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). In addition, observations of telluric standards and/or flux standards, flat field calibrations, biases, and and arc lamps for wavelength calibration are usually needed.
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. Due to additional bright columns at either side of the chip gaps, a minimum dither step of 10 arcsec in the X-direction (p-direction on the OT) is recommended. Please also note the updated recommendations on dither strategies to deal with the GMOS-N and GMOS-S detector artifacts.
(The gap width for the old GMOS-N e2v DD devices was 2.8 arcsec, and the recommended minimum dither step in X-direction 5 arcsec.)
For unbinned observations the pixel scale is 0.0807 arcsec/pixel (GMOS-N Hamamatsu) and 0.080 arcsec/pixel (GMOS-S Hamamatsu). (The pixel scale of the old GMOS-N e2v DD devices was 0.0728 arcsec/pixel.) If you are requesting imaging in image quality 70-percentile or worse, consider binning the CCDs 2x2 giving an effective pixel scale of 0.161 arcsec/pixel (GMOS-N) and 0.160 arcsec/pixel (GMOS-S). This cuts the overheads from the readout from ~83 sec to ~24 sec per frame(*).
The original GMOS EEV CCDs had strong fringing in the z' and i'-filters. The peak-to-peak amplitude was about 5% for the z'-filter. The new GMOS-N and GMOS-S Hamamatsu CCDs have less fringing in both the i' and z' filters. The peak-to-peak amplitude is about 2.5% for the Y-filter. For sparsely populated fields, fringe frames can be constructed from the science images. For crowded fields or extended objects, you may consider obtaining additional sky observations for the construction of fringe frames. Further information about fringing can be found here. To construct a reasonably good fringe frame you will need a minimum of six dithered science exposures.
Are the Baseline Calibrations sufficient for your program? If you need photometry to better than 5%, you will need to add standard stars to your observations to determine extinction. Photometry with the GMOS Hamamatsu detectors is complicated by the fact that the three CCDs in each of the GMOS detector arrays have different quantum efficiency characteristics. The implications for photometry are outlined here, together with an overview of calibration options and recommended observing strategies.
(*) Final characterization of the read-out overheads for the GMOS-N Hamamatsu detectors is ongoing.
The gaps between the three detectors in GMOS cause gaps in the spectral coverage, see the example. The size of the gaps in wavelength space depends on the grating used, but is typically a few nanometers. If continuous spectral coverage is essential for your program, consider using two central wavelength configurations of the grating. The recommended wavelength dither steps for the Hamamatsu detectors are >~ 20 nm (R150 grating), >~7.5 nm (R400 grating), and >~ 5 nm for all other gratings. Please also note the updated recommendations on dither strategies to deal with the GMOS-N and GMOS-S detector artifacts.
The GMOS long-slits have bridges that cause gaps in the spatial coverage, see the example. The bridges are approximately 5 arcsec wide (Hamamatsu) and 3 arcsec wide (e2v). If continuous spatial coverage of your long-slit observations is needed, consider dithering in the Y-direction (q-direction on the OT). A minimum dither step of 5 arcsec is recommended.
If you are requesting long-slit observations in image quality 70-percentile or worse, consider binning the CCDs by two in the spatial (Y) direction giving an effective pixel scale of 0.1614 arcsec/pixel at GMOS-N and of 0.160 arcsec/pixel at GMOS-S.
You may also consider binning in the spectral (X) direction depending on how well-sampled your spectra need to be. For example, with the B600 grating and a 1" slit, the resolution is about fwhm=0.54 nm, which is equivalent to ~10 unbinned pixels (Hamamatsu) or 12 unbinned pixels (e2v). Use the grating information to derive this information for other configurations.
If your science target cannot be detected in imaging mode in one of the GMOS filters in about 1 min of exposure time (corresponding to 2-3 min of through-slit exposures), you will need to define a blind offset acquisition. Under average conditions (IQ85, CC70, SB80, airmass 1.5), point sources brighter than 21 mag (Vega) in any of the g'r'i'z' filters are typically suitable for direct acquisitions. For fainter or extended sources you will need to supply coordinates for a nearby brighter target (R<19 mag. and preferably a point source) and accurate offsets between that brighter target and your science target. For GMOS-S, the accuracy of blind offsetting is better than 0.1 arcsec for offsets less than 20 arcsec. For GMOS-N, the accuracy is better than 0.1" independent of distance. For the blind offsetting to work it is essential that the same guide star can be reached for the bright object and the science target.
If your science target is fainter than about R=18, you will have to supply a finding chart at the time of Phase II submission.
If you are observing very faint objects in the red, you may want to consider if your program would benefit from using GMOS in Nod-and-Shuffle mode.
Are the Baseline Calibrations sufficient for your program? If you need accurate telluric line removal, you will need to add telluric standard stars to your program. If you need radial velocity standards, then these need to be added to your program.
The GMOS MOS masks must be designed either from images acquired with GMOS, or from object catalogs with very accurate relative astrometry. MOS PIs are encouraged to submit their mask designs as early as possible in order to increase the chance that the MOS observations will be completed. Detailed information on the deadlines for the mask design submission for GMOS-S and GMOS-N masks can be found in the Phase II instructions for the current semester.
The pixel positions of the spectroscopic targets are obtained from a GMOS image. Due to distortions over the GMOS field of view, this image has to be taken with the same pointing and orientation on sky as planned for the MOS observations. If no prior GMOS image is available, PIs can request pre-imaging in queue mode prior to the classical or queue mode MOS spectroscopic observations. Observing time for pre-imaging will be charged against the program's allocated time and must be requested during Phase I. The observations should be defined as part of the Phase II process as early as possible during the semester, so that the images can be obtained once the target field becomes accessible. PIs may want to access carefully the observing conditions needed for the pre-imaging. Requesting very good observing conditions may lower the chance of getting pre-imaging data early during a semester, and therefore negatively impact the overall chance of getting the MOS observations completed within a given semester.
