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GMOS OT Details

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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):

GMOS Component

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.

Selecting a filter

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. 

Setting the exposure time

The exposure time is set typing the number of seconds in the "Exposure Time" field. The exposure time must be a non-zero integer.

Selecting a disperser and the central wavelength

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.

Selecting the CCD manufacturer

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". 

Selecting a focal plane unit

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.

Setting the position angle

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.

Setting CCD readout details

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.

Setting Translation stage details

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.

Setting the region of interest (ROI)

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
The central 300pix x 300pix. Imaging of single standard stars.
The central 300pix x 308pix. Imaging of single standard stars.

Selecting the ISS port

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.

Nod-and-Shuffle specifications

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.

gmos NStab

The following items need to be defined by the user:

  • A valid nod-and-shuffle configuration has two offset positions. As default the first [p,q] offset position is set to (0,0) and the second to (0,10) arcsec. An offset in q means a nod along the slit, regardless of instrument rotation angle, giving the possibility of having the target on the slit for both nod positions.
  • If the distance between the two nod positions is less than 2 arcsec then electronic offsetting can be used to reduce overheads (see below). When using electronic offsetting it is best to offset around (0,0) as in the example above.  During electronic offsetting the telescope motion is compensated electronically so that the OIWFS probe arm does not have to move.
  • Set the guide configuration as needed. You may guide at both offset positions, if the OIWFS probe arm can reach the guide star for both offset position. Or you may "freeze" the OIWFS probe arm on for position #1 (beam B) - usually only used if you are nodding off to sky rather than along the slit. 
  • Set the "shuffle". By default the shuffle offset is set to 1392 (112.61 arcsec) for the Hamamatsu detectors (1536 rows - equivalent to 111.67 arcsec - for the previous e2v detectors). This is appropriate for longslit observations. For the GMOS-S IFU the shuffle should be 256 rows. For MOS the "shuffle" must match the mask design. You can define the shuffle either in arcsec or in detector rows and the other is calculated automatically. The shuffle must be an integer number of rows in unbinned pixels; a warning is issued if the number of rows is not exactly divisible by the Y-binning. If the offset is entered in arcseconds then the OT will calculate the nearest offset in pixels that is an integer of the Y-binning. 
  • Set the number of nod-and-shuffle cycles. The total open shutter time on position #0 (beam A) will be cycles*exposure time. The total open shutter time on position #1 (beam B) is also cycles*exposure time. Thus if you are nodding along the slit(s) and you have the targets in the slit(s) for both nod positions, the total open shutter time on the target will be 2*cycles*exposure time.

Note that when using nod-and-shuffle, an observation cannot also contain an offset iterator.

GMOS 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

GMOS iterator - imaging GMOS iterator - imaging
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

GMOS iterator - spectroscopy GMOS iterator - spectroscopy
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.

Viewing the GMOS On-Instrument Wavefront Sensor Field

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.

gmos oiwfs

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