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 The new Spectral Instruments Series 600 camera mounted on the Ritter échelle spectrograph. The silver hoses are the refrigeration lines to the Cryotiger. |
Ritter Observatory is located on the campus of The University of Toledo. The telescope is a 1.06-m, f/8 Ritchey-Chrétien reflector manufactured in 1967 by the Warner & Swasey Co. of Cleveland, OH. In the 1980's, it was retrofitted with computer-controlled stepper motors in an open-loop control system. Star acquisition and manual guiding on the fiber are carried out by means of a CCD camera, SBIG model ST-9. The spectrographs are connected to the Cassegrain focus of the telescope by fiber optic cables with core diameters of 200 µm. At any time, either a high-resolution échelle or a low-resolution, single-grating spectrograph can be selected.
The échelle spectrograph was built in the mid 1970's after a design by D. W. Latham (Harvard-Smithsonian CfA). Nowadays, it is usually employed at a spectral resolving power of approximately 26,000 in the H alpha region.
With support from the NSF-PREST program, a large-format CCD has been acquired. It entered routine operational status on 14 April 2007. This device is a CryoTiger-cooled Series 600 camera from Spectral Instruments, Inc. (Tucson, AZ) with a front-illuminated Imager Labs IL-C2004 4100 x 4096 pixel sensor (15-micron pixels). With 2 X 2 on-chip binning, the spectrograph entrance slit projects to a resolution element about 3 pixels in width. A Lumigen dye coating enhances the CCD's sensitivity to blue photons.
Compared to the old camera (described below), wavelength coverage is greatly extended, covering about half the spectrum from about 4600 Å to beyond Hα at the grating settings normally in use. There are about 21 useful échelle orders, each about 150 Å in extent. In hot star spectra, Hα, Hβ, and He I λ6678 are all included, as well as many other lines of interest. With adjustment of the grating tilt settings, the entire CCD spectral range is accessible. More details about the wavelength coverage will be provided here after the wavelength calibration is completed.
Here is the first scientific image taken with the new camera, displayed in a partial screen grab from Spectral Instruments' image acquisition software, SI Image. The grating tilt settings have since been changed, the échelle slightly and the cross-disperser a lot.At R = 26,000, the entrance slit of the échelle projected to a width of about 4.3 pixels, and therefore the data are oversampled. Between 1997 April 8 and 1997 July 25, échelle observations were made with a narrower entrance slit, which gave R = 50,000 (2 pixels) and a corresponding loss in throughput.
At the usual grating settings, H alpha is in the center of the lowest-numbered order on the CCD. Eight additional orders are included on the chip as well, with a free spectral range of about 70 Å per order. The table below gives the wavelength coverage of this CCD at this grating setting, and the figure [to be added later] shows a raw spectrum of an A-type supergiant star with prominent lines labeled.
| Échelle Order | Wavelength Range (Å) |
|---|---|
| 34 | 6527 - 6594 |
| 35 | 6340 - 6406 |
| 36 | 6165 - 6230 |
| 37 | 5998 - 6060 |
| 38 | 5840 - 5901 |
| 39 | 5691 - 5750 |
| 40 | 5550 - 5606 |
| 41 | 5413 - 5470 |
| 42 | 5285 - 5339 |
Prior to early May 1996, the échelle raster was shifted downward by one order relative to the CCD. That is, échelle order 42 was not included and order 33 was included on the CCD.
In a one-hour integration in good conditions, spectra with signal-to-noise ratios of 100 or better per pixel are routinely obtained for stars as faint as V = 5.5. Fainter stars can be observed with correspondingly lower signal-to-noise ratio.
At R = 6,000, the useful spectral range is about 700 Å, e.g., at H alpha, 6100 to 6700 Å. Currently we are obtaining spectra of stars down to V = 8 in an hour's integration, but experience is still too limited to quote a typical signal-to-noise ratio.
