CCD Mosaic Imager Progress

Since the September Newsletter deadline, the CCD Mosaic Imager has had first light at the Mayall 4-m telescope. The run was a success: many aspects of the Mosaic were tested, and a number of areas for further work were identified and prioritized. We are happy to report that there was significant progress since the 0.9-m testing runs in early June. For example, the ARCON hardware and software has advanced markedly from that time. In addition, the Mosaic's large pneumatic shutter is operating well now. Dome-flat tests indicate that it has better than 1% accuracy in a 1-second exposure.

The new 4-m corrector also underwent its first tests. Although the new corrector includes optics for atmospheric dispersion compensation, we did not have time to test their performance. We hope to verify the operation of the ADCs during the next engineering run. Overall, the corrector performed quite well, producing subarcsecond images (when atmospheric conditions allowed) over the entire Mosaic field (diagonal of 50'). The wide field also provides the optical feed for a pair of guide TV cameras; these were tested successfully as well. Since we did not have ample time to evaluate the properties of the new corrector (e.g., UV and blue performance), we will continue to use the old doublet corrector for PFCCD imaging runs until we have demonstrated that the new corrector is a suitable replacement.

We have measured preliminary count rates for the Mosaic at the 4-m, which will likely be of use to proposers considering the use of Mosaic. The count rates in e-/second of a 20th magnitude star measured through a 4" diameter aperture, are:

                     B      V      R      I
                    63     156    217    187

Further Mosaic testing is planned at the 4-m in November and January, and at the 0.9-m in October and January.

We have now received the dedicated Mosaic computer, a Sun UltraSparc2 with 40 GB of disk storage, 512 MB of RAM, and two 20inch monitors. Integration with the Mosaic has begun.

In our last Newsletter article (No. 47, page 27), we described how interested users could request access to Mosaic in the spring semester, provided that it becomes ready for users in that time frame. The first method, allowing users to request T2KB at the 4-m or T2KA at the 0.9-m with a contingent "upgrade" to Mosaic, was selected by seven proposals (four for 4-m time and three for 0.9-m time). The second method, Mosaic expressions of interest, resulted in 11 requests (eight of which asked for 4-m time and four of which asked for 0.9-m time, with one proposal asking for both telescopes). We are reviewing these requests and comparing the project's progress with the scientific needs of the requests. We will contact proposers for further information as required. It is our sincere hope that we will be able to provide one or more Mosaic blocks for a subset of these proposals.

The Loral CCDs, which we are using initially in the CCD Mosaic Imager, have a number of serious shortcomings. They are thick, and so they peak at only about 40% QE and have little sensitivity in the blue. They have numerous bad columns, both dark and bright, and regions of overall poor charge transfer efficiency. Because of a processing error in the CCD fabrication, they cannot be run at temperatures colder than -60C, causing a significant dark count.

It is our plan to replace these CCDs with chips of better quality. We have placed a large order with Scientific Imaging Technologies, Inc. (SITe) for 2K X 4K CCDs (same format and pixel size as the existing CCDs). Eight of these CCDs will be used in the CCD Mosaic Imager. These SITe chips will be thinned for high QE, even in the U band. They should have lower noise and better cosmetic quality than the Loral CCDs. They will be flat so as not to degrade the image quality by departing from the flat focal plane. We expect delivery of these CCDs by late in 1997, and the existing chips will then be replaced as soon as feasible.

For updates on the progress of the Mosaic project, check out the Mosaic Web page at http://www.noao.edu/kpno/mosaic/mosaic.html.

Taft Armandroff, Todd Boroson, George Jacoby,
Rich Reed (for the Mosaic Team)