Dr. Robert Mutel and his astronomy students at the University of Iowa designed and built a GRISM spectrograph for remote observations that they installed on the new 0.51-meter Gemini PlaneWave telescope in Arizona. The GRISM system replaces the previous Transmission Grating Spectrograph (TGS) that was on the smaller Rigel Telescope, which the Gemini Telescope replaces. Like the TGS the GRISM is a low-resolution spectrograph. However, unlike the TGS system, it produces an in focus linear spectrum throughout the entire 380nm to 850nm range with a resolving power of R = 300 (about 2nm resolution).
Figure 1 shows the entire GRISM assembly that was designed to fit into one of the filter wheel openings.
Figure 2 shows a sample spectrum of an 8.9 magnitude G6V star (HD 20023) that shows pronounced absorption lines.
Figure 3 shows a sample spectrum of WR5, a 10th magnitude WC6 Wolf-Rayet star, with pronounced broad carbon emission lines.
Finally Figure 4 is an animated GIF Dr. Mutel sent to me with GRISM spectra of 20 stars ranging from late O to middle M types. Here is Robert’s description of the images:
Attached is a spectral sequence of 20 stellar spectra, ranging from late O to mid-M, taken with the low-resolution GRISM spectrometer at Gemini on Jan 27. [Spectral types and star ID’s are in the titles). The intensity calibration was done using a catalog of standard stellar spectra from KPNO (Jacoby et al. 1984). Most stars were between magnitude 8 and 10, and the exposure time was 60 sec per spectrum. The format is an animated GIF.
It’s fun to see the gradual appearance, then disappearance of the Balmer lines, the emergence of the molecular bands of TiO, and the shift in the continuum spectrum shape as the surface temperature drops.
The Gemini GRISM system is a fantastic tool for education and research project. It is also more straightforward to use than a TGS system. Give it a try. It’s a great way to break into and learn spectroscopy.\
- Rich Williams