TGS Spectroscopy

The Transmission Grating Spectrometer (TGS) is a low-resolution spectrometer that uses a high-efficiency transmission grating installed in the Rigel telescope filter wheel. The grating is a high-quality commercial  (Thorlabs GT50-06V) 600 line per mm, 50 mm2 square transmission grating with a peak transmission efficient near 60%.  Two gratings are currently installed, both 600 lpmm. The first (filter code '6') has no slit, while the second (filter code 'S') has a 3 mm occulting slit near one edge of the grating. 

The spectral resolution is R ~ 200 across the visible band, i.e. 2 nm at 400 nm to 3 nm at 650 nm. The imaging CCD sensor is used as the detector. Since the dispersed spectrum focusses along a curved surface, this means that a wavelength-dependent focus offset must be specified for each target wavelength range. In addition, for slitted observations, the telescope must be offset so the star is centered on the slit. These procedures are performed automatically for Rigel web-based schedule inputs if filter '6' or 'S' is specified.

TGS spectra can be calibrated and plotted either using any software package that can analyze strip-image spectra. For Windows-based options, a very nice commercial program is Rspec, which does both polynomial wavelength-to-pixel calibration and amplitude normalization using a library of standard calibrated spectra.

For Mac and Linux-based computer users, we have written two user-friendly Python-based programs, TGS-spec and TGS-stitch. TGS-spec calibrates TGS spectra using  built-in algorithms for wavelength-to-pixel conversion and wavelength-dependent CCD and grating response functions for amplitude calibration. It has many options e.g. plotting reference lines, changing minimum and maximum plotted wavelength, and median-window averaging. TGS-stitch overcomes the limited wavelength-resolution problem by creating a 'stitched' spectrum from 2-5 input spectra, each of which has a different focussed waveband. Examples of these programs are shown below. 

Documentation

  1. The TGS User Guide contains detailed technical information about how the TGS system works, and how to calculate position offsets and focus offsets for a given source and target wavelength.
  2. The TGS software bundle [tar archive format] contains Python programs TGS-spec, TGS-stitch, installation instructions, and example FITS image with TGS spectra and plot output files.  In addition, the scheduling planning tool RST is included.
  3. TGS software documentation. This page illustrates how to use RST, TGS-spec, and TGS-stitch, with installation instructions and example of use. 

Gallery of TGS spectra


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