At the end of the present observing season the Burrell Schmidt will no longer be available through KPNO, as is discussed in the Director's section of this Newsletter. At the same time installation of the CCD-mosaic imager at the 0.9-m telescope will actually improve the wide-field imaging options offered to KPNO observers. Despite the loss of the Schmidt, there will be a net gain for programs that had relied on its capabilities.
Previous to the installation of Mosaic on the 0.9-m, the Burrell Schmidt was unique in offering an extremely large, 69' x 69', field-of-view, compared to other CCD cameras on the mountain. Although the Mosaic on the 0.9-m actually has a smaller field, 59' x 59' (at f/7.5), or 0.73 of the Schmidt's imaging area, when the telescope plus CCD throughput is folded in, the Mosaic-0.9-m combination gets to a given depth over a given area faster. For example, in the case of source-dominated applications, that is those where the object is much brighter than the sky, the Mosaic-0.9-m is 1.6x faster to a given flux limit in the R-band, or still 1.2x faster when the its smaller field is considered. The Mosaic is slightly slower in V and I, but even at B with the present thick-CCDs, it is just as fast as the Schmidt. As thinned-CCDs are installed, these ratios will improve further (see the lead article in this section).
The real gain of the Mosaic-0.9-m combination over the Schmidt occurs for faint sources, where the sky dominates. The Mosaic-0.9-m combination offers 0.43" pixels, compared to the 2.03" pixels of the Schmidt, which with the improved corrector and seeing at the 0.9-m, allows for much better rejection of the sky background. For sky-dominated images in the R-band, for example, in 1" FWHM seeing the mosaic is already ~ 4x faster to a given area-flux limit than the Schmidt, and still ~ 3x faster in B even with the thick CCDs. The sharper images and finer pixel scale also allow analysis applications, such as star-galaxy classification, PSF-fitting, and so on, to be conducted to much fainter limits than is possible with the Schmidt.
The only major deficit of Mosaic-0.9-m imaging compared to the Schmidt is the loss of objective-prism spectroscopic applications. Such work represented a small, but important minority of the Schmidt programs that we have supported in the past. It is possible, however, that we will be able to offer similar capabilities at the 0.9-m. George Jacoby has experimented with gratings within the camera to offer low-resolution spectroscopic imaging. The results of the experiment are encouraging and may lead to an understanding of how best to add this capability to the Mosaic.
Lastly, there are many "secondary" attributes of the 0.9-m that make it a superior imaging platform over the Schmidt. Unlike the Schmidt, the 0.9-m features a complete telescope-control system, is completely operable from a control room, and so on. Such features offer greater efficiency at the telescope, and greater ease in data reduction away from the telescope.
Tod Lauer