Wavelength and flatfield calibration is obtained using the GCAL, the facility calibration unit. Baseline calibrations will be obtained for all observations, queue and classical. Additional calibrations must be requested as part of your program.
For all queue observations a set of standard calibrations (the "baseline calibrations"), shown in the tables below, 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. 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. All calibration data are available through the Gemini Observatory Archive.
The time required for any nighttime baseline calibrations has to be included in the "Total Time" requested during Phase I. The required "Night Basecal Time" per observation is automatically calculated by the Phase I Tool (PIT). 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, as nighttime partner, or program calibrations. Note that all calibrations must be included in the Phase II science program.
|Bias levels||Bias images are taken during the day in all modes used for science and calibrations, and are used for bias subtraction. The biases levels over time can be found here ***link to webpages with noise levels for each amplifier.***|
|Bad pixel mask||New bad pixel masks are derived using dome flats, and bias images each semester. The variation of the noise levels of the detector over time can be found here ***link to webpages with noise levels for each amplifier.***|
|Flat field corrections||Dome flats are taken in the respective resolution and read modes of science during the day using a 100W lamp. All dome flat-field observations are taken in dual target mode, with the two IFUs illuminated evenly. These are sufficent to reduce single target observations taken in matching instrument modes.|
|Wavelength calibration||Wavelength calibration is conducted using arc exposures taken during the day with a ThAr lamp in the respective resolution and read modes to match science. There is minimal flexure in the Cassegrain unit, and no nighttime arcs images are taken. All arc lamp images are taken in dual target mode, with the two IFUs illuminated evenly. These are sufficent to reduce single target observations taken in matching instrument modes.|
|Twilight flats||Twilight flats, to define the slit illumination function need to be defined and requested by the PI. A twilight flat in each resolution mode, will be obtained once per semester. Twilight flats are considered baseline calibrations and are not charged to the program.|
|Internal flats||Internal flats mixed with night time science observations, taken using the 100W lamp in the position of the IFUs for science are not taken as part of the baseline calbration program. Such observations, if desired by the PI must be requested as part of nighttime partner calibration.|
|Radial velocity standards||Radial velocity standards are observed as part of the instrument monitoring program, but are not guaranteed to be obtained during the same nights as the science observations of a program. If required for science, please include a chosen radial velocity standard as part of your time request, as nighttime partner calibration. This will be taken in conjunction with your science.|
|Telluric standards||Wavelength longward of 700nm are contaminated by telluric lines. If your science region of interest requires accurate telluric subtraction, please include a telluric standard closely matching the airmass of your target, so that it can be scheduled before or after the science observation in your time request. These observations are taken as part of the nighttime partner calibrations.|
|Spectrophotometric standards||Given the aperture limit and the fiber relay, absolute spectrophotometric calibration is not possible unless taken in seeing limited clear conditions (i.e., IQ70 or better). Although spectrophotometric standards are taken as part of the regular instrument monitoring program, these observations are not guaranteed to be obtained during the same nights as the science observations for the program. If absolute flux calibration is required for science, please include a chosen spectrophotometric standard in your time request, which will be charged to the nighttime program calibration. This will be taken in conjunction with your science.|
Observations of spectrophotometric standard stars are required in order for spectra obtained to be flux-calibrated. For spectra at wavelength longwards of about 680nm, calibration spectra of hot stars are needed if cancellation of telluric features in the spectrum is required.
Spectrophotometric standard stars for performance monitoring are chosen from the sources listed in the following table, and include the HST white dwarf primary standards, and Hamuy standards. If you require an additional standard star not present in this list, please let us know.
|Spectrophotometric standard stars|
|Hamuy et al. (1994, PASP, 106, 566)||-68 deg to +11 deg||330-1030 nm||5 nm|
|Bohlin et al. (1995, AJ, 110, 1316)||+15 deg to +52 deg||100-30000 nm||0.06 nm|
Observers interested in features in the red part of the spectrum, may benefit from telluric corrections depending on the wavelengths of the lines of interest. The selection of a telluric standard depends on the science spectrum, and an appropriate standard may be found here. The telluric standard should match closely the airmass of the target during observations, and is usually taken before the science spectra. Early type stars are preferred, however these have broad lines that needs to be subtracted. Instead, users may also use solar type stars for telluric correction to avoid introducing noise if those are the regions of interest.