Aperture photometry lab report

AST 311: Observational Techniques

Determining magnitudes of astronomical objects

Missouri State University

Joshua W. Kern

Abstract

On September 30, 2015, we used an 8-inch telescope at Baker Observatory to take 10 images of Saturn using the ST-I monochromatic CCD cameras without filters. Darks, flats, and sky darks were taken to reduce the science images which has since been completed. We then used aperture photometry codes to determine the instrumental magnitude of Saturn in each of our ten images. Using images taken of Vega by Matt Zerilli on the same evening, we were able to calculate the instrumental magnitude of Vega using the aperture photometry code, we researched the apparent magnitude, and consequently used it as a calibration star. After calibration and using an aperture override equal to four, our calculated apparent magnitude of Saturn is 0.387 ± 0.067.

Experiment

The purpose of this experiment is to become familiar with performing aperture photometry and applying the necessary codes (supplied by Dr. Plavchan) in order to determine apparent magnitudes for Saturn. The aperture photometry code finds objects with luminosities over a certain threshold and prints to the screen the x and y coordinates, instrumental magnitudes, errors, etc. This code was ran on each of our ten images of Saturn which consequently gave use ten instrumental magnitudes. Each one was converted to an apparent magnitude using Vega as a calibration star and an equation derived from the distance modulus. The average was then taken of the ten calculated apparent magnitudes while the error was determined by taking the standard deviation of the apparent magnitudes.

Results & Discussion

The science images we obtained of Saturn were taken with an integration time of 0.05 seconds and no filters were used. The integration time of Vega's images were 0.1 seconds and were also taken without a filter. This scaling of integration times effects our calculations for the apparent magnitude of Saturn, and was corrected for in the apparent magnitude equation that was given in lecture. Examples of the images used are given below with Vega on the left and Saturn on the right. 

However, since Saturn is an extended source the magnitude given by the photometry code using the standard aperture size does not accurately represent the magnitude of the object. Therefore, an aperture override equal to four was used to obtain magnitudes that are reasonable with respect to Vega. On average the instrumental magnitudes of Saturn were in the range of roughly 10.5 -10.7 where as the instrumental magnitude of Vega was found to be 11.06. Using this information, the average apparent magnitude of Saturn could be calculated and was determined to be 0.387 ± 0.067.

The scientific accuracy of our measurements, however, is lacking due to many reasons. First, for precise measurements of astronomical magnitudes the range of wavelengths in which you image the target must be controlled, which we did not do. Secondly, Vega was not a good choice for a calibration star due to the fact that Saturn and Vega, on the night of observations, were being viewed through drastically different airmasses. Finally, the images obtained of Vega and Saturn were taken using two separate telescopes and imaging cameras meaning that the quantum efficiency of each system was assuredly different. However, the process that was needed to perform the apparent magnitude calculations was accurate which can be applied in the future using more rigorous imaging constraints.