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How to deal with coma?

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#1 salehest6990

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Posted 25 June 2019 - 05:47 PM

I finally got a clear sky with minimal wind and I took my first sub last night from the lagoon nebula. The sub can be accessed here: https://drive.google...iew?usp=sharing

 

As you can see, I think I got a lot of coma specially when looking toward the bottom left corner. I use a Explore Scientific 80mm telescope and I use a 2" Explore Scientific field flattener with a 55mm back focus (I made sure the back focus is at 55mm +/- 1mm). Still, it seems I got a lot of coma with my stars. I doubt that it would be due to bad tracking and autoguiding because the stars at the center look better. Does anyone know why I might be getting such a large coma effect and how to deal with it? Thanks!

 

Edit: I took this image using ZWO 1.25" Ha filter.

 

Edit2: I use off axis guiding. I made sure the prism is not blocking the sensor even a bit.


Edited by salehest6990, 25 June 2019 - 05:53 PM.


#2 Gipht

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Posted 25 June 2019 - 11:07 PM

Most often a flattener/reducer helps improve an image right out of the box.   In your case, at least with the OAG and Ha-filter, you were not so lucky. There is quite a bit of star elongation and especially so in the corners. Some of the possible issues are spacing, tilt, reflections, tracking error,  or imperfections in the optics and/or flattener.  Since you have a somewhat complicated imaging train with the OAG and possibly a filter wheel, you may want to simplify to help find the problem.  To keep  tracking problems to a minimum,  a fairly bright star field would be ideal, so exposures can be short.  You may want to contact tech support from ES to be sure of the spacing.  For example, some back focus spacings are measured from the top of the thread of the corrector.  Have you included the distance from the face of the camera to the sensor in your measurement?

 

Wish I could help more.  Perhaps if you stretch the picture and post it, you will get more of a response.



#3 Kevin Ross

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Posted 26 June 2019 - 02:04 AM

The left half of your image is much worse than the right half. I think you might have some tilt or sag somewhere in the imaging train.



#4 Stephen Kennedy

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Posted 26 June 2019 - 05:05 AM

Off-axis coma is an aberration found in all telescopes.  Some types like the Ritchey- Cretien reflectors are especially designed to reduce it to an imperceptible level but removing all off-axis coma is impossible,  The amount of off-axis coma is the result of the quality  of the optics of the telescopes.  Well figured mirrors and lenses will show some coma at the extreme edges but a large area around the center of the FOV will have perfect stars.  Poorly made mirrors and lenses will only have an extremely small area at the center of the FOV that will be coma free. 



#5 Gipht

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Posted 26 June 2019 - 08:20 AM

Tilt or sag can be created by a weak focuser,  or if you have a weak spot in the spacing assembly.  For example,  a filter installed in line as part of the spacing without adequate threads.




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