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Early learning Astro centre Coma issue?


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So, just getting started in imaging. shooting the Orion nebula at the moment. This is my second attempt. a vast improvement on my first try, yet quite the way to go.

I've googled the issue i'm having with the stars here. could someone confirm to me that the issue here is "coma aberration"?

i think that is what is going on here but just want someone to confirm before i purchase a flattener.

I'm using a canon EOS M3 attached to a hyperion zoom MK3 on an F5 Newtonian.

sorry if these are basic questions.



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+1, Peter. You beat me to it.

It's coma, exaggerated by the eypeice projection method you seem to be using. You will get better results when you remove the eyepiece from the imaging train and go for prime focus astro imaging. Use a T-adaptor

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53 minutes ago, ElCatski said:

so, is having a zoom lens in the light path not the done thing?

No, the best and easiest approach is to image at prime focus, without any optics (other than the coma corrector as suggested by Louise, and possibly filters) between the scope and sensor.

The Newton replaces the camera's optics.

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Thank you all so much for the help.


"do get a Bahtinov mask to set and check focus"

I have one, but couldn't fget it to work through the zoom lens, I think I can see why now.

"Is the image you posted cropped?"

Yes, in the uncropped version the coma is equal all around the edge.

I have now ordered a T-adaptor to try prime focus.

A further question though. I would imagine with prime focus the photo would have a massive FOV, to get just the Orion, do you use post processing to crop just the area you want?

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1 hour ago, ElCatski said:

do you use post processing to crop just the area you want?

You can, but quite often the surrounding space gives your target a nice frame. It depends upon the focal length of your telescope. The longer, the more of the frame it will fill. An idea: M42 with my 750 focal length, I kept as much of the frame as possible. HTH.

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Have a look here to see what FOV you'd get.


Yes, I assume the FOV will be much larger. You can of course crop the image as you suggest. Orion has loads of lovely nebulosity surrounding it so it'll still be a nice image - mainly in Ha.


Edited by Astrosurf
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When you extend the focal length with an eyepiece or Barlow you also slow the F ratio down and this causes the exposure time to increase vastly. You got away with it in this image because the Trapezium is incredibly bright. I just imaged it in 15 second subs to avoid that part burning out, but I used 10 minute subs for the main part of the image. On most targets you'd get next to no signal using the method you used here.

Deep sky imaging is almost invariably done at prime focus or with a focal reducer, the opposite of a Barlow.


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