Instrument Description

A full description of the VISTA telescope and the instrument is given in the VIRCAM @ VISTA user manual and summarized on this page later. Some information can be found at the WEB pages provided by the VISTA consortium, but the ESO VIRCAM page is the definitive source of information about the instrument.

Modes

VIRCAM is a wide field imager for surveys, and therefore, it offers only imaging observations. Observations of moving target with non-sideral rates are not possible.

Filters and Focal Field Geometry

The filter wheel is the only moving part in the camera. It has seven slots for scientific filters, and one more position reserved for a SUNBLIND (i.e. closed or dark) filter. Each filter is a set of sixteen pieces, one for each detector, mounted on a tray.
Currently, the camera carries a complement of Z, Y, J, H, Ks, NB980, and NB118 filters. Note that the NB980 position actually holds two subsets of filters - NB980 and NB990, - each covering eight positions. So, to obtain full coverage with these two NB filters the survey area has to be observed twice, with rotator at two complementary positions separated by 180 deg.
The list of available filters is given in the table below, followed by a plot with the filter transmission curves, the detector quantum efficiency curves, and a theoretical model for the atmospheric transmission for various airmasses and water vapor content of the atmosphere.

Filter Wavelength FHWM Comment
  micron micron  
Z 0.88 0.12 in camera
Y 1.02 0.10 in camera
J 1.25 0.18 in camera
H 1.65 0.30 in camera
Ks 2.15 0.30 in camera
NB980 0.98 0.01 in camera, sharing a slot with NB990
NB990 0.99 0.01 in camera, sharing a slot with NB980
NB118 1.18 0.01 in camera

A tar file with all curves shown in this plot in machine readable form is available here.

The sixteen 2048x2048 pixel IR detectors (Raytheon VIRGO HgCdTe 0.84-2.5 micron) in the camera are not buttable and are arranged as in the following figures which shows a diagram of the focal plane as would be seen looking directly down the camera body (down the Z-axis which on the telescope points towards the sky). On the sky (in the default instrument rotator position) +Y corresponds to N, and +X to West:


A single Integration of length DIT secs (or a co-added series of these known as an Exposure) produces a sparsely sampled image of the sky known as a "Pawprint". The area of sky covered by the pixels of a "pawprint" is 0.6 deg2. Full almost uniform sky coverage of a "tile" of 1.501 deg2 can be achieved with six "pawprints", offset by ±47.5% in y at two respective x-positions offset by 95% of the detector size. Any sky position of a "tile" will fall at least on two of these six pawprints.

Instrument Performance

The instrument performance is summarized in the following table:

pixel scale 0.34 arcsecs/ pxl
best image quality achieved 0.6 arcsecs including seeing, optics and sampling
estimated image qualityParanal seeing convolved with the instrument PSF of 0.51 arcseconds
image distortion <15% in the corners
photometric calibration+-2% RMS in respect to 2MASS in J,H,Ks
photometric calibration+-2% RMS internally
sky concentration/ illumination <5% absolute, can be corrected down to <2%
detector16 Raytheon VIRGO HgCdTe arrays, sensitive over 0.84 to 2.5 micron, high quantum efficiency (click here for the QE curve), large number of hot pixels, some dead areas on detector 1

The system characterization and the optimization of the calibration strategy continue during the operation phase.

Last modified: 2013-03-07, vivanov