From: "milton_aupperle" <milton@outcastsoft.com>

Date: June 6, 2010 4:15:51 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Gradient Removal


Hi Folks;


Here is the difference that Gradient Removal while stacking can make in for Astro IIDC 4.06.00 Alpha 06:


http://tech.groups.yahoo.com/group/Astro_IIDC/files/Other/GradientRemoval.jpg


This is half size Messier 3 Globular Cluster I shot in early May as an LRGB image. The source movies have been dark frame subtracted and flat framed for each color channel. Post processing using log sqrt curves to stretch the image brightness has compounded and exaggerated the slightest amount of background image unevenness.


The difference may not be immediately visible, although you should be able to pick out that the bottom left corner of the upper image looks redder and brighter than the other three corners. If you use a tool (like Apples "DigitalColor Meter") to measure the image brightness you can really see the gradient changes across the image.


The upper image has a gradient of about 4000 (out of 65535) red, 2700 Green and 2000 blue form the bottom left corner to the right side of the image. I've had to do a lot of fiddling with curves and cutoffs to reduce it to this level of unevenness too.


In the lower image we have a gradient of < 300 (out of 65535) across the image from left to right and it's color consistent too, so we have no discolored areas. So the Gradient Leveling / removal has cut it back to barely detectable levels.


Today I have changed the algorithm for the image leveling code, which give better control of it via your percentage value. This change will be rolled into the Alpha 7 release.


TTYL..


Milton Aupperle