OSIRIS Hieroglyph The OSIRIS User's Manual
[Contents]

Spectroscopic Observing

This section describes basic spectroscopic observing setup and techniques. The subjects covered are: We recommend familiarizing yourself with the Spectroscopy Overview before reading through this section.


Putting OSIRIS into Spectroscopic Mode

From imaging mode, OSIRIS is put into spectroscopic mode with the SPMODE command. SPMODE performs the following operations, with typical times for completion given in ()'s:
  1. Retracts the prefilter wheel and pupil mask mechanism (~2 minutes).
  2. Flips the diffraction grating into the beam, replacing the imaging flat mirror (~5 seconds)
Once started, this operation cannot be interrupted, and it will take the same amount of time to switch back to imaging mode (via the IMMODE command).

After selecting SPMODE, you will need to select the camera, camfocus, grating tilt, slit, and filter required for the desired spectroscopic mode as described in Table 1 of the Spectroscopy Overview. To make this more convenient, SPMODE can take additional command-line arguments to select the filter, camera, etc. in one line. For example, typing:

   SPMODE FILTER=3 CAMERA=0 CAMFOCUS=245 SLIT=0
will put OSIRIS into SPMODE, put filter 3 into the beam, select camera 0 and slit 0, and set the camera focus to 245. Which settings you actually use, however, will depend on the configuration you want (warning: these values are only placeholders not actual examples).

[ Top of Page | Contents ]


Wavelength Region Selection

You also need to select the grating tilt for the spectroscopic region of interest. Once selected, the grating tilt will stay fixed until you change it again. To keep calibration tractable, it is a good idea to keep the number of different settings to a minimum, as dicussed in the Strategies section below.

Low-Res and X-Disp Modes

In Low-Res and cross-dispersed modes the grating tilts are fixed as given in the Wavelength Selection section of the Spectroscopy Overview. The commands provided for selecting these preset grating tilts are as follows:
F3SPEC band
Selects the optimal tilt for low-res long-slit spectra in the given band (one of J, H, or K).

XDSPEC
Selects the optimal tilt for the cross-dispersed mode. The order-sorting filter for cross-dispersed mode is integrated into the X-disp slit.

In general, you should never need to make any adjustments to the preset tilts for either of the two low-res spectroscopic modes (see Low-Res Long-Slit Mode for details).

High-Res Mode

For High-Res spectra, you need to select the tilt by wavelength for the order of interest using GRATING commands as follows:
GRATING tilt
Tilts the grating to encoder value tilt. This requires that you first determine the grating tilt corresponding to a particular wavelength. [ Top of Page | Contents ]


Spectroscopic Target Acquisition: ACQMODE

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:

Table 1: Typical Acquisition Mode Parameters
Spec
Mode
SLIT Mask FILTERs
Low-Res f/2.8 Imaging J, H, or K
Hi-Res f/7 Imaging J, H, or K
X-Disp f/2.8 Imaging J, H, or K

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.

Table 2: Slit Centers in Pixels
Mode/Slit Center (X,Y)
Low-Res (535,585)
Hi-Res (495,560)
X-Disp (520,585)

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:

  1. Switch to ACQMODE, selecting the appropriate imaging mask and filter.
  2. Find your object, and move it to the nominal slit center.
  3. 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).
  4. Verify centering of the target after the guider "locks."
  5. 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.

[ Top of Page | Contents ]


Sky Chopping

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:
  1. At the end of an object spectrum integration, disable the autoguider using the -GUIDER command.
  2. Chop off to some distant sky position using the OFFSET command.
  3. Take sky spectra of comparable integration time.
  4. Return to the object by using OFFSET with the reverse vector.
  5. After the telescope has settled down, move the guide star back into position, and resume guiding using the +GUIDER command.
  6. Once the guider has locked, start the next object spectrum integration.
[ Top of Page | Contents ]


Spectroscopic Calibration

Wavelength Calibration

Wavelength calibration can be done in one of two ways:
  1. Sky Spectra using the OH airglow lines.

  2. 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)

Spectroscopic Standards

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

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. [ Top of Page | Contents ]


Spectroscopic Observing Strategies & Caveats

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.

[ Top of Page | Contents ]


Updated: 1999 December 22