RE: [APML] Orion Mosaic (finally done)

["Scott Hammonds" <shammonds@creatorsview.com>]

John,
Great explanation and a very useful tool. Thanks for sharing.
Scott Hammonds 

-----Original Message-----
From: astro-photo-bounces@seds.org [mailto:astro-photo-bounces@seds.org] On
Behalf Of john laborde
Sent: Wednesday, June 08, 2005 3:49 AM
To: Discussion of Film Astrophotography
Subject: Re: [APML] Orion Mosaic (finally done)

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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