Reduced Image- Lab Report

AST 311: Observational Techniques

Reducing Astronomical Images

Missouri State University

Joshua W. Kern

Abstract

We reduced the science images using IDL and, having an exposure time of 0.05 seconds for our science images, we obtained counts of ~14,000 for Saturn. However, negative counts were given in all other areas of our reduced science image not associated with Saturn itself. This is a manifestation of the fact that while obtaining our science images an automatic dark correction was made on the image. Since our code subtracted the darks as well we, in effect, subtracted two sets of dark images. This will be corrected for in future imaging endeavors. Also, due to tracking errors with the telescope, Saturn drifted in our images.  This made creating a final stacked science image impossible without correcting for the drift. Therefore our final product is 10 separately reduced science images.

Experiment

The purpose of this experiment was to reduce the astronomical images we obtained on September 30, 2015 of Saturn at Baker Observatory. We used the 8-inch telescopes with the ST-I CCD cameras to obtain science images as well as all of the necessary correction images. An example of our raw science image is given below with counts of ~16,000 corresponding to Saturn itself.

Examples of our darks, biases, and flats used for image reduction are given below respectively.

Results and Discussion

To reduce our science images we used IDL to subtract off the excess counts caused by “noise” from the sky and the instruments. We also adjusted the counts for pixel to pixel variations by taking flats. Since no filter was used to obtain the science images, no filter was used when obtaining flats. Exposure times for darks and flats were both 2 seconds, and exposure time for our sky-dark and science images were 0.05 seconds.

We then used a code written by Dr. Peter Plavchan that utilizes this information to reduce our raw science image. An example of the reduced science image is given below with counts of ~14,000 corresponding to Saturn. These counts are slightly lower than the raw science image which is to be expected.

However, in areas of the image which don’t correspond to Saturn the counts have negative values. This is due to the fact that an automatic dark correction was made while obtaining the raw science images without our knowledge. Therefore, when we reduced our images using Dr. Plavchan’s code we consequently subtracted off the dark images a second time leaving negative values where the counts were already much smaller than the counts associated with Saturn. This can be easily corrected for future image reduction by slightly modifying the image reduction code.

A compilation of all ten images of Saturn was attempted in order to increase our signal to noise ratio. However, due to the fact that the tracking systems of our telescopes is fickle at best, Saturn drifted on our CCD. Consequently, when a final compilation image was created, the image was dominated by noise and no distinguishable object could be seen. Therefore, each science image was reduced and stored separately.