Redcat, but has the benefit of variable aperture (if one wants to use it for daytime). The 85, maybe is similar to the WO Redcat 71.

Bit disappointed it's all SD glass though, even the 55 has one ED glass and one SD.

😂😂😂 it can’t be any worse than the pile of crap FSQ85 I owned, it was awful…my Esprit walks all over it in every way….the optical design is way outdated and not good with modern small pixel cameras, even this Askar 71f is better at less than a sixth of the cost…

[Elp]

8 minutes ago, Stuart1971 said:

😂😂😂 it can’t be any worse than the pile of crap FSQ85 I owned, it was awful…my Esprit walks all over it in every way….the optical design is way outdated and not good with modern small pixel cameras, even this Askar 71f is better at less than a sixth of the cost…

Maybe says more about keeping up with the times. Would be interesting to note if the three new Taks announced this year will fare any better (or maybe just the FCT).

But Askar have certainly shown others what can be done within the market. Haven't got one yet but very likely to be my next one if I decide to buy.

Edited Wednesday at 17:33 by Elp

[vlaiv]

1 hour ago, Phillyo said:

I wish I understood all the different charts and spot diagrams etc.

I can explain what those charts mean, but it is not easy to interpret them in terms of absolute quality.

This is MTF vs distance from center in sagittal and meridional direction (S and M respectively). 10 and 30 stands for 10 line pairs per mm and 30 line pairs per mm.

This is what above diagram "speaks" about. MTF of 1 is perfect image. Anything below 1 is contrast loss to some degree. Meridional direction is "in direction of sensor center" and Sagittal direction is perpendicular to that. 10 line pairs per mm represents level of detail that is order of 50um on sensor. 30 line pairs per mm represents detail that is about 16.66um - or rather should be equated with pixel size.

In another words - above diagram shows that if you have line pairs - meaning black / white lines that go towards the center of the image - like this target:

at the edge of the sensor at 22 mm with lines being 16.66um wide - you'll get only 80% contrast - so you won't image black and white 0 and 1, but rather 0.1 and 0.9 for example (in 0-1 range or perhaps 25 and 225 in 0-256 range).

From what I can see from the graph - lens has rather good correction all the way to the edge of full size sensor as it has the same MTF so star size won't change much.

However, in order to asses absolute performance of the lens - we should compare it to 85mm perfect aperture and that would mean creating ideal MTF of 85mm aperture and seeing what sort of MTF we get for 10 LP per mm and 30 LP per mm.

I could do that, but it would take me some time to perform the calculation and simulation.

This graph is rather simple one:

It represents "profile" of flat frame. It is level of illumination from center all the way to 22mm away from optical axis (I think Y field in degrees is mislabeled and should be distance from optical axis - same as in first image).

This one is result of "ray trace" sort of thing. A lot of rays are cast and traced and their end position is plotted. Now, this does not give us full picture as it does not include interference effects of light - but it does give us some information.

First - we can see the same thing like in MTF diagram. Star image is roughly the same in center of the field as is on the edge of full frame (22mm away from optical axis). It does not blow up significantly nor distort.

Second thing that we can do - is compare RMS radius of ray trace to actual RMS radius of star for perfect lens.

RMS radius of actual star profile is about 2/3 of airy disk radius. For F/4.8 system linear airy disk radius is 3.2um. RMS of that will be about 2um. In above spot diagram we see that RMS radius is below this value (good thing) all the way up to 18mm. Only on far edges stars will be a bit larger than with diffraction limited scope.

I have only one objection and that is the choice of 200um for box size in spot diagram. I think that is due to marketing - but it hides actual spot diagram shape. By choice of box size we "zoom in" or "out" the diagram. It has nothing to do with performance of the optics - but people like to see "tight" points rather than some weird scatter plots - although latter are more informative.

 

 

[Phillyo]

Amazing thanks vlaiv I actually understood most of that!  

Edited Wednesday at 18:43 by Phillyo