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NIR Baseline Calibrations

For all queue observations, a set of baseline standard calibrations will be taken by Gemini Staff to ensure the long-term utility of data in the archive. The baseline calibration set varies from mode to mode. Applicants must put ALL baseline calibrations that they require into their Phase II files. These will typically include flat field and standard star observations. They may also include arc lamp spectra and darks (note that a set of short dark frames are the only ones automatically taken each day).

Baseline calibration data are shared between all programs. Any time used to obtain baseline calibrations between nautical evening twilight and nautical morning twilight will not be explicitly charged to the program but will be charged to the partner country whose NTAC recommended time (i.e. it is part of the time accounting used to balance partner usage). The time will be charged in proportion to the number of programs that receive the calibration data. Any time required for additional calibrations requested by the PI will be charged to that particular program.

	Imaging 1-3 um	Imaging 3-5 um	Spectroscopy 1-3 um	Spectroscopy 3-5 um
Bad pixel mask	Can be derived from the daily GCAL flats and short darks
GCAL Flat Fields	GCAL flats specified in the Phase II will be taken the morning after the science observations	GCAL cannot be used for L and M-band imaging due to saturation	GCAL spectral flats will be taken immdediately before or after the science observations
Sky Flat Fields	Not included as part of the baseline calibration set. If desired, sky flats must be derived from the program data. Dark current stability can limit the accuracy of the sky flats, so you may wish to specify dark frames.
Darks	Short 1s darks are taken daily to help identify bad pixles. Darks for science observations are NOT normally taken, and the PI should request daytime darks with the correct exposure times if they are desired.
Wavelength calibration	Not applicable		Argon arc lamp measurements are made each time the slit is repositioned and for each grism and camera used. Arc lamp examples are linked to the NIRI grism list table.	Sky lines are used to construct the wavelength calibration and no arc lamp spectra are taken. Sky spectra examples are linked to the NIRI grism list table.
Telluric standard	Not applicable		We will observe one telluric standard per science target for every 1.5 hours of integration, including integrations up to 3 hours in length bracketed by two telluric standards. PIs should include two standards in every observation group: one suitable for observation before the science target, and one suitable for after the science target.	We will observe one telluric standard per science target for every 1 hour of integration, including integrations up to 2 hours in length bracketed by two telluric standards. PIs should include two standards in every observation group: one suitable for observation before the science target, and one suitable for after the science target.
Flux standard	We will observe one photometric standard for every 2 hours of science integration. The PI should supply one standard before and one after the science observation in order to give a good airmass match. The standards are typically selected from the OT Library which uses stars from the UKIRT faint standard list, although Hunt or Persson standards may also be used. Photometric accuracy is limited to ~10% by the uncertainty in the (unmeasured) atmospheric extinction. Stars should have 10 < K < 14 or 3 < M' < 9.		Not included as part of the baseline calibration set. Include imaging observations in your program if this is required. (See the Observing Strategies page.)
Atmospheric extinction	Not included as part of the baseline calibration set.
Point Spread Function	Not included as part of the baseline calibration set.
World coordinate system	Automatically included in the header of all images. The WCS is accurate to approximately one arcsec.
Focal plane mask image	Not applicable		The slit will be imaged without the disperser as part of the acquisition sequence.
Radial Velocity Standards	Not applicable		Not included as part of the baseline calibration set.

* IMPORTANT NOTE regarding GCAL flats and flatfield accuracy:

Flats taken with the calibration unit at 1-2.5um make use of shutter open and closed images to explicitly correct for dark current and thermal emission from the camera optics. The calibration unit produces a beam that matches the telescope pupil very nicely except that it contains no central obscuration. The light path between GCAL and the instrument excludes the primary and secondary mirrors as well. The GCAL illumination is therefore subtly different from that of sky flats.

GCAL flats are reproducible from night to night to about 0.3% i.e. the sensitivity of a given pixel varies by 0.3% over many nights as measured by GCAL. For a star that subtends many pixels, the photometric accuracy will be approximately 0.3% divided by the square root of the number of pixels.

Alternatively, sky flats can be constructed from data that sees exactly the same pupil. Sky flats, however, cannot make use of the equivalent "shutter closed" images to subtract dark current and thermal emission, which may be significant at K-band. Dark current subtraction is essential for making sky flats, and our experience shows that dark current variations over periods of a few hours limit the accuracy of sky flats. Dark current images really must be taken at the same time as the data to be used to construct sky flats. The difference between a given pixel in a sky flats vs. a GCAL flat can be 2% to 3%. The large-scale illumination pattern differences are completely swamped by the dark current variation, which has a spatial pattern of its own.

Baseline calibrations include only GCAL flats. Sky flats can be constructed from the science data itself. If sky flats are essential, be sure to include adequate measurements of the dark current in your proposal.

Twilight flats taken with concurrent dark images have not yet been tested.

Last update 2006 June 10 by Andrew Stephens & Bernadette Rodgers