EFOSC2 Grisms
Note: (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have been replaced
by Gr#18, Gr#16, and Gr#17 respectively.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-efosc@eso.org.
Content
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)6) | Tables6) | Comments | |
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| Name | [Å] | gr/mm | wavelengh [Å] | [Å/pixel] | FWHM [Å] * | Efficiency |
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Throughput [Å] | |
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3) >9280Å | |
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3) >6280Å |
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html | 4) |
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replaced by Gr#18 | |
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Gr#10 decommisioned |
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replaced by Gr#16 | |
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4) | |
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replaced by Gr#17 | ||
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3) >7170Å | |
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3) >7000Å | ||
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5) | |
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1070 |
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3)
>6900Å 5) |
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1The pixel mentioned in the dispersion
column is the 0.13
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.
5Grism
Dispersion and Resolution measured using a 0.5" slit and 1x1 binning.
In an independent test Vik Dhillon found Grism dispersions of 0.
287 Å/pixel and 0.480 Å/pixel for Gr#19 and
Gr#20, respectively. The corresponding resolutions are: 1.08 Å
(Gr#19) and 1.77 Å (Gr#20). This test was performed
using a 0.5" slit.
6All the plots and tables linked from this table are still relative to EFOSC2 @ 3P6
Grism Response
All the efficiency and response funcion reported below are still relative to EFOSC2 @ 3P6
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).