CCD Mosaic Imager Status Report
(from KPNO, NOAO Newsletter No. 50, June 1997)
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. This R Band image of Virgo is
a sum of five 300 seconds exposures where each is astrometrically and
photometrically corrected, and shifted to a common frame. The image
reduction was performed by Frank Valdes. The sky level differences
between individual CCDs is emphasized by the contrast chosen to make
fainter objects visible. The peak-to-peak range of the median sky
values is about 3.5% and the RMS range is about 1% of the sky level.
[The full 155 Mbyte image can be downloaded via anonymous FTP from
madrona.tuc.noao.edu, cd pub, get m84.fits.Z (64 Mbytes) 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.]
Mosaic Imager Status
During the past report period, we have been proceeding with the
commissioning of the 8Kx8K 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.
Data Acquisition Software
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 ever before,
and is very close to a final user configuration in its appearance and
stability. Significant advances have been made to incorporate the IRAF
message bus 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.
New 4-m Corrector
Considerable testing of the ADCs demonstrated that they offer a real
image quality improvement. In 1 arcsec 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 degrees). We
offer 3 user modes with the ADCs. TRACK mode is the simplest and most
likely to yield good consistent images and will be the default. The 2
prisms rotate to their 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 5000 and 7000 Angstroms. 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.
TV Guider
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.
Data Tapes
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 3 nights of data. As a backup, we will continue to
offer two Exabyte 8505 drives with the Mosaic system as well.
Science Grade CCDs
As discussed in the last Newsletter, we are awaiting the science grade
CCDs from SITe. To date, none has been delivered, but the first
shipment is expected by June.
Proposals
We are pleased to report that we received 12 proposals requesting the
CCD Mosaic in Fall.
User Manual
A user manual for the Mosaic is under construction and will be made
available on the world-wide web off the KPNO Mosaic web page.
We also have initiated
an internal training program for system use and hardware installation
in anticipation of visitor use of the Mosaic system.
For updates on the progress of the Mosaic project, check out the
Mosaic web page
at http://www.noao.edu/kpno/mosaic/mosaic.html
George Jacoby gjacoby@noao.edu
Taft Armandroff tarmandroff@noao.edu
Todd Boroson tboroson@noao.edu
For the Mosaic Team ...