The VISTA Autoguider: predicting magnitudes from 2MASS photometry

The suggestion is that we might be able to predict optical colours for point sources from their near infrared 2MASS colours alone, and thus provide a reliable alternative to the optical catalogues currently used to select VISTA guide stars.

A cross-match between SDSS (DR7) and the 2MASS Point Source Catalogue for regions of high Galactic latitude (|b|>30 degrees), resulted in 540,000 stars common to both catalogues, with a 2MASS SNR>20.0 in J,H and Ks, and SDSS photometry with PSFMAG errors<0.05 in ugriz.

The resulting diagram of i-J versus various 2MASS IR colours is shown here on the left. White dots are all of the half a million stars, while blue dots are the bright subset of all stars with J<13.5, and red dots are the red subset of all stars with J-K>0.7.

The first panel appears to have the least scatter , albeit with a hockey stick turn up for the reddest stars, where i-J becomes almost independent of J-Ks.

We fit the bluer stars only, i.e. objects with J-K<0.75 with a simple linear relation which is shown in yellow. We then take out the linear relation and fit the residuals against H-K, for those stars with H-K>0.15.

The next figure shows the results of removing the J-K linear relation from Figure 1, together with an additional residual H-K dependence for stars with H-K>0.15. This succesfully removes the turn up seen above. The results show a very nice and fairly colour-independent set of residuals. The rms for all stars (the white dots) is 0.08 mag. This means we can succesfully use 2MASS to predict i-band magnitudes to better than 10% using very simple linear relations.

For completeness, the equations are:

i = J + (J-K)*1.175 + 0.459 (for H-K <= 0.15)

i = J + (J-K)*1.175 + 0.459 + (H-K-0.15)*3.529   (for H-K >  0.15)

NB: this gives the transformation to SDSS. A further transformation is required to turn these into VISTA guide camera magnitudes. This will be added in the future.