William optics Gran Turismo 81 reducer for full frame - Forums

Topic starter

Sep 13, 2022

I am pretty new here, but seeing all the great work on this site I thought someone might be able to help me out with a dilemma. I am looking for a travel (and wide field) scope and was originally going for the redcat 71, but am now trying to figure out if the Gran Turismo 81 would be a better choice. I want something around 350mm focal length, so here the Redcat would be spot on. But with the GT and a 0.8 reducer you kind of get two scopes in one (478 and 382). My problem is that WO calls this reducer (6AIII) «near» full frame, with good illumination of 97% of the full frame. Which bugs me a little. So I was wondering if anybody knows if the more expensive full frame reducer (ikke the 7A) will work with this scope? Or is the Redcat 71 better at 350mm anyway?

also looked at the Askar Fra500 but feel the reviews are a bit mixed.

it will be used with a AM5 and a 6200mm pro.

Really appreciate any help 😊

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Jonny Bravo

Sep 13, 2022

The 6AIII will _not_ give you a properly corrected field on a full frame camera. As far as illuminating… flats will probably take care of the light falloff. To be honest, I think you're going to struggle getting a properly corrected full frame on any 80mm scope. I suppose a lot depends on your tolerance for misshapen stars.

There are a number of threads over on Cloudy Nights discussing this very thing.

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Chris-Alexander N. Lunder

Topic starter

Sep 13, 2022

Jonny Bravo:
The 6AIII will _not_ give you a properly corrected field on a full frame camera. As far as illuminating... flats will probably take care of the light falloff. To be honest, I think you're going to struggle getting a properly corrected full frame on any 80mm scope. I suppose a lot depends on your tolerance for misshapen stars.

There are a number of threads over on Cloudy Nights discussing this very thing.

Thanks for the reply! Ok. And I guess any scope bigger then 80mm will be a longer focal lengt..? Cant seem to find any that wide over 80.. besides the Askar FRA500 with a 0.7 reducer. Would that be better you think. Been searching cloudynights but not super easy when you dont know exactly what to search for 😅

Jonny Bravo

Sep 13, 2022

Here's one: https://www.cloudynights.com/topic/798933-wo-gt81-full-frame-sensor-round-stars/

The OP (dcm_guitar) has some other threads in the experienced imaging forum with contributions by folks with a lot more knowledge of optics than I'll ever have

. The general consensus is that the wide field scopes will not properly correct/flatten on a full frame sensor, and that it is further exacerbated by the IMX455 (which is what is in your 6200) because of the small pixel size. If I recall correctly, they were up over 100mm of aperture before scopes were capable of producing flat, well corrected images on the IMX455.

My own GT81 has trouble getting a properly corrected flat frame out to the edges of a 4/3 sensor (my 294MM Pro). I both love and hate NINA's hocus focus analysis tools for making me discover that LOL.

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Chris-Alexander N. Lunder

Topic starter

Sep 13, 2022

Jonny Bravo:
Here's one: https://www.cloudynights.com/topic/798933-wo-gt81-full-frame-sensor-round-stars/

The OP (dcm_guitar) has some other threads in the experienced imaging forum with contributions by folks with a lot more knowledge of optics than I'll ever have . The general consensus is that the wide field scopes will not properly correct/flatten on a full frame sensor, and that it is further exacerbated by the IMX455 (which is what is in your 6200) because of the small pixel size. If I recall correctly, they were up over 100mm of aperture before scopes were capable of producing flat, well corrected images on the IMX455.

My own GT81 has trouble getting a properly corrected flat frame out to the edges of a 4/3 sensor (my 294MM Pro). I both love and hate NINA's hocus focus analysis tools for making me discover that LOL.

Thank you! I will check out the thread. So no way around this then. Kind of missleading that WO say the RC71 will produce a flat field across a full frame sensor. Al least for those of us a bit new in the game 😉 really appreciate the help!

andrea tasselli

Frequent contributor

Sep 13, 2022

Chris-Alexander N. Lunder:
Thank you! I will check out the thread. So no way around this then. Kind of missleading that WO say the RC71 will produce a flat field across a full frame sensor. Al least for those of us a bit new in the game 😉 really appreciate the help!

If you really want a really flat (meaning with no discernible aberrations) field with a FF sensor with those 3.8 um pixels at similar FL then you need to look elsewhere, as in: the Epsilon 130ED.

Björn Arnold

Sep 13, 2022

I've checked the cloudy nights thread that was mentioned above. One of the screenshots shows aberrated stars in the image corners which match the shape of the spot diagrams that William Optics provides for the scope and reducer combination. From that point of view, the systems apparently behaves as designed. If one finds this acceptable is a different question of course but it's not that something is wrong.

Nowadays, we don't have too many options about pixel size and so in my opinion the resolution of modern sensors leads to the impression that good optics look bad. Another example for a wide-field and full-frame setup, adding to what @andrea tasselli has mentioned would be a Takahashi FSQ.

For the RedCat71, I recommend browsing AstroBin and see results from people using it with a FF camera:
https://www.astrobin.com/search/?d=i&sort=-likes&q=%22William%20Optics%20Redcat%2071%22
From a quick browse of these images, there are very good ones amongst them. Of course, do the same for other scopes and the GT81 and see what results people produce.

A general question I am always asking: if you image with a large sensor, do you usually look at the image at 100% zoom? In a picture gallery, I usually don't see people smelling at the canvas. I made the observation that with shrinking pixels, people start to analyze the images on atomic levels and forget a bit about the whole picture. -- but that's just my opinion on it.

Björn

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Chris White- Overcast Observatory

Sep 13, 2022

You will not find a reducer for any budget scope that will produce a field that will correct a full frame sensor. By correct, I mean round stars to the corners.

The real question is whether you care. If you are just looking for a wide FOV and getting round stars to pixel peep is not important than you can make a decent image with just about anything. If you want to have great stars to the corners at 1:1 zoom with full frame, expect to spend thousands of dollars. If you want a short focal length your options become even more limited.

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Ruediger

Sep 13, 2022

A general question I am always asking: if you image with a large sensor, do you usually look at the image at 100% zoom? In a picture gallery, I usually don't see people smelling at the canvas. I made the observation that with shrinking pixels, people start to analyze the images on atomic levels and forget a bit about the whole picture. -- but that's just my opinion on it.

Björn

Hi Björn,

a valid point. Especially when people try to find arguments why your image is really, really bad. Then very often these slightly elliptic stars in the corner, close to the edge, are the reason the image sooooo terribly bad. If the electron microscopic inspection reveals some distortion, it does not deserve any batch. No way! 😜😜😜🫣

Sorry for the sarcasm, but I share your opinion. This obsession for technical perfection sometimes completely overshadows the sense for esthetic and beauty.

Chris-Alexander N. Lunder

Topic starter

Sep 13, 2022

Thank for all the replies. And I agree. A bit elliptical stars does not bother me. But I deffinetly want to aim for a high quality result as I love printing. That's why I went with the 6200mm. But looking at example shots with etc a RC71 and a full frame, I am totally fine with the shape of the stars. Contrast, sharpness and composition is way more important in my mind

I think the RC will be a good choice for me. At least I really like the images I've seen taken with it.

Thanks again everyone! This really helped

Wish you all a 100 days of clear skies ;)

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