|
|||
| Joint Astronomy Centre |
|
|
|
| Show document only |
IntroductionWFCAM is a near-IR wide field camera for UKIRT.WFCAM has been designed specifically to carry out large-scale survey observations. Most WFCAM observing is devoted to the UKIRT Infrared Deep Sky Survey (UKIDSS) and to Campaigns Projects, but normal proposals for WFCAM through PATT occupy some 15% of the available WFCAM time. The raw data products from WFCAM total around 200GB per clear night. On line processing at the telescope provides near real time data quality assessment and initial science results, though the raw data are transfered to CASU at Cambridge in the UK for full off-line processing, and from there to the WFCAM Science Archive at the WFAU in Edinburgh. The WFCAM Science Archive is the primary data source for both UKIDSS results and PATT project data. Focal plane layout The focal plane layout is shown in Figure 1.1. The camera consists of 4 Rockwell Hawaii-II (HgCdTe 2048x2048) arrays spaced by 94% in the focal plane, such that 4 separately pointed observations can be tiled together to cover a filled square of sky covering 0.75 square degrees with 0.4 arcsecond pixels. A fixed auto-guider array is installed in the centre. The cameras are numbered from 1 to 4 from the bottom-right in counterclockwise direction. A single exposure covers an area of 0.19 sq. degrees, while 4 exposures cover an area of 0.75 sq. degrees. Additional details about the focal plane and the optical layout can be found here. Note that due to WFCAM's large field of view, there is significant distortion over the field. Although this can be handled during the astrometry calibration stage of data reduction, a second order effect known as differential distortion is important in determining offset sizes. In general, it is safer to keep offset sizes small, around 10 arcsec. A typical image from WFCAM look like this:  FiltersThe science filters available in WFCAM are summarised in Table 1.1. WFCAM contains 8 filter paddles, one of which holds blanks for taking dark frames and blanking off the arrays, whilst each of the 7 remaining paddles contains a set of 4 science filters and a clear autoguider filter.Table 1.1. WFCAM science filters
The profile of broad-band filters are shown in Figure 1.2, and narrow-band filters are shown in Figure 1.3. To see the profile of a specific filter, follow the links within Table 1.1. The J, H, and K filters are specified to conform to the specification of the Mauna Kea consortium. The design of the MK consortium filter set is described by Tokunaga et al. (2002, PASP, 114, 180). The WFCAM Z filter has a similar effective wavelength to the SDSS z' filter. For details, see below. Note - all the filter profiles here were measured at room
temperature and are not corrected for the wavelength shifts induced by
cooling to 120K and the 10-degree angle of incidence of the beam on the
filter. For a description of the final transmission curve, corrected for detector quantum efficiency, angle of incidence and
Figure 1.2. Broad-band filter summary  Figure 1.3. Narrow-band filter summary  Autoguider CCDWFCAM contains an on-instrument autoguider used to send tip-tilt commands to the UKIRT secondary mirror unit. The autoguider detector is an optical CCD that is mounted in the centre of the instrument focal plane between the Hawaii II science arrays. The autoguider CCD is rotated 45 degrees with respect to the science arrays, as shown in Figure 1.1, “WFCAM focal plane layout”.The pixel scale of the autoguider CCD is 0.251"/pixel. The autoguider works by holding the centroid of an image of a guide-star at the point where 4 super-pixels meet. This geometry results in a grid of possible guide centres as shown in figure Figure 1.4, “WFCAM autoguider geometry ”. Figure 1.4. WFCAM autoguider geometry 
Diagram illustrating the geometry of the WFCAM autoguider superpixels and possible guide locations Unlike the off-instrument autoguider used with the other ukirt instruments, which is mounted on an X-Y crosshead stage which can be moved to arbitrary positions within it's movement range, the on-instrument autoguider in wfcam canot physically move. Instead, the guide position is selected by selecting the position of the 12×12 pixel area of the CCD which is used for autoguiding. Although this allows for very fast, time-efficient offsets, it means that it is only possible to hold the guidestar at discreet positions within the autoguider field of view. This means that telescope offsets are essentially quantised to the grid of possible guide centres as described in Figure 1.4, “WFCAM autoguider geometry ”. Note that MSB prepared using the UKIRTOT and the WFCAM template libray do automatically provide suitable offsets that make use of this geometry, so that the user does not need to worry about this. In addition, the ukirt off-instrument guider has a filter wheel enabling the operator to select neutral density filters to prevent the CCD saturating when using bright guide stars. The wfcam on-instrument guider has no choice of filter, and this cannot use guidestars that will saturate the CCD in the minimum exposure time. The autoguider wavelength bandwith is approximately 0.6-1.0µm, which corresponds vaguely to R band up to Z band. The WFCAM autoguider is specified so that under ideal conditions, we can guide on stars as faint as V=18. Ideal conditions in this sense means dark sky, and seeing <0.5". Because the WFCAM autoguider has no neutral-density filter capability, guide stars which are too bright will saturate the guider CCD and guiding will fail. In general, guide stars should not be brighter than V=7. |
