ASTR306

Data Reduction

To make the data readily available, all of the image files were downloaded from the Nassau station to one of the CWRU Astronomy Department PC's. From there, IRAF's rfits package was used to change every fits image file from the observatory into a .imh file on a local directory accesible to all group members. After naming and organizing the files, they were ready to be zero subtracted.

Zero Subtraction

IRAF's zerocombine package was employed to create two zero frames. Each zero was the average of all zeroes taken on a single observing night. The appropriate zero image was then subtracted from every other image with the IRAF ccdproc package.

Dark Correction

We also attempted to correct for the effects of dark current (fixed pattern noise arising in long exposures) and cosmic rays. To this end, a series of dark frames were taken along with the actual image frames for each target. (Dark frames are exposures of the same length as the data exposure except that no light is allowed to reach the CCD. They help to determine the level of dark current, i.e., noise, present in the CCD.) These darks were combined (with IRAF's darkcombine procedure) into three dark images, corresponding to the three different exposure lengths called for by our four targets. Every image was then dark subtracted with a second run of ccdproc.

The counts in the dark frames were much less than the counts in the zero frames, indicating that the dark current in this particular CCD is not very big. Therefore, we decided to forgo the dark subtraction for all images taken on September 29 (images for DYPeg and NGC7789). All images taken on October 2 (HXLac and the Dumbbell) were dark corrected, since it improved the overall quality of the data, which was hamped by the presence of ice crystals on the CCD structure.

Flat Fielding

To account for the varying quantum efficiencies of the CCD pixels, it was necessary to divide out a flat field from every image. (A flat field is an exposure taken of a uniformly illuminated white spot on the inside of the dome.) Flat fields were taken each night for every filter used during that night's observations. These flats were combined, filter by filter, using IRAF's flatcombine procedure. A third run through ccdproc (again, filter by filter) yielded our flat-fielded data.

At this point, every useful image file was zero-subtracted, (dark corrected), and flat-fielded.

NOTE: The flat frames for images taken on October 2 showed evidence of ice crystals on the CCD. Thus some aberrations appeared on the final flat-fielded images of HX Lac and M27. This was most severe for the HX Lac data, taken in the clear filter. Because of these aberrations, the HX Lac images were trimmed to the immediate vicinity of the star before doing any photometry. Since no photometry was to be done on the M27 images, these were left as they were.