Using vphot and ptgp to Generate Transformation Coefficients (Abstract)

Volume 43 number 1 (2015)

Ken Menzies

318A Potter Road, Framingham, MA 01701; kenmenstar@gmail.com

Gordon Myers

5 Inverness Way, Hillsborough, CA 94010; GordonMyers@hotmail.com

Abstract

(Abstract only) The AAVSO web site hosts two useful tools, vphot and ptgp, to help develop your transformation coefficients. They can be used together to simplify a tedious process involving standard comparison star selection, image reduction, and spreadsheet analysis. The process necessary to generate Transformation Coefficients involves: (1) measurement of instrumental magnitudes of Standard Comparison Stars, (2) measurement of instrumental magnitudes for a set of images and filters (UBVRI), and (3) downloading of comparison star magnitude files in a standard format. The subsequent process involves: (4) importing the set of standard comparison star magnitude files into ptgp, and (5) the automatic calculation of transformation coefficients and transformation plots. During testing in vphot and ptgp, simple steps and alternatives have been identified to generate accurate transform coefficients. The first vphot step is the obvious need to upload multiple images in multiple standard filters to vphot. The second step is to open each image, overlay all standard comparison stars (currently M67 or NGC 7790), view the photometry table, and download the comparison star magnitude data files. Two alternatives involve either the use of individual image files (for example, 4B, 4V, 4R; 4I) or the stacking of all images for each filter (for example, 1B, 1V, 1R; 1I). The latter improves the SNR. In ptgp after the selection of telescope, standard fields, and data reduction software, one selects all comp star magnitude files. The “Calculate Transform Set” button calculates the applicable filter magnitude and color index coefficients. Testing of this process included an evaluation of the impact of several alternatives including: (1) choice of individual filter images or stacked filter images, (2) choice of all standard comparison stars or a selected subset of standard comparison stars, and (3) choice of a minimum SNR. Each of these alternatives affects the transformation coefficients to a small extent.