ALFOSC grism #17
As of September 2005 we have a new grism available for ALFOSC. The
grism was designed by Michael Andersen (AI Potsdam), with the aim of doing
medium-resolution lithium studies in brown dwarfs, in the spectral
region around H_alpha (approx 6350-6850 Angstrom).
The resolution of the grism is highest of all ALFOSC grisms (including
the Echelle grisms), and results in R=10000 for a 0.5 arcsec slit (2.5
detector pixels), with a corresponding dispersion of 0.254
Angstrom/pixel. Due to the VPH technology used the grism has very
high efficiency, with the highest peak efficiency of all ALFOSC grisms.
The grism is mounted in a special holder designed and made by Dennis
Wistisen at Copenhagen University. The grism can only be mounted to
give the dispersion along the rows of the CCD, and can hence only be
used with our set of 'vertical' slits.
A drawing of the grism , and another .
A picture of the front side , and of the back side of the grism.
| Prism glas || OHARA S-NPH2 |
| Grism straight-through wavelength || 6580 |
| VPH ruling frequency per mm || 2400 |
| Prism angle || 47.1 |
| Slit width (arcsec) || 0.5 |
| Resolution || 10000 |
The throughput of the grism was measured using wide-slit (10 arcsec)
standard star observations. The peak system efficiency, including
atmosphere, telescope, instrument and detector, is well above 30%, but
falls down quite steeply away from the central wavelength. A plot of the system efficiency for grism #17.
We do not yet have this new grism added to our Exposure Time
Calculator. Given the efficiencies of grism #8 and grism#17, one will
get about 20% more counts/Angstrom for grism#17 with a dispersion
(A/pix) that is 5 times smaller. See the example spectrum below for
counts/pixel/second for a standard star.
A ghost is seen in flatfield images taken with any of the halogen
lamps. The ghost appears in flats as a near-vertical streak at the
1-2% level, that crosses the object spectrum position in the middle of
the CCD. In the object spectra the ghost is sufficiently far away
from the object. The X position of the ghost can be tuned by slightly
changing the slit position, but this will also affect the sampled
wavelength range (see below) and the alignment of the slit.
Arc lines are heavily curved, and care should be taking when aligning
the slits. Below a full frame Neon spectrum (see arc
Tuning the wavelength range and peak efficiency
In principle, the VPH grism
allows the wavelength at which the peak efficiency appears to be
changed. This can be done by using offset slits, and this will also
change the wavelength region that is sampled on the CCD. Currently
we have the following
vertical offset slits.
We investigated this VPH feature using a pinhole rather than a slit.
The pinhole was moved in steps of 2000 aperture wheel units.
One can see that for a given wavelength the optimal grism efficiency can be tuned
by choosing the best pinhole position.
Note that the dome-flat lamps have increasing brightness towards the
red, which is the reason that the overall countrates in this plot
increase towards the red.
As one can see in the above plot, fringing starts around 6400
Angstrom, and reaches 10% peak-to-peak levels at 6800 Angstrom. This
is a feature of CCD#8.
In September 2005 the grism saw first starlight. An example spectrum
showing one of the first science frames taken with this grism, of the
spectral flux standard SP0644+375 (He 3, V=12.0, spectral type DA),
showing counts/second in the extracted spectrum (dispersion 0.26