HBS - 229mm

Updated on June 15, 2021, 7:36 am

The Hydra Bench Spectrograph with the 229mm Air Schmidt Camera

The CTIO Hydra bench spectrograph (HBS) is located in the so-called "Small Coudé Room" of the Blanco telescope building, where the Argus spectrograph was located before Argus was retired. The HBS borrows design elements of the Argus Bench Spectrograph, the 1.5M BME Spectrograph and the WIYN Hydra Spectrograph.

The optical design of the HBS is shown here. Its GUI can be seen here.

The tips of the fibers are arranged in a fanlike array, at the focus of a corrector-less Schmidt collimator mirror of 1200mm focal length. The grating is placed at the center of curvature of the collimator. The tips of the fibers are set on the curved focal surface. This locates the pupil on the grating for maximum efficiency.

The grating is tilted about a vertical axis at an angle of nine degrees to the side in a "quasi- Littrow" configuration, directing the light eighteen degrees off-axis to the camera. The camera is a classical Schmidt with the CCD mounted in a cold finger holding it at the focal surface.

Until recently, the spectrograph used a Loral 1Kx3K CCD in the "Air Schmidt" camera of 229mm focal length, soon to be replaced by a 400mm Bench Schmidt camera with a SiTe 2x4K CCD. The new camera/CCD combination is expected to have much better performance than the old one. For information on the new camera please click here.

The Loral CCD has 15u square pixels, about 3 electrons readout noise and good QE in the visible, near IR and UV. For more details see the CCD section of the CTIO WWW site. The Air Schmidt camera gives an optical reduction of 5.24:1 in the spectrograph, producing a theoretical projected fiber size of 3.5 pixels for the 300u fibers. The fiber images are separated by approximately 8 pixels on the CCD so this camera/CCD combination only covers approximately 120 of the 138 Hydra fibers.

The optical quality of the Air Schmidt camera with the Loral is not very good. The fwhm of the images is ~3.6-4.5 pixels. The theoretical resolution is 3.3 pixels, so the best attainable resolution is fairly good. However, the CCD is not perfectly flat so there are random variations in image size over the field. Additionally, the images have quite wide wings so the spectra from adjacent fibers overlap and the PSF varies depending on where a line is located on the CCD. Alternate fibers must be used, which means that only about 60 indepedent targets can be selected. A special Hydra "concentricities" file is available which automatically disables every other fiber and all non-functional ones. This concentricities file should always be used when making target assignments with Hydra and the Air Schmidt. The Air Schmidt also has a quite large central obstruction and there is about 35% vignetting at the edges of the field. Image quality deteriorates markedly towards the edge of the field.

The Loral/Air Schmidt canera/CCD combination is also the one currently used in the R/C Spectrograph. The resolution and coverage of both instruments are thus approximately the same though the dispersion of the HBS is slightly higher because of the quasi-Littrow design of the spectrograph. This list of the Hydra gratings details the options available.

The 300u (2 arcsec) fibers are fully operational, though a few have low transmission, are broken or cannot be seen on the Loral CCD, making the actual number of good fibers approximately 130. The 200u (1.3 arcsec) fibers are functional, but unfortunately the fibers used have proven to be quite fragile. As a result, about half of the small fibers are either broken or have low transmission. The small fibers have thus been decommissioned and are not available.

The HBS shutter can be seen in the optical diagram. It is a large rotating disc in front of the corrector of the Schmidt camera. Although the shutter rotates relatively slowly and exposures of less than one second are not possible, it cuts the entire beam evenly and symmetrically so that exposures are quite uniform and accurate.

Click here for an Exposure Calculator for use with this camera/CCD combination. The result is only approximate, so use this estimate with care.

 

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T.Ingerson
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