Sky Subtraction

Using the Leuschner telescope, we took images of 3 stars. In order to do photometry on our data, we had to clean up our images a bit to isolate the stars themselves. First, we had to subtract out the dark current and sky background light from our images. We did this by taking a set of nine images of a single star, each image slightly offset from the other so that the star is in a different place in each image, and extracted out the sky and dark data by finding the median value of the composite images at each pixel. Assuming that the sky is uniformly lit and the dark frame is consistent between all nine images, this method will isolate any moving sources of light. Any star or other source of light that is brighter than the sky and moves relative to the sky will be disregarded by taking the median of the composite image pixels (so long as they do not occupy the same space in 4 or more images). If we subtract this sky and dark frame from any of the original images and then divide by the flat field image to correct for the relative gain, we will then have an isolated the star in our image. We find that some of our sky background remains because our approximation that the sky is uniform and the same between all nine frames is not exact. However, to the first order it is a very good estimate because the variance from zero that we get for our remaining sky is at most about 15 DN.