Tilts the grating to encoder value tilt. This requires
that you first determine the grating tilt corresponding to a particular
wavelength.
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Because it takes 2 minutes to switch between full imaging and spectroscopic
modes, a simple quick-look imaging configuration, called
"Acquisition
Mode", is provided via the ACQMODE command.
This will replace the grating with the imaging flat (leaving the grating
tilt unchanged). It takes approximately 5 seconds to switch from
SPMODE to ACQMODE and back, compared to 2 minutes to switch from
SPMODE to IMMODE and vice versa.
After selecting ACQMODE, you will also have to select a filter and
imaging mask appropriate to your spectroscopic setup. Like
SPMODE,
the ACQMODE also accepts optional command-line arguments
to set the SLIT and FILTER to be used for acquisition imaging. The typical
selections are described in Table 1 below:
Thus, if you are in Hi-Res long-slit mode, you need to select the
f/7 Imaging mask for ACQMODE target acquisition, etc.
If your target is bright at visible wavelengths, the J filter is usually
the best choice for target acquisition since J has the lowest
background. If your target is very reddened, H or K will be best, but since
ACQMODE works without a pupil mask, the background will be higher
than in full imaging mode (this is why ACQMODE should not be
used for normal photometric imaging - it is only intended for quick-look
spectroscopic target acquisition).
Table 2
below gives the approximate pixel coordinates (X,Y) of the slit centers.
The precise location of the slit center should be verified when moving to
new objects at substantially different telescope positions. For a discussion
of object centering in the slit, see the
IRS manual.
Note that these pixel centers assume that you have the appropriate camera
in place for each slit mode (see Table 1 of
the Spectroscopy Overview Section).
The typical target acquisition strategy is as follows:
- Switch to ACQMODE, selecting the appropriate imaging mask and filter.
- Find your object, and move it to the nominal slit center.
- Setup the autoguider on nearby field stars (the guide probe is warm
and must not be in the OSIRIS field of view as it will greatly increase
the thermal background).
- Verify centering of the target after the guider "locks."
- Switch back to SPMODE and select the appropriate slit, filter/grism,
etc. for your spectroscopic observations.
For particularly bright targets, an optional step is to image your target
through the slit while still in ACQMODE to verify that it is centered.
This is done by selecting the slit, and then "peaking up" the object by
taking a series of images with slight adjustment of the pointing (using the
NORTH, SOUTH, EAST, and WEST commands as appropriate) so that the maximum
broad-band light gets through. Once you are peaked up, lock the guider
onto the guidestar and switch back to SPMODE. In general, you will find
that if you can center the object at the location nominal slit center in
ACQMODE, you will have the object well-centered in the slit.
Note that if your object is very bright, the first few spectra may have
residual image artifacts due to the previous imaging observations. The
solution is take a few quick exposures before starting your science frames
to clean off the residual images. The section on the
HgCdTe Detector Array provides more information on the array
properties.
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Yes, you should sky chop.
Point Sources
The most reliable way to sky chop when observing point sources is to
chop along the slit, so that you are always taking object and sky spectra
simultaneously, giving 100% on-source dwell time.
In general, this strategy is most useful if the point source is very bright
and easy to see on the array in a single integration. If you need to
build up S/N to see a faint point source, you can still use this "slit
chopping" trick, but you need to be very careful to make sure your object
does not wander out of the slit.
Extended Sources
For extended sources that fill the entire length of the slit, you need
to sky chop away from the object to take separate sky frames, then go
back to the object, much in the same way as is done for imaging extended
sources. The main difference is that in order to build up sufficient
S/N, you need to be certain of returning the object to the same location
on the slit each time. You do this as follows:
- At the end of an object spectrum integration, disable the
autoguider using the -GUIDER command.
- Chop off to some distant sky position using the
OFFSET command.
- Take sky spectra of comparable integration time.
- Return to the object by using OFFSET with the reverse
vector.
- After the telescope has settled down, move the guide star back into
position, and resume guiding using the +GUIDER command.
- Once the guider has locked, start the next object spectrum integration.
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Wavelength calibration can be done in one of two ways:
- Sky Spectra using the OH airglow lines.
- Lamp Spectra using the lamps in the 4-meter guide/acquire module
(He+Ar and Ne gas-discharge lamps).
All sky spectra will contain bright OH airglow lines that may be used for
spectroscopic calibration. Adding together all sky spectra from a given
object will usually provide a good S/N reference spectrum. The only
problems are some blending of OH lines together, and the general scarcity
of OH lines in the red half of the K-band. Both IRAF and XVista provide
lists of identified OH airglow lines, and you can download
examples of OSIRIS OH airglow spectra with
the brightest unblended lines identified. A list of the OH airglow
lines in the near-infrared can be found in
Oliva & Origlia [1992, A&A, 193, 327]
OSIRIS spectra of the He+Ar and Ne lamps are available:
K-band HiRes Lamp Spectrum (Tilt=950)
H-band HiRes Lamp Spectrum (Tilt=950)
J-band HiRes Lamp Spectrum (Tilt=950)
The near-infrared region has many strong absorption features due to various
molecules in the atmosphere. One of the best ways to remove these features
is to ratio the spectrum of the target object with the spectrum of a
featureless source observed with the same instrumental setup and
airmass. In the J and K band, A stars provide good atmospheric standards,
since they have only H absorption features at 2.17 microns (Br-gamma) and
1.28 microns (Pa-beta). In the H band, A stars have many Br-series
absorption features that make them somewhat problematic. Note that Kurucz
models of A stars reproduce the near-infrared spectrum of
these stars reasonably well, so these may be used to correct for the
intrinsic absorption in the stellar atmosphere. Another technique for
correcting for atmospheric absorption in the H band is to obtain standards
of both G and A stars as atmospheric standards. G stars have relatively few
spectra features in this regime and can be used to correct for the Brackett
absorption lines in the A stars.
Spectroscopic flats should be obtained using the dome flat field lamps. A series
of exposures with the lamps off should be subtracted from a similar series
with the lamps on or with the dark filter in.
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Multiple Spectroscopic Modes
Since each grating tilt setting requires a separate set of flat-field,
wavelength, and flux calibrations. The best strategy is to keep the number
of different settings for a given program to an absolute minimum and to take
calibration frames as often as possible.
Spectroscopic mode setup can get involved, if you are going to be
switching often between spectroscopic and imaging modes, it is a good idea
to learn how to define command aliases or scripts in Prospero and
use these for setting different modes. These can greatly simplify your
setup issues, and eliminate loss of time due to careless mistakes late at
night. See the Prospero Observer's Guide for OSIRIS for a
description of how to use aliases and scripts for simplifying instrument
configuration.
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Updated: 1999 December 22
