AstroArt Plugin for Scaled Darks with the MX7C

Self-guided images captured with the MX7C CCD camera from Starlight Xpress often show a gradient caused by amplifier glow. Standard dark subtraction does not always accurately remove this glow. Robert Vanderbei recently published a manual process for scaling a dark frame to better match an image (Click here to go to his page). While this gives good results, it is a bit tedious to accomplish. This plugin automates the method (with some minor variations).

Download the plugin.

 
This is a 600 second self-guided MX7C image of M51 (scaled to half size). Note the amplifier glow and hot pixels.
This is the matching Dark Frame - 600 seconds with guiding enabled.

The result of a simple dark subtraction leaves some residual amplifier glow and hot pixels.

Simply scaling the dark frame is better, but still would not completely remove the glow and hot pixels since the two components require individual scaling factors.
The glow is really evident in this color image created using the MXC color synthesis and Trichromy routines with no rescaling or additional processing.
To correct for this, the dark frame's glow component needs to be separated from the hot pixels and each part scaled independently then recombined. This is accomplished by doing a median filter on the dark frame then subtracting that "Glow Dark" from the original dark frame to create a "Hot Dark."
The Glow Dark
The Hot Dark

The software now subtracts the median value in the in the lower right corner from the median value of the brightest part of the left edge of the Glow Dark. The difference is saved and divided into to the same value as generated from the Image to give the glow scale factor.

The software then locates the hottest hot pixel in each of nine areas in the Hot Dark. The average of the median values of the areas around these hot pixels is subtracted from the average value of the hot pixels. This value is divided into the same value as derived form teh same hot pixels in the Image to give the hot pixel scale factor

The Hot Dark is multiplied by the hot pixel scale factor. The Glow Dark is multiplied by the glow scale factor. The two darks are then added to create the Matched Dark Frame.
The Matched Dark Frame.
The result of subtracting the Matched Dark Frame from the Image. Note tha tthe glow is gone and most hot pixels are removed.
The resulting unprocessed color image shows a much more even background and better contrast.