HW7: Image Reduction Report

Image Reduction Report

Joe Regan

10/23/15

Abstract/Introduction:

On Wednesday, October 21, 2015, we used previously written code to reduce images we had taken at Baker Observatory a few weeks prior on September 28, 2015. Each student took images of a different object of their choosing, myself choosing the star Altair. In my final reduced images, the counts for the star reached up to roughly 24,000 counts at the brightest point. Due to imperfections in the image reduction process, we were left with some negative final pixel values in the reduced images, which were later corrected. 

Procedure:

This experiment's purpose was to reduce the images taken at Baker Observatory, and to familiarize students with the 8-inch CPC telescopes along with the reduction process. First, ten darks and flats were taken each at ten second exposure times, in order to detect the dark current, caused by electrical current, in the detector, and to evenly distribute the light in the final image, respectively. Next, ten science darks were taken as an extra precaution. Finally, in my case, ten science images were taken of the star Altair, each at 0.5 second exposures due to the brightness of the star.

Fig. 1: Example raw image. Highest value: 29516, Lowest value: 970

The following examples of darks, science darks, and flats were taken to subtract the dark current and bad pixels from the image.

Fig. 2: Example Dark image. Highest value: 20657, Lowest value: 891

Fig. 3: Example Science Dark. Highest value: 13620, Lowest value: 966

Figure 4: Example Flat image. Highest value: 33041, Lowest value: 13230

Results and Discussion:

The program we used was, in this case, reducestinb.pro, since there were no bias images taken at Baker. The program was written to subtract the bad pixels caused by the dark current, and to adjust pixel counts using the flat images. The program is not perfect, and was giving negative pixel values as low as roughly -24000 counts. Adjusting the program to account for these negative values, and adding 200 counts to the overall image to adjust for a negative average outside of the target area, the final reduced image was produced.

Fig. 5: Example reduced image. Highest value: 28614, Lowest value: -299.899

A notable difference is shown from the raw image, as the reduced image shows far less variation in brightness of the overall image, and the target stands out a bit more visibly due to the scaling of the image. The brightness of the target hardly changed, only slightly darker in the final reduced image. 

The negative counts are still there, possibly due to either an error in the reduction process or unexpected initial subtraction of the dark current, automatically produced by the detector. Both issues can be corrected with a slight change in the reduction code.