We are shortly about to enter the fourth semester that the Big Throughput Camera (BTC, http://www.ctio.noao.edu/pfccd/btc_arw.html) has been available for wide-field imaging at the prime focus of the Blanco 4m telescope. During this time the percentage of BTC time requested compared to that for the total instrument complement has steadily risen, from 15% for semester I 1997 to 30% for semester II 1998. This has partially been at the expense of the prime focus CCD imager as we increase the 6 × 6 inch filter complement, but also represents a decrease in the requests for the R-C spectrograph. Interestingly, BTC proposals have always done better than average in the TAC. As an example, the BTC is scheduled for 25% of the available nights in the present semester whereas it corresponds to only 20% of the total requests.
Upgrades to the BTC planned for May 1998 include replacement of the #1 CCD with a new science-grade device, and installation of a more compact VME chassis. The present #1 CCD has very poor blue response and a large thinning defect in the center of the format, so a higher quality replacement will be welcome. There is also a possibility that the #2 CCD, which apart from poor blue response is very non-linear, will be replaced at the same time.
The longer-term future of wide-field imaging on the Blanco 4-m involves the use of the NOAO Mosaic II Imager. This instrument is a clone of the Mosaic I, used at KPNO on the Mayall telescope (http://www.noao.edu/kpno/mosaic/mosaic.html) and will have an 8K × 8K mosaic of thinned SITe CCDs as detector. The instrument itself is undergoing final tests in Tucson while the pacing item, the CCDs, are being delivered regularly by SITe. The first eight CCDs will be used to replace the Loral CCDs in NOAO Mosaic I during June, and the present schedule calls for the installation of the CCDs in Mosaic II later in the year, with commissioning on the Blanco telescope taking place in March 1999.
Given the high scientific demand for wide-field imaging at the Blanco, a goal is to make the transition from the BTC to Mosaic II as seamless as possible. Extensive testing of the instrument (less CCDs) has begun in Tucson, and has included tests at the Kitt Peak 0.9-m telescope. The Arcon CCD controllers are being built at CTIO with final system integration scheduled for January and February 1999. Extensive modifications to the prime focus pedestal and cage will also occur during this period. Since the instrument is a clone, with almost all hardware and software identical to that for Mosaic I, a prolonged commissioning and verification period should not be required. However schedule details depend critically on the rate of delivery of the SITe CCDs, so details of instrument availability for Semester I 1999 must be postponed to the September Newsletter. If there are delays, the BTC loan will be extended into Semester I 1999.
Finally, we are slowly building up our complement of large (146mm sq.) filters, with Washington c and m, DDO 51, Sloan ugriz, [OIII] 5007, [OIII] continuum, Ha 6563 and [SII] 6728 on order, together with an improved Johnson U presently under construction in Tucson. Suggestions for future filter purchases are welcome.
Alistair Walker (awalker@noao.edu)