Hi All,
New to AAVSO and still a beginner with photometry. My friends and I have a telescope in Spain, primarily used for pretty pictures, but equipped with a QHY600 and Sloan g’r’i’ filter set. During winter we provide photometric data and assist Undergrad students at the Uni of Glasgow. We have never gone downthe road of transforming magnitudes, but with T CrB teeterring on the edge, I thought its time to use the T coefficients.
I took some data of M67, probably several hours per filter with 120s subs, and combined in to a single master image for each filter.
I have downloaded and got working the Transform Generator, and so I proceeded to upload my M67 data.
With a quick play in VPhot, I tried to import Standard Stars (having told the uploader that my filter is ‘SG’ etc). However, it says that no standard stars exist for that filter.
I hope I am wrong, but M67 has to have been imaged in SDSS filters surely.
Am I doing something wrong, how do I use TG with Sloan filters?
and specify the constraint that the name of the entry’s cluster is “M 67”.
One could, of course, query the SDSS Archive for its measurements of the stars, either one by one (tiresome) or based on some table of positions (requires some arcane knowledge). But that’s not easy. I hope that the small set of 53 stars mentioned above might help you a bit.
A quick check of a few stars in the VizieR ATLAS refcat2 catalogue shows that the g and r mags differ by only about 0.03 mag from the UCAC4 mags displayed in Tonny Vanmunster’s Phoranso package. After solving a FITS image in Phoranso, UCAC4 stars are marked with a green aperture (if this catalogue is selected as the default photometric catalogue). I think flagged stars are only those with B-V between 0.5 and 1.0. Hovering the cursor over a flagged star shows both the SLOAN and Johnson magnitudes. VSX variables are also automatically labelled down to a user-selected minimum mag in the image.
For what it is worth, the UCAC4 g,r photometry is copied from APASS DR6. To produce refcat2, APASS data taken from CTIO (i.e. south of about +20 Dec) was re-reduced. See section 3.3 of the refcat2 paper:
…for a description of what was done, and the size and nature of the systematic problems encountered (much larger than 0.03 mag).
Not sure if it’s worth continuing the discussion, but my previous post was stimulated by the Phoranso labels of the UCAC4 stars in a particular field (around GY Pup), 32 x 22 arc min taken with my equipment. As noted previously, if UCAC4 is chosen as the photometric catalogue in Phoranso, solving the image flags UCAC4 stars with B-V from 0.5 - 1.0, and allows pop up labels of the star ID and J2000 position, and griBV mags with error flags. Of the 25 stars labelled, only 3 had g or r or B or V error flags of 0.1 or greater. All the others had g or r error flags of 0.04 or less, and B or V flags of 0.05 or less (I think one had a B error flag of 0.06). Most stars are 11th to 14th V mag. There is only one AAVSO comp in the field (AUID 000-BPW-316), a 10.6 mag star with a V error flag of 0.111. I don’t know why the latter is the case (maybe it’s an APASS error flag, but I haven’t checked), because the Tycho BT and VT errors in VizieR are 0.074 and 0.073, and the B-V error is 0.085.