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M 13 (NGC 6205) Using Photometric Filters

Jun - July 2012

Messier 13 is one of the most popular objects for public star parties.  I must have looked at it hundreds of times with my own and with scopes I run for the public.  As a star cluster  my narrow band filters will not work.  Thus I targeted it for my first test of my revised Photometric Filter technique.  This technique extrapolates the star colors by using a color filters that are more narrow than RGB filters to sample the colors.  From this sample I can assign the star a color that is representative of the star type.  This does not try to capture true star colors, in part, since one of the components is a Near Infrared (NIR) filter.  Red M stars will appear brighter (and redder).  G and K stars may have more of a red component since more of their curve in NIR was captured.  A and brighter stars will be bluer since they are dimmer in NIR than they are in red.  The goal however, we be to come as close to correct star colors as I can.

How close are the  resulting colors to LRGB? See the discussion on my Using Photometric Filter to Overcome Light Pollution.

click on the images below for a larger image

M13 closeup M13 vicinity using NIR-synG-pB

Annotated Map

Here are two annotated Maps.  One emphasizes the NGC objects and the other the B-V classes of various stars. 

Click for a larger image

NGC Objects in image


Stars and their B-V values

click for a larger version. 


Processing Details

The data was captured as two High Dynamic Range Images each constructed from 3 exposures

Johnson/Cousins B Filter (pB) 32 x 030 seconds

30 x 120

28 x 600

Near Infrared Luminance (NIR)
32 x 030

30 x 120

36 x 600

Of course this is only what met the image quality standards. 

The images were processed with PixInsight.  The individual images were combined using HDR Composition.  An RGB image was then created using PixMath

Copyrights for Photos

Creative Commons License
Except as noted, all work on this site by Robert J. Hawley is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. This permits the non commercial use of the material on this site, either in whole or in part, in other works provided that I am credited for the work.