The Accuracy of Differential Photometry of Star Magnitudes

Star magnitude data is most valuable scientifically when it can be measured over time, in different geographic regions, and on-demand.  The desire for this detailed information has heightened professional astronomers’ dependence on observations by amateur astronomers.  Amateurs, in their quest to accurately measure the apparent magnitudes of stars, have increased their reliance on digital single lens reflex (DSLR) cameras for astrophotometry.  To improve the accuracy of their DSLR photometric images, amateur astronomers often utilize image integration, the process of “stacking” multiple images to produce a single composite image with a comparatively high signal-to-noise ratio (SNR).

The purpose of the experiment was to examine how the number of DSLR camera images integrated into a composite image affects the accuracy of differential photometry of the apparent visual magnitudes of stars.  Three composite types – Two-Image, Three-Image, and Five-Image – were tested.  It was hypothesized that if the accuracy of differential photometry of low variability stars’ visual apparent magnitudes is affected by the number of integrated DSLR images in the composite image, then the Five-Image Composite would generate the most accurate apparent visual magnitudes when compared to the Two-Image Composite and Three-Image Composite. 

The procedure was to use a DSLR camera to capture 150 “light” star field images, plus calibration images. Groups of five "lights" were integrated into 30 Two-Image, 30 Three-Image, and 30 Five-Image composites. For each composite, instrumental magnitudes for the Target Star and six Comparison Stars were recorded, and the Target Star’s measured magnitude was calculated.  The control was the Target Star’s catalogued visual magnitude. 

The experimental data showed the Two-Image, Three-Image, and Five-Image means (in magnitudes) were .065, .056, and .062, respectively.  The Three-Image Composite produced the lowest mean difference between the Target Star’s measured magnitude and the control, therefore the stated hypothesis was not confirmed. 

In summary, differential photometry using integrated DSLR images demonstrated the Three-Image Composite generated more accurate magnitude measurements than the Two-Image and Five-Image types.  Application of this research could contribute to the refinement of astrophotometry practices and to the improvement of the quality of photometric data utilized by professional astronomers.