#1  
Old 26-06-2012, 07:40 PM
pvelez's Avatar
pvelez (Pete)
Registered User

pvelez is offline
 
Join Date: Aug 2006
Location: Sydney
Posts: 1,250
colour gradient

I have an issue that is driving me bananas

I've taken a few images of M4 in Scorpio. I have found with each image that there is a noticeable colour gradient. To the left of centre, blue is accentuated while to the right I have a surfeit of green.

No dramas I thought, that must be some of the emission nebulae in that area.

Turning to the region around Eta Carinae - same issue - magenta to the left, green to the right. The bright areas are fine, its in the background that it is noticeable.

Must be dodgy flats - so I reshot them. And reshot them. And reshot them. No change.

Am I missing something? Its not light pollution - the colour gradation looks the same irrespective of which area of the sky I am imaging.

The flats look fine. I'm setting them at 20,000 but have also tried 25,000 and 15,000. I've checked that they are in fact flat by test calibrating a flat - even illumination results. I've checked it with the graph and info tools in Maxim.

I'm shooting RGB and LRGB. I'm median combining 20 - 30 flats taken with a lightbox. I've rotated the box in case its not flat - no change. Darks and bias frames are applied and I'm median combining in Maxim. There's no difference using average or sigma combine (though I wouldn't expect any difference).

So what else am I doing wrong?

Pete
Reply With Quote
  #2  
Old 26-06-2012, 10:06 PM
coldlegs's Avatar
coldlegs (Stephen)
Chopped its rear end off!

coldlegs is offline
 
Join Date: Jul 2009
Location: adelaide, sth aust
Posts: 330
Pete
I'm finding a similar thing when the target is only a couple of hundred counts above the noise floor (see attached single processed frame). Some of the problem is solved by eliminating outlier bias/flats and darks. By that I mean watching the histogram max/min as you scan through them and deleting any that aren't close to what you think the average is going to be or ones that suddenly jump 40+ counts etc. This seems to eliminate any skewing of the average. Of course the flats need to be treated as lights i.e. subtract darks and bias from them before use. Maxim may automatically do that, I'm not sure as I use Nebulosity. Anyway I deal with the gradient with this procedure on youtube .

Astrophotography: Gradient Removal when GXT won't hack it
http://www.youtube.com/watch?v=yTEVMH_WE80

Good luck
Stephen
Attached Thumbnails
Click for full-size image (bad gradient small vers.jpg)
71.9 KB20 views
Reply With Quote
  #3  
Old 27-06-2012, 07:00 AM
pvelez's Avatar
pvelez (Pete)
Registered User

pvelez is offline
 
Join Date: Aug 2006
Location: Sydney
Posts: 1,250
Thanks Stephen

Maxim subtracts dark and bias frames. I have been relying on Maxim to properly scale the darks tp match the exposure time of the flats - I wonder if that is contributing to this.

Pete
Reply With Quote
  #4  
Old 27-06-2012, 07:56 AM
Oscar in Bin's Avatar
Oscar in Bin (John)
Registered User

Oscar in Bin is offline
 
Join Date: Apr 2011
Location: Melbourne
Posts: 48
Colour Gradient

Pete,
I use CCDStack and that has a nice simple gadient removal feature. However, I was having problems with colours when I combined RGB images - for example bad coloured halos. On investigation I found that I had negative ADU values after dark subtraction and prior to combining the three channels. Using pixel maths to get rid of the negative values by adding a positive floor worked very well in getting back to sensible colours.
John
Reply With Quote
  #5  
Old 30-06-2012, 05:20 PM
OzRob's Avatar
OzRob (Rob)
Registered User

OzRob is offline
 
Join Date: May 2009
Location: Thailand
Posts: 446
Gradientxterminator is a great tool for removing gradients. It needs to be used carefully though. Here is what it did with Stephen's image. You need to have Photoshop to use it.
Attached Thumbnails
Click for full-size image (bad gradient small vers.jpg)
98.2 KB17 views
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 09:22 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
Celestron Australia
Advertisement
NexDome Observatories
Advertisement
Lunatico Astronomical
Advertisement
SkyWatcher Australia
Advertisement
Bintel
Advertisement
OzScopes Authorised Dealer
Advertisement
Meade Australia
Advertisement
Astronomy and Electronics Centre
Advertisement