The Faint-object Nasmyth Spectrograph of the Tautenburg 2m-Telescope

Nasmyth spectrograph The principle components of the Nasmyth spectrograph.


The Nasmyth spectrograph is a single object spectrograph in combination with a focal reducer camera. It was developed and built at TLS for the spectroscopy of faint objects at low dispersion. First light was in 2001.

The imaging optics of the focal reducer consists of the collimator (f = 482.2mm) which re-images the entrance pupil of the telescope and the camera optics (f = 135 mm) which focusses the beam onto the CCD detector. Direct imaging mode is useful only for aquisition in order to position the object on the slit. (There is no field flattening lens used and there is not enough space to insert a filter wheel.)

For spectroscopy a longslit of tunable width is used in combination with a grism wheel. The grism wheel allows to select one out of five grims or, alternatively, a grism-free position ("white") for aquisition.

The detector is a nitrogen-cooled SITe CCD with 1024x800 pixels. The pixel size is 15 µm, corresponding to 0.53 arcsec.

A graphical user interface allows simple and quick handling of the spectrograph.

General Data

  • field of view: ~5 arcmin (no field flattening!)
  • longslit of 20 mm (~3 arcmin) length and tunable width
  • five grisms with dispersions between 100 and 230 Å per mm
  • wavelength coverage: 3500 Å to 10000 Å (depending on grism)
  • wavelength calibration: night sky lines

Available grisms

grism lines per mm wavelength range Å Å per pix Å per mm resolution (2 px) in Å
B200 300 1210 - 7490 3.56 231 580 (at 4000 Å)
V200 300 2310 - 9350 3.38 225 960 (at 6500 Å)
R200 300 5480 - 11560 3.06 204 1310 (at 8000 Å)
B100 600 2160 - 5730 1.64 109 1220 (at 4000 Å)
V100 600 4670 - 7690 1.52 101 2140 (at 6500 Å)


  • First, determine the accurate slit focus. Set a marker at the position of the centre of the slit and save the marker.
  • Take a (short) aquisition frame to make sure that your target is near the field-centre (press the button "Aquisition" and select the grism="white" on the spectrograph-GUI).
  • Determine the position angle of the slit (double exposure in aquisition mode with telescope shift in delta).
  • Compute the offset in alpha and delta relative to the slit centre and correct the telescope position accordingly.
  • (Eventually a second iteration of the last step will be necessary.)
  • If you want to check the target position relative to the slit, take another aquisition frame and load the slit marker again.
  • If you believe that your target is on the slit, select the grism and the slit width on the spectrograph-GUI,...
  • the CCD parameters,... and start the exposure (CCD-GUI). Good luck!

Example spectrum

NGC 1275
Figure: Emission line spectrum (continuum subtracted) of the nucleus of the active galaxy NGC 1275 with grism V200, 600s exposure time.

Helmut Meusinger, Holger Lehmann