Imaging obtained with one GMOS in a prior semester may be used for the mask design for the same instrument if the observations were taken with exactly the same pointing and orientation on the sky as is planned for the MOS observations. For a given pointing and orientation, the location of the OIWFS patrol field on the sky depends on whether GMOS is on the up-looking port or one of the side-looking ports on the telescope. Imaging data obtained with GMOS on the up- or side-looking port may be used for a mask design for the other type of port only if a suitable guide star is available without changing the pointing and orientation of the field on the sky. GMOS North was on the up-looking port in 2001B, and has since been on the side-looking port. GMOS South is on the side-looking port. If you are in doubt about whether existing pre-imaging can be used for your mask design, please contact the Gemini HelpDesk.
GMOS-N images cannot directly be used for GMOS-S mask designs and vice versa. If a PI has prior imaging from the respective other GMOS instrument or other telescopes, pre-imaging with the GMOS instrument used for the MOS observations is required. However, if there are existing images of the field, it may be possible to base the mask design on new pre-images obtained with quite short exposure times and/or in worse seeing conditions than needed for the spectroscopy. If the GMOS images contain a sufficient number of objects (> 50) with good signal-to-noise then these objects may be used to "boot-strap" the pixel positions of the science targets. This would be useful if, for example, the relative astrometry provided by the WCS is not accurate enough to construct a reliable object catalog.
The mask is designed from accurate relative coordinates (RA and Dec) for the spectroscopic targets from a catalog. In this case, known transformations are used to calculate the GMOS pixel positions from these coordinates. The PI must have an object catalog with accurate relative coordinates for spectroscopic targets and alignment stars. It is crucial that all coordinates in the object catalog be given in the same astrometric system, as systematic offsets between science targets and acquisition targets for example will produce a fatally flawed MOS mask. In most cases these coordinates should be known with a relative accuracy < 0.1 arcsec. Gemini recommends that masks with slit widths narrower than 1 arcsec be designed from GMOS direct imaging rather than solely from object catalogs. The PI must also create a "pseudo-GMOS" image of the field, using the gmskcreate task available as part of the Gemini IRAF package. This pseudo-GMOS image must be submitted via the OT along with the mask design as it is required during the mask checking process. This image is created from a PI-supplied image taken with another telescope and the gmskcreate task transforms that image onto GMOS pixels when you specify fl_getim=yes. As with MOS masks designed from GMOS direct imaging, the PI designs the mask for a specific field center and position angle on the sky. Once the mask design is submitted these cannot be changed, therefore the PI must verify that a suitable guide star exists for that field center and position angle. The OT can be used for this. The MOS observations defined in the OT must use the same coordinates and position angle as those used for the mask design.
The gaps between the three detectors in both GMOS cause gaps in the spectral coverage, see the data examples. The size of the gaps in wavelength space depends on the grating used, but is typically a few nanometers. If continuous spectral coverage is needed, the recommended minimum shifts are ~20 nm for the R150 grating, ~7.5 nm for the R400 grating, and ~5 nm for all other gratings. Please also note the updated recommendations on dither strategies to deal with the GMOS-N and GMOS-S detector artifacts.
The width of each fiber in the IFU is only 5 unbinned pixels. Thus, it is recommended to always have the Y-direction of the detectors unbinned. The spectral resolution of the IFU is equivalent to a slit with 0.31 arcsec width (3.88 unbinned pixels for Hamamatsu, 4.26 unbinned pixels for e2v), thus you may consider binning in the spectral direction if your object is very faint.
You will have to decide if your science targets are best observed in IFU 2-slit mode or IFU 1-slit mode. The 2-slit mode gives you a larger field on the sky at the expense of spectral coverage. The 1-slit mode gives you larger spectral coverage, but half the field of view on the sky compared to the 2-slit mode. In 2-slit mode you will have to use one of the color filters in order to avoid overlap between the spectra, see the spectral overlap page for details. In 1-slit mode you may have to use a filter to avoid 2nd order contamination.
In 1-slit mode the central wavelength you specify will be interpreted as the desired wavelength at the center of the detector array. In 2-slit mode, the central wavelength you specify will be the wavelength at the location of the two pseudo-slits.
If your science target cannot be detected in imaging mode in one of the GMOS filters in about 1 min of exposure time (corresponding to about 3 min of through-IFU exposures), you will need to define a blind offset acquisition. Under average conditions (IQ85, CC70, SB80, airmass 1.5), point sources brighter than 21 mag (Vega) in any of the g'r'i'z' filters are typically suitable for direct acquisitions. For fainter or extended sources you will need to supply coordinates for a nearby brighter target (R<19 mag. and preferably a point source) and accurate offsets between that brighter target and your science target. The accuracy of blind offsetting is better than 0.1 arcsec for offsets less than 20 arcsec. For the blind offsetting to work it is essential that the same guide star can be reached for the bright object and the science target.
If your science target is fainter than about R=18, you will have to supply a finding chart at the time of Phase II submission.
Are the Baseline Calibrations sufficient for your program? If you need accurate telluric line removal, you will need to add telluric standard stars to your program. If you need radial velocity standards, these need to be added to your program. Also if you need very accurate velocity calibration and you are observing in the blue where not many skylines are available from which to bootstrap your daytime CuAr, you may need to request CuAr calibrations be taken on the sky with your science observation. On-sky arc lamp exposures for setups which do not cover at leat the 5577 A sky line are provided as part of the baseline calibrations (details can be found here).
IFU Target acquisition is done by taking a GMOS image of the field using the sub-region of the CCDs that contains the IFU fields. An IRAF task in the Gemini package can overlay the IFU fields on the image and calculate the offsets to the desired position in the IFU field-of-view. Another task does rapid IFU image reconstruction to verify the placement of the target in the IFU. The following information is needed to define an IFU observation:
In one-slit mode the grating and filter combinations may be selected as for long slit spectroscopy.
