EFOSC2

ESO Faint Object Spectrograph and Camera

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EFOSC2 Grisms

(14/06/2007) Grism Trouble: Gr#13 has been repaired, Grisms #9, #12, and #15 have been replaced by Gr#18, Gr#16, and Gr#17 respectively. Gr#10 suffered a major damage and is no longer offered. In case of questions, please contact ls-spectro@eso.org.

Observing Issues
The optical elements

Observing Issues

Grisms are mounted on the grism wheel in the parallel beam of EFOSC2 and so they do not introduce any wavelength dependent optical aberrations in the spectra.

Alignment

The slit and grism orientation is always such that the spectral dispersion is along the vertical axis (y-axis) of the CCD, with the longer wavelength further towards the top. The slits and grisms are aligned so as to have the spectrum aligned with the CCD y-axis to better than a tenth of a pixel. Important It takes 10-20 minutes to align the grism (and slits). So observers should not request grism/slit changes during the night. All such changes must be completed the previous day or latest by the afternoon before the start of observations.

Fringing

Some of the red grisms - notably Grism #12 - produce fringing beyond 8000 Ang. It has been noticed that afternoon dome flats do not always adequately remove all the fringing and this may be due to a small amount of CCD flexure with telescope pointing. One solution to this is to obtain a quartz lamp internal flat "on location".

Ghosts

Helium - Argon arc spectra often show a variety of unwanted features including extra lines which are often highly curved, big splotches of light, etc. All of these are due to a combination of reflections in the optics, the very bright emission lines in the arc lamp spectra and possible higher order spectra. At first glance these features look quite terrible as they are displayed on the Real Time Display (RTD) but in fact they are quite faint and the problem is with the display cuts automatically selected by the RTD. These have never been a problem during actual observing where the spectral lines are much fainter and such features are much fainter than the noise.

The Optical Elements

Grisms

Grisms	Range	Grating	Blaze angle	Dispersion ¹	Resolution ²	Plots (PS Gzipped)		Tables	Comments
Name	[Å]	gr/mm	wavelengh [Å]	[Å/pixel]	FWHM [Å] *	Efficiency	Response	Throughput [Å]
Gr#01	3185-10940	100	4500	6.66	40.0	gzipped ps	gzipped ps		³⁾ >9280Å
Gr#02	5100-11000	100	6700	6.60	41.3	gzipped ps	ps		³⁾ >9850Å
Gr#03	3050-6100	400	3900	1.50	9.7	gzipped ps	ps	html
Gr#04	4085-7520	360	4700	1.68	10.5	gzipped ps	gzipped ps
Gr#05	5200-9350	300	6700	2.06	12.8	gzipped ps	gzipped ps	html
Gr#06	3860-8070	300	5000	2.06	12.9	gzipped ps	gzipped ps	html	³⁾ >6280Å
Gr#07	3270-5240	600	3800	0.96	6.2	gzipped ps	gzipped ps	html	⁴⁾
Gr#08	4320-6360	600	5300	0.99	6.2	gzipped ps	ps	html
Gr#09	4700-6770	600	5600	1.0	6.3		ps	html	replaced by Gr#18
Gr#10	6280-8200	600	6500	0.95	5.9	gzipped ps	ps	html	Gr#10 decommisioned
Gr#11	3380-7520	300	4000	2.04	13.2	gzipped ps	ps	html
Gr#12	6015-10320	300	7900	2.12	13.2		ps	html	replaced by Gr#16
Gr#13	3685-9315	236	4400	2.77	17.7	gzipped ps	gzipped fits	html
Gr#14	3095-5085	600	4000	0.93	5.8	gzipped ps	ps		⁴⁾
Gr#15	6895-8765	600	8300	0.86	5.4		ps		replaced by Gr#17
Gr#16	6015-10320	300	7900	2.12	13.2	gzipped ps			³⁾ >7170Å
Gr#17	6895-8765	600	8300	0.92	5.4	gzipped ps			³⁾ >7000Å
Gr#18	4700-6770	600	5600	1.0	6.3	gzipped ps

¹The pixel mentioned in the dispersion column is the 0.157 arcsec unbinned pixel of CCD #40.
²The resolution depends on the slit used. Those quoted here are for a 1.0" slit.
³2nd order contamination has been found for these grims at wavelengths longer than the indicated one.

⁴Grisms 7 and 14 are almost identical; see the addendum to the manual for a comparison.

Grism Response

Efficiency curves for all grisms, 2004 (JPG 55 kb).

Response curves for all grisms, 2003 (PDF 36 kb).
Response curves for all grisms, 1999 (PS 162 kb).

Prisms

There are 2 low dispersion prisms available for spectroscopy. They do not show the zeroeth order and so are useful for slitless spectroscopy, particularly for use with narrow-band filters. The resolution depends on the external seeing.