Why do I get coma here?

[msacco]

Hi, I bought a C8 Edge HD and been imaging with that for the past few months, I'm imaging with full focal length/reducer/hyperstar, but I'll refer to using reducer in this thread.

I'm trying to understand if my issue is backfocus issues, colimation issues, or maybe both or something else.
Here is a recent image I took of M16:

This is only the stack after ABE, as you can see there is coma, the C8 Edge HD should be pretty much flat field edge to edge as far as I'm aware.

I know that the required backfocus for the 0.7x reducer is 105mm, my backfocus was 105.1mm, which in F7 should be fine I believe.

I also worked for really long time on the collimation and thought it was very well collimated. Still, I'm getting this coma and not really sure why.
Would love to hear some thoughts about how that can be improved and what I could check.

Thanks for the help!

Edited May 4, 2020 by msacco

[iapa]

TBH my eyesight is not up to seeing your coma, I just see some slight stretch in top right

could it be camera tilt?

[TerryMcK]

I can see the coma. It is very apparent. I'm not familiar with the scope so cannot really make any suggestions but no doubt there are other here who can.

[msacco]

8 minutes ago, iapa said:

TBH my eyesight is not up to seeing your coma, I just see some slight stretch in top right

could it be camera tilt?

This is the right bottom side of the image for example:

I think the coma is quite visible, might not be completely horrible, but it is something I should really love to resolve.

As for camera tilt, not really sure tbh...Any way to check that? I don't see any reason for the camera to tilt.

3 minutes ago, TerryMcK said:

I can see the coma. It is very apparent. I'm not familiar with the scope so cannot really make any suggestions but no doubt there are other here who can.

Thanks for the comment

[iapa]

3 minutes ago, msacco said:

 

As for camera tilt, not really sure tbh...Any way to check that? I don't see any reason for the camera to tilt.

Thanks for the comment

Try here: https://www.cloudynights.com/topic/600764-analyzing-field-flatness-tilt-etc-with-ccd-inspector/

 

 

[msacco]

11 minutes ago, iapa said:

Try here: https://www.cloudynights.com/topic/600764-analyzing-field-flatness-tilt-etc-with-ccd-inspector/

 

 

Seems like the CCD inspector costs money though?

[iapa]

30day free trial

 

[alacant]

Hi

The coma is the same in all 4 corners so unlikely to be tilt. Is the reducer advertised to be cover aps-c? How is it at f10?

My only (not too good to say the least!) experience with this type of telescope was with an orange coloured c8 and the 0.63 reducer. Without the reducer, it was ok but we still needed a refractor field flattener to get anything like.

JTOL but HTH.

Edited May 4, 2020 by alacant

[msacco]

58 minutes ago, iapa said:

30day free trial

 

Guess the trial was already over for me, but I simply did that in another machine, this is the result I got in a few images(is there any way to get an avg?):

I'm not really sure what it means though...Does it means that the camera is tilted?

30 minutes ago, alacant said:

Hi

The coma is the same in all 4 corners. I don't think the reducer is gonna cover aps-c. How is it at f10?

My only (not too good to say the least!) experience with this type of telescope was with an orange coloured c8 and the 0.63 reducer. Without the reducer, it was ok but we still needed a refractor field flattener to get anything like.

JTOL but HTH.

Well I imaged only once on F10, but it was very bad due to many reasons, so I can't really say anything about that right now.

Also, according to the reducer in celestron's site: "Optimized for APS-C sized sensors, including most DSLR cameras and Nightscape CCD cameras" 

[alacant]

20 minutes ago, msacco said:

Optimized for APS-C sized sensors

Hi. Yeah, that's good news. The elongation is corner to centre. It it were a corrector, I'd say move it away from the sensor to shorten the aberration. As it's a reducer only, and the ff is a (non adjustable?) part of the optical train already, i'm not certain. A few mm may be all you need. worth a try?

Edited May 4, 2020 by alacant

[msacco]

2 minutes ago, alacant said:

Hi. Yeah, that's good news. The elongation id corner to centre. It it were a corrector, I'd say move it away from the sensor to shorten the aberration. As it's a reducer only, and the ff is a (non adjustable?) part of the optical train already, i'm not certain. A few mm may be all you need. worth a try?

It is totally possible, but I'm not really sure about that...

I'm imaging with the ASI071MC pro, so the backfocus to the sensor itself is 17.5, the reducer requires 105mm of backfocus to the sensor, so that should be:
17.5 + 50(Celestron T adapter), + 35(Extension tube) + 1.2&-1.4(spacer rings) which is equal to 105.1mm. It's 0.1mm more, but with F7 I believe that shouldn't make much of a difference.

Is there any chance that this is not correct?

[iapa]

16 minutes ago, msacco said:

Guess the trial was already over for me, but I simply did that in another machine, this is the result I got in a few images(is there any way to get an avg?):

I'm not really sure what it means though...Does it means that the camera is tilted?

Well I imaged only once on F10, but it was very bad due to many reasons, so I can't really say anything about that right now.

Also, according to the reducer in celestron's site: "Optimized for APS-C sized sensors, including most DSLR cameras and Nightscape CCD cameras" 

I would suggest that there is an amount of tilt, how much is too much I couldn't say.

You can select multiple images to get an over all measure for your stack

 

 

[iapa]

BTW - this is my first foray into CCDI, so, blind leading the sighted here I think.