In two-slit mode the broad-band filters can be used to prevent the two banks of spectra from overlapping on the detector. The useful combinations of gratings and filters that allow the use of two slits with no or minimal overlap are given in the table below. The calculated separations between the slits in unbinned pixels for the various grating/filter combinations are also given. Differences in the slit separations are due to anamorphic magnification. For most of the configurations below, the complete filter passband fits on the detector for both slits (typically with a few nm lost due to overlap at the center). With the B600 (or R600) grating + g filter at 475nm, 11.5nm of the g passband is lost at either end of the detector, so the blue slit has slightly less blue coverage and the red slit slightly less red coverage. In general, one should set the grating central wavelength to that of the filter for two-slit mode (+/- 11.5nm for B600+g), to avoid losing spectral range at the ends of the detector unnecessarily. Small adjustments may be needed to account for the CCD gaps. For the R150 + GG455, a central wavelength of ~680nm will cover roughly 460-900nm at both slits (truncating the very red end of the red slit; longer settings will reach >=1000nm at the expense of the blue end of the blue slit).
Note that with the R150 grating in 2-slit mode, there is a small amount of second order contamination from the blue slit overlapping the red slit (eg. for a central wavelength of 630nm, this peaks at the level of ~4%, dying away in the red). See the gratings and GMOS-N and GMOS-S filters page for more information on GMOS gratings and filters.
When using the IFU 2-slit mode together with the R150 grating, a part of the spectrum from the blue IFU pseudo-slit is affected by a zero-order contamination from the red IFU pseudo-slit. The affected wavelength region has been measured based on extracted, wavelength-calibrated, and distortion-corrected arc frames observed with GMOS-N and GMOS-S for different central wavelength settings. The pseudo-slits are not perfectly straight, but show slight positional shifts between individual fibers and between fiber blocks. Consequently, the effective wavelength of the contamination in the blue-slit spectrum shows small variations from fiber to fiber. The plot below illustrates the total width of the zero-order contamination in wavelength space after summing over all fibers from the blue pseudo-slit. The central wavelength setting of each data set is marked in the legend. (The absolute and relative scale of the counts in this plot is arbitrary.)
The data sets show that the effective wavelength of the zero-order contamination is similar for GMOS-N and GMOS-S. The longer the central wavelength setting, the shorter the effective wavelength of the contamination. Using a large central wavelength dither can potentially help with mitigating the effect from the contamination in the combined spectra. However, the benefit from large central wavelength dithers needs to be evaluated against the fact that unusually large or short central wavelengths will shift a part of the R150 wavelength coverage beyond the borders of the detector.
The recommendations and performance numbers given in this section are based on observations done during the commissioning and System Verification in August and September 2002. For the Hamamatsu CCDs, which have a higher sensitivity in the red, the considerations below still apply, however the exposure times may be ~1/3 shorter.
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.
For more exact calculation of the overheads where these are critical to your observing program (for example time-resolved observations), the detailed listings on this page may be used. The detailed overhead information is also useful for the Phase II planning of your observing program. The various overheads can be broken into the following categories:
The setup times below include slewing to a new target, starting guiding, and accurately centering objects on slits (if appropriate).
GMOS mode | Setup time |
Imaging | 6 min |
Long-slit | 16 min |
IFU | 18 min |
MOS | 18 min |
Applicants for GMOS time should allow for one acquisition for approximately every 2 hours observed (including overheads and calibrations). Shorter observations may also be split in order to accommodate them in the queue planning.
The acquisition times listed above are based on average statistics from the last two years of GMOS observations. Note that if acquiring particularly faint targets requiring long exposure times (> 2 min) the actual acquisition times will be longer. PIs should take this into account when filling out their Phase I proposals and Phase II programs.
Below are listed approximate configuration times for various components within GMOS (the exact times depend on the details of the positions between which a particular component is being moved). It is possible (and usual) to reconfigure GMOS while slewing to a new target, but it is not currently possible to reconfigure GMOS while reading out the detector. This is a limitation of the current software that is used to execute sequences.
GMOS component | Config Time |
Filter change | 20 sec |
Grating change | 90 sec |
Mask move in or out | 60 sec |
Below are listed the readout times, including the overhead added by the sequence executor software, which is used to execute sequences. It is unfortunately not possible to reconfigure GMOS, dither the telescope position or begin setup on the next target during readout.
The table shows the overheads for the GMOS-N E2V DD detectors (6-amplifier read out) and GMOS-S Hamamatsu detectors (12-amplifier read out). Please note that the GMOS-N E2V DD detectors were replaced with Hamamatsu CCDs in March 2017. Until the measurement of the new overheads for the GMOS-N Hamamatsu detector array (12-amplifier read out) is complete, PIs should assume the GMOS-S (12amp) overheads.
ROI* | Binning | Read speed | Readout Time (s) | |
GMOS-N (6amp) | GMOS-S (12amp) | |||
Full frame | 1x1 | slow | 76 | 83 |
Full frame | 1x1 | fast | 36 | 35 |
Full frame | 1x2 | slow | 46 | 41 |
Full frame | 2x1 | slow | 49 | 50 |
Full frame | 2x2 | slow | 31 | 24 |
Full frame | 2x2 | fast | 20 | 11 |
Central spectrum** | 1x1 | slow | 25 | 20 |
Central spectrum | 1x2 | slow | 18 | 11 |
Central spectrum | 2x1 | slow | 18 | 15 |
Central spectrum | 2x2 | slow | 14 | 9 |
Central spectrum | 4x4 | slow | 11 | 4 |
Central spectrum | 4x4 | fast | 11 | 3 |
* For more details on Regions of Interest (ROIs) see the GMOS OT component page.
** The "Central spectrum" ROI corresponds to the central 1024 (unbinned) rows of the detector array in the spatial direction and the full width of the array in the spectral direction.
