Instrument Description


OmegaCAM is the wide-field imager for the Cassegrain focus of the VLT Survey Telescope (VST) on Paranal, a 2.6m modified Ritchey-Chretien alt-az telescope designed specifically for wide-field imaging. It is the only instrument on this telescope. In principle all observations are carried out in service mode.

The VST/OmegaCAM system is designed to critically sample the best seeing at Paranal over a wide field. The telescope has an actively controlled meniscus primary mirror, an active secondary, and an image analysis system. It contains two interchangeable correctors: one is a high-throughput two-lens corrector which provides high throughput from the u to the z band, the other contains an Atmospheric Dispersion Corrector (ADC) for observations at lower elevations. The throughput of the ADC is very low in the u band. The dewar entrance window of the cryostat of OmegaCAM is the third and final optical element.

The VST provides a 1 degree unvignetted field of view, which OmegaCAM samples with a 32-CCD, 16k x 16k detector mosaic (Fig. 2) at 0.21 arcsec per pixel (0.214 for the two-lens corrector, 0.215 for the ADC configuration). The CCDs are thinned, blue-sensitive, 3-edge buttable CCD44-82 devices from e2v of high cosmetic quality. Image quality is specified such that in the absence of seeing 80% of the energy from a point source should fall within a 2x2 pixel area over the full field. The field distortion is very low, so that the image scale is virtually constant over the whole field. There are narrow gaps between the CCDs: the overall geometric filling factor of the array is 91.4%.

In addition to the 32 CCDs making up the science array, OmegaCAM also contains four auxiliary CCDs around the edges of the field. Two of these are used for autoguiding, so that both field position and rotation can be tracked accurately. The other two auxiliary CCDs are mounted 2mm outside the focal plane (one in front, one behind), and are used for recording defocused star images for curvature wavefront sensing and controlling the active optics system of the VST.

Detector array
There are narrow gaps between the individual CCDs, resulting in 1.5mm wide, insensitive strips across the focal plane. In addition there are two broader insensitive bands, each 5.7mm wide, which is where the CCD bond wires are. The gaps translate to 25" and 85" on the sky respectively. Reflection of starlight off the wires is prevented with black metal strips mounted above them.
Focal Plane Segmented
The scientific detector array covers 1x1 degree on the sky, with gaps of 25-85" between chips. Each chip subtends approximately 7.3x14.6' on the sky. The cross shaped vignetting occurs for the Johnson B and V filters, H_alpha, and the segmented calibration filter u_g_r_i.

OmegaCAM contains a 12-filter exchange mechanism. Currently the available filters include the Sloan ugriz set, Johnson B and V filters, several narrow-band filter mosaics, a Stromgren v filter, and a special calibration filter (see Table below). OmegaCAM data are taken in the context of a calibration plan that ensures that all data can be photometrically and astrometrically calibrated to 0.05 magnitudes and 0.1 arcsec rms precision, respectively.

Compared to the Wide-Field Imager on the ESO/MPG La Silla 2.2m telescope, OmegaCAM on the VST offers
1. a four times larger field
2. better cosmetic quality CCDs
3. a 1.4 times primary mirror collecting area
4. year-round operation in service mode
5. better image quality due to active telescope optics, and astro-climate
6. a data flow pipeline for automatic reduction of data
7. the Sloan filter set

Below some selected transmission curves for OmegaCAM and a table of the available filters. Recall that this link provides a GUI of the fitler transmission curves, the telescope and CCD efficiencies and more as part of the exposure time calculator. Information is also given on the tools part of the OmegaCAM webpages.

OmegaCAM/VST throughput
Filter name System Lambda DLambdaa ZP [ADU]b P2PP ID Commentc
u SDSD 350 60 22.9 u_SDSS IF,M
g SDSS 480 140 24.8 g_SDSS IF,M
r SDSS 625 140 24.7 r_SDSS IF,M
i SDSS 770 150 24.2 i_SDSS IF,M
z SDSS 910 120 22.8 z_SDSS IF,M
24.7 B_JOHN
24.5 V_JOHN
z=0.3 H_alpha --- 852,861,869,878 13   NB_852_861_869_878 IF,4Q
Calib SDSS u, g, r, i ---   u_g_r_i_SDSS CG,4Q

a FWHM of filter throughput x mean CCD QE curve.

b These are the zeropoints derived in the first round of commissioning in April/May 2011. There is a  chip-to-chip variation in the ZPs of ~0.1 mag. The detector gain is ~ 2.5 e-/ADU, RON is 5-6 e-.

c IF=Interference Filter; CG=Coloured Glass filter; M=Monolithic; S=Segmented; 4Q=4 quadrants with di fferent passband. Because of the diff erent manufacturing processes, the Calib Filter bandpasses di ffer in detail from the monolithic Fi lters. The z=0.3 Halpha is a private filter of the Munich University Observatory for the first five years of OmegaCAM operations.