 

 

[msacco]

4 minutes ago, iapa said:

I would suggest that there is an amount of tilt, how much is too much I couldn't say.

You can select multiple images to get an over all measure for your stack

 

 

This seems to be the average result:

So in case there is tilt, why is happens, and how to fix it?

Is it an issue with the camera itself? Or an issue with the way the camera is set up with the gear?

Thanks btw

[iapa]

as far as I have reenable to determine - starting for zero knowledge this am - it is most likely to be in the imaging train just because mechanics are not as accurate as a light beam.

If everything was manufactured to sub nanometre tolerances, probability if tilt would be reduced. As it is, one suggestion I have seen is to rotate the camera 180deg.

 

[iapa]

Here's a stack of 76 x 30s Lum images.

From C8 XLT, 6.3FR and ASI1600MM-cool - I know that there is some flexure in my spacers as I've not tightened them up fully.

 

[msacco]

2 minutes ago, iapa said:

as far as I have reenable to determine - starting for zero knowledge this am - it is most likely to be in the imaging train just because mechanics are not as accurate as a light beam.

If everything was manufactured to sub nanometre tolerances, probability if tilt would be reduced. As it is, one suggestion I have seen is to rotate the camera 180deg.

 

I'll try that! Actually, when trying to think about this more logically, it could make a lot of sense that this is a tilt issue! I've experienced coma for really long time now in many different situations.
With hyperstar for example, I also experienced that, but I had an incorrect adapter which means my back focus wasn't 100% accurate, which with F2 should make a big difference I'd say.

Still, even when I was supposed to be in the correct back focus, it happened. One more thing here is that the coma is mostly in the right side of the image, which is matching the tilt data going from right to left.

Here is the full image:

What I do wonder about though, is why the difference in the tilt is so big between different frames? Even if the mount is moving and somehow affects that a bit, I would not expect such differences?

[iapa]

Does the EDGE HD eliminate mirror flop that the C8 XLT has? As the OTA changes angle the mirror shifts slightly.

Perhaps the camera is not as secure as you think and is sagging under our friend Gravity?

 

We need someone who knows what they are talking about

 

Edited May 4, 2020 by iapa

[Ricker]

ASTAP has a free ccd inspector feature. 

[msacco]

1 minute ago, iapa said:

Does the EDGE HD eliminate mirror flop that the C8 XLT has? As the OTA changes angle the mirror shifts slightly.

Perhaps the camera is not as secure as you think and is sagging under our friend Gravity?

 

We need someone who knows what they are talking about

 

I'm not really sure about the first part honestly....I'm pretty sure the camera was very secure though.

1 minute ago, Ricker said:

ASTAP has a free ccd inspector feature. 

I'll give it a try, thanks

[msacco]

17 minutes ago, Ricker said:

ASTAP has a free ccd inspector feature. 

Is it increasing the thought of camera tilt?

[Xplode]

To get images to analyze you should take images of the same spot all the time, i suggest an area without a nebula/galaxy or star cluster, analyze the raw images, not the stacked one.

Use shortish images and make sure they have good guiding.
Don't trust your readings unless you get 5+ images with pretty much the same results, a variance of 5% isn't usual from an image to the next.

 

To check if it's collimation, if possible rotate the camera 180 degrees to see if the abberation rotates with the camera or not.
If it's sag/tilt because of weight check images before and after meridian flip, abberation will rotate 180 degrees.

If you want good input on what the problem is you should upload some of your fits images.

I have found that the numbers on spacing given by the manufaturer is rarely to be trusted, i try to always have spacers to adjust +/- 2mm available.

 

[msacco]

42 minutes ago, Xplode said:

To get images to analyze you should take images of the same spot all the time, i suggest an area without a nebula/galaxy or star cluster, analyze the raw images, not the stacked one.

Use shortish images and make sure they have good guiding.
Don't trust your readings unless you get 5+ images with pretty much the same results, a variance of 5% isn't usual from an image to the next.

 

To check if it's collimation, if possible rotate the camera 180 degrees to see if the abberation rotates with the camera or not.
If it's sag/tilt because of weight check images before and after meridian flip, abberation will rotate 180 degrees.

If you want good input on what the problem is you should upload some of your fits images.

I have found that the numbers on spacing given by the manufaturer is rarely to be trusted, i try to always have spacers to adjust +/- 2mm available.

 

Thanks for the input! How many images should I upload?

I'm meanwhile uploading everything, here is a link: https://drive.google.com/drive/u/1/folders/1UtvWJQREP8sz93EJgDyFksz8G4znrH4b

Edited May 4, 2020 by msacco

[Xplode]

Something else to avoud when creating images to check collimation/tilt/etc is low altitudes, especially for color cameras there will be an effect from atmospheric dispersion that could skew the measurements.

By looking at your images it's either tilt or collimation which is the main issue.

You might want to take a startest to check collimation, use Sharpcap with short exposures, defocus a bright star and see how the donut looks.

[msacco]

1 minute ago, Xplode said:

Something else to avoud when creating images to check collimation/tilt/etc is low altitudes, especially for color cameras there will be an effect from atmospheric dispersion that could skew the measurements.

By looking at your images it's either tilt or collimation which is the main issue.

You might want to take a startest to check collimation, use Sharpcap with short exposures, defocus a bright star and see how the donut looks.

I've spent around an hour working on collimation, and I really think I got it quite good this time(still not very epxerienced with collimating)