The time to offset the telescope as part of a dither sequence is currently ~7s (from the time of turning off guiding at one position to guiding at the next position). At present it is not possible to offset the telescope while reading out the detector when these operations are parts of a sequence.
A portion of the overhead for taking calibrations is the time it takes to move the science fold mirror, which sends the beam either from the sky or from GCAL into GMOS. Again, when using the sequence executor to run sequences, this move cannot be done while reading out the detector array.
With GMOS on side port 5, the relevant science fold moves take about 20 seconds each. Therefore the total overhead to move to and from the calibration position is about 40s. (This does not include the time to actually take the calibrations).
When observing with Nod & Shuffle the same GMOS observing overheads as for classical long-slit and MOS spectroscopy target acquisition and detector readout are still applicable. However, there are additional Nod & Shuffle overheads which are substantial and must be considered in the Phase I observing proposal. The "rule of thumb" given below can be used for a large fraction of Nod & Shuffle programs. Details are also given, in case the "rule of thumb" does not apply to your program.
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):
The detailed component editor for GMOS is accessed in the usual manner, by selecting the GMOS component in your science program. Select the GMOS-N component if your program is scheduled for Gemini-North; select GMOS-S component if your program is scheduled for Gemini-South. The example below shows GMOS-N component. This window looks similar for GMOS-S.
The filter is chosen by clicking on the pull down list (i.e. the down-pointing arrow) and selecting the desired filter. The filter names match those listed on the GMOS filter pages. In spectroscopic mode, all filters can be used as blocking filters as required by your science program. There is no requirement to use a filter in spectroscopic mode.
The exposure time is set typing the number of seconds in the "Exposure Time" field. The exposure time must be a non-zero integer.
Choice of grating (for spectroscopy) or mirror (for imaging) is made by clicking on the pull down lists (i.e. the down-pointing arrows) and selecting the desired item. The grating names match those listed on the GMOS grating pages. If a grating is chosen, the grating central wavelength must also be set and the order chosen. Currently all gratings are only used in first order.
GMOS-S and GMOS-N were upgraded with red-sensitive Hamamatsu CCDs in 2014 and 2017, respectively. Therefore, the CCD manufacturer should be set to "HAMAMATSU".
Choice of one of the built-in focal plane units is made by clicking on the pull down lists (i.e. the down-pointing arrows) and selecting the desired unit. The built-in focal plane units include various longslits and the IFU. For imaging, the focal plane unit should be set to "None". For MOS observations a custom Mask Definition File (MDF) is needed. Select the custom mask MDF by clicking the "radio"-button. Then type the MDF label supplied from the Gemini Observatory for your observation(s) in the field to the right.
Note that the longslit length for nod-and-shuffle observations is approximately 1/3 of the regular longslit (to provide sufficient un-illuminated areas on the detector within which to shuffle the charge). Make sure that you have selected the correct slit for your observations.
If you display a view of the field with the position editor and have selected the "science area" button, then this will reflect the choice of focal plane unit (displayed as a blue box). Note that because of their small field of view, the two integral field unit 'apertures' can be quite hard to see. Currently, all custom mask MDFs are shown as the largest field of view within which slit-lets can be cut.
The facility Cassegrain Rotator can rotate the instrument to any desired angle. The angle (in conventional astronomical notation of degrees east of north) is set by typing in the "position angle" entry box. The view of the science field in the position editor will reflect the selected angle. Alternatively the angle may be set or adjusted in the position editor itself by interactively rotating the science field.
GMOS users may elect to use the average parallactic angle in order to minimize the effects of atmospheric differential refraction. This is strongly recommended for all long-slit observations where the position angle is otherwise unconstrained. When the "Average parallactic" option is selected, the nighttime observer will use the "Set To" drop-down menu to calculate the airmass-weighted mean parallactic angle. It is recommended that PIs set the PA to 90 for observations using the average parallactic angle and verify that a guide star is available. The OT will automatically search for the best guide star at PA=90 and PA=90+180. If no guide star is available with the OIWFS, the default guider should be changed to PWFS2, keeping in mind that flexure may require reacquisitions for observations longer than ~45 minutes. Note that the average parallactic angle option is only available for longslit and IFU observations.
Select the binning in the X and Y direction by clicking on the pull down lists (i.e. the down-pointing arrows). No binning (1), and binning by 2 and 4 are supported. For imaging mode, the binning in X and Y should be set identically. For the spectroscopic modes, binning in X is equivalent to binning in the spectral direction, while binning in Y is equivalent to binning in the spatial direction.
Select the desired read-out mode/gain combination. The available modes are those listed as suggested settings on the GMOS Detectors web pages. For the Hamamatsu CCDs twelve amplifier read-out is the default and the number of amplifiers is not user-selectable.
The GMOS detector is mounted on a translation stage to compensate for flexure. The default setting, "Follow in X and Y", currently gives the best flexure compensation for GMOS-N. For GMOS-S, the default setting is "Follow in X, Y and Z". For daytime calibration such as arcs and mask images taken when the telescope is stationary, the translation stage should be set to "Do Not Follow" for GMOS-N and "Follow in Z Only" for GMOS-S.
The DTA-X option allows the PI to shift the spectral image on the detector array by an integral number of unbinned rows, allowing for averaging of pixel features not adequately removed by flat fielding. The number of DTA-X pixels selected should be an integral factor of the detector y-binning to avoid shifting by partial binned rows. Note: DTA-X offsetting was implemented to aid in the suppression of linear charge traps affecting Nod & Shuffle data in the original EEV detectors. With the upgraded GMOS-N and GMOS-S Hamamatsu detectors DTA-X offsetting is no longer as advantageous.
The GMOS detector can be configured to either read out the full frame or read out a smaller region of interest. The default is to read out the full frame. To change this, click on the "Regions of Interest" tab and then click the "radio"-button on the preferred region of interest.
