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Image Processing

Updated: 10/02/2007


My Imaging Steps (3/5/2006): This describes how I currently go from data acquisition to final result for my imaging.

Favorite Books: The Handbook of Astronomical Processing version 2  is a technical and practical guide to the theory of CCD imaging. The accompanying program, AIP4WIN, provides some functions not available elsewhere and is continually being improved and augmented by its authors. The New CCD Astronomy is a practical guide to equipment, imaging and processing that exists nowhere else in the field, as nearly as I can tell. I believe these two books are essential and will provide much grounding in the art and a ready source of reference.  PhotoShop for Astrophotographers is an eBook by Jerry Lodriguss.  Although written primarily for film imagers, there is a lot of great background information and the chapter on Photoshop is worth the price alone.  It illustrates various techniques clearly.

Color Calibration (8/13/2002): I used the information from some very interesting emails to determine the exposure ratios of my ST-8E.  SBIG recommended RGB exposure ratios are 1:1:1.6.  After calibrating using the procedures outlined by Bart Declercq in "Calibrating RGB Color Part 1" and "Calibrating RGB Color Part 2" along with the warnings noted by Stan Moore in "True Color and Ratio", I found my exposure ratio to be more like 1:0.9:1.4.  This is for a star at the zenith and should be adjusted for atmospheric extinction.  Al Kelly described the calibration process for AIP4WIN in "Solar Analog Stars".  It is relatively easy to develop the ratios for your own camera following the approach outlined in the above links.  Also, Don Goldman has done an incredible amount of comparative measurements on color camera and filters.  See his page on "Color Exposure".

Light Box Design (5/21/2003): Here is a page on a light box I built for use with my RC.  You may be able to get some ideas for your own needs.  The high intensity white LED's provide a good balanced light source to keep the exposure times for the blue flat field reasonable.

LRGB Processing in AIP4WIN (6/21/2002): Al Kelly has recently described his rationale for, and how to use AIP4WIN color combining. I have found the results to be very pleasing. He also graciously processed my recent (6/19/2002) M27 image using these steps. His mastery of AIP4WIN is clearly demonstrated in the smooth luminance processing he achieved. Here is the result of his processing. Clearly Al has mastered the nuances of AIP4WIN. Note the fainter stars and more stars with diffraction spikes, indicating excellent low-level processing. Compare this to my M57 page and my full size image. My RGB exposure ratio is 1.0:0.9:1.4 and all sub-exposures were 5 minutes except for blue at 7 minutes.

Processing Techniques: There is probably no end to ways to process images but two recent developments have been particularly interesting and helpful to me.

Rob Gendler has documented his LLRGB process that improves the SNR of color information and is documented on his site. Here is a summary of the method from one of Robís emails:

RGB and Luminance saved as 16 bit and brought up in PS where curves, levels applied and converted to 8 bit.
First LRGB created using layers in PS. Opacity brought to 50% and image flattened. Additional saturation (30%) and gaussian blur (1.7 pixels) applied. This image was then used as the "new" RGB and two more additional rounds of the same were performed which very significantly enhanced the color data.

Bruce Johnston proposed a method of correcting star bloat that can be used to remove elongation due to tracking errors, if the camera is orthogonal to the tracking axes. Chris Peterson has documented a way to implement this process in Photoshop. Note that he has also provided a link to some Photoshop actions to automate the process.  Here is a brief summary of his Photoshop approach:

The image is imported into Photoshop and resampled by 2x or 4x.
The background layer is duplicated four times (for a total of five layers).
One of the new layers is marquee selected (control-T) and shifted one or more pixels.
This is repeated with each of the new layers, for up, down, left, and right.
The layers are displayed using the Darken option.
The layers are flattened.
The image is resampled back to its original size.

I used this technique on a 4-hour luminance image of M51. I cropped the image severely to show a part of M51 shown here. The only processing of the image was reduce, align and combine. The left image is the unprocessed one; the one on the right had a 0.5 pixel shift in x (DEC direction) and a 2 pixel shift in y (RA direction). The improvement on not only star size but also trailing is apparent.