Question about Dark Edges in photos

[JohnnyD333]

Hey all,

Ive been dabbling in AP for a few months now. Im slowly gathering all the necessary equipment to build a descent rig. While Ive been waiting, I figured I would experiment with the equipment I currently have. My autoguider being the big piece Im waiting for. However, my question is about the pictures Ive taken during this experimenting. Going Prime Focus with my D3100 using a CG5 mount and C6 Newtonian, I have gotten some results. But I consistently have darker edges in the pictures with more noise and light in the center. Is there something to correct this (coma corrector perhaps) or is it something I am doing wrong (ISO 1600 too high) Ill attach a pic of M101, guided sidereal with the mount only no autoguiding, at ISO 1600 5min exposure. I added some darks in DSS as well. Thanks guys.

[Psychobilly]

Basically the best way to sort this out is to take flats and use them in the processing there are quite a few different methods for taking flats...

couple of different methods in this thread...

http://stargazerslounge.com/imaging-tips-tricks-techniques/140012-method-taking-flats.html

Peter...

[JohnnyD333]

Thanks Psych, off to take some flats and give it a go.

[Psychobilly]

NO worries

[cgarry]

But what causes this?

Thanks,

Chris

[Psychobilly]

Uneven illumination of the sensor - vignetting...

It can also be processed out...

Here's a link to one method...

http://www.covingtoninnovations.com/dslr/vignetting.html

[cgarry]

Uneven illumination of the sensor - vignetting...

It can also be processed out...

Here's a link to one method...

Correct Vignetting with Photoshop

Many thanks. I have not really worried about it as PixInsight's DBE tool mostly just removes it. I will try using flats at some point to see how that improves the final image.

Another artefact I get in some subs with my 450D is a dark band at the bottom of the image. Any idea at the cause of this?

Below are a couple of very stretched subs from some auto guiding tests, the first without the banding and the second with the banding.

Sub without banding.

Sub with banding.

Cheers,

Chris

[swag72]

Once you do flats, I think you will be surprised at how much of an improvement that makes. Much better to get it right prior to processing I think.

[cgarry]

Once you do flats, I think you will be surprised at how much of an improvement that makes. Much better to get it right prior to processing I think.

Yes, I agree. The less processing that is needed the better I think. Now just to figure out how to take flats with my C9.25 as my laptop screen is not big enough!

Cheers,

Chris

[ollypenrice]

You could try sky flats using a few layers of white T shirt over the scope on an even sky. Trouble is, your camera and focus must not change at all between the lights and the flats so the camera really has to stay on the scope. If you are obsy based, not a problem.

Olly

[cgarry]

.

[ATTACH]59645[/ATTACH]

Sub without banding.

[ATTACH]59646[/ATTACH]

Sub with banding.

I am still curious as to the cause of this banding I get with my DSLR. Can anybody shed any light on it?

Cheers,

Chris