CFHT Information Bulletin, number 39, Second Semester 1998

Three New CCDs for CFHT

Tim Abbott, CFHT, Resident Astronomer,

In December 1996, CFHT placed an order with EEV to purchase a single EEV CCD 42-80 (plus an engineering-grade device for "smoke testing"), primarily for use on MOS. EEV CCD 42-80's have 2048×4096 pixels, each of which are 13.5µ square. These devices will just fill the available MOS imaging field, with no mask holder installed, and will provide greater spectral coverage than currently available CCDs when oriented with the vertical direction of the CCD parallel with the dispersion direction. The increased QE and lower readout noise of this device will significantly enhance MOS's capabilities.

In 1997, given EEV's production schedule and to accelerate delivery of the device, the order was amended to be for a CCD 42-90. These are identical to CCD 42-80's, but have another 512 rows, thus measuring 2048×4608 pixels.

Also, given CFHT's restricted resources, it was decided to contract-out the adaptation of a standard CFHT dewar to accept this new CCD. The design and fabrication of the new mount was carried out by Ron Johnson of the University of British Columbia. The engineering grade device, named EEV1e, was received by CFHT in May 1998 and was used to "baptise" the new CFHT cleanroom (see Wilcox, this Bulletin). It was immediately sent to Ron Johnson who installed and tested it in the new dewar. Ron found that this engineering grade device was of rather better quality that we anticipated - preliminary results suggest a read noise of <3 electrons/pixel, a well depth of ~100,000 electrons and excellent CTE. Although there are several very bright hot columns and a number of fainter ones, there appear to sufficient good regions to make this device suitable for use on Gecko. At the time of writing, the completed dewar and EEV1e have been received back at CFHT and are awaiting the availability of a controller for optimisation and tests.

Shortly before the delivery of EEV1e, EEV informed CFHT of the availability of another CCD42-90, from the same wafer as EEV1e, which was "grade 1" in all respects except for a rather poor serial CTE of 0.99995. The device has two functioning readout amplifiers, with readout noise of each measured at less than 3 electrons/pixel. The well depth is greater than 200,000 electrons and vertical CTE is 0.999997, cosmetically, it is near-perfect. Reasoning that this device would make an excellent spectroscopic detector, either for Gecko or OASIS, CFHT purchased it for a price significantly reduced over that of an uncompromised grade 0 detector. It was received in Waimea in June 1998 and has been named EEV2. It currently resides in the cleanroom, awaiting the completion of tests on EEV1e in the new dewar. At this time, we are anticipating first light for EEV2 in September 1998 for the scheduled OASIS run.

The device originally ordered, EEV1, has yet to be received, but CFHT must still obtain two more dewars for this relative glut of CCDs! We are moving ever closer to our goal of a detector dedicated to every instrument. When that goal is reached, instrument changes and maintenance will be rendered considerably more efficient. One argument against such a goal is that observers typically request the best detector for their use, regardless of the instrument, and may therefore be frustrated by such restrictions if the devices are significantly different. However, CCD manufacturers improve all of the time and the salient properties of these three EEV CCDs will be either so similar as to make no difference, or optimally matched to the instruments they will serve. For example, all three QE curves will be very similar; EEV1e can only be reasonably used at Gecko and EEV2 at OASIS (and Gecko, if EEV1e is destroyed in testing) leaving EEV1 for MOS.

Editor: Dr. Timothy M. C. Abbott,
Copyright © 1998, CFHT Corporation. All rights reserved.