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NIR Baseline Calibrations

A set of baseline calibrations will be taken for all Gemini queue observations in order to ensure the long-term utility of the data in the archive. This baseline calibration set varies from mode to mode, but includes all the data deemed necessary to produce scientifically useful data.   PIs are free to request additional calibrations beyond the baseline set, however, all calibrations, baseline or additional, must be specified by the PI in the Phase II definition.  The difference is that baseline calibrations are not charged to the program, while any additional calibrations are charged to the program.  The table below describes various calibrations for each wavelength regime and delineates which are included in the baseline set.   Instrument-specific variations are noted.

Baseline calibration data have no proprietary period and are publicly available in the archive as soon as they are uploaded.  These observations may be executed from a shared "Basecal" program (for shared configurations) or from within each science program.  Time used to obtain baseline calibrations between nautical evening twilight and nautical morning twilight is charged to the partner country associated with the program or programs (i.e., "Nighttime Partner Calibration"). Time between morning and evening twilight is not charged (i.e., "Daytime Calibration").  Any time required for additional calibrations requested by the PI is charged to the program (i.e., "Nighttime Program Calibration").  To ensure proper time accounting, please set the Observation Class correctly on all calibration observations.  Examples of baseline calibration observations are available in the OT libraries for each instrument.

Imaging 1-3 mu Imaging >3 mu Spectroscopy 1-2.5 mu Spectroscopy >2.5 mu
Bad pixel mask Can be derived nightly from GCAL flats and short darks; a "canned" BPM may be available in the instrument IRAF package.
GCAL Flat Fields GCAL imaging flats specified in the Phase II will be taken the morning after the science observations; these should include shutter-open and shutter-closed exposures to allow for correction of thermal emission, dark current, and hot pixels. GCAL cannot be used for L and M-band imaging due to saturation. GCAL spectroscopic flats specified in the Phase II will normally be taken the morning after the science observations1; exceptions are NIRI, Phoenix, NIFS coronographic, and GNIRS IFU and high-resolution (R=18000) flats which are taken immediately before or after the science data (before the instrument is re-configured).  NIFS flats should include "lamp off" flats to remove dark current.
Sky Flat Fields Not 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.2 Not applicable
Darks If desired, darks for science exposures can be defined and will be taken as daytime calibrations (not during the night).   These are recommended for NIFS observations, to identify hot pixels.  It is expected that most PIs will use "sky" frames to remove dark current. Short darks (~1s) are taken daily for all instruments to assess readnoise and identify bad pixels.
Wavelength calibration Not applicable Argon arc lamp measurements should be specified in the science observation for each spectral setting.  In some cases,3 a corresponding "lamp off" exposure should be included. The narrow wavelength coverage of Phoenix does not always include any argon lines. In such cases sky emission lines, telluric absorption lines, or lines in the calibration star must be used.
Telluric absorption lines present in the science data are used to determine the wavelength calibration.
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. 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.  See our telluric standards page for more information.
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.   See our photometric standards page for more information. Photometric accuracy is limited to ~10% by the uncertainty in the (unmeasured) atmospheric extinction. Not part of the baseline calibration set.
Atmospheric extinction Not part of the baseline calibration set.
Point Spread Function Not part of the baseline calibration set.
World coordinate system   Written in the header of all images. The WCS is accurate to approximately one arcsec.
Spatial Rectification mask (i.e., for S-distortion correction) Not applicable GNIRS:for cross-dispersed data an appropriate pinhole mask spectrum should be defined with the flatfields; for long slit, a pinhole can be specified if desired; no pinhole is available for IFU data;
NIFS: a "Ronchi" calibration mask spectrum is provided for each wavelength setting.
Focal plane mask image Not applicable The slit or IFU will be imaged without the disperser as part of the acquisition sequence.
Radial Velocity or Line index Standards Not applicable Not part of the baseline calibration set.


1 Where necessary, flats taken with the calibration unit make use of shutter open and closed images to allow correction of dark current and hot pixels. NIRI spectroscopic flats typically only include shutter-open exposures, as the contribution from the background is negligible, and the dark current can be removed using daytime darks.

GCAL spectral flats with GNIRS are reproducible (after instrument configuration changes) to ~2-3%; if higher precision is desired for non-IFU, moderate resolution data, the PI should specify that flats be taken with the data.  However, this is not currently possible for cross-dispersed flats, which must be taken in the morning in a sequence of increasing exposures in order to illuminate the higher orders (6-8) but which saturate the lower orders (3-5).

2 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.  Sky flats can be constructed from data that sees exactly the same pupil.  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. Therefore, dark current images 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.

GCAL imaging 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. Obviously, 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.

3 Arc lamps with high background: All NIFS arc lamps; GNIRS lamps in K band with resolution R=1800 and exposure times > 30secs.

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Last update 2007 September 2; Andrew Stephens, Tom Geballe & Bernadette Rodgers