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NGC6781 - Pk 41-2.1    Click on image for Ha image.
Aquila
Penryn, California
July 2003 / August 2010
FS-128  (ag, ST-4)  ( rgb - 2003)
M 250 @ f9.3 (ag, ST-4) ( Lum - 2010)
ST-10XME LRGB 10 minute subs.

NGC 6781 is a 1.8 arc minute mag 11.1 planetary.  The disk is approximately 2 ly in diameter. 

I'm seeing a number of images on the internet with an intense blue center so I decided to calibrate the color.  I could not
find a G2v star in the 2003 color data so resorted to using SBIG's  published transmission data for the filters and the QE of the ST-10XME chip.

Filter                Mean transmission frequency                QE at mean transmission frequency               Color ratio

Blue                450 @ 80%                                            60                                                                1.41
Green              525 @ 90%                                            77                                                                1.00
Red                 650 @ 90%                                            85                                                                0.91

The result is the color above.