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Still seeing coma with a coma corrector?

imaging reflector accessories beginner
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#1 bobbyzomo

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Posted 03 August 2020 - 12:52 PM

Hello!

 

I recently received my TPO 2" Coma Corrector and I noticed I'm still seeing coma around the edges of my images. Here's an example taken last night.

 

I have an 8" f/4 newtonian 800mm telescope.

 

I'll say it's an improvement over not using a coma corrector but I'm still not happy. Am I missing something? Any guidance/tips would be highly appreciated!



#2 StephenW

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Posted 03 August 2020 - 01:30 PM

Spacing is often critical with coma correctors - have you tripled checked how yuo have it configured?  And maybe try (if you can) adjusting the spacing to be shorter or longer to see what affect it has?

 

In the end though, you may not be able to remove all coma, just improve on what was there.


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#3 bobbyzomo

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Posted 03 August 2020 - 04:20 PM

Spacing is often critical with coma correctors - have you tripled checked how yuo have it configured?  And maybe try (if you can) adjusting the spacing to be shorter or longer to see what affect it has?

 

In the end though, you may not be able to remove all coma, just improve on what was there.

By adjusting the spacing do you mean tightening the focuser while the coma corrector is not fully in?



#4 billdan

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Posted 03 August 2020 - 05:36 PM

The specs for your coma corrector shows it has 70mm back-focus, which implies that the camera sensor must be 70mm from the coma corrector. Not sure if its 70mm from the glass optics or from the shoulder of the CC, you should have received some instructions explaining this.

 

Complying with this spec, could mean it pushes the camera further away from the focuser, so you may have to wind the focuser all the way in to achieve focus.

Which then presents the problem of the possibility of the focuser drawtube casting a shadow on the primary mirror.

 

Most of the other coma correctors available today have a back focus spacing requirement of 55mm which makes focusing a lot more convenient.



#5 StephenW

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Posted 03 August 2020 - 05:40 PM

Your TPO corrector requires 70mm of back focus, so your camera sensor should be places 70mm from the back surface of the corrector.

 

If that spacing is off it could affect the quality of the coma correction you are getting.



#6 bobbyzomo

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Posted 03 August 2020 - 06:23 PM

Ah I see! It did come with a ring with a T-thread but i don't think it's long enough to get 70mm...

 

I'm assuming this is something that I would need?



#7 bobbyzomo

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Posted 04 August 2020 - 05:15 AM

Thank you guys for your help, I really appreciate it. I went to a local telescope shop and picked up some spacers. I'm seeing much better results and wanted to share it with you.

 

Tips, suggestions and critique are welcome.

 

Ring Nebula (Messier 57)

 

60 minute total exposure time

60x 60s lights

25 flats

25 darks

25 bias

autoguiding on


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#8 StephenW

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Posted 04 August 2020 - 08:37 AM

There is definitely improvement!, but as you can see there is still coma present.  It also looks like you have some tilt - the stars in the lower left corner have significantly more coma than in your other corners.

 

Did you measure your spacers to get to the magic 70mm separation?

 

And as I said in my first reply, getting rid of all the coma may not be possible so at some point you will have to decide what's acceptable to you and just enjoy the images! :)


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#9 bobbyzomo

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Posted 04 August 2020 - 10:54 AM

There is definitely improvement!, but as you can see there is still coma present. It also looks like you have some tilt - the stars in the lower left corner have significantly more coma than in your other corners.

Did you measure your spacers to get to the magic 70mm separation?

And as I said in my first reply, getting rid of all the coma may not be possible so at some point you will have to decide what's acceptable to you and just enjoy the images! smile.gif


I did the best I could, I wasn’t too sure where exactly I was supposed to start measuring from the camera’s sensor since the first object is a diagonal mirror in the camera, so I did a lot of trial and error using a combo of different spacers until I settled with the best result.

For the tilt, how do I fix this if everything seems to be snug and locked in tight? Or did you mean tilt on the secondary mirror?


Edited by bobbyzomo, 04 August 2020 - 01:10 PM.


#10 StephenW

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Posted 04 August 2020 - 01:58 PM

>I did the best I could, I wasn’t too sure where exactly I was supposed to start measuring from

 

Which camera are you using?   Most camera specs will tell you exactly how much back focus the camera eats up.   If you have nothing else in your imaging pipeline (no filter wheel, oag etc) then you simply subtract the camera's back focus form the 70mm your reducer requires and that tells you how big a spacer you need.

 

I would get your spacing as close as possible to the required 70mm first, before trying to address tilt.


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#11 bobbyzomo

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Posted 04 August 2020 - 03:45 PM

>I did the best I could, I wasn’t too sure where exactly I was supposed to start measuring from

 

Which camera are you using?   Most camera specs will tell you exactly how much back focus the camera eats up.   If you have nothing else in your imaging pipeline (no filter wheel, oag etc) then you simply subtract the camera's back focus form the 70mm your reducer requires and that tells you how big a spacer you need.

 

I would get your spacing as close as possible to the required 70mm first, before trying to address tilt.

You know what.. I decided to return the TSO CC, I also just got back from returning the T spacers i bought yesterday and I just ordered the Explore Scientific CC online.




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