Introduction

The Wide Field Camera (WFCAM) is mounted in a forward Cassegrain position in front of the UKIRT primary mirror on its lifting plug. The vacuum vessel is approximately 1 m in diameter and 3 m long. The overall length to the field lens is ~5 m and the overall mass of the instrument is ~1.5 tonnes. A two stage closed cycle cooler is used. The first stage cools the majority of the instrument (radiation shield, support structure, tertiary mirror) to ~120 K. The second stage cools the focal plane unit to ~70 K. Figure 1 shows the instrument as mounted in its final position on the telescope. Key operating parameters and specifications of WFCAM are listed in Table 1. The four IR detectors and autoguider CCD are positioned at the focal plane and closely packed. This can be seen in Figure 2. This presented a challenge due to the lack of space around the detectors for positioning components and connectors. The allowed footprint for each IR detector PCB is only slightly larger than the ZIF socket holding the detectors themselves. A flexi-rigid design was used for each detector PCB with flexible sections linking the main board section to connectors for the outputs/biases and clocks.

Table 1. Key instrument design parameters

Detector

4 x Rockwell HAWAII-2 PACE, 2x2 array, 94% spacing of active areas

Pixel scale

0.4 arcsec/pixel

Field of view

0.933° (single exposure, diagonal)

Image Quality

MTF > 0.5 at 1.2 cycles/arcsecond

Broad band - Y (1.00-1.07 |am), J98 (1.17-1.33 |am),

Filters

H98 (1.49-1.78 |am), K98 (2.00-2.37 |am)

Narrow band - H2 (2.122 |am)

Figure 1. WFCAM mounted on telescope. Figure 2. The final focal plane assembly.

The four IR detectors are each controlled using their own 32 channel ARC-III controller. The controllers are highly synchronized using a master clock and frame start pulse scheme to ensure that they clock the arrays at exactly the same time to avoid clock coupling between detectors. However, the autoguider CCD is operated asynchronously to the IR detectors. We typically run this E2V47-10 device at 100-200Hz windowed frame rates to feed corrections to our tip-tilt secondary mirror system. We do not see any clock coupling between the autoguider clocks and the IR detectors even though the devices are mounted in close proximity to each other. We did mount the CCD completely inside a "Faraday" enclosure, even coating the window with Indium Tin Oxide on both sides. We mounted the window using an electrical gasket to the rest of the enclosure. We reduced all the CCD clock swings to the minimum voltage required for correct operation. This does reduce full well, but this is not an issue for our autoguider system.

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