Calibrations
Wavelength calibration of GRACES is obtained using GCAL, the facility calibration unit. Flat-fielding is done with GCAL. Baseline calibrations will be obtained for all observations, both queue and classical. Note that GRACES calibrations are obtained during the day. There is no gain and it takes too much time to obtain them during nighttime.
Baseline Calibrations
Baseline calibrations include:
Flat/Arc Configuration and Exposure Times
Flat field images and ThAr spectra do not have to be planned during phaseII, as they will be observed in the day time as baseline calibrations. These calibrations will be passed along with the data observed during the night in the corresponding spectral mode.
Examples of Calibration Data
- ThAr spectra for wavelength calibration
- Flat fields for GRACES + GCAL
- Bias images from ESPaDOnS' detector
ThAr spectra
Components
Detector Characteristics (ESPaDOnS)
GRACES is using the ESPaDOnS detector array, which consists of a 2kx4.5k deep-depletion e2v device called Olapa. The table below gives a summary of the current detector/controller characteristics.
Capability
GRACES performs high-resolution echelle spectroscopy between 400 and 1,000nm. It is offered in two spectroscopic modes.
Spectral Range and Resolution
The full GRACES spectral is 400-1,050nm. The useful range is 420-1,010nm, and the GRACES sensitivity is at its best between 490 and 950nm (see the Sensitivity page for more details)
Sensitivity page for more details).Throughput and Sensitivity
We present here measured sensitivity and throughput of GRACES. Note that this was obtained after the GRACES fiber was installed permanently in the ESPaDOnS spectrograph at CFHT in preparation for the 2015B semester (GRACES Phase II), and represents current performance. Note that this performance shows a clear improvement compared to what was presented for GRACES phase I.
Guiding Options
The guiding is performed on the Gemini-North telescope side. Therefore, the guiding options are the same as for GMOS-N; i.e., two guiding options are available, the GMOS On-Instrument Wavefront Sensor (OIWFS) and the telescope's Peripheral Wavefront Sensors (PWFSs). It is strongly recommended that the GMOS OIWFS be used for all but non-sidereal objects. This is because there is significant flexure between GMOS and the PWFSs.
Polarimetry
The spectropolarimetric mode IS NOT OFFERED with GRACES.
ESPaDOnS' polarimetric device is at the Cassegrain focus of the CFHT telescope. Therefore, the polarization of the beam is analyzed before it enters ESPaDOnS' spectrograph. To do the same with GRACES, the light would have to be analyzed with a polarimetric device in the Gemini telescope instrument support before it is sent into the long 270m fibers.
Data reduction
Data Format
This section describes both the data format for all Gemini instruments, including GMOS, and the Gemini IRAF Package, which supports imaging, as well as long-slit, multi-object, and IFU spectroscopic data from both GMOS-N and GMOS-S.
Photometry with the GMOS Hamamatsu CCD mosaic
Flat fielding and photometric calibration of the GMOS Hamamatsu CCD mosaics is complicated by the fact that the individual CCDs have different quantum efficiency (QE) characteristics.
The conversion from instrumental to standard magnitudes is provided by:
mstd = mzero - 2.5 log10 (N[e-]/exptime) - k (airmass-1.0) + CT (color1 - color2)
Wavelength zeropoint checks using sky lines
If CuAr arc exposures were obtained in the form of daytime baseline calibrations - the default for any GMOS spectroscopic configuration covering sky emission lines until the end of 2024A - flexure effects between the on-sky science data and daytime arcs might result in wavelength zeropoint offsets.
Fringing
The GMOS-N and GMOS-S EEV detectors had significant fringing in the red. The GMOS-N DD E2V detectors, which were in operation until February 2017, and the new GMOS-N and GMOS-S Hamamatsu detectors exhibit much less fringing than the original EEV detectors. The pages in this section show examples of fringe frames in imaging mode as well as science data before and after correction for fringing.
