You are here: Home / Surveys & Projects / VISTA / Technical Information / The VISTA Autoguider: predicting magnitudes from 2MASS photometry

The VISTA Autoguider: predicting magnitudes from 2MASS photometry

The results of comparing 2MASS (JHK) and SDSS (i') photometry


ijv2mThe 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.