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Observing Strategies

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The gaps between the three detectors in both GMOS cause gaps in the spectral coverage, see the data examples. The size of the gaps in wavelength space depends on the grating used, but is typically a few nanometers. If continuous spectral coverage is essential for your program, consider using two configurations of the grating with central wavelengths 3-5 nm different.

The width of each fiber in the IFU is only 5 unbinned pixels. Thus, it is recommended to always have the Y-direction of the detectors unbinned. The spectral resolution of the IFU is equivalent to a slit with 0.31 arcsec width (3.88 unbinned pixels for Hamamatsu, 4.26 unbinned pixels for e2v), thus you may consider binning in the spectral direction if your object is very faint.

You will have to decide if your science targets are best observed in IFU 2-slit mode or IFU 1-slit mode. The 2-slit mode gives you a larger field on the sky at the expense of spectral coverage. The 1-slit mode gives you larger spectral coverage, but half the field of view on the sky compared to the 2-slit mode. In 2-slit mode you will have to use one of the color filters in order to avoid overlap between the spectra, see the spectral overlap page for details. In 1-slit mode you may have to use a filter to avoid 2nd order contamination.

In 1-slit mode the central wavelength you specify will be interpreted as the desired wavelength at the center of the detector array. In 2-slit mode, the central wavelength you specify will be the wavelength at the location of the two pseudo-slits.

If your science target cannot be detected in imaging mode in one of the GMOS filters in about 5 min of exposure time, you will need to supply coordinates for a nearby brighter target (R<19 mag. and preferably a point source) and accurate offsets between that brighter target and your science target. The accuracy of blind offsetting is better than 0.1 arcsec for offsets less than 20 arcsec. For the blind offsetting to work it is essential that the same guide star can be reached for the bright object and the science target.

If your science target is fainter than about R=18, you will have to supply a finding chart at the time of Phase II submission.

Are the Baseline Calibrations sufficient for your program? If you need accurate telluric line removal, you will need to add telluric standard stars to your program. If you need radial velocity standards, these need to be added to your program. Also if you need very accurate velocity calibration and you are observing in the blue where not many skylines are available from which to bootstrap your daytime CuAr, you may need to request CuAr calibrations be taken on the sky with your science observation. On-sky arc lamp exposures for setups which do not cover at leat the 5577 A sky line are provided as part of the baseline calibrations (details can be found here).

IFU Target Acquisition and Observation Definition

IFU Target acquisition is done by taking a GMOS image of the field using the sub-region of the CCDs that contains the IFU fields.  An IRAF task in the Gemini package can overlay the IFU fields on the image and calculate the offsets to the desired position in the IFU field-of-view. Another task does rapid IFU image reconstruction to verify the placement of the target in the IFU. The following information is needed to define an IFU observation:

  • One-slit or two-slit mode.  If in one-slit mode, then one can choose which slit to use (IFU Right Slit (Red) recommended for both GMOS-N and GMOS-S).
  • Coordinate to be centered in the desired IFU field.
  • Position angle (PA) to track.  Note that the long axis of the Object field changes by 90 degrees between one and two-slit modes.
  • Whether beam-switching is needed
  • Blocking filter
  • Grating
  • Central wavelength at the position of the pseudo-slits.
  • CCD binning (no binning recommended).
  • Exposure time

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