The CTIO Hydra bench spectrograph (HBS) is a new design, 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. 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.

At this time, the spectrograph uses 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.

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 ~3.6-3.8 pixels and they have quite wide wings.  The images from adjacent fibers overlap so that 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.

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.

As previously mentioned, the Loral/Air Schmidt is scheduled to be replaced with the SiTe/Bench Schmidt before the next scheduled Hydra run in late November, 1999.  Because it cannot be guaranteed that this changeover will take place smoothly as scheduled, observers MUST come prepared to use either camera until the changeover has occurred.

The new camera is a 400mm f.l. bench Schmidt.  The SiTe chip has 2048x4196 pixels 15u square.  The new combination is expected to have minimal vignetting and cover all the fibers with little or no image overlap.  It has a much smaller central obstruction and higher resolution.    The dispersion will be 1.75X higher and the spectral coverage 76% of the Loral with the Air Schmidt.   The quantum efficiency of the SiTe is approximately the same as that of the Loral except in the UV where the SiTe is significantly less efficient.  Once the new camera is operational, the Loral/Air Schmidt combination will no longer be available, even in the UV.

IMPORTANT!  Persons planning to use Hydra must keep in mind that until the SiTe/400mm combination has been officially commissioned, the Loral/Air Schmidt remains the Hydra CCD and camera.   We expect that the new camera will be commissioned in March and available after that on a shared risk basis, but until it is ready the Air Schmidt/Loral 3K with a maximum of 60 fibers will have to be used.

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.   As a result, the small fibers have been decommissioned and are not available.

Once the 400mm camera is available, higher resolutions will be achievable by the use of slit plates which will be put in front of the line of fibers to reduce their diameter in the dispersion direction,  with some loss of light.  200u and 100u slits are available for the large fibers and a 100u slit plate is available for the small fibers.  With the 200u slit plate, the 300u fibers can  be masked down to the width of the small fibers, preserving resolution with a loss of only about 20% of the light.  Under seeing conditions worse than about .7 arcsec, more light will be gained by use of the large fibers than will be lost by use of the slit.  The use of even narrower slits will further increase the resolution of the spectrograph with a corresponding loss of light.  In theory, the maximum resolution with the 100u slits is approximately 50K in the echelle mode, but it remains to be demonstrated that the new camera is capable of images this good.   The slit plates make no significant improvement in resolution with the Loral because of the poor images.

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.