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Dark subtract the images

Some or all of the images may already be dark-subtracted. Usually the image name will indicate if it has been processed in this way. The raw image might be ``m51.fits'' and the dark subtracted image might be ``m51d.fits''. If not, on the command line in a terminal window and in your image directory (use cd to navigate), an ls command will remind you of the file names. You may look at the notes.txt file to see information about each image. We need to remove the dark background by subtracting from each raw image of the sky an image taken - with the same exposure time - with the shutter closed. This is a ``dark'' frame. It records the bias in each signal element that results from the thermal transfer of charge. These losses very nearly are independent of exposure to light, so we can simply subtract them and get an image that is free of dark signal.

There is a program on the system that will subtract two fits image files. On the command line, type fits_subtract and you will be prompted for the name of the first image file $A$ of the pair to be subtracted to make a difference file $A-B$. Use the dark frames that are closest in time to the image files and make a dark-subtracted image file for each region we recorded. It may be helpful to use a similar file name such as ``neptuned.fits'' for a dark-subtracted neptune image file. The important thing is to use a naming sequence that is easy for you to remember. The system does not care what names you use, but do not overwrite your original files.

In the following you will use only the dark-subtracted files. Often these will have been done for you and you can go immediately to the next step.


next up previous
Next: Display and study processed Up: Reduction of 16-inch Telescope Previous: Display the Fits Images
John Kielkopf
2004-11-30