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ALFOSC grism #16

As of December 2003 we have a new grism available for ALFOSC. The grism was designed by Michael Andersen and John Telting, with the aim of doing R=2000 radial velocity studies on compact stars in the blue part of the spectrum, covering the strong Balmer and Helium lines.
The resolution of the grism is covering the gap between grism #6 and the Echelle grisms.
The grism was ordered from RGL Spectra-Physics and was glued in a holder made in the NOT mechanical workshop by Peter Brandt. Glue: 3M Scotch-Weld 2216.

Design parameters

Prism glas PBM2Y
Resin regular
Grism straight-through wavelength 4250
Groove frequency 1000
Groove angle 41
Best illumination angle of grating -10
Prism front angle -12
Camera focal length 146.3
Slit width (arcsec) 0.5
Resolution 2216
Grism angle 40.57
Prism index at grism wavelength 1.64
Resin index at grism wavelength 1.61
Best prism index 1.72
Dispersion 51.21
0-order angle with horizontal -36.02
0-order position on detector -7090.29
Blaze angle in min order -1.42
Blaze position on detector -240.95
Blaze wavelength (nm) 406.87
Anamorphic magnification 0.94
Angle of incidence on facet -6.86

A drawing of the grism .
A plot of the grism efficiency.

On-sky efficiency

The throughput of the grism was measured using wide-slit standard star observations. The system efficiency is somewhat lower than for grism #6 (factor 0.85), but still much better (almost factor 2) than for the Echelle combinations.

A plot of the efficiency of grism #16 compared with that of grism #4 . Note that this plot was obtained when the system efficiency was affected by a 4500 Angstrom dip caused by the camera optics that needed replacing (for comparison: the measurement with grism #4 with the old CCD #7 is not affected by the dip).

Wavelength coverage and effective resolution

The wavelength coverage is approximately 3500-5060 Angstrom (see extracted ThAr spectrum). On the left, a 100 second ThAr exposure is shown.

The spectrograph was focussed in the spectral direction using arclines with Hartmann masks, the 0.5 arcsec slit and the grism in the beam: the best focus was found to be approx 1770. Arclines obtained with a slitwidth of 0.5 arcsec are imaged onto approximately 2.4 pixels FWHM giving an approximate resolution of R=1800. The 0.75 arcsec slit is imaged to 3.3 pixels and the 1.0 arcsec slit to 3.7 pixels. For comparison: the resolution of grism #6 and the Echelle mode are R=980 and R=4500 respectively (for 0.5 arcsec slit). The dispersion is about 0.8 Angstrom per pixel.


A small ghost was seen in flatfield images taken with the internal halogen lamp, and a mask was fitted on the holder of the grism to try to resolve this problem. The mask makes the grism aperture at the entrance about 2mm in diameter smaller than the diameter of the grism (the grisms are sufficiently larger than the beamsize).
Internal flatfield without (left) and with the mask mounted.

Example spectrum

In January 2004 the grism saw first starlight. An example spectrum showing one of the first science frames taken with this grism, of an sdB star.


Thanks to Michael, Niels, Roy, Johannes, Pedro, Peter, Thomas, who all contributed to what is displayed in this report.

cheers, John

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