The GMOS-S and GMOS-N Hamamatsu detector arrays are 6144 pixels by 4224 pixels. The regions read out for the different choices of region of interest (ROI) are given in the table below as [x1:x2,y1:y2], where x1 and x2 are the starting and ending pixel in the X direction and y1 and y2 are the starting and ending pixel in the Y direction. The ROIs are given in unbinned pixels. All ROIs can be used with all binnings; the software takes care of any needed conversions. The table below also lists typical use for the different ROIs. Currently the chosen ROI is not visualized in the Position Editor.
Name of ROI | [x1:x2,y1:y2] | Common use |
Full Frame Readout | [1:6144,1:4224] | Most science observations. |
CCD2 | [2049:4096,1:4224] | Longslit with color filters when useful wavelength range can fit on 2048 pixels. |
Central Spectrum | [1:6144,1625:2648] | The central 1024 rows. Longslit spectra of point sources (standard stars). |
Central Stamp GMOS-S Central Stamp GMOS-N |
[2923:3222,1987:2286] [2923:3222,1987:2294] |
The central 300pix x 300pix. Imaging of single standard stars. The central 300pix x 308pix. Imaging of single standard stars. |
The location of the GMOS OIWFS patrol field on the sky depends on which ISS port on the telescope has GMOS mounted. Normally GMOS is mounted on the "side-looking" ISS port. The OT has this selection as its default. It is important to make sure that the correct ISS port is selected for the current semester, since it affects whether the selected OIWFS guide star can be reached.
First you must select nod-and-shuffle mode using the radio-button in the upper right hand corner of the GMOS component. Once that is selected, the nod-and-shuffle tab becomes active. (Depending on the size of your window, you may have to use the arrows to the right of the tabs to locate it). The content of the nod-and-shuffle tab is shown below.
The following items need to be defined by the user:
Note that when using nod-and-shuffle, an observation cannot also contain an offset iterator.
The GMOS Iterator is a member of a class of instrument iterators. Each works exactly the same way, except that different options are presented depending upon the instrument. Below some of its features are shown in two examples. The first example is for imaging, the second example for longslit spectroscopy. The iteration sequence is set up by building an Iteration Table. The table columns are items over which to iterate.
The items that are available for inclusion in the iterator table are shown in the box in the upper right-hand corner of the GMOS Sequence Components figures below. Selecting one of these items moves it into the table in its own column. Each cell of the table is selectable. The selected cell is highlighted blue. When a cell is selected, the available options for its value are displayed in the box in the upper left-hand corner. For example, when a cell in the filter column is selected, the available filters are entered into the text box. When a cell in the exposure time column is picked, the upper left-hand corner displays a text box so that the number of seconds can be entered. When a cell in the "disperserLambda" column is picked, the upper left-hand corner displays a text box so that the central wavelength in nanometers can be entered.
Note that when using nod-and-shuffle, an observation cannot also contain an offset iterator.
Rows or columns may be added and removed at will. Rows (iteration steps) may be rearranged using the arrow buttons.
Example 1: GMOS Iterator imaging example
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GMOS iterator example for imaging | The resulting sequence with one observe element and no offset iterator |
In this example the sequence iterates over filters and exposure times so there are two corresponding columns in the table. Table rows correspond to iterator steps. At run time, all the values in a row are set at once. Since there are two steps in this table, an observe element nested inside the GMOS iterator would produce an observe command for each filter using the specified integration times.
Example 2: GMOS Iterator longslit spectroscopy example
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GMOS iterator example for longslit spectroscopy | The resulting sequence with one observe element and no offset iterator |
In this example the sequence iterates over the central wavelength. Table rows correspond to iterator steps. At run time, all the values in a row are set at once. Since there are three steps in this table, an observe element nested inside the GMOS iterator would produce an observe command for each of three steps. Iteration of the central wavelength may be useful due to the gaps between the three detectors.
GMOS is equipped with an on-instrument wavefront sensor (OIWFS). The region accessible to the OIWFS is a rectangular field, partly inside the imaging field of view, see the GMOS OIWFS page for details. The OIWFS arm vignettes a small part of the field of view if a guide star inside the imaging field of view is chosen. The vignetting and the region accessible to OIWFS can be viewed with the position editor by using the view...OIWFS FOV item in the position editor menu bar. An example of this is shown in the figure below; the accessible patrol field is outlined with the red dashed line and the projected vignetting of the OIWFS is shown shaded in red. If multiple offsets are defined the green dashed line delineates the region within which valid guide stars may be selected, showing the intersection of the accessible patrol field for all the offset positions. The example is shown for GMOS on the side-looking ISS port.
In cases where the science orientation is not critical, it is possible to rotate the instrument position angle to reach stars at other locations in the field.
This page contains useful information for configuring GMOS observations which should help reduce the number of iterations on the Phase II file between the PI and the NGO. It also contains an overview of the required OT observations and components for programs of different types. Before submitting your Phase II program to Gemini, use the GMOS Phase II checklist to eliminate common mistakes.
A detailed description of all features related to the use of the Observing Tool (OT) template observations is provided on the OT help pages.
For each type of program, there are different requirements to which OT observations and components the user has to define in the Phase II. The table below gives an overview over the requirements. "Required" means that a Phase II will not be accepted without this type of observation or component being defined. "As needed" means that the user can add this type of observation or component as needed by the science goals of the program. Use the GMOS OT library as a source of template and example observations.
CuAr arcs mixed with the science data, i.e. taken at night, and any special standard stars are charged towards the program's allocated time. Baseline standard stars, GCAL flats, twilight flats, baseline CuAr arcs, and mask images are not charged. The time for the acquisition observations is already included in the science observation overhead. No additional time is charged. However, the acquisition observations have to be defined as separate observations in the Phase II.
Each "Observe" is given a Class (see more details about Classes).
The details of each of the required observations and components are explained in the sections below.
