GMOS North
GMOS Optical Properties
The optical elements of GMOS (e.g. filters, gratings and slits) are described in detail on the main GMOS pages.
The filter menu includes all available filters (for GMOS North or South as appropriate). For imaging, select one of the filters u', g', r', i', z', Y, Z or narrow-band. For spectroscopy, select a filter appropriate for the grating or select "none" if no order sorting or limitations on the spectral range is required.
To cover different wavelength regions with a given grating, change the input central wavelength. In general, the gratings can be use over a larger wavelength range than can be covered simultaneously, see the GMOS gratings page for details.
A change in central wavelength corresponds to a change in the instrument grating angle. The GMOS Integration Time Calculator does not apply constraints on the input central wavelength, but the user should in general not attempt to specify central wavelengths very far from the blaze wavelengths given on the GMOS gratings page. For example, the GMOS Integration Time Calculator (ITC) will allow the user to specify a central wavelength of 400nm and either grating R150_G5306 or grating R400_G5305. However, in reality this is not a useful setup.
The wavelength range returned by the ITC corresponds to a centrally located slit. In the MOS mode the location of the spectrum on the detector depends on the position of the slit-let. Thus, the wavelength range for a specific slit-let may be limited by the spatial extent of the CCDs.
The hexagonal field of view of an individual IFU element is approximated as a square aperture (size 0.21 arcsec) which gives the same effective area.
GMOS Detector Properties
GMOS is equipped with three CCDs. For computational simplicity the average properties of these CCDs are used in the ITC. For details on the individual CCDs, see the GMOS North detector or GMOS South detector pages.
In the spectroscopic mode of the GMOS ITC, the signal at the wavelengths corresponding to the gaps between the CCDs is set to zero.
The GMOS CCDs can be used in either low gain or high gain mode. Further, they can be read out in a fast or a slow mode. For the purpose of the ITC, it is assumed that the detectors are operated in slow readout mode. An average read-noise of 3.6 electrons was assumed. This is the average read-noise for the two gains used in the slow readout mode. The ITC warns about possible saturation for the low and high gain modes (software saturation), and possible hardware saturation. The mode of operation is not a user input parameter for the ITC, but the saturation warnings will be sufficient for the user to assess if the CCDs should be operated in low or high gain modes for the proposed observations.
The GMOS CCDs can be binned. The GMOS ITC allows binning in the spatial and spectral direction independently. The results from the ITC are derived using integer pixels. Thus, in some cases of strong binning the use of integer pixels will give a larger (or smaller) aperture than requested. In general, the binning in the spatial direction should be chosen such that the point-spread-function is still sufficiently sampled for the chosen image quality. Binning in the spectral direction may result in degradation of the resolution.
Please note that for IFU modes, spatial binning is discouraged. This is because the GMOS fiber traces on the detector blend together if the detector is binned spatially and the fibers cannot be extracted reliably using the Gemini IRAF data reduction package.
In the imaging mode, the spectral binning is ignored and the spatial binning results in square (binned) pixels.
Spectroscopic resolution and sampling
The ITC samples the spectral distributions at 0.25nm. However, many of the input spectra have a lower resolution, see description of the spectral distributions used by the ITC. Therefore, the output spectra from the ITC does not correctly reflect the resolution that can be achieved with GMOS.
GMOS South
GMOS Optical Properties
The optical elements of GMOS (e.g. filters, gratings and slits) are described in detail on the main GMOS pages.
The filter menu includes all available filters (for GMOS North or South as appropriate). For imaging, select one of the filters u', g', r', i', z', Y, Z or narrow-band. For spectroscopy, select a filter appropriate for the grating or select "none" if no order sorting or limitations on the spectral range is required.
To cover different wavelength regions with a given grating, change the input central wavelength. In general, the gratings can be use over a larger wavelength range than can be covered simultaneously, see the GMOS gratings page for details.
