Re: [APML] Orion Mosaic (finally done)

[john laborde <jlaborde@ix.netcom.com>]

all

Here is a useful procedure for removing red halo around stars. I applied 
this procedure on my Orion mosaic after I posted it and it made a big 
difference. The reddish tint on the Orion mosaic is not due to color 
cast but to hundreds of red halos around brighter red stars. Removing 
this without affecting the faint nebulosity made for a more color 
balanced image.

see this image for reference: 
http://aisig.sdaa.org/astroblogDetail.asp?imgID=543&UserID=67

In PhotoShop with the eyedropper set to 3x3 or 5x5, Select>ColorRange 
and pick the center of a bright star.
Set fuzziness to a high value, 150 to 180. Set Select>Modify>contract to 
a number of pixels that will deselect the smaller stars.
Set Select>Modify>expand to a number of pixels to encircle the red halo 
around the remaining stars.
Open the channels palette and select the red channel only.
Use the Minimum filter set to 1 or 2 pixels.

The image on the right is before red halo reduction. The image on the 
left is after red halo reduction. Notice that the red nebulosity is 
unaffected. Copy this image to your computer and make the image smaller 
on the screen. You can see readily that the left image has better color 
balance.

John Laborde

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