Scattered light
Both GMOS-N and GMOS-S suffer fairly significant scattered light in spectroscopic mode. The six gratings exhibit different levels of scattered light, with some gratings being more affected by red scattered light than blue (some examples shown below). The scattered light affects both science observations and GCAL lamp (flat and CuAr) calibration exposures.
Nod & Shuffle performance and examples
The data shown on this page were taken during the commissioning of Nod & Shuffle on GMOS-N in August 2002.
![[NS raw longslit]](/sciops/instruments/gmos/NS/NSraw.jpg)
Air bubbles in the GMOS lens interfaces
Air bubbles in the index-matching oil that is used to fill the interfaces of the different lens groups in the collimator and camera have been a long-standing problem affecting both, GMOS-N and GMOS-S. The air bubbles develop when oil is lost from the lens interfaces and can have a measurable impact on the light transmission and image quality in the affected parts of the field of view.
R150 grating at GMOS-N
A new R150 grating (R150_G5308) was installed in GMOS-N in December 2016 (semester 2016B). This grating replaces the previous R150 grating (R150_G5306) which had developed an issue with the coating causing decreasing throughput in the blue part of the spectrum (see details below). The old R150 grating was available until June 2016.
Exposure Time Estimation
The Integration Time Calculator (ITC) can be used to determine limiting magnitudes, exposure times, S/N ratios, background levels, etc. for a wide range of source properties, observing conditions, filters, and GMOS configurations.
Warning: ITC results for the GMOS-N B600 grating currently overpredict the grating sensitivity (see this GMOS-N announcement for further details).
GMOS-N Integration Time Calculator
In the four sections of this form, select the appropriate astronomical source, telescope and instrument configuration, observing conditions, and observation parameters.
GMOS-S Integration Time Calculator
In the four sections of this form, select the appropriate astronomical source, telescope and instrument configuration, observing conditions, and observation parameters.
Observation preparation
This section mainly describes how to prepare and check GMOS observations. Setting up acquisition and observing sequences at Phase II is not intuitive until one is quite experienced (and maybe even not then). An important key to successful observation preparation is starting from the GMOS OT Library. Preparing observing sequences involves much more than setting up the observation of the target; the complete observing sequences must include "acquisition observations" (e.g., for finding the target and putting it in the spectrograph slit).
Imaging Observing Strategies
The gaps between the three detectors in GMOS-S cause gaps in the imaging field about 4.9 arcsec wide. For GMOS-N, the inter-chip gaps between the three Hamamatsu detectors are 0.5 arcsec wider (5.4 arcsec). If continuous coverage of your field is needed, you will need to obtain multiple exposures and dither between the exposures.
Overheads
For the Phase I observing proposals that are not using Nod-and-Shuffle and do not require very detailed timing, you can use the telescope setup times given below, plus 2 min per exposure to cover readout, filter changes etc.
When using Nod-and-Shuffle, additional overheads apply: these overheads should be added to the overheads from the telescope setup and the readout etc., whenever Nod-and-Shuffle is used.
GMOS OT Details
This page explains how to configure the Gemini Multi-Object Spectrographs (GMOS) in the Observing Tool. There are separate components and iterators for GMOS-N and GMOS-S (because the list of filters, gratings etc may differ):
Calibrations
Wavelength calibration of GMOS is obtained using GCAL, the facility calibration unit. Flat-fielding is done with GCAL or on the sky during twilight. Baseline calibrations will be obtained for all observations, both queue and classical.
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.
Flat/Arc Exposure Times
GCAL configurations and Exposure Times for GMOS calibrations: Quartz Halogen flats and CuAr arcs
Note the following:
Photometric Standards
Spectroscopic Stds
Observations of spectrophotometric standard stars are required in order for spectra obtained by GMOS 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.
Lick Standards
The following Lick secondary standard stars (Worthey et al. 1994) have been observed and the reduced data is available using the links below. The data reduction and Lick calibration are described in Puzia et al. (2013).
