Grism Spectroscopy


10 October 2017 Note that the grism spectrometer needs to be modified for the current camera: The focusing lens is incorrect and needs to be replaced. The scheduled replacement is early December. Unfortunately for the next 6-8 weeks, It cannot be used.

The Gemini telescope has two grism spectrometers installed in the filter wheel, equipped with 300 line per mm and  600 line per mm gratings. The gratings are enclosed in a 3-d printed housing designed to fit into a 50 mm round filter slot on a filter wheel. There are also two prisms to redirect the dispersed beam, along with collimating and refocusing achromatic lenses. The optical ray of the grism is shown below.


The dispersed grism spectra are recorded on the main imaging camera. The grism spectra are wavelength and amplitude calibrated using calibration tables derived by observing standard stars  (Jacoby et al. 1984). A complete description of the grism system, can be found in the preprint by Ludovici and Mutel 2017 (submitted to Amercian Journal of Physics).

Sample calibrated spectra.


Grism spectrum of emission-line B star HD76868. The Halpha line is in emission, while the H beta line is partially in emission and partially in absorption, and the remaining Balmer series are in absorption. This is a result of hot circumstellar gas which is optical think longword of about 500nm. Note the inset H beta line, which shows that the spectral resolution is about 1 nm.


Spectra of two low-redshift quasars, 1E0754+395 (z = 0.096) and 3C273 (z = 0.159). Each spectrum had an exposure time of 15 minutes.


Spectral sequence of stellar spectra from hot to cool stars: (a) HD158659 (B0V), (b) HD111608 (A1V), (c) HD107132 (F7V), (d) SAO 62808 (M5III)

Copyright 2018 University of Iowa