Cassegrain Twin Spectrograph

The spectrograph has two separate channels ("blue" and "red"). Light enters these channels via a dichroic beam splitter (behind the entrance slit). Each of the two channels contains all the elements of a spectrograph (collimator, grating and filter equipment, camera detector and shutter) necessary for independent operation. The diameter of the collimator beam (146mm) and the camera's f-ratio are the same for both channels. The optical image quality and the transmission were optimised separately for the blue (320 - 550nm) and the red (420 - 1100nm) channels. Exchanging the beam splitter by a simple mirror makes the whole area accessible in the red channel (apart from the UV).
The selected angle of the grating (154mm x 206mm) defines the spectral range imaged on the detector of the respective channel. The shape and size of the spectrograph's entrance aperture are determined by means of an aperture wheel with 8 positions of different slit width (240" in length) A decker mask is not available. The position angle of the slit is determined by turning the spectrograph at the instrument flange of the telescope. The object is set and guided at the slit via TV remote guiders (with autoguider and offset guiding facility). The spectrograph (aperture slit, grating angle, comparison and flat field lamps, shutter) is operated remotely from the control room. Calar Alto personnel will change the gratings, beam splitters and detectors manually.

The CCD-detectors in use are SITe-CCD's with 2000 x 800 Pixel à 15 µm especially dedicated to the TWIN-spectrograph (see also CCD detectors).

Slit apertures

Width ["]: 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 3.5
Length: 240"

Position angle:

adjustable by rotating the spectrograph at the cassegrain flange

Beam splitters:

available at 450, 550, 680, and 750 nm

Order sorting

by beam splitter and/or colour filters

Cameras (folded Schmidt type)

Focal length:230mm
Scale:1" = 26.8 micrometer (slit direction)
Field:7mm x 40mm

Detectors

SITe-CCD's with 2000 x 800 Pixel à 15 µm (CCD)

Comparison lamps

Ne
He/Ar
Fe/Ne
Th/Ar

Flat fields

Quartz-halogen bulbs (+ various filters) (built-in)
Dome light

Performance

Spectral resolution: At 36Å/mm and slit width 1.2" (2.0") line width is < 0.9Å (1.3Å)
Grating angle: Adjustment accuracy: approx. 0.5Å at 36Å/mm
Sensitivity: Tests with an RCA CCD-chip gave a S/N = 32 (18) for a star with m = 17 (18) and 1h exposure time, at a resolution of 1Å in the region around Halpha (36Å/mm)

Grating list

No. grooves blaze angle blaze recommended application
1. order range order dispersion channel
[lines/mm] [degrees] [Å] [Å] [Å/mm]
T01 830 19 7700 6000-10000 1 52 red
3200-5000 2 26 blue
T03 600 17 9500 7500-11000 1 72 red
3800-5500 2 36 blue
T04 600 13 7100 5500-9000 1 72 red
3200-4500 2 36 blue
T05 1200 17 4700 3700-6500 1 36 blue
T06 1200 26 7100 5500-9000 1 36 red
3200-4300 2 18 blue
T07 400 9 8000 5500-11000 1 108 red
3300-5000 2 54 blue
T08 600 9 4700 3500-6500 1 72 blue
T09 600 22 12000 8400-11000 1 72 red
5500-7500 2 36 red
3500-4500 3 24 blue
T10 830 30 11500 8400-11000 1 52 red
5500-7000 2 26 red
4500-5500 2 26 blue
T11 270 5 7200 5500-11000 1 160 red
3200-4500 2 80 blue
T12 600 7 3800 3100-4800 1 72 blue
T13 300 4 4700 3500-5500 1 144 blue
5500-7000 1 144 red

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Ulli Thiele