GNIRS Baseline Calibrations

For all queue observations, a set of standard calibrations (the "baseline calibrations") will be taken by Gemini Staff to ensure the long-term utility of data in the archive. The baseline calibration set varies from instrument to instrument and from mode to mode.

Applicants should not include time for baseline calibrations in their proposals. If additional calibrations are deemed necessary by applicants, for example to achieve a precision beyond that achievable via the baseline set, then these must be included explicitly as part of the Phase I proposal and Phase II science program. Baseline calibrations (telluric standards, flats, and arcs) need to be included in Phase II programs. Telluric standard observations must be defined by the PI in the Phase II program (as described below), for ease of observing and to ensure that appropriate "standard" stars are selected.

Baseline calibration data may be shared between programs and will be distributed to the users as part of their dataset. Any time used between the hours of nautical sunset and sunrise to obtain baseline calibrations 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.

2006: All Phase II (OT) files must include observations for all baseline calibrations: telluric standards, flats and arcs. See table below for details, and examples in the OT help pages.

Measurement Type	Notes
Bad pixel mask	Derived nightly from GCAL flats and short darks.
S-distortion pinhole spectrum	A pinhole mask is taken for the cross-dispersed modes at least once per run. Pinhole spectra are not usually taken for long-slit configurations; PIs can request one (at no extra charge) if desired.
Calibration unit flat field	GCAL flats are taken nightly before or after each program for each prism/mirror/grating and camera used. Flats are taken at each grating wavelength setting, usually before or after the target observation (i.e, without moving the grating), but not always. If high precision flatfielding is critical, the PI should request this. (As a result of acquisition, the slit will be repositioned but this mechanism is repeatable to <0.5pix). Flats are normalized in post-processing. Long slit flat fields for wavelengths < 2.0um use the GCAL quartz halogen lamp; K band flats (2.0-2.5um) use the GCAL IR lamp, and thermal wavelengths (>2.5um) use the ambient thermal background (no lamp). Median S/N in combined long-slit flats is ~400. Cross-dispersed (XD) flats are done at the end of the night only (until better GCAL filters are obtained). They involve a sequence of increasing exposures in order to get sufficient signal in the shorter wavelength orders (up to order 6), while saturating the longer wavelength orders (3-5), since the counts vary dramatically between and within orders 3 (K) through 7 (~1um). Target S/N is 300-400 in orders 3-6; orders 7 and 8 (0.7-1.0um) have almost no signal. If flatfielding is required in these orders, PIs must request this. See the short camera/32 l/mm grating page for an example XD flat. The sequence of XD flats with exposure times is outlined here.
Dark	Short darks (~1 sec) are used to help identify bad pixels. Darks for the flatfields are only taken for exposure times of 2seconds or greater. (Very short flats do not require dark subtraction.) Darks are not normally taken for the exposure times of target observations; PI should request this if needed.
Wavelength calibration	For wavelengths < 2.5um, arc lamp measurements are made at each grating position, immediately before or after the target observation i.e. without moving the grating. At L and M, sky lines are used to construct the wavelength calibration and no arc lamp spectra are taken as part of the baseline calibration set.
Telluric standard star	An appropriate telluric standard should be chosen by the PI and included in the Phase II program. To maximize flexibility, standards before and after each observation should be defined for observations longer than ~15 minutes. When the observation is executed in the queue, the astronomer will select one of the two for the Baseline calibration. The PI is not be charged for one telluric standard for each observation. When the same standard can be used for multiple targets, the PI should note this in the Phase II program. The PI should take care to select a star of the appropriate spectral type and brightness at an airmass that will match the science target. For help selecting telluric standards, see the spectroscopic standards page.
Absolute flux calibration	Not included as part of the baseline calibration set. Observations with a wide (3") slit can be made for this purpose, but must be explicitly requested (and allocated time) by the PI.
Atmospheric extinction	Not included as part of the baseline calibration set.
Point Spread Function star	Not included as part of the baseline calibration set.
World coordinate system	RA and Dec information are automatically recorded in the WCS information in the raw data. This information only makes sense in the un-dispersed acquisition images and in the spatial direction for long slit data. Wavelength calibration information will be added to the WCS header information when the data are processed with the GNIRS IRAF scripts.
Focal plane mask image	The slit will be imaged without the disperser as part of the setup. All setup images are passed to the PI along with the spectral data.
Special standards	Radial velocity standards are not included as part of the baseline calibration set.