VISTA sensitivity and Colour terms

VISTA throughput

From the Bohlin 2007 Vega spectrum it is possible to work out how many photons  will be incident on the VISTA collecting area after modifying for  atmospheric transmission and assuming the effective collecting area is PI*(3.7/2)^2- PI*(1.63/2)^2.   For this initial exercise approximations to the filter transmission, detector  sensitivity and mirror reflectivity, and glass transmission were used (these will be updated shortly since all the information is to hand).

The measured  ZPs in a Vega system were converted to photons for a star with ZYJHK=0, (assuming the ZP is equivalent to 1ADU/sec, and an average gain of 4.2.

The ratio of the scaled photons measured for a 0th magnitude star compared to the expected photons for Vega is then the throughput. and yields the following values (Z was not included in this initial analysis but has been factored in later).  This does not yet include an analysis of any covariance in the detector noise properties, though earlier measurement on lab data suggested this was a much smaller effect than present in Rockwell Hawaii II devices.  The WFCAM throughput is shown for comparison.  The ETC columns uses all the latest data tables to directly compute the throughput ab-initio and compares favourably with the initial values deduced from the measured zeropoints.

ETC limiting magnitude estimates

The ETC can also be used to estimate  VISTA sensitivity figures for, say a range of  total exposure times in 0.8 arcsec seeing using a 2 arcsec diameter aperture, with airmass 1.2 and extinction 0.05.  Aperture corrections (0.35mag) to/from other data assuming a Moffat profile with beta=2.

Derived on-sky zeropoints

2nd pass (improved) colour equations and zeropoints, derived from to 2MASS from a compilation of data measured on good (photometric) nights, with good seeing, and for fields with E(B-V)<0.1. The nominal ZPs are unchanged, but the colour equations have been adjusted slightly to give robust fits to the data.

In deriving the photometric zero-point per field we also make a correction for Galactic extinction toward each star used proportional to  E(B-V)' ,  the Bonifacio, Monai & Beers (2000) correction to the Schlegel et al. extinction map values i.e.

 if  E(B-V) > 0.1  then  E(B-V) -> 0.1+0.65* [E(B-V) - 0.1]

where E(B-V) is computed using bilinear interpolation from the 4 nearest  Schlegel map pixels.

The coefficients used are: 0.37, 0.14, 0.010, 0.015, 0.005 for Z, Y, J, H, Ks respectively in the sense of increasing the inferred zero-point. The maximum offset is currently limited to E'(B-V) = 10 pending further investigations of the dependency in high extinction regions.

Comparison with WFCAM

Our simple analysis suggests that VISTA is a factor 2.5 (at YJHK) to 3.5 (at Z) times more efficient than WFCAM. VISTA has a smaller collecting area though, by around 20%, which means the differences in zeropoints between the instruments is more like a factor 1.6 to 3 (0.5 mags at K, 1.2 mags at Z).