For a list of the comparison and check stars used see this link:
Comparison and Check Stars for the Luminous Stars Survey
Instrumental magnitudes (I) computed from the PSF fit were transformed to the standard magnitude system (M) via the equation:
M = I + Z + K*(C)
The zero-point (Z) was computed as the unweighted average of the difference between standard and instrumental magnitudes for an ensemble of comparison stars in each image. C is the color of the target subtracted from the average color of the comparisons in the ensemble used to compute Z. The color term (K) for the telescope/filter/camera combination is computed from separate observations of M67 done at least once a year. There has been no significant change in those terms between 2012-2016. The relevant time averaged terms are given here.
Comparison stars were selected by the method of Toone (2005JAVSO..34...76T) from the APASS survey (2014CoSka..43..518H, 2016yCat.2336....0H). Stars selected for use as photometric comparisons are fainter than 12 mag, have colors 0 < (B-V) < 0.8 and magnitudes with the smallest errors relative to others in the field. Ideally the comparison stars span the same range of brightness as expected targets in each image. Stars between 13 -- 16 mag are well represented. It was difficult to identify suitable comparison stars fainter than 16 mag.
Stars with neighbors separated by than 1.5 PSF FWHM are rejected as comparisons for that image. The photometric solutions for each image were also checked for comparison stars with significant systematic errors in their published brightness. Those comparison stars which are consistently offset more than 2.5 standard deviations from the average zero-point across all epochs were removed from use on any image.
In addition to APASS there are a few other sources for good comparison stars in M31 and M33. We exclusively used APASS to take advantage of its all-sky calibration so that our photometry was uniform as possible across all fields and epochs. Here are some of those other sources for comparison star photometry in M31 and M33:
Henden & Munari (2006A&A...458..339H)
Henden & Honeycutt (1997PASP..109..441H)
Henden & Honeycutt (1995PASP..107..324H)