16 de Agosto 2003
Michael Richer
During the past year and a half,
observations were obtained in order to measure the efficiency of the MES-SPM
spectrometer, identify the lines of its Th-Ar lamp,
and characterize its spectral stability.
The efficiency measured
for extended sources is high, up to 11% for the H alpha and [
For the uninitiated, the MES-SPM is a
long-slit echelle spectrometer optimized for
observing faint, extended sources (Meaburn et al.
1984, MNRAS, 210, 463; Meaburn et al. 2003, RMxAA, in press).
Instead of a cross-disperser, an interference filter is used to isolate
the spectral order of interest. The
spectral resolution depends upon the slit width, and a variety of slits of
different widths are available.
The observations upon which this
report is based were obtained on the nights of
The following table lists the
efficiencies measured for both resolved and point sources. The definition of efficiency that is adopted
is the fraction of photons incident upon the primary mirror that are
subsequently detected by the CCD. To
calculate the efficiency of the spectrometer, the standard star spectra were
extracted to one-dimensional spectra and calibrated in wavelength using
identical extractions of spectra of the Th-Ar arc
lamp. I correct the measured flux per
pixel for atmospheric extinction and compare the result with the expected flux
for the standard, based upon the tabulated AB magnitude (from Jeremy Walsh's
compilation at ESO in this case).
The atmospheric extinction correction is based upon the San Pedro Mártir
extinction curve published by Schuster & Parrao
(2001, RMxAA, 37, 187).
To make use of these efficiencies, one
computes the expected photon flux incident upon the primary mirror and
multiplies by the efficiency to obtain the photon flux that should be detected
by the CCD.
|
wavelength |
efficiency: resolved sources |
efficiency: point sources |
|
4686 |
|
0.029 |
|
5007 |
0.093 |
0.043 |
|
6550 |
0.129 |
0.050 |
|
6716 |
0.131 |
0.060 |
The efficiency at H alpha should be about
75% of that found at 6550Å (Meaburn et al. 1984).
The efficiencies for resolved sources
are based upon the slitless observations obtained on
The efficiencies for point sources are
derived from the other observations that were made through a 150 micron
slit. Compared to the efficiency for slitless observations, the efficiency for point sources is
a factor of 1.6-2.6 lower when the 150 micron slit is used. Obviously, an even larger reduction would be
found for narrower slits. The image
quality, object centering, and telescope guiding can all modify the efficiency
for point sources substantially, and it would not be unusual to find
efficiencies differing from those quoted by 50%, depending upon the observing
conditions. For this reason, the
efficiencies for point sources are only indicative. Nonetheless, I have verified that these
efficiencies predict the actual performance of the MES-SPM reasonably well for
spectroscopic observations of extragalactic planetary nebulae that were
obtained with the [O III]5007 filter and the 150
micron slit during an observing run in September 2001.
Finally, please note that these
efficiencies are very different from those of the spectrometer itself (Meaburn et al. 1984, Meaburn et
al. 2003), since they involve many components besides the spectrometer. Indeed, these efficiencies are dominated by
other factors than the spectrometer: the reflectivity of the telescope optics,
the optical quality of the order-selecting filter, the CCD quantum efficiency,
and, for observations of point sources, the image quality, telescope focus,
object centering, and the light losses at the slit. For example, the drop in efficiency observed
for [O III]5007 and He II 4686 is principally due to
the response of the SITe3 CCD.
The following table presents line
identifications for the Th-Ar lamp in two ways. First, there are plots of the lamp for the
MES-SPM alone. Second, there are plots
comparing the Th-Ar lamps in the MES-SPM and that of
the coudé spectrograph at the 2.2m telescope in
|
line of interest |
identification of the Th-Ar lamp |
comparison of MES-SPM and |
|
He II 4686 |
||
|
[O III]5007 |
||
|
5200A |
||
|
He
I 5876 |
||
|
[O
I]6300 |
||
|
H I 6563 |
||
|
[S II]6716,6731 |
These line identifications cover only
approximately half of a spectral order, since the CCD intercepts only this
fraction of an order.
The He II 4686, He I 5876, and [O I]6300 filters are from the galactic nebular I
filter set and are not normally installed in the spectrometer. The 5200A filter is from the extragalactic
filter set and likewise is not normally installed in the spectrometer. Any 50 mm round filter should fit in the
spectrometer's filter holders, but the image quality will depend upon how
closely the filter matches the optical thickness of the MES-SPM's
filter set. As a very general rule, the spectrometer is NOT re-focussed for different filters. For more information, please contact José
Alberto López.
The night of
I thank Serguei
Zharikov for obtaining the slitless
spectra upon which the efficiencies for resolved sources are based. I acknowledge the help and patience of
Gabriel García, José Alberto López,
Gustavo Melgoza, and Salvador Monrroy
during various observing runs. I thank
Leonid Georgiev for his collaboration in the spectral
stability tests and Sandra Ayala for her collaboration with the line
identifications for the 5200A filter. I
thank Will Henney and Teresa García
for loaning me one of their [O I]6300 spectra. I thank John Meaburn,
Alan Watson, and José Alberto López for various
useful and constructive discussions regarding the derivation of efficiencies
that contributed to substantially improve this report.