Observation or Component |
Program Type | Class | |||
Imaging (details) Pre-imaging for MOS |
MOS (details) | Longslit (details) | IFU (details) | ||
Science Obs. | Required | Required | Required | Required | Science |
Offset Comp. | As needed | As needed | As needed | As needed | N/A |
GCAL flats | Do not add | Required Mix w/ science |
Required Mix w/ science |
Required Mix w/ science |
Nighttime Partner Calibration |
Twilight flats | Do not add | As needed Separate observation |
As needed Separate observation |
Required Separate observation |
Daytime Calibration |
CuAr arcs | N/A | Required Baseline: Separate observation |
Required Baseline: Separate observation |
Required Baseline: Separate observation |
Daytime Calibration |
Charged if mixed w/ science | Charged if mixed w/ science | Charged if mixed w/ science | Nighttime Program Calibration |
||
Acquisition Obs. | N/A | Required Separate observation |
Required Separate observation |
Required Separate observation |
Acquisition |
If for baseline standard | If for baseline standard | If for baseline standard | Acquisition Calibration |
||
Mask image | N/A | Required Separate observation |
N/A | N/A | Daytime Calibration |
Baseline standard stars | Do not add | Required Separate observation |
Required Separate observation |
Required Separate observation |
Nighttime Partner Calibration |
Nod & Shuffle Darks | N/A | As needed (for Nod & Shuffle only) Separate observation |
As needed (for Nod & Shuffle only) Separate observation |
As needed (for Nod & Shuffle only) Separate observation |
Daytime Calibration |
Special standard stars Charged |
As needed Separate imaging observation |
As needed Separate longslit observation Follow requirements for longslit |
As needed Separate longslit observation Follow requirements for longslit |
As needed Separate IFU observation Follow requirements for IFU |
Nighttime Program Calibration |
Detailed information about overheads for acquisitions (and reacquisitions), as well as readout and configuration times can be found on the GMOS overheads page. PIs can now use the calculated planned execution times in the OT as reasonable approximations of the actual time that will be required, with the exception that reacquisitions must be included when integrations times are long. If you do not take this into consideration, it is likely that your Phase II will be overfull. Gemini queue observers will stop executing your program when the allocated time has been depleted, regardless of whether or not there are still unexecuted observations.
If your observation classes are set correctly, the OT will not add any planned time for GCAL flats, twilight flats, baseline CuAr arcs or other daytime calibrations (eg mask images or Nod & Shuffle Darks). The support staff from your National Gemini Office and/or your Gemini Support Scientist will work with you if you have questions about your OT calculated program execution time.
All of the gratings included in the OT are available for science use. Only three of the gratings can be mounted in GMOS simultaneously. Grating changes will not be done during the night.
It is recommended that groups be used as much as possible to keep the Phase II organized. Some recommended organizational practices are:
If full-frame readout is being used then an offset iterator should be used to define at least two offset positions separated by at least 10 arcseconds in p in order to fill in the chip gaps. The first offset position should always have p=0, q=0.
Twilight flats are baseline calibration and are handled by Gemini staff. Twilight or GCAL imaging flat observations should not be included in the Phase II.
Observations in the z'- or Y-bands are susceptible to fringing. Ideally, fringe frames can be derived from the science observations. If this is not possible or the baseline calibration (taken a few times per year) is not sufficient then blank sky observations need to be defined. The class should be 'Nighttime Program Calibration' and the time will be charged to the program. These observations should be like the science observations but the target component should be blank or removed. Gemini staff will add the appropriate blank sky field.
Programs that contain MOS observations require masks designed either from GMOS pre-imaging taken for all separate fields or from object catalogs. PIs with MOS programs are encouraged to submit designs as soon as possible, either early in the semester if designed from object catalogs or shortly after pre-imaging has been taken.
MOS programs should contain the final Phase II information for the pre-imaging observations when it is first submitted. The MOS observations should also be included in the program. Small adjustments to the MOS observations are allowed when the mask design has been done. However, the pointing and PA of your target cannot be changed between the pre-imaging and the MOS observations. PIs with MOS programs will be contacted by Gemini when the pre-imaging data is available. Revised Phase II MOS programs and mask designs should be submitted asap. See GMOS mask deadlines (linked from the relevant semester's OT instructions page) for Classical runs. Mask designs should be submitted directly through the OT, instructions will be sent to you when your pre-imaging is available.
PIs with pre-imaging from previous semesters should use the mask naming scheme (below) for their current program. Upload the original pre-imaging through the OT so that masks can be checked. Please also include a note listing the previous program number.
The focal plane unit for the MOS observations should be specified as Custom Mask MDF, and the field for the name should contain your program ID and a running number for the mask within the program, e.g. for program GN-2010B-Q-27 the mask names should be GN2010BQ027-01 and GN2010BQ027-02 for the first and the 2nd mask, respectively. Note the leading zero on the program ID and mask numbers.
The total time used for both pre-imaging and MOS observations must not exceed the allocated time.
It is recommended that pre-imaging observations are dithered by +/-5 arcsec in both directions, e.g. 4 exposures in a square pattern with size 5 arcsec will work ok. (Examples are available in the GMOS OT library). Pre-imaging exposures should be taken in the broad band filter closest to the central wavelength coverage of the MOS observations.
GCAL flats should be mixed with your science MOS observations. The baseline GCAL flats include one flat for each hour of open-shutter time, though a minimum of two flats is always taken. Make sure you get flats for all spectral configurations. GCAL flats within these guidelines have class 'Nighttime Partner Calibration' and are not charged to the program. Any additional GCAL flats should have class 'Nighttime Program Calibration' and will be charged to the program. The smartGCAL flats will automatically determine the best exposure times for the instrument configuration.
CuAr arcs taken during the day are baseline calibrations. These calibrations are not charged to the program, but the PI has to define them in the Phase II. Define these observations as separate from the science observations and set the class to 'Daytime Calibration'. Make sure the instrument configuration matches the science observation. If you copy the science observation in order to edit it for the CuAr arc, make sure to remove the guide star from the target component, remove any science exposures, and change the class. Arcs taken as part of a science MOS observations should have class 'Nighttime Program Calibration' and will be charged to the program. The smartGCAL arcs will automatically determine the best exposure times for the instrument configuration.
