It's time for a new set of transformation coefficients. I've been looking at the Landholt equatorial fields in VSX <http://www.aavso.org/standard-stars-vsp> which have photometry available to VPHOT. As my fov is rather small (~20 arcmin), many of these fields have only a few usable stars. I'm wondering how effective it would be to combine the photometry of several such fields into one analysis batch? Arne has recommended doing the transformations on different fields and averaging the results. Wouldn't combining the photometry from the different fields give the same (or similar) result?
If I can get some clear skies, I'll give it a try and report back but I would like to hear any caveats while I'm waiting for those rare skies.
Jim Roe [ROE]
Jim:
As an experiment, I did just that within the past month. I used three henden and landolt fields. I collected 3 images for all filters within a relatively short period of time. One field after the other. Obviously, each field had a different air mass but I managed to select and use fields with an airmass of less than 1.4.
All data, for each filter, fit tightly on the same line (SD < 0.02 or so). Results were reasonable. That is, similar to last year.
Additionally, to get first order and second order extinction transforms, I imaged M67 continuously from 30 degrees to 80 degrees (altitude) on one night. Just for the heck of it! The separate filter transforms from M67 near meridian, agreed "well" with the three field procedure.
I still find that transforms measured in my normal "poor skies" have a wide error range. I find it requires a lot of luck to get a good night/sky! I think my sky near Boston will NEVER be photometric! This observation is supported by the difficuly I have getting "reasonable" transforms. IMHO.
BUT, differential photometry saves the day. It is amazing what good science one can do with a CCD camera in an urban/suburban environment.
Ken