Accuracy and Precision in Amateur Photometry

Volume 50 number 1 (2022)

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Edward O. Wiley

Live Oaks Observatory, Stellar Skies, Pontotoc, TX; ewiley@suddenlink.net

Kenneth Menzies

Tigh Speuran Observatory, Framingham, MA; kenmenstar@gmail.com

Abstract

Photometric accuracy and photometric precision were determined using the average magnitude and standard deviation of 7 to 10 images of 63 Landolt stars taken from 11 Northern Landolt fields by two amateur observers using CCD sensors in B, V, and Ic. Similar measures were taken for two of these Landolt fields using a CMOS sensor from the AAVSO Bright Star Monitoring NH2 observatory. A series of analyses were performed on observed average magnitudes compared to known Landolt magnitudes of the pooled data under different treatments that included both transformed and untransformed analyses under both single comparison star and ensemble treatments using observed minus known magnitude values (O–K analysis). A variety of non-parametric tests of magnitudes resulting from different treatments using absolute O–K values was used to assess the statistical differences between treatments. Regression analysis using untransformed (“raw”) O–K values and B–V color indexes for each star were used to assess the differences between transformed and untransformed treatments for each filter and test for any statistical differences. Correlation analysis was used to assess the relationship between accuracy and precision. In most cases, transformed magnitudes are statistically more accurate than untransformed magnitudes. Even when there is no statistical difference in median values between transformed and untransformed results there is a statistically significant difference in the regression analysis indicating that transformation improves accuracy for the data as a whole in each filter. There were no statistical differences between the 16-bit CCD results and the 12-bit CMOS results for the two fields analyzed. Both were capable of a median accuracy of 0.02 magnitude or less, which is similar to the accuracies of the same APASS secondary standard stars in four of the fields included in the study. We detected no statistical difference between using a single versus small ensemble of comparison stars but prefer ensembles for reasons given. Precision is not correlated with accuracy nor need it be for some studies.