improve kband error

[boada]

Add some quality flags to improve the photometry in the 2MASS bands.

Before:

boada$ pp_calibrate -cat 2MASS test.fits (16547, 25) (sources, columns) read from test.fits query Vizier for 2MASS at xxx.xxx/ +xx.xxx in a 0.99 deg radius 2060 sources retrieved. 2060 sources downloaded from 2MASS 1133 sources with accurate magnitudes in K band zeropoint for test.ldac: 26.028+-0.170 (134/265 reference stars) write calibrated data into database files

After:

boada$ pp_calibrate -cat 2MASS test.fits (16547, 25) (sources, columns) read from test.fits query Vizier for 2MASS at xxx.xxx/ +xx.xxx in a 0.99 deg radius 1280 sources retrieved. 1280 sources downloaded from 2MASS 952 sources with accurate magnitudes in K band zeropoint for test.ldac: 26.024+-0.079 (115/226 reference stars) write calibrated data into database files

From the 2MASS documentation:

Three character flag, one character per band [JHK], that provides a summary of the net quality of the default photometry in each band, as derived from the Read Flag (Rflg), measurement uncertainties ([jhk]cmsig), scan signal-to-noise ratios ([jhk]snr), frame-detection statistics (Ndet), and profile-fit reduced chi-squared values ([jhk]psfchi).

And:

A = Detections in any brightness regime where valid measurements were made (Rflg="1","2" or "3") with [jhk]snr>10 AND [jhk]cmsig<0.10857.

B = Detections in any brightness regime where valid measurements were made (Rflg="1","2" or "3") with [jhk]snr>7 AND [jhk]cmsig<0.15510.

C = Detections in any brightness regime where valid measurements were made (Rflg="1","2" or "3") with [jhk]snr>5 AND [jhk]cmsig<0.21714.

Sure it reduces the number of sources that we have to choose from, but it also reduces the zeropoint error by a factor of 2.

[mommermi]

Yep, that's a useful addition. Thanks!