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

Annotated Map

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

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

• Red = NIR * 1.5
• Green = min( NIR*1.5, pB)
• Blue = pB
  

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