The BTC (Big Throughput Camera), formerly known as LACCD, is a CCD mosaic imager built by Tony Tyson (Lucent Technologies) and Gary Bernstein (Michigan). The BTC system includes a dewar with four thinned SITe 2048 CCDs plus all readout electronics, and a Sparc 10 computer with fast disk and tape (DLT) storage. It was successfully used at the CTIO 4-m prime focus during May 1996, and is scheduled to be used again in December. In combination with the 4-m telescope, BTC is the highest- throughput mosaic imager anywhere, and the only one to use thinned, high QE CCDs. A single exposure covers 0.25 square degrees. Visit the WWW site to see some of the results from the May observing run: http://www.astro.lsa.umich.edu/btc/btc.html.
BTC will be available to visitors as a supported instrument for semesters I and II 1997, and possibly longer. It will be scheduled in two or three block runs per semester at the 4-m prime focus. We will continue to schedule the CTIO prime focus imager, which contains a single SITe 2048 CCD, for programs that do not need the large field or which require special filters. The information below should help you decide whether or not the BTC will be advantageous for your 4-m imaging program. Please contact the author if you have further questions.
The BTC mounts at the 4-m prime focus, behind the usual Atmospheric Dispersion Compensator (ADC), and it uses the standard PFCCD CCDTV guide camera. There is a four-position computer-controlled filter bolt holding 6-inch square filters, followed by a "near-focal-plane" shutter. The CCDs live in a large dewar behind lenses that correct for their curvature. Since the CCDs are mounted in standard packages, they are not butted together, and there is a cross shaped band approximately 16 mm wide between CCDs. Normal operation is to take many exposures in a shift-and-stare mode, to produce a large panoramic picture with no gaps. However, this is not essential. The pixel scale is 0.43"/pixel, and the ADC produces images better than 0.5" over the whole field. Work to minimize PSF variation (due to the non-flat CCDs) is on-going. The CCDs themselves have good (but not perfect) cosmetics, and eventually will be replaced by flatter devices, each with two working amplifiers.
There is only a limited selection of filters. BTC comes with 6 x 6 inch Bj, R and I (see ApJS, 99, 281, 1995). Anything significantly smaller will vignette. R and I are similar to the CTIO 4 x 4 R and I filters, although the BTC I filter has a long wavelength tail, which is not present in the CTIO I response. We will also provide UBV filters.
The data acquisition computer and electronics read the array (32 Mbytes per composite frame) to disk in approximately two minutes. There is a simple-to-use user interface, which may even be a GUI by 1997. Each mosaic image is written as four individual IRAF image files, one per CCD. Initial processing of the data (trimming, flat-fielding etc.) is straightforward, using the standard IRAF routines. Software for combining mosaic frames (distortion corrections, etc.) is still in its infancy, although some will be available by next year. In general, we believe that data handling issues are under control and do not severely affect observing efficiency. It IS recommended however, that there be two observers, one to concentrate on data acquisition and the second to handle image verification and processing.
The data storage system consists of 18 GB of disk storage and a DLT tape drive. Each DLT tape drive cartridge holds 15 GB uncompressed. For those without facilities to read DLT cartridges, it will be necessary to copy to exabytes or DATs. We will provide a DLT drive in La Serena, so that this copying can be done after an observing run. Observers should plan to spend 1-2 days in La Serena after their run, if much data copying is contemplated. Of course DLT tapes may be verified on the mountain, and we may have save-the-bits archiving backup for the BTC as well.
Alistair Walker (awalker@noao.edu)