Quote:
Originally Posted by Gama
Oh, i think the fact you have no darks/flats IS "A BIG REASON"!.
Not only you got some heavy vigneting, but as you said a little noisy too.
You need to do these two tasks when using a DSLR.
Theo.
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Thanks Theo and Frank. It probably sounds like heroic ignorance, but until I fully understood flats and darks I wasn't planning to use them. I could get my head around using darks to clean up sensor noise, but didn't appreciate what a difference flats could make.
I always thought vignetting was something that happened at the edges of a photo as optics or focuser aperture impinged on getting a full image. I think I understand a bit better now why both are required - have just been reading up on flats at:
http://www.waid-observatory.com/articles.html
I must find and devour a good astrophotography text.
Quote:
Originally Posted by spearo
Rob,
good on you for delving into DSOs.
The glow around NGC253 is always tricky to remve i think.
Theo is right, the flats will help with vignetting.
technically to remove the left over glow without affecting the galaxie too much, there are a couple of ways of doing it.
The one i tend to use is :
copy the image twice.
blur copy 1 in gaussian blur maximally
blur copy 2 just a bit and invert (black becomes white
copy and paste copy1 onto original (you would of course want to have tucked away a copy of the original so it is saved safely without alterations)
in layers select the mask icon, Alt click on it copy and paste into that window the copy 2
you're now pasting a blur onto the original with a mask of everything but the galaxy
select the type of blending works best and that should help
you'll need to experiment with size/intensity of mask, blending mode, blending transparency etc
but that should help remove some of the glow around the galaxy without touching the galaxy itself.
hope this helps
frank
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Frank - many many thanks for your Photoshop advice. I surmised it would be possible to do some sort of masked cleanup, but had no idea where to start. I'll try as you suggest, but must admit I struggle to keep up with much of what you've described - that means I'll learn more though!
And in the future will try to avoid collecting noisey data that needs excessive post-treatment ideally.
Quote:
Originally Posted by Bassnut
As Theo says, use flats/darks, and 5 min subs.
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Fred - I know it probably sounds primitive, but this was done without any guiding, so I thought I better limit the length of my subs. Even with guiding I understood sky glow/pollution to become more problematic with increasing exposure time. Must confess still try to come to grips with difference between say 5 x 1min subs and 1 x 5 min sub. Again - I guess I've got plenty of learning fun left ahead of me in the future.....
