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GMOS Baseline Calibrations

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.

Applicants should not include time for baseline calibrations in their Phase I 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. Note that all calibrations, including baseline arcs, flats, spectral response standards and Nod & Shuffle Darks must be included in the Phase II science program. A table of recommended GCAL settings and exposure times for spectroscopic flats and arcs can be found here.

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.

For Nod-and-Shuffle observations all baseline calibrations (with the exception of the Nod & Shuffle Darks) will be taken without Nod-and-Shuffle, e.g. spectrophotometric standard stars will not be taken with Nod-and-Shuffle.

All calibration data is available through the Gemini Science Archive. All raw data is available as well as processed biases, imaging twilight flats, and some standards.

GMOS Imaging Mode

Measurement Type Notes
Bad pixel mask Derived each semester from dome flats and bias images. May be derived more often if changes in the detector array are found. The bad pixel mask for the imaging mode will show the areas outside the imaging field of view as bad pixels, even though the pixels are not strictly bad, just not illuminated by the imaging field of view.
Flat field Imaging twilight flats are taken each day as the weather allows. All twilight flats from a 10-14 night dark period are combined in order to built a high S/N flat field. The typical noise in the avarage of the twilight flats that can be obtained in a given run is 0.75 percent.
Dome flats may also be taken, but are not guaranteed to be part of the baseline calibration set.
All flat fields are obtained in slow read, low gain mode. Other readout modes can use the same flat fields when the ratios of the gains for the different readout modes are taken into account.
Fringe Frames GMOS South images show strong fringing in both i' and z' bands, and GMOS North images to a lesser extent in z' band. One set of blank sky images in i' and z' bands are taken once per semester for fringe frame generation for GMOS South, and one set in z' band for GMOS North. These fringe frame images are taken as part of baseline calibration. However, users may find that better fringe frames can be generated from the science imaging data and if possible should consider including multiple dithers for their imaging science data.
Dark / Bias The dark current for GMOS is <1.0e-/hour. Dark images are normally not taken. Bias images are taken regularly in the readout modes and binnings used for the science observations. The bias is very stable and all bias images obtained during a month can be used to create a high S/N bias image.
Wavelength calibration Not applicable
Telluric standard star Not applicable
Flux standard stars Selected from Landolt (1992, AJ, 104, 340). Each observed field contains 3-5 standard stars. A minimum of one field is observed each photometric night during which imaging programs are carried out. Observations of standard stars are only obtained in broadband filters used for the science observations. Approximate zero points are also available. Photometric accuracy is limited to ~5% by the uncertainty in the (unmeasured) atmospheric extinction.
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 Automatically included for each GMOS image. Relative accuracy is better than 0.2 arcsec on average over the field. Absolute accuracy is about 5 arcsec.
Focal plane mask image Not applicable.
Special standards Not applicable.

 

GMOS Long-slit Mode

Measurement Type Notes
Bad pixel mask Derived each semester from dome flats and bias images. May be derived more often if changes in the detector array are found. The bad pixel mask for the long-slit mode will show the bridges in the long-slits, even though the pixels are not strictly bad, just not illuminated by the long-slit observations. 
GCAL flat field GCAL flats are taken mixed with the science observations. For science programs one GCAL flat is obtained for each hour of open shutter time, though a minimum of two are always obtained. For example, for an observation that consists of 6x 30min of exposure time, a total of 3 GCAL flats would be taken. For an observation that consists of 2x15min a total of 2 GCAL flats would be taken. For standard star observations, only one GCAL flat is obtained. This is sufficient since the standard star observations have significantly higher S/N than most science observations.
Dark / Bias The dark current for GMOS is <1.0e-/hour. Dark images are normally not taken (except for Nod & Shuffle Darks). Bias images are taken regularly in the readout modes and binnings used for the science observations. The bias is very stable and all bias images obtained during a month can be used to create a high S/N bias image. Nod & Shuffle Darks should be defined by the PI and will be taken on a best-effort basis when the telescope is closed for bad weather.
Wavelength calibration CuAr arc exposures matching the configuration (grating, filter, slit width, CCD binning) are obtained as part of daytime baseline calibrations. One exposure is obtained each observing run during which science observations are gathered. Arc exposures obtained at the elevation of the science observations are not included as part of the baseline calibration set. All arc exposures are obtained in fast-read, low gain mode.
Twilight flat field Twilight flats, which provide information about the slit function, are usually not taken for longslit programs, but can be provided if requested (and defined) by the PI. Each twilight flat exposure, matching the science configuration, is obtained once per semester. Twilight flats are considered baseline calibrations and are not charged to the program.
Telluric standard star The flux standard stars may be use for approximate removal of telluric lines but the observations of the flux standard are not guaranteed to be obtained during the same nights as the science observations for the program.
Flux standard stars Selected from the sources listed on the GMOS spectrophotometric standard star page. For each configuration (grating, filter, slit width, CCD binning) used by a science program, one spectrophotometric standard star is observed per semester. Observations are not guaranteed to be obtained during the same nights as the science observations for the program. Observations to provide absolute spectrophotometric calibration are not included as part of the baseline calibration set.
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 Automatically included for each GMOS image. Relative accuracy is better than 0.2 arcsec on average over the field. Absolute accuracy is about 5 arcsec.
Focal plane mask image Images of the long-slits are obtained regularly for monitoring the quality of the slits. The images are not distributed as part of the baseline calibration set.
Special standards Radial velocity standards are occasionally obtained and distributed. A library of radial velocity standards observed with GMOS will be built and made available. Radial velocity standards are not observed routinely as part of baseline calibrations. Any program that requires radial velocity standards should include these observations in the phase II for the program.

