HW 9
Okay, we’re in the home stretch now! For this week’s assignment, you’ll perform aperture photometry on the three primary stars in each of the observations. Aperture photometry involves summing up the counts in the pixels on which the stellar image is formed. To do this, you will need to perform the following steps:
1) Extract a 50x50 sub image centered on each star.
2) Use an annular mask centered on the star to determine the median background level near the star (ones between an inner and outer radius, zeros everywhere else).
3) Subtract this background level from the sub image.
4) Multiply the sub image by a circular pixel mask centered on the star’s centroid with a width wide enough to include the entire, smeared out stellar image.
5) Sum up the counts from the masked sub image.
The circular mask should have a radius of 15 pixels, and the annulus should have inner and outer radii of 15 and 20 pixels, respectively. (These radii are determined from a curve of growth, which is a technique taught in the Astro Lab. If you are interested about how to do this, I can show you during office hours.)
Write a function called eb_phot.pro that loops over each science exposure and measures the brightness (number of counts) from each of the three brightest stars on the frame (the brightest star in the center is OO Aql). Use the positions determined from eb_findstar.pro to center your mask and background annulus. Be sure to divide each science frame by either the median dome or sky flat. (Which flat type gives the best results?)
Store the derived counts in the CTS field of the STARS structure.