Schmidt Telescope Upgrade (1Jun95)

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Schmidt Telescope Upgrade (1Jun95) (from CTIO, NOAO Newsletter No. 42, June 1995) On 28 February 1995 the CCD field-of-view at the Curtis Schmidt telescope increased by a factor of 4.9, with the commissioning of the new 4-inch shutter-filter assembly and a STIS 2048 X 2048 CCD. The STIS CCD was obtained for CTIO by Richard Green from Bruce Woodgate, the PI of the STIS Project. The Thomson 1024 X 1024 CCD, which has operated with a prototype Arcon controller for more than two years, will be retired. The shutter-filter unit was designed at CTIO and built at the University of Michigan. The filter bolt can hold up to five filters, each four inches square. Due to time pressure, filter operation at the moment is in "semi-manual" mode, but during April and May both the filter-bolt and the telescope focus will be brought under motor control. The user will then be able to focus the telescope and move the filters from the Arcon user-interface, thus greatly improving efficiency. Visits upstairs will then be restricted to the occasions when it is necessary to move the telescope or the dome. A set of 4 X 4 UBVRI filters has been purchased for the Schmidt; other 4 X 4 filters are shared with the 4-m PFCCD. Sets available include Washington, Stromgren, Gunn (only r,i,z) and some narrow band filters. The STIS 2048^2 CCD is a front-illuminated device made for the Space Telescope Imaging Spectrograph program by Tektronix. It has 21 um pixels (scale is 2.03"/pixel) and is coated with Metachrome to provide UV response. The QE is thus almost identical to that of the Thomson CCD which it replaces (18% 3000-4700A, rising to 50%by 7000A, then falling to 10% by 10,000A). It is not cosmetically perfect, having some partially hot columns, some of which subtract out. All four amplifiers work at low noise, but for the first month of observing we used a single amplifier (lower right), which delivers an impressively low 3.8 e- rms noise at 2.3 e- / adu gain. Full-well is some 150,000 e- but linearity rolls off above 90,000 e-. Work on implementing quad readout and attempting to improve the linear range is also scheduled for April-May. The readout time using quad is expected to be 30-35s; at present it is four times longer. Given that exposures at the Schmidt are typically only a few minutes in duration, the disks fill up at a alarmingly rapid rate. At present the Schmidt has almost 5 GB of disk space and a DAT drive. [Photo not included] 500s V exposure of Centaurus A, taken in moonlight. The efficiency of the Schmidt in detecting low-surface brightness extended structures is clearly seen. Performance at the telescope is very satisfactory. Pat Seitzer (Michigan), assisted at times by Eileen Friel (Maria Mitchell ) and the REU students, spent several nights painstakingly adjusting the tilts of the focussing assembly, with the result that image quality over the whole 69 X 69 arcmin field is very uniform, with FWHM 1.7 pixels. There is some vignetting due to the Newtonian flat being under-sized, but the pattern is centered and amounts to only 15% by the CCD corners. Twilight sky flats successfully produce a flat sky background. In these days of tight budgets it is difficult to make substantive improvements to our small telescopes. This project was partially financed by the University of Michigan, and the ability to divide the work between CTIO and Michigan advanced the implementation date by many months. Gabriel Pe‚rez designed the filter-bolt and Scott Webster (Michigan) built it. Ricardo Schmidt commissioned the STIS CCD. Pat Seitzer and the under-signed orchestrated the project, with much advice and help from many others. A report on the motor-control implementation and the CCD work mentioned above will appear in a future Newsletter. Alistair Walker (awalker@noao.edu)

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