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7. Real-life image processing

The basic steps of image processing have already been explained in the section entitled

“Introduction to image processing”. Here, we will outline the procedure again (but will not

repeat all the details), and add a few steps that make your job more successful or easier. In

any real-life situation, it will be necessary to align a number of pictures with each other, and

often to sharpen the images using special software. The color composition routines of

CCDOPS and CCDSOFT are very primitive, we must use Adobe Photoshop for quality work.

I. Image processing work is meant to be done under close supervision of an instructor. In

practice, students do a few steps at a time, then ask their instructor to see what they are

doing, and get advice. As each further step relies on the careful and correct execution of

the previous steps. ‘Skipping’ such constant evaluation will probably result in having to

redo earlier steps, and all that follows those, again. It usually takes three-four or more

trials to get the pictures into acceptable shape.

II. Keeping the work well organized is very important. By the end of the process more than

a hundred different files may be generated; it will be almost impossible to remember

what each file contains. It is important to keep the images that were produced in the

intermediate stages, both for evaluation as well as to return to them when a further step

goes wrong. Experience shows that it is a good idea to keep files in each step in a

different folder; an example of the resulting set of folders is shown in the following table.

Notice the numbering; it helps to indicate the order of the steps taken.

An example of a well-organized set of folders

Folder Names

Subfolder Names

00_OriginalsUsed

LightImages; FlatsUsed; DarksUsed

01_DarkSubtracted

02_FlatFielded

03_PixelsFixed

04_Aligned

Red; Green; Blue; Hα;

05_Combined

06_Sharpened

07_ColorComposition

08_Finalized

III. Students will do their image processing on the computers in the astronomy lab in Lewis

Hall. You need to run an emulation of Windows XP on these Mac computers. The raw

images are located on the AstroX server; a shortcut in the shape of a little camera, named

“CCD”, has been placed on the Windows desktop of the lab computers, which points to the

raw files. They are found in the “CCD/Originals and comments/ST10/…” folders,

together with the “Note” file that contains some more information. There is a dark library

under “CCD/DarkLibrary (Processed)”. It will take some detective work to find the

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