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APASS magnitudes as standards

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APASS magnitudes as standards

Hello, is there any reason why one shouldn't use APASS magnitudes as 'standards' when computing such things as transformation coefficients?

Thanks, Jesse

MZK's picture
Transformation Comps

Not really BUT the best comps to use are those provided by Arne/AAVSO for M67 and NGC 7790. These are the two open clusters that are recommended for calculating your transformation coeffs. The comps cover a large range of color and have accurate magnitudes in UBVRI so they help generate good transform coeffs. APASS comps only have BV and sloan mags without other calculations.

The M67 and NGC7790 standard fields are provided in VPhot. So even if you don't regularly use VPhot, upload your images of either cluster and use the standard fields. You can alternatively, find the standard comps for these clusters by entering the RA/DEC of the clusters in VSP. At this moment you cannot use the name (e.g., M67) but perhaps HQ will make that work in the near future? Look at the new transformation software products shown under Data/Data Analysis/Transformation in the AAVSO web page.

Hope this helps even if it is more info than you wanted!   ;-)


It so happens we're using

It so happens we're using Sloan filters. I'm trying to automate things and the APASS format is convenient. While I'm using APASS data for M67 to test my code that generates the transformation coefficients, I was wondering why not use it anywhere, anytime. Thanks, Jesse

MZK's picture

Note that most comps used in recent sequences are from APASS. Arne would conservatively call them secondary standards but they are certainly better than most, if not all, available catalogs in the applicable magnitude range. Error perhaps +-0.02 mag at this point?

Of course, they continue to undergo further refinement with each data release so over time the values may change slightly.

Arne's BVRI values for M67 are not specifically from APASS but have been collected by Arne and refined based on his photometry.

(Arne - Speak up if I'm stretching the truth too far!)


HQA's picture
APASS for Soan calibration

Hi Jesse,

APASS is fine for comparison stars.  It is NOT at the level that I would use for determining transformation coefficients.  For Sloan, the only set of true standards are those by Smith, et al.  Note that there is a systematic diffference between ugriz (the actual survey) and u'g'r'i'z' (the standards defined by Smith, et al.).

If you are only interested in a first-order correction to your data, so that your data will lie on top of someone else's data, who are also using the same survey for determining their coefficients, APASS is ok.  If you are using your photometry to, for example, determine the SED of a high-z supernovae, where precise matching to a model is essential, the systematics become important and you need to be on whatever is the appropriate system.

When the final APASS release is available (about 18 months), I will feel much more comfortable about claiming high accuracy, as well as high precision, for APASS.  At that time, it will also be the defining standard for bright stars and for z-short/Yband photometry.  For now, you could use it for determining coefficients, but put an explanatory note in any photometry derived in that way.


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