During the past report period, we have been proceeding with the commissioning of the 8K × 8K CCD Mosaic Imager, concentrating on user level functionality. Engineering runs at the 4-m in February and April enhanced the end-user status of the instrument significantly.
An example of an image taken during the April 4-m run is shown in the figure. The giant Virgo cluster galaxies M84 and M86, as well as many other smaller galaxies, are evident. (The full 155 Mbyte image can be downloaded via anonymous FTP from madrona.tuc.noao.edu, cd pub, get m84.fits.Z to get an idea what it's like to work with a full Mosaic frame. Also, the image may have scientific value; contact G. Jacoby or T. Armandroff for details about the image and an appropriate credit line if you find it scientifically useful to your work.)
The data acquisition and reduction software continues to develop and mature. We have added several new features, particularly in the use of the atmospheric dispersion compensator (ADC) and documentation of the images through new and more accurate header keywords. Most importantly, though, the software is far more robust and reliable than before, and is close to a final user configuration. Significant advances have been made to incorporate the IRAF message bus (see the IRAF section of this Newsletter) for data capture. Much of the software work has been directed toward reducing the readout time. Formerly, close to 6 minutes elapsed before the picture was written to disk. Now, with the picture descrambling and formatting occurring on a Sun UltraSparc, it takes only 2 minutes.
Considerable testing of the ADCs demonstrated that they offer a real image quality improvement. In 1" seeing, it is evident that the image quality is improved in the B-band when using the ADCs during observations at high zenith distances (greater than 40°). We offer three user modes with the ADCs. TRACK mode is the simplest and most likely to yield good consistent images and will be the default. The two prisms rotate to the appropriate positions that minimize atmospheric dispersion as the telescope tracks. PRESET mode places the prisms at a position where the correction is optimized for the moment of mid-exposure. The prisms do not move during an exposure. This mode is offered for those who may be concerned about image motion or PSF changes during the exposure, but still want a good correction. (We have looked for but found no evidence that the images move or change in any way, but we will continue to offer this mode anyway). NULL mode can be selected to place the prisms in a non-correcting fixed configuration, although we have no reason to believe there is any advantage to this mode.
As reported in the previous Newsletter, we found evidence for a ghost image of the telescope pupil due to an internal reflection in the corrector. Formerly, the ghost image exhibited a maximum magnitude above sky of 3-5% with narrow-band filters. We have reduced that level by a factor of 4 to 5 by applying a carefully tuned Sol-gel AR coating to the offending surface. The coating minimizes reflections between 5000Å and 7000Å. Furthermore, on-sky testing demonstrates that any residual ghost image is effectively eliminated by flatfielding with twilight or dark sky flats. See the article on the Prime Focus CCD System in this Newsletter for further details.
The relay optics for the guide TV system have been revised so that the image quality (and presumably guider stability) is much improved. We verified that we can guide on images fainter than V = 20.
With the CCD Mosaic, every exposure produces 128 Mbytes of raw data. After a good night, an observer may have 100 images, or 12.8 Gbytes of data. Writing this volume of bits consumes a great deal of time and tape, comparable to more than 10 hours and 3 Exabyte 8505 tapes. We have considered the market for a new direction in tape drives and have decided to experiment with the digital linear tape technology. A DLT-7000 tape drive is now on order, and a second drive will be purchased for use downtown for those needing to transfer tapes to other technologies. The DLT-7000 can write tapes about 5 times faster than the Exabyte 8505 and the tape cartridge holds 7 times the data volume in an uncompressed mode. While tape cartridges cost about $100, they can hold roughly three nights of data. As a backup, we will continue to offer two Exabyte 8505 drives with the Mosaic system as well.
As discussed in the last Newsletter, we are awaiting the science grade CCDs from SITe. The first shipment is expected by June.
We are pleased to report that we received 12 proposals requesting the CCD Mosaic for fall 1997. A user manual for the Mosaic is under construction and will be made available under the KPNO Mosaic web page (http://www.noao.edu/kpno/mosaic/mosaic.html ). We also have initiated an internal training program for system use and hardware installation in anticipation of visitor use of the Mosaic system.
George Jacoby, Taft Armandroff,
Todd Boroson, for the Mosaic Team