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 [6]. Its GUI can be seen here [4].

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 [1].

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 [7] 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.

The gratings shown below can all be used with Hydra. The dispersions shown are those obtained when the gratings are used with the 400mm Bench Schmidt camera and the 2x4K SiTe CCD.

200 and 100 µ wide slit plates are available to put in front of the output of the fibers.

With the 400mm camera and no slit plate the images will be approximately 7 pixels wide. With 200µ and 100µ slits they will be about 4.5 and 2.5 pixels wide respectively. Thus, with (say) KPGL3 which has a dispersion of .70A/pixel at 5500A, the resolution would be .70X7=4.9A (R=1100) with a bare fiber, .70x4=2.8A (R=1900) with the 200µ slit plate and .70x2.5=1.75A (R=3000) with the 100µ slit plate. The efficiency of the system will be reduced by about 25% when the 200µ slit plates are used and by roughly 60% with the 100µ slits.

In selecting a grating, you should take into account the tabulated grating efficiencies ((grating for 4m RC spectrograph)) or those shown in the charts at the bottom of this page. These charts were made with the R/C spectrograph. For Hydra the wavelength scale should be shifted by about 9% towards the red.

The 229mm Air Schmidt camera with the 1Kx3K Loral CCD is now available a backup only. With the Air Schmidt the dispersions are 1.75X the values shown and the images are approximately 4 pixels wide. The slit plates do not improve the resolution with the Air Schmidt because of the poor images and thus cannot be used with this camera.

Gratings

GENERAL: All the gratings used in the R/C spectrograph will fit on Hydra. However, gratings 250 and 400 have been left off the list because their dispersion is too low to be useful. Grating 420 is not included because it is redundant with KPGLF and less efficient.

Dispersions will be half the values shown when the gratings are used in second order.

Sometimes it is necessary to use an order-separating filter. Available filters are shown here. Don't use a filter unless you must, since they cause a slight loss even at their best wavelengths and can cause suprious reflections.

Looking for spectra taken with the comparison lamps? [8]

NOTE #1 Care must be taken when using gratings 510, 181 and KPGL2. They have too much wavelength coverage for the CCD. Second order contamination limits the coverage to less than the full width of the chip and a blocking filter is normally required.

For example, if KPGL2 is used with a central wavelength of 6000A, coverage from 4000-8000A can be obtained but a filter blocking wavelengths below 4000A will be needed. Wavelengths beyond 8000A will be seen on the CCD but will be strongly contaminated by second order blue light.

Grating 181 as currently used in the Hydra spectrograph has very low throughput, and should not be used until further notice. (KAO 1/25/06)

NOTE #2  KPGL1 should always be used with a blocking filter, EVEN in first order. Tests done by P. Massey and K. Olsen in March 2005 demonstrated that a few percent of the light in the red appears in the blue at half the wavelength (not to be confused with second order diffraction, which goes the other way). The problem was likely introduced by the machine that ruled the grating. None of the other gratings have been found to have this problem.

NOTE #3  Grating 380 is not very efficient in second order and produces very bad ghost reflections at some wavelengths.

NOTE #4  Blaze wavelength is shown in "Littrow", i.e. normal incidence. This is how they are used in the spectrograph and is approximately the peak wavelength.

NOTE #5  See the echelle mode [9] documentation for more information about how to use the echelle grating.

Relative Efficiency of R/C Spectrograph Gratings
(Wavelegths for Hydra = scale x 1.09)

Blue - relative to grating 250

Red - relative to grating 400