A modular dewar design and detector mounting strategy for large-format, astronomical CCD mosaics

Abstract

We present our mechanical design for a very-large-format, 4096x4096 pixel chargecoupled device (CCD) mosaic constructed from a 2x2 array of two-edge-abuttable, 15 pm pixel 2048X2048 CCDs. Each 2048x2048 quadrant is mounted in a custom abuttable package, thus allowing us to assemble the mosaic from the best, separately tested 2048 X 2048 imagers, while keeping the gaps between imaging areas less than 500 pm. We describe the advanced manufacturing techniques used to fabricate our package and mounting structure so that the assembled mosaic is flat ( ± 1 pixel), and the rows and columns of the individual quadrants are aligned. We have also packaged monolithic 4096x4096 CCD arrays cut from the 100 mm silicon wafers that contained four functional 2048x2048 edge-abuttable imagers. The quadrants of the monolithic array are perfectly aligned and flat, and the gaps, as designed on the wafer, are just slightly larger (approximately 550 pm). We describe our design for a modular dewar built to house the 4096x4096 CCD mosaic. The dewar splits into two parts-a camera head housing the mosaic array, and a cryogenic back containing a 3 liter capacity liquidnitrogen (LN 2 ) can, the associated LN 2 fill and vent structures, and the vacuum connections. Thermal connection to the detector is made through a spring-loaded copper contact that allows the two dewar halves to be separated easily. The dewar can be used in any orientation (look-up, look-down, or horizontal) with nearly full LN 2 capacity, by simply inserting a vent extension tube for look-up applications. With a fully wired 4096x4096 mosaic installed and operating at -100 C, the hold time of the dewar exceeds 24 hr.

Wide-Field CCD Imaging at CFHT: The MOCAM Example

“…Since the UH 4K×4K camera (Luppino & Miller 1992) was designed for the CFHT prime focus, it was decided to simply duplicate the dewar and camera head, as well as the Loral 2K×2K two-edge buttable CCDs initially used in this camera. The standard CFHT GenIII CCD acquisition system based on a San Diego State University (SDSU) controller (Leach 1988) was adapted by the Toulouse group to a parallel multi-output system, and the Pegasus software was upgraded to allow the acquisition of images as large as 33 Mbytes with the multi-output feature.…”

MOCAM: A 4k × 4k CCD Mosaic for the Canada-France-Hawaii Telescope Prime Focus

“…The objective of this project is to produce a reliable facility instrument for the whole scientific community in the shortest time. The dewar developed by G. Luppino from the University of Hawaii was chosen because its design had been fully developed and optimized (Luppino and Miller 1992) as a prototype of the 8k χ 8k mosaic that is currently near completion (Luppino et al 1994).…”

“…The dewar has been fully described by Luppino and Miller (1992). It was designed to handle a C C D mosaic of four 2k χ 2k two-edge buttable L O R A L 15 μιη pixel CCDs.…”

“…1 gives the focal plane characteristics at the C F H T prime focus. The larger gap between the CCDs is 500 μιη and thanks to the recent development that uses new techniques to cut the packages (Luppino and Miller 1992) onto which the CCDs are mounted, the alignment between the different CCDs is kept within two pixels and the mosaic use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900056461 is kept flat to about two pixels, introducing no defocusing effect at the the f/4 prime focus.…”

First results from the UBC etch-alignment mosaic CCD

“…The largest existing CCD mosaic (8K x 8K pixels) uses a mounting scheme developed by Luppino et al 1 . It uses custom packages constructed for each CCD from a low expansion alloy, which are attached to a machined mounting block.…”