 

GMOS multi-object spectroscopy mode

Measurement Type Notes
Bad pixel mask Not included as part of the baseline calibration set. The user may use the bad pixel mask for the long-slit mode.
GCAL Flat field GCAL flats are taken mixed with the science observations. For science programs one GCAL flat is obtained for each hour of open shutter time, though a minimum of two are always obtained. For example, for an observation that consists of 6x 30min of exposure time, a total of 3 GCAL flats would be taken. For an observation that consists of 2x15min a total of 2 GCAL flats would be taken.
Dark / Bias The dark current for GMOS is <1.0e-/hour. Dark images are normally not taken (except for Nod & Shuffle Darks). Bias images are taken regularly in the readout modes and binnings used for the science observations. The bias is very stable and all bias images obtained during a month can be used to create a high S/N bias image. Nod & Shuffle Darks should be defined by the PI and will be taken on a best-effort basis when the telescope is closed for bad weather.
Wavelength calibration CuAr arc exposures matching the configuration (grating, filter, MOS mask, CCD binning) are obtained as part of daytime baseline calibrations. One exposure is obtained each observing run during which science observations are gathered. Arc exposures obtained at the elevation of the science observations are not included as part of the baseline calibration set. All arc exposures are obtained in fast-read, low gain mode.
Twilight Flat field Twilight flat observations, if requested (and defined) by the PI, are taken through each MOS mask with the configuration (grating, wavelength, CCD binning) matching the science observations. The twilight flat can be used to derive the slit function for the mask. The exposure is obtained once per semester. Twilight flats are considered baseline calibrations and are not charged to the program.
Telluric standard star The flux standard stars may be use for approximate removal of telluric lines but the observations of the flux standard are not guaranteed to be obtained during the same nights as the science observations for the program.
Flux standard stars Selected from the sources listed on the GMOS spectrophotometric standard star page. For each configuration (grating, filter, slit width, CCD binning) used by a science program, one spectrophotometric standard star is observed per semester. Observations are not guaranteed to be obtained during the same nights as the science observations for the program. Standard stars are observed using the standard set of long-slits. If a MOS program uses a non-standard slitwidth, the standard star observations will be obtained through the long-slit with a slit width closest to the one used by the MOS program. Observations to provide absolute spectrophotometric calibration are not included as part of the baseline calibration set.
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 Automatically included for each GMOS image. Relative accuracy is better than 0.2 arcsec on average over the field. Absolute accuracy is about 5 arcsec.
Focal plane mask image Each MOS mask is imaged once using GCAL as the illumination source. The imaging is done with the r'-filter. The image is supplied as part of the baseline calibration set.
Special standards Radial velocity standards are occasionally obtained and distributed. A library of radial velocity standards observed with GMOS will be built and made available. Radial velocity standards are not observed routinely as part of baseline calibrations. Any program that requires radial velocity standards should include these observations in the phase II for the program.

 

GMOS IFU mode

Measurement Type Notes
Bad pixel mask Not included as part of the baseline calibration set.
GCAL Flat field GCAL flats are taken mixed with the science observations. For science programs one GCAL flat is obtained for each hour of open shutter time, though a minimum of two are always obtained. For example, for an observation that consists of 6x 30min of exposure time, a total of 3 GCAL flats would be taken. For an observation that consists of 2x15min a total of 2 GCAL flats would be taken.
Dark / Bias The dark current for GMOS is <1.0e-/hour. Dark images are normally not taken (except for Nod & Shuffle Darks). At least 20 bias images are taken each month in the readout modes and binnings used for the science observations. The bias is very stable and all bias images obtained during a month can be used to create a high S/N bias image. Nod & Shuffle Darks should be defined by the PI and will be taken on a best-effort basis when the telescope is closed for bad weather.
Wavelength calibration CuAr arc exposures matching the configuration (grating, filter, IFU configuration, CCD binning) are obtained as part of daytime baseline calibrations. One exposure is obtained each observing run during which science observations are gathered. Arc exposures obtained at the elevation of the science observations are not included as part of the baseline calibration set. All arc exposures are obtained in fast-read, low gain mode.
Twilight flat field Twilight flat observations are taken through the appropriate IFU with the configuration (grating, wavelength, CCD binning) matching the science observations. The exposure is obtained once per semester. Twilight flats are considered baseline calibrations and are not charged to the program.
Telluric standard star The flux standard stars may be use for approximate removal of telluric lines but the observations of the flux standard are not guaranteed to be obtained during the same nights as the science observations for the program.
Flux standard stars Selected from the sources listed on the GMOS spectrophotometric standard star page. For each configuration (grating, filter, IFU configuration, CCD binning) used by a science program, one spectrophotometric standard star is observed per semester. Observations are not guaranteed to be obtained during the same nights as the science observations for the program. Observations to provide absolute spectrophotometric calibration are not included as part of the baseline calibration set.
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 Automatically included for each GMOS image. Relative accuracy is better than 0.2 arcsec on average over the field. Absolute accuracy is about 5 arcsec.
Focal plane mask image Images of the IFU are obtained regularly for monitoring the quality of the fibers. The images are not distributed as part of the baseline calibration set.
Special standards See the long-slit mode for information regarding radial velocity standards.


Previous version: July 3, 2009: Kathy Roth; March 6, 2006: Percy Gomez, Bryan Miller, Kathy Roth