Description of WFCAM

Description of WFCAM


WFCAM is a near-IR wide-field camera for UKIRT. WFCAM has been designed specifically to carry out large-scale survey observations.

Initially, most of the WFCAM observations were devoted to the UKIRT Hemisphere Survey (UHS), UKIRT Infrared Deep Sky Survey (UKIDSS), observations for S. Korean partners, and other monitoring and backup projects.

The raw data products from WFCAM total around 160GB per clear night. Online processing at the telescope provides near real-time data quality assessment and initial science results, though the raw data are transferred 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

Figure 1.1a – Focal Plane Layout (diagram)

The focal plane layout is shown in Figure 1.1a. The camera consists of four Rockwell Hawaii-II (HgCdTe 2048×2048) 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 center. The cameras are numbered from one to four from the bottom-right in the 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 is shown in Figure 1.1b.

Figure 1.1b – Focal Plane Layout (images)


The science filters available in WFCAM are summarised in Table 1.1. WFCAM contains eight filter paddles, one of which holds blanks for taking dark frames and blanking off the arrays, whilst each of the seven remaining paddles contains a set of four science filters and a clear autoguider filter.

Table 1.1. WFCAM science filters

FilterProfileData50% Cut-On50% Cut-OffBand WidthNotes
ZPlotText0.830.9250.095Similar passband to SDSS z’
YPlotText0.971.070.1Similar passband to SDSS Y
JPlotText1.171.330.16Mauna Kea Consortium Spec
HPlotText1.491.780.29Mauna Kea Consortium Spec
KPlotText2.032.370.34Mauna Kea Consortium Spec
H2 1-0 S1PlotText2.1112.1320.021
Br gammaPlotText2.1552.1770.022Currently unavailable
1.205nbJ1.1991.2110.12Currently unavailable
1.619nbH1.6171.6380.021Currently unavailable

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 
the operating temperature of the filters, please see Hewett et al. 2006.

Figure 1.2. Broad-band filter summary

Figure 1.3. Narrow-band filter summary

Autoguider CCD

WFCAM 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 center 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 1.1a.

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 centers as shown in figure Figure 1.4, “WFCAM autoguider geometry ”.

Figure 1.4. WFCAM autoguider geometry

Figure 1.4 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 its movement range, the on-instrument autoguider in WFCAM cannot 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 guide star at discreet positions within the autoguider field of view. This means that telescope offsets are essentially quantized to the grid of possible guide centers as described in Figure 1.4.

Note that MSB prepared using the UKIRT OT and the WFCAM template library 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 guide stars 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.