A change in central wavelength corresponds to a change in the instrument grating angle. The GMOS Integration Time Calculator does not apply constraints on the input central wavelength, but the user should in general not attempt to specify central wavelengths very far from the blaze wavelengths given on the GMOS gratings page. For example, the GMOS Integration Time Calculator (ITC) will allow the user to specify a central wavelength of 400nm and either grating R150_G5306 or grating R400_G5305. However, in reality this is not a useful setup.
The wavelength range returned by the ITC corresponds to a centrally located slit. In the MOS mode the location of the spectrum on the detector depends on the position of the slit-let. Thus, the wavelength range for a specific slit-let may be limited by the spatial extent of the CCDs.
The hexagonal field of view of an individual IFU element is approximated as a square aperture (size 0.21 arcsec) which gives the same effective area.
GMOS Detector Properties
GMOS is equipped with three CCDs. For computational simplicity the average properties of these CCDs are used in the ITC. For details on the individual CCDs, see the GMOS North detector or GMOS South detector pages.
In the spectroscopic mode of the GMOS ITC, the signal at the wavelengths corresponding to the gaps between the CCDs is set to zero.
The GMOS CCDs can be used in either low gain or high gain mode. Further, they can be read out in a fast or a slow mode. For the purpose of the ITC, it is assumed that the detectors are operated in slow readout mode. An average read-noise of 3.6 electrons was assumed. This is the average read-noise for the two gains used in the slow readout mode. The ITC warns about possible saturation for the low and high gain modes (software saturation), and possible hardware saturation. The mode of operation is not a user input parameter for the ITC, but the saturation warnings will be sufficient for the user to assess if the CCDs should be operated in low or high gain modes for the proposed observations.
The GMOS CCDs can be binned. The GMOS ITC allows binning in the spatial and spectral direction independently. The results from the ITC are derived using integer pixels. Thus, in some cases of strong binning the use of integer pixels will give a larger (or smaller) aperture than requested. In general, the binning in the spatial direction should be chosen such that the point-spread-function is still sufficiently sampled for the chosen image quality. Binning in the spectral direction may result in degradation of the resolution.
Please note that for IFU modes, spatial binning is discouraged. This is because the GMOS fiber traces on the detector blend together if the detector is binned spatially and the fibers cannot be extracted reliably using the Gemini IRAF data reduction package.
In the imaging mode, the spectral binning is ignored and the spatial binning results in square (binned) pixels.
Spectroscopic resolution and sampling
The ITC samples the spectral distributions at 0.25nm. However, many of the input spectra have a lower resolution, see description of the spectral distributions used by the ITC. Therefore, the output spectra from the ITC does not correctly reflect the resolution that can be achieved with GMOS.
NIRI
NIRI Optical Properties
The optical elements of NIRI (e.g. cameras, filters, grisms and slits) are described in detail on the NIRI pages.
The filter menu is divided into broad- and narrow-band filters, in order of increasing wavelength. For spectroscopy, select the "grism order sorting" filter from the menu, the appropriate grism (disperser) and slit.
The wavelength coverage of the J and L grisms can be shifted bluewards by selection of the blue offset slit (see the NIRI grism page for more details). For computational simplicity, the slit width is assumed to define the spectral resolution (i.e. it is assumed that the source fills the slit for calculating resolution but not for calculating slit throughput).
Note that even though specific capabilities are listed in the ITC, they may not be available in every semester. For the latest information on the delivery of specific narrow band filters, please check the NIRI filter page.
NIRI Detector Properties
Detector bias: the bias is adjustable to provide an increased well depth for high-flux observations such as imaging at thermal wavelengths (3-5um). Even with a deeper well the high background flux typically means that multi-NDR (non-destructive readout) is not possible otherwise the detector will saturate.
The exposure times for 50% of full-well depth on the typical background are given on the NIRI imaging exposure times page.
Detector read noise: typical single-read and NDR noise characteristics, which vary with the number of non-destructive reads and digital samples, are given on the NIRI science detector page. The ITC currently adopts a fixed value of 13e- for multi-NDR, low-noise observations (typically at wavelengths <2.5um) and 50e- for fast-readout, high-noise observations (>2.5um), which will normally be background noise limited in any case. These values are derived from lab measurements with the science detector.
