Image of the Moon taken with the CCD Mosaic Imager and the Mayall 4-m telescope using the new 4-m PF wide-field corrector. The field of view is 36 arcmin on a side. The image was processed only through dark subtraction and flat fielding in order to show the locations of the bad columns. Prior Mosaic experience shows that combining multiple shifted images eliminates the defects and fills the inter-CCD gaps.
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 sub-arcsecond images (when atmospheric conditions allowed) over the entire Mosaic field (diagonal of 50 arcmin). The wide field also provides the optical feed for a pair of guide TV cameras, and 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 counts rates for the Mosaic at the 4-m. These will likely be of use to proposers considering the use of Mosaic. The count rates, electrons/second normalized to a 20th magnitude star measured through a 4-arcsec diameter aperture, are:
Filter electrons/sec B 63 V 156 R 217 I 187
Image of M33 taken with the CCD Mosaic Imager and the Mayall 4-m telescope. The image was processed as for the Moon above. The exposure time was 300 seconds.
In our last Newsletter article (Number 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 7 proposals (4 for 4-m time and 3 for 0.9-m time). The second method, Mosaic expressions of interest, resulted in 11 requests (8 of which asked for 4-m time and 4 of which asked for 0.9-m time, where 1 proposal asked for both telescopes). We are reviewing these requests and comparing the project's progress with the 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 to enable a subset of these proposals.
The Loral CCDs with which we are commissioning 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 (poor CTE) and bright, and regions of overall poor CTE. Because of a processing error in the CCD fabrication, they cannot be run at temperatures colder than -60 C, and this results in 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 be replaced as soon after that as it is 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