Mask images are taken of all MOS masks before they are used for science. These calibrations are not charged to the program, but the PI has to define them in the Phase II. Define these observations as separate from the science observations. By applying the OT templates, a mask image observation is automatically created for each target. For a complete example see the GMOS OT Library.
Acquisition observations need to be defined for each MOS mask. No extra time is charged for these observations, as the overhead for setting up is already included in the science observation. However, the acquisition observations should be defined as separate observations in the Phase II. By applying the OT templates, an acquisition observation is automatically created for each target. The acquisition observation needs to have the same target, guide star, and PA as the science observation. Make sure that the exposure time (typically 30-90 sec) and filter match the setup required for the mask acquisition stars. Long-pass filters cannot be used for MOS acquisitions. Narrow band filters should in general not be used for MOS acquisitions.
The acquisition template "MOS: Acq starting with the mask in the beam" should only be used if the MOS mask was designed from pre-imaging obtained with the same GMOS instrument and using the same CCDs. MOS masks designed from other data (catalogs, pre-imaging using the previous GMOS CCDs, etc.) will need to be acquired using the "MOS: Acq starting with the mask out of the beam" template. The support staff from your National Gemini Office and/or your Gemini Support Scientist can provide further details regarding the use of the two different templates. See also the example in the GMOS OT Library.
Twilight flatfields are taken for each MOS mask if requested. No extra time is charged for these observations. However, the twilight flatfields should be defined as separate observations in the Phase II. Make sure the instrument configuration matches the science observation. If you copy the science observation in order to edit it for the twilight flatfield, make sure to remove the guide star from the target component, then edit the science exposures to have only one exposure with 30sec exposure time. The class should be 'Daytime Calibration.' If doing small wavelength dithers to fill in the chip gaps then only one wavelength setting will have a twilight flatfield taken, so you need to select the wavelength at which you want the twilight flatfield. If there are no twilight flats in the submitted Phase II for a MOS program, it will be assumed that they are not needed. See also the example in the GMOS OT Library.
GCAL flats should be mixed with your science observations. The baseline GCAL flats include one flat for each hour of open-shutter time, though a minimum of two flats is always taken. Make sure you get flats for all spectral configurations. GCAL flats within these guidelines have class 'Nighttime Partner Calibration' and are not charged to the program. Any additional GCAL flats should have class 'Nighttime Program Calibration' and will be charged to the program. The smartGCAL flats will automatically determine the best exposure times for the instrument configuration.
CuAr arcs taken during the day as baseline calibrations. These calibrations are not charged to the program, but the PI has to define them in the Phase II. Define these observations as separate from the science observations and set the class to 'Daytime Calibration'. Make sure the instrument configuration matches the science observation. If you copy the science observation in order to edit it for the CuAr arc, make sure to remove the guide star from the target component, remove any science exposures, and change the class. Arcs taken as part of a science longslit observation should have class 'Nighttime Program Calibration' and will be charged to the program. The smartGCAL arcs will automatically determine the best exposure times for the instrument configuration.
Acquisition observations need to be defined for each longslit target. No extra time is charged for these observations, as the overhead for setting up is already included in the science observation. However, the acquisition observations should be defined as separate observations in the Phase II. For Point Sources a ROI of Central Stamp (300x300 unbinned pixels) should be used to measure the slit center and to confirm if the science target is within the slit. For Extended Objects, Double Sources and Off-axis Sources a ROI of CCD2 should be used to measure the slit center and confirm if the object is within the slit. By applying the OT templates, an acquisition observation for each target will be automatically created. The acquisition observation needs to have the same target, guide star, and PA as the science observation. Make sure that the exposure time and filter match the setup required for the target. Long-pass filters cannot be used for longslit acquisitions. Narrow band filters should only be used if your target is an emission line source with no continuum. For a complete example see GMOS OT Library.
Twilight flatfields Twilight flats, which provide information about the slit function, are usually not taken for longslit programs, but can be provided if requested. No extra time is charged for these observations. However, the twilight flatfields should be defined as separate observations in the Phase II. Make sure the instrument configuration matches the science observation. If you copy the science observation in order to edit it for the twilight flatfield, make sure to remove the guide star from the target component, then edit the science exposures to have only one exposure with 30sec exposure time and class 'Daytime Calibration'. If doing small wavelength dithers to fill in the chip gaps then only one wavelength setting will have a twilight flatfield taken, so you need to select the wavelength at which you want the twilight flatfield. See also the example in the GMOS OT Library. If there are no twilight flats in the submitted Phase II for a longslit programs, it will be assumed that they are not needed.
IFU observations can be done either in "two-slit mode" or "one-slit mode". For GMOS observations in "one-slit mode" the "IFU Right slit (red)" must be chosen. (The Blue slit is not offered due to a larger number of broken fibers). The change between the two modes is a manual process that involves taking the IFU out of the instrument. This will not be done during the night. GMOS is used for blocks of time in either mode, and changes between the modes are done infrequently.
The OT visualization of the GMOS IFU shows the larger of the two IFU fields at the target position. Thus, if switching between fpu=none and the IFU, the user will see the OIWFS field patrol field move -- this is as expected, since the larger of the two IFU fields is approximately 30 arcsec from the center of the imaging field of view.
In one-slit mode the central wavelength you specify will be interpreted as the desired wavelength at the center of the detector array. In two-slit mode, the central wavelength you specify will be the wavelength at the location of the two pseudo-slits.
In two-slit mode you will have to use one of the color filters in order to avoid overlap between the spectra, see the grating/filter combinations page for details. In 1-slit mode you may have to use a filter to avoid 2nd order contamination.