Examples of Calibration Data
- Plots of CuAr spectra to assist with wavelength calibration
- Imaging flat fields for GMOS-N and GMOS-S
- Bias images for GMOS-N and GMOS-S
The GMOS-N DD E2V detectors, which were in operation until February 2017, and the new GMOS-N and GMOS-S Hamamatsu detectors exhibit much less fringing than the original EEV detectors. See fringing for examples.
Components
Detector Array
Detector characteristics are given in this section. Note that the GMOS-South and GMOS-North detectors have been upgraded to Hamamatsu CCDs in 14B and 17A, respectively. Note also that the telescope optics are currently silver-coated, reducing the system throughput in the blue.
GMOS-N Hamamatsu Array
The upgraded GMOS-N detector array consists of three ~ 2048 x 4176 Hamamatsu chips of two different types arranged in a row. The central CCD (CCDg) has an enhanced response between ~450 and 650 nm compared to the left- and right-most CCDs (CCDr and CCDb, which probe the red and blue end of the spectral dispersion). CCDr and CCDb are both of the same type and have an enhanced response below ~450 nm and above ~650 nm. In the ITC, the central CCD is referred to as "HSC", while the two outer CCDs are referred to as "BB".
Decommissioned detector Arrays
GMOS-N Array (e2v DD)
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Filters
The GMOS filters u', g', r', i' and z' are similar to the filters used by the Sloan Digital Sky Survey (SDSS). A description of the SDSS photometric system is presented in Fukugita et al. (1996, AJ, 111, 1748).
Gratings
GMOS-N and GMOS-S use identical gratings and all gratings are available with each GMOS. However, some gratings are more heavily used than others and therefore spend more time in the instruments; see the grating statistics for recent semesters. Note in particular that the R600 grating has historically had very limited use and is now restricted to Classical programs only.
Long-slit Dimensions
Both GMOSs have a number of 'permanent' masks containing slits suitable for long-slit spectroscopy, for both standard and Nod-and-Shuffle observations. The slits have small bridges, approximately 3 arcsec long, to keep the mask flat in the focal plane (see the image in the table below).
Capability
Imaging
NEW! GMOS-S detector array has been upgraded. CCD1 and CCD2 have been replaced by new Hamamatsu chips (of similar type to the original ones). The commissioning is ongoing, as it had to be interrupted due to the cybersecurity incident. It will be resumed once the telescope is back online.
Spectroscopy
Three types of spectroscopy are possible with each GMOS and its selection of gratings.
- Long-slit spectroscopy, which covers the full extent of the CCD package (3 x 2048 pixels) with two small gaps.
- Multi-object spectroscopy, which uses custom-designed, laser-milled masks.
- Integral field spectroscopy, which uses 0.2 arcsec fibers covering a 35 square arcsec field of view.
Nod & Shuffle
The GMOS Nod & Shuffle mode applies techniques adopted for the infrared to optical MOS, long-slit, and IFU (Gemini-South only) spectroscopy whereby the sky is sampled with the same pixels used to observe the science target.
The advantages of observing with Nod & Shuffle include:
- Improved sky subtraction
- Potential increased density of slits
Disadvantages of observing with Nod & Shuffle include:
Sensitivity
Imaging
This page presents results from using the GMOS Integration Time Calculator (ITC). The table lists the estimated brightnesses of point sources and uniform surface brightness sources that give a signal-to-noise ratio (S/N) of 5 in a one-hour integration.
GMOS performance monitoring
Guiding Options
Two guiding options are available: the GMOS On-Instrument Wavefront Sensor (OIWFS) and the telescope's Peripheral Wavefront Sensors (PWFSs). It is strongly recommended that the GMOS OIWFS be used for all but non-sidereal objects. This is because (1) the PWFSs vignette a large part of the GMOS science field and (2) there is significant flexure between GMOS and the PWFSs.