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Astigmatism with 50mm focal lens in conjunction with astronomy cooled camera (yet not with a DSLR)

Equipment CMOS Help Needed
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Mau_Bard avatar
Mau_Bard avatar
Dear AB Friends,
during a backfocus tuning session for a new Sigma 50mm 1.4 Art lens, I found out that when it was used together with a cooled astro camera (I used a Touptek 2600 Color), it generated visible astigmatism in the corners (Picture 1). This happened always in the backfocus range of 44 to 45.5 mm. I tried removing the filter that was placed between the lens and the camera, but the astigmatism persisted.

Instead, when I mounted the lens on a modded Canon 77D body, the astigmatism in the corners disappeared (Picture 2).

Please note that my Touptek works correctly with 135mm focals.

What is happening? I suspect that, at such short focal lengths, the protective glass of the camera (which in my case is a UV/IR filter) is enough to cause this astigmatism. Or maybe I am doing something wrong.

Have you had a similar experience with such short focals? It seems to me that that only viable solution with a 50mm lens is to use a DSLR rather than a cooled camera.
Thank you in advance for your advice.

Mau


Picture 1: Image corner with Touptek TS2600 Color + Sigma 50 1.4 Art



Picture 2: Image corner with Canon 77D + Sigma 50 1.4 Art
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andrea tasselli avatar
andrea tasselli avatar
It isn't astigmatism, I bet it is some quirk of the AR coatings in those cameras (which, given your experience with them, seem best avoided).
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Mau_Bard avatar
Mau_Bard avatar
Grazie Andrea, thank you.
Good point. After your remark, I can try and check it with an ASI camera.
Thank you again.
Ciao! Mau
John Stiner avatar
John Stiner avatar

I have had similar results with the same lens and a QHY128C.

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Mau_Bard avatar
Mau_Bard avatar

John Stiner · Nov 11, 2025, 04:03 AM

I have had similar results with the same lens and a QHY128C.

Hi John, thanks for your post.

In the meantime, I've been field-testing the Sigma 50 with a Canon DSLR to minimize this astigmatism (for example, in this photo). The overall optical result is impressive, but the stars in the corners show this flaw. And, especially in summer, cooled cameras are essential.

For this reason, I've returned to trying to optimize a cooled camera with the Sigma 50.

A friend who's an astrophotography expert recommended reducing the lens aperture to 1.8 or 2.0, that, in his experience, worked.

I initially attached aperture reduction rings to the front of the lens, but the internal aperture definitely offers much better control.

In conclusion: I'm now working on a setup with a cooled camera, where I set the Sigma's internal aperture to 1.8-2.0. I don't have a test case yet; it's a work in progress.

The Sigma doesn't allow mechanical aperture adjustment. Unless you're using an Astromechanics controller, the aperture must be set using a DSLR with an external power supply.

The procedure is as follows: with the camera turned on, set the aperture to the desired value. Then, with the camera turned on, unplug the power cable and you're done.

Simply removing the battery doesn't work, because the camera recognizes what you're doing and performs a controlled shutdown, including resetting the aperture.

Please let me know if you in the meantime have found a solution.

Regards,

Mau

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John Stiner avatar
John Stiner avatar

I tried stopping the lens down to f/2.8 (I removed my QHY128C and attached an old Canon 6D). It helped a bit; however, I am still encountering the astigmatism along with some coma and chromatic aberration. I had hoped to avoid reducing the aperture too much. I’m trying to image unguided with a SkyGuider Pro and don’t want to push the tracking beyond its capabilities it requires longer exposures. I have become accustomed to the quality of images using an FSQ-106EDXIII and understood that imaging wide open at f/1.4 would not necessarily yield those kind of results. However, I am not too happy with what I am seeing at this point.

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andrea tasselli avatar
andrea tasselli avatar
This is how I expect a proper 50mm lens to perform (at this image scale) and @f/2.8:

https://app.astrobin.com/u/andreatax?collection=7793&i=6ah5fh#gallery
Mau_Bard avatar
Mau_Bard avatar

andrea tasselli · Nov 14, 2025, 03:43 PM

This is how I expect a proper 50mm lens to perform (at this image scale) and @f/2.8:

https://app.astrobin.com/u/andreatax?collection=7793&i=6ah5fh#gallery

I perfectly remember this image of yours, Andrea, I commented it already at time of issue. It is definitively an excellent flat field.

John Stiner avatar
John Stiner avatar

https://app.astrobin.com/i/1a2ho8

Here’s a link to my Sigma 50mm DG HSM Art Lens @ f/2.8 on a full-frame sensor (QHY128C). File may be a bit large for lens view but perhaps a download would allow you to pixel peep a bit more.

andrea tasselli avatar
andrea tasselli avatar
Yes, it is pretty bad, even close to the center. And pretty soft. Are you sure you focused it tight?
John Stiner avatar
John Stiner avatar

This is a new setup so I have not perfected my focus routine. However, I used a ZWO EAF focuser and Sequence Generator Pro’s autofocus routine. The autofocus curve appeared fine. I double checked by focusing in and out to verify that I obtained the lowest HFR possible (not always an exact science when the focuser uses a belt to adjust a camera lens). The QHY128C has fairly large pixels and the lens has a short focal length so I expected a soft image. I’ll try again tonight. However, that astigmatism really bothers me and may relegate this lens to daytime use.

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andrea tasselli avatar
andrea tasselli avatar
You can get a used Nikon as the one I used in the post above for a ridiculous price, less then a pricey adaptor!
John Stiner avatar
John Stiner avatar

I am also going to experiment tonight with backfocus adjustments. QHY sells a rather complex set of adapters to mate its full frame camera with Canon EF mounts (in my case the D2 adapter set). If the backfocus were off, Sequence Generator Pro’s focus routine would show me the best focus I could achieve with the incorrect backfocus. However, it would be much better if spacing were correct. I’ll give it a try tonight.

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John Stiner avatar
John Stiner avatar

I started a topic on CN and think that, at least in my case, it may be a backfocus issue. I’ll try some adjustments when the sky clears.

https://www.cloudynights.com/forums/topic/985045-astigmatism-in-sigma-50mm-14-art-lens/#comment-14417972

andrea tasselli avatar
andrea tasselli avatar
A Canon EF lens should have 44 mm of backfocus to operate optimally. Is that what you have?
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John Stiner avatar
John Stiner avatar

The lens is a Canon EF mount. I am using a QHY128C camera and QHY’s D2 adapter set which purports to provide exactly 44 mm of backfocus.

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Mau_Bard avatar
Mau_Bard avatar

Hi John, I had a look to the very interesting parallel conversation you initiated on Cloudynights.

It seems that definitively some spacing rings have to be added in your train to reach the correct backfocus distance (that has to be incremented in case you have filters on the optical path).

The point that remains open is whether these lenses with extremely short focals can be used with cooled astro cameras or instead the usage of a DSLR (designed for shorter focals) is recommended to reach optimal performance. I personally would prefer to stick to cooled cameras, yet the doubt that the frontal AR glass or UV/IR filter is not working with strongly inclined light rays remains.

If I find the time, I will perform some further tests. I’ll update here if I land to some usable conclusion.