Help with optimizing imaging train - Forums

Topic starter

Jan 22, 2023 · Edited: Jan 22, 2023

Hi fellow astrobin'ers,

I have just acquired an ASI2600MC Pro, and as expected, I have a bit of optimization to do.

The scope is a Skywatcher 130pds with an ES coma corrector.

Most of the frame is ok, but the corners show some 'comet' shapes towards the center. I have tried with 0.1mm more and 0.1mm less back focus. Less makes it worse (as expected, right?), But 0.1 mm more seems to go in the other direction and I start getting oblongness perpendicular the comets shown here, but this comet shape does not disappear. The result is astigmatism like crosses.

I know the FWHM is terrible. High clouds gave really bad seeing.

Can someone help me diagnose what could be the problem here?

Thanks!

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andrea tasselli

Frequent contributor

Jan 22, 2023

1. It is well possible that your scope isn't collimated well and that adds to the pile, sort to say…
2. You have tilt in the image train and you need to correct for that.

Brian Puhl

IOTD Staff

Jan 22, 2023

Add some backfocus. You should also consider using single subs to evaluate your imaging train issues. Stacks aren't the best for it.

Dale Penkala

Jan 22, 2023 · Edited: Jan 22, 2023

Defiantly tilt in your imaging train. I use this for a guide when making back focus adjustments.
While collimation will affect the appearance of your image I think if you get your tilt and back focus corrected you will find your collimation is probably fine. I had the very same appearance in an 80mm setup and found it to be tilt and back focus adjustments that corrected the issue. At least thats been my experience. Good luck!

Dale

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Morsing

Topic starter

Jan 22, 2023 · Edited: Jan 22, 2023

Thank you for the replies! I appreciate you taking time to help me with this!

I also see signs of tilt, as both Andrea and Dale point out - the corner abberations are not symmetrical to all 4 corners - lower right looks ok, while the other 3 are quite bad. But when I put a sub through ASTAP image analysis, it said there was negligeble tilt. I also though I should see an effect on the FWHM plot, but that looks very symmetrical - that is why I did not pursue it further. But I guess it is bad enough to cause trouble and I will have to see if I can improve the situation!

Brian, as you suggest, and as Dales figures show, it does look like more back focus is needed. This was also my original conclusion, only when I added more distance between the CC and the camera, I got these astigmatism-looking issues instead. But I guess non-perfect collimation combined with tilt could make this hard to judge. To many effects at once!

Regarding collimation, I checked it again, and it was indeed slightly off, though not by much. I am by no means an expert in collimation even an easy scope like a 'slow' Newt, but I think my secondary is well centered with respect to the focuser and both a collimating eyepiece and a laser tells me I am pretty close on the collimation of the two mirrors. I don't have any more advanced collimation tools, but I was hoping this would be sufficient for a slow parabolic mirror.

As I said, I have re-collimated and also added a bit of back focus (and can add more) so next clear night I will have to work on the tilt. I will update when I have the chance to try that - but clear nights are few and far between at the moment!

If there are any other suggestions in the meantime, they are of course very welcome!

BR
Morsing

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andrea tasselli

Frequent contributor

Jan 22, 2023

Looking at your picture it seems obvious to me that the optical axis lays near the lower mid side so it would make sense to move the optical axis of the telescope so that that location now is found in the middle of the sensor. You can check the status of your collimation by going to a patch of sky rich in brightish stars and defocus significantly the image so that all stars look like fat donuts. The optical axis will be where the most symmetrical ones are. Hopefully you'll find it within the sensor's field.

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Morsing

Topic starter

Jan 22, 2023

Hi Andrea,

Thank you, that is helpful! That is the kind of interpretation I am not experienced enough to make myself, at least not confidently.

Would it be best to start by trying to adjust the focuser tilt adjustment or go straight for the camera tilt plate?

BR
Morsing

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andrea tasselli

Frequent contributor

Jan 22, 2023

Hi Andrea,

Thank you, that is helpful! That is the kind of interpretation I am not experienced enough to make myself, at least not confidently.

Would it be best to start by trying to adjust the focuser tilt adjustment or go straight for the camera tilt plate?

BR
Morsing

Collimation screws on the primary cell should be your next port of call. Once this is done you can adjust tilt with the camera tilt plate.

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Dale Penkala

Jan 23, 2023 · Edited: Jan 23, 2023

andrea tasselli:
Hi Andrea,

Thank you, that is helpful! That is the kind of interpretation I am not experienced enough to make myself, at least not confidently.

Would it be best to start by trying to adjust the focuser tilt adjustment or go straight for the camera tilt plate?

BR
Morsing

Collimation screws on the primary cell should be your next port of call. Once this is done you can adjust tilt with the camera tilt plate.

My suggestion for collimation of the newt would be to refocus a star in the middle of the FOV and as @andrea tasselli states adjust the primary mirror cell collimation screws on the back of the cell until your rings/secondary shadow is perfectly centered. Each time you make an adjustment be sure to recenter the star and check again and readjust. Do this until your happy with the results.

If you really want to dial in your collimation there is a free tool/software called MetaGuide. It’s an autoguiding software but it has an excellent collimation tool in it that really allows you to dial in your collimation based on the airy disk of the star. If you’re interested in it here is the info on the program. Just a heads up, it does have a learning curve to the software getting things setup correctly but once you get it it’s excellent. https://smallstarspot.com/metaguide/

Dale

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Morsing

Topic starter

Jan 23, 2023

Dale Penkala:
andrea tasselli:
Hi Andrea,

Thank you, that is helpful! That is the kind of interpretation I am not experienced enough to make myself, at least not confidently.

Would it be best to start by trying to adjust the focuser tilt adjustment or go straight for the camera tilt plate?

BR
Morsing

Collimation screws on the primary cell should be your next port of call. Once this is done you can adjust tilt with the camera tilt plate.

My suggestion for collimation of the newt would be to refocus a star in the middle of the FOV and as @andrea tasselli states adjust the primary mirror cell collimation screws on the back of the cell until your rings/secondary shadow is perfectly centered. Each time you make an adjustment be sure to recenter the star and check again and readjust. Do this until your happy with the results.

If you really want to dial in your collimation there is a free tool/software called MetaGuide. It’s an autoguiding software but it has an excellent collimation tool in it that really allows you to dial in your collimation based on the airy disk of the star. If you’re interested in it here is the info on the program. Just a heads up, it does have a learning curve to the software getting things setup correctly but once you get it it’s excellent. https://smallstarspot.com/metaguide/

Dale

Thank you, both! I will do so at the next given opportunity. Hopefully I will be able to report some success 😉.

BR
Morsing

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Paul Wilson

Jan 27, 2023

I have the RASA 8 and tried using the ASI2600s with it. My results with that big sensor camera looked similar or worse than yours. I couldn't get rid of the stretched and shredded stars.
I finally downsized to a ASI294 which has made a noticeable difference. But my edge stars are still not perfect.
Getting your backfocus right is not easy. I've tried 4 different camera adaptors, none of them helped. I tried shims. It's really a can of worms.

For a lot of people, there is ultimately no apparent solution other than installing a tilt corrector on your camera and using ASTAP or NINA to help you adjust out the errors. I don't think this should be necessary, but the equipment makers are not doing enough to ensure that users of their expensive equipment get a nice flat field. Correctors also have the disadvantage of limiting your camera to one scope. And really, to 1 camera since most recommend that once you install them, that they should remain attached to your scope to maintain the precise adjustments.

Im not happy about the lack of flat fields.

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