GCAL flats should be mixed with your science observations. For IFU observations it is particularly important to take GCALflats frequently as flexure can impact the fiber tracing. The baseline GCAL flats include one flat for each hour of open-shutter time, though a minimum of two flats is always taken. Make sure you get flats for all spectral configurations. GCAL flats within these guidelines have class 'Nighttime Partner Calibration' and are not charged to the program. Any additional GCAL flats should have class 'Nighttime Program Calibration' and will be charged to the program. The smartGCAL flats will automatically determine the best exposure times for the instrument configuration.
CuAr arcs taken during the day as baseline calibrations. These calibrations are not charged to the program, but the PI has to define them in the Phase II. Define these observations as separate from the science observations and set the class to 'Daytime Calibration'. Make sure the instrument configuration matches the science observation. If you copy the science observation in order to edit it for the CuAr arc, make sure to remove the guide star from the target component, remove any science exposures, and change the class. Arcs taken as part of of a science IFU observation should have class 'Nighttime Program Calibration' and will be charged to the program. As flexure can have a noticeable impact on IFU observations, it is recommended to include arcs in the science IFU observation, especially if the wavelength coverage does not include any sky lines. The smartGCAL arcs will automatically determine the best exposure times for the instrument configuration.
Acquisition observations need to be defined for each IFU target. No extra time is charged for these observations, as the overhead for setting up is already included in the science observation. However, the acquisition observations should be defined as separate observations in the Phase II. By applying the OT templates, an acquisition observation for each target will be automatically created. The acquisition observation needs to have the same target, guide star, and PA as the science observation. Make sure that the exposure time (typically 2-5 min.) and filter match the setup required for the target. Longpass filters cannot be used for IFU acquisitions. Narrow band filters should only be used if your target is an emission line source with no continuum. See also the example in the GMOS OT Library.
Twilight flatfields are taken for IFU observations. No extra time is charged for these observations. However, the twilight flatfields should be defined as separate observations in the Phase II. Make sure the instrument configuration matches the science observation. If you copy the science observation in order to edit it for the twilight flatfield, make sure to remove the guide star from the target component, then edit the science exposures to have only one exposure with 90sec exposure time with class 'Daytime Calibration'. For small spectral dithers, twilight flats are typically only taken for one wavelength setting. See also the example in the GMOS OT Library.
Imaging standards sufficient to obtain flux calibration at the 5% level are base calibrations and are taken by Gemini staff. If better calibration is needed then observations for additional standards must be included in the Phase II. The class should be 'Nighttime Program Calibration' and the time will be charged to the program.
Spectroscopic flux standards sufficient to determine the spectral response function, not absolute flux calibration, are baseline calibration and are not charged to the program. Observations for the baseline flux standard need to be defined in the Phase II. All MOS flux standards are taken in longslit mode (using the longslit closest in width to the width of the MOS slitlets in MOS mode). All standard star observations should have the observing conditions set to 'Any' and should have the target component deleted and the PA set to 'average parallactic'. The observer will fill in the target and align the PA to the parallactic angle when taking the standard. Baseline standard observations consist of the following components:
Spectroscopic observation with class 'Nighttime Partner Calibration': Please select an exposure time long enough to obtain good signal-to-noise (i.e., 120 seconds). The instrument configuration should match the science observations (using the longslit closest in width to the width of the MOS slitlets in MOS mode). For longslit and MOS programs, the spectroscopic observations should have the ROI defined as central spectrum. For MOS mode, if the grating is not the R150, then a GMOS sequence should be used to define three wavelength settings that will bracket the expected wavelengths of the MOS observations. The three recommended wavelength settings for each grating are shown in the table below. GCAL flats at each wavelength setting should be included. Examples can be found in the GMOS OT Library.
Daytime arcs with class 'Daytime Calibration'. All wavelength settings used by the nighttime observation should be included. All other guidelines for daytime arcs apply.
Grating | First Setting | Second Setting | Third Setting |
R400 | central wavelength - 150 nm | central wavelength | central wavelength + 150 nm |
B/R600 | central wavelength - 100 nm | central wavelength | central wavelength + 100 nm |
R831 | central wavelength - 80 nm | central wavelength | central wavelength + 80 nm |
B1200 | central wavelength - 30 nm | central wavelength | central wavelength + 30 nm |
Recommended wavelength settings for standard star observations obtained for MOS programs
Additional spectroscopic standards --- absolute flux standards, velocity standards, line-strength standards, telluric standards, etc.--- must also be defined. If absolute flux calibration is desired then the 5arcsec-wide longslit should be used and at least three wavelength settings should be used, as in the MOS spectral response calibration above, to cover the full wavelength ranges of most gratings. Recommended settings are the same as the ones given in the table above.
The guidelines are the same as for normal longslit observations except that the classes for the on-sky observes should be 'Nighttime Program Calibration' and the time will be charged to the program. The associated acquisition observations should have class 'Acquisition' and any GCAL flats are still 'Nighttime Partner Calibration'. Daytime arcs are always 'Daytime Calibration' and are not charged.
Some observations cannot be completed within one night and therefore will require multiple acquisitions. Exactly how such an observation will be split in several observations over several nights will normally be determined by the Gemini Staff at the time of scheduling the night's observations (see the overheads page for guidelines). The user should define such observations as one observation, and add a comment to explain which assumptions were made about the number of reacquisitions and the resulting overheads.
The observing conditions are specified as percentiles in image quality, cloud cover, sky background (and water vapor). Refer to the observing conditions page for details about the meaning of these percentiles. If the percentiles do not give sufficient information to the queue observer about the observing conditions required for a given observation, the user should add a comment to the observation detailing the observing conditions, e.g. "Need fwhm better than 0.75 arcsec in r". Such comments cannot be used to request better observing conditions than approved by the time allocation process.
GMOS Baseline calibrations that are not specifically mentioned above should not be included in the Phase II programs prepared by the users.