Dear all,
I do a lot of astrometry of narrow and under-observed double stars (using plate-solving and Astro-ImageJ). Most of these pairs have really awfull quoted magnitudes. The data in the WDS-catalogue are almost exclusively visual magnitudes, most of them are old visual estimates.
I want to add more precise differential photometry to my measurements, using only one (photometric grade) V filter. How do I go about to make transforms, if I want to use a photometric V filter ONLY? I just added the V-filter into the image train and so, I do not need a filter wheel. I'm studying B.D.Warner (A practical guide to lightcurve photometry and analysis) but I only find methods using at least TWO filters (usually B and V). Please help me clarify the situation.
Thanks for your suggestions. Serge
To get a nominal color transformation only one filter is required if you observe a field containing standards with a wide range in color-index, such a Landolt region or one of the open cluster fields given by the AAVSO. The problem then is for 'unknowns', where you do not really know the color-index for individual stars. In that case one does need to have data in two filters (at least).
Are there really under-observed double stars any longer? It seems nearly all pairs that one could observe with ordinary backyard equipment are contained in GAIA, even if those separations and position-angles are not yet in the WDS. (And the sep/pa values are likely to be of far higher accuracy than one can measure.) One would look at the photometry (not always good) and transform the GAIA BP, G, and RP photometry to standard V (say), for which transformations exist. Roughly speaking, for ordinary stars GAIA 'BP' is similar to Sloan g, 'G' is very close to Sloan r, and 'RP' is close to Cousins I. Thus getting from one to the other is straightforward if there is not much interstellar reddening.
Another place to look before taking new data (or to confirm one's results) is the Pan-STARRS ATLAS 'refcat2' catalogue, which is item j/apj/867/105 at the CDS VizieR catalogue-query utility. This shows Sloan g,r,i,z for nearly a billlion stars sky-wide down to mag 20 or so. The photometry was derived from new and existing data and has been shown to be free of systematic errors at the ~0.01 mag level. The Pan-STARRS flavor Sloan data can be transformed to BVRI using the Kostov & Bonev (2018) relations here:
https://arxiv.org/abs/1706.06147
\Brian
Dear Brian, thank you for your rapid answer. According to WDS, there are 65552 doubles with less than 5 observations. However, in many cases, the spectral type is known. From that spectral type, (B-V) could be inferred. Would that not help to calculate transforms obtained form images of say NGC7790 with one photometric V filter? Yours truly, Serge
Yes, you could simply make a rough guess as to the spectral type or color, and go from there. You could also make a very good color estimate from the GAIA photometry. It would be best to carry along some reasonable uncertainties in all this, so that realistic errors could be reported with your measurements.
\Brian