Dark current: lab characterisation indicates a value of 0.25e-/s at the adopted operating temperature. This may include some internal cryostat background (e.g. a low-level light leak).
Last update August 28, 2000; Phil Puxley
GNIRS
GNIRS Optical Properties
The optical elements of GNIRS (e.g. cameras, gratings, cross-dispersing prisms and slits) are described in detail on the GNIRS spectroscopy pages.
The choice of camera, grating and slit width set the spectral resolution. Not all combinations are viable. Use of the cross-dispersing prism may be toggled on/off (again, only some combinations of prism, grating and camera are viable). Given a specific central wavelength, the choice of "blue" or "red" optimised camera, grating order and order sorting filter is made automatically. A broad-bandpass filter is used with the cross-dispersing prisms.
Note that even though specific capabilities are listed in the ITC, they may not be available in every semester.
Caution: the atmospheric transmission and emission files currently used by the ITC have a resolution of 1nm and therefore produce telluric features substantially broader than the actual instrumental resolution, especially at R=5900 and 18000. The inter-line signal and noise calculations are correct.
GNIRS Detector Properties
The four combinations of well depth and read mode (more specifically, the number of non-destructive reads) define the minimum exposure time, read noise and saturation limit. At this time, there is no checking that the specified exposure time is more than the minimum allowable.
Values of the read noise and minimum exposure times are given on the GNIRS science detector page.
Dark current: laboratory characterisation indicates a value of 0.1e-/s at the adopted operating temperature. This may include some internal cryostat background (e.g. a low-level light leak).
GNIRS On-Instrument Wavefront Sensor
At this time, the GNIRS OIWFS is not available. The facility Peripheral Wavefront Sensor is required for tip-tilt image stabilization.
GSAOI
GSAOI is described in detail on the GSAOI web page.
The camera has an 85" x 85" field-of-view with a plate scale of 0.02" per pixel.
The filters are divided into broad- and narrow-band, in order of increasing wavelength.
The detector read mode sets the read noise at the expense of longer readout overheads.
GSAOI must use Canopus as the wavefront sensor.
The Strehl ratio is estimated from the selected filter and image quality:
| IQ | J | H | K |
| 20%-tile | 10% | 15% | 30% |
| 70%-tile | 5% | 10% | 15% |
| 85%-tile | 2% | 5% | 10% |
T-ReCS
T-ReCS Optical Properties
The optical elements of T-ReCS (e.g. windows, filters, gratings and slits) are described in detail on the T-ReCS instrument pages
T-ReCS Detector Properties
v3.1 of the T-ReCS ITC includes the most recent detector characteristics derived from the pre-ship Acceptance Testing. These tests indicated an "extra low frequency noise" (ELFN) which effectively multiplies the Poisson noise on the source and background flux by a factor of ~4. Further measurements will be made during instrument commissioning.
The individual exposure (a.k.a "frame") time is fixed and not a user-changeable parameter (unlike the total integration time). During normal operation the frame time is set to optimally fill the wells and achieve sensitive performance. For the purposes of ITC calculation the frame times used are somewhat arbitrarily set to 0.02, 0.1 and 0.5s for imaging (all filters), low-resolution spectroscopy (10 and 20um) and high-resolution spectroscopy, respectively.
Spectroscopic resolution and sampling
The ITC samples the spectral distributions at 10nm. However, the input source, background and transmission spectra may have a lower resolution, see description of the spectral distributions used by the ITC. Therefore, the output spectra from the ITC may not correctly reflect the resolution that can be achieved with T-ReCS.
Acquisition Camera
Acq Cam Optical Properties
The optical elements of Acquisition Camera (e.g. filters) are described in detail on the Acq Cam pages.
Acq Cam Detector Properties
Detector read noise: typical noise characteristics are given on the Acq Cam pages. The ITC currently adopts a fixed value of 6e-.
Dark current: varies with the temperature of the TE-cooled detector. The final operating temperature has not yet been determined. The ITC assumed 20e-/s appropriate for -20C.