The camera will be constructed in three steps. Our first step was to build two test cryostats which could in principle hold a 3 x 3 array of close-packed OTAs for 200 Mpixels, but we are using these cryostats for device testing.
The OTAs are four-side buttable, so for testing we normally mount them in a handling bracket. The test cryostats can carry a loose-packed 2 x 2 array of OTAs (90 Mpixels) on handling brackets.
The next step is "Test Camera 3" (TC3), which is almost an exact quarter of the "Giga-pixel Camera 1" (GPC1). This camera will be a 4 x 4 array of OTAs, closely packed onto a silicon carbide (a good CTE match to silicon, thermal conductivity, and stiffness) focal plane, which has been figured to the curvature required by the optics. The close packed array of devices connects via flexprint to a rigidflex board below. Four devices in a line are bussed out together, and the rigid flex passes through a slot in the cryostat wall with epoxy potting making a vacuum seal. One of the main advantages of the potted rigidflex is the elimination of hand wiring.
TC3 will have two controller boxes, each holding 4 controller boards. Each board provides 16 channels for 2 OTAs, so the system will have a total of 128 readout channels and 360 Mpixels. A CTI cryocooler will cool the focal plane instead of the Cryotigers used by TC1 and TC2. We will gain experience in the thermal management of the focal plane and devices, which is extremely important because at 1 ^m the QE will vary by about 1% per degree.
TC3 will also incorporate both wavefront sensors, the deployable Shack-Hartmann apparatus and the continuous curvature sensing optics. TC3 will be used for first light on PS1, and will be populated with the development Lot 1 OTAs which are already in hand. The data from TC3 will provide an excellent data stream for shakedown of the Image Processing Pipeline (IPP). When the PS1 telescope is operating reliably (~June 2006) we will replace TC3 with GPC1.
The full-scale Gigapixel camera, illustrated in Fig. 4, with a focal plane of some 1.4 billion pixels spanning 40 x 40 cm, is essentially four close-packed copies of TC3. The final lens in the optics is the cryostat window.
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