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OT Details

This page guides you through the main steps and considerations for configuring Michelle observations in the Observing Tool (OT).

The best starting point for defining your observations is the Michelle OT Library which can be obtained following the instructions at this link: Michelle OT library. The OT Library contains examples and templates of all typical Michelle observations and should be the basis of your phase II (the example best matching your desired observation can be 'copied' and 'pasted' into your program in the OT, and then modified to your requirements). A new OT Library is released with each call and may contain extra instructions that are specific to the current semester.

See below for information on the

  • Michelle Component - to define 'static' configurations:
    • Filter
    • Observation Time
    • Focal Plane Mask
    • Position Angle (Instrument or Slit)
    • Disperser and Wavelength
    • Chop/Nod Angle and Direction
    • Polarimetry Mode
  • Michelle Iterator - to sequence different instrument configurations
  • Position Editor
    • Visualisation of chopping and nodding observations
    • Guide star selection

Other help:

Important note: Phase II files for Michelle programs should include definitions of all baseline calibrations (and extra calibrations if needed). Examples of observations (containing these calibrations) can be found in the Michelle OT Library.

Michelle Component

The detailed component editor for Michelle is accessed in the usual manner, by selecting the Michelle component in your science program, and is shown below:

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 Michelle filter page. The appropriate spectroscopy filter is set by the instrument itself, overriding the choice made in the OT.

Setting the observation time

The exposure time (i.e. the duration of each individual frame, typically a few tens of milliseconds) is not set explicitly because the value for optimum sensitivity of mid-IR detectors depends on the incident flux. Hence the Michelle software adopts an individual exposure time based on the filter, disperser and other parameters. The PI only defines the total on-source time.

Likewise, the PI does not define the time between nods in the ABBA sequence or the number of ABBA's. The former is typically set to about 40 seconds, and the latter calculated internally so that the total on-source time requirement of the PI is met. Note that this means that the "Observe" component is normally set to 1X in Michelle, at least for brighter targets (including standard stars). For long integrations on a target we recommend defining the time on source to be no more than 900 seconds, and using repeats or multiple steps in the Michelle iterator to get the desired total time. This does not affect the actual observation, except that it is stopped to write out the files at intervals, but it means that a file is written to disk about every 1 hour or so. If this is done, should there be a problem with the observation and the data are lost (which is unusual but has been known to happen) one would not lose all the data from an observation of several hours duration. Splitting an observation into several files with repeats or iterator steps barely changes the data reduction process, since all the frames from all the files in an observation of a target can just be combined.

The Total On-Source Time is the time requested in the (Nod A Chop A) + (Nod B Chop B) beams. That is, when chopping and nodding off-chip it is equal to the total time spent exposing on the source; however, when chopping and nodding on-chip it is half of that time. The total time spent collecting (source and sky) photons is (Nod A Chop A) + (Nod A Chop B) + (Nod B Chop A) + (Nod B Chop B), which is twice the user-defined value. This is because only half of the observations are guided in chop-nod mode; this is discussed in more detail here. In stare-nod mode (for medium- and high-resolution spectroscopy), all observations are guided and the total on-source time is for Nod A + Nod B.

When running an actual observation the total on-source time will be slightly larger than requested in order to allow an integer number of ABBA nod cycles to be completed. For a bright source one ABBA cycle will always be done even if the time on-source is set to a very small value. Thus a 2-second on-source time observation will not take less time to execute than if the value were set as 30 seconds.

The observation sequence timeline displays the estimated total time required for each Observe command. This is typically 4 times the time spent exposing on the source when chopping off-chip or twice that time when chopping on-chip, because it includes the current overheads associated with chopping and nodding (e.g., of the current nod and chop delay and chop duty cycle of 80%). We are working to reduce these overheads.

For spectroscopy useful individual observations times are limited by the time it takes for the target to drift out of the slit. OT spectroscopic observations should be limited to the following values: lowN and lowQ gratings obtained in chop/nod mode) - no more than 600 sec per observation (on-source);
medN1, medN2, and echelle (obtained in stare/nod mode) - no more than 1800 sec per observation (on-source).

For longer spectroscopic integration times, multiple observations must be defined.

Selecting a focal plane mask

The choice of one of the built-in focal plane masks is made by clicking on the pull down list.

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 mask (displayed as a blue box). The chop beams are also displayed (see more details about the Michelle position editor display, below).

Setting the position angle

The facility Cassegrain Rotator can rotate the instrument to any desired angle. In imaging mode the angle (in conventional astronomical notation of degrees east of north) is set by typing in the "position angle" window. 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.

In spectroscopy mode the "position angle" (which must be 0-180 degrees) determines the slit position angle on the sky rather than rotation of the instrument as a whole. The desired angle is achieved by rotating the slit mechanism and/or a field rotator inside the instrument.

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. If a grating is chosen, the grating central wavelength must also be set and the order chosen.

Setting chop angle, chop throw, and nod direction

It is important to note that guiding while chopping is only possible on one side of the chop. Thus, when observing a point source in conditions of less than ideal seeing and windshake, the unguided image will be noticeably less sharp than the other image and should not generally be considered useful for science (except perhaps for photometry). Extensive testing of chopping with the secondary mirror has only been carried out at a chop frequency of about 3-4Hz.

The chop angle may be set independently of the instrument position angle (thus, for example, you can set the spectrograph slit at a specific angle and chop either along the slit or off the slit). The chop angle is defined in conventional astronomical notation of degrees east of north. The chop beams are displayed in the position editor (see more details about the Michelle position editor display, below). Currently the maximum allowable chop throw is 15 arcsec.

The default nod orientation is beam-switching (nod parallel to and same amplitude as chop). Please submit a helpdesk request to inquire about other nod patterns.

Polarimetry Mode

Since semester 2006B, imaging polarimetry is available with Michelle. To configure an observation in polarimetry mode simply set the "Polarimetry" radio button to "yes". Setting this option to "yes" is sufficient to set up Michelle in polarimetry mode and no further special configuration is required (although the appropriate calibrations must still be defined). See the imaging polarimetry examples in the Michelle OT Library.

Saving changes

The save button accepts the latest changes and stores the program to the local database on your hard disk, the undo/redo button (and, transiently, the edit pencil) toggles pending and saved changes and the close button closes the science program editor (saving any changes to the local database). Note that none of these actions affects the version of our program in the Gemini observing database; that version is only updated when the program is stored to the database.

Michelle Iterator

The Michelle 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. 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 Michelle Sequence Component.(Note that "Exposure Time" is not used at this time and should not be confused with Time On Source.) 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 green. 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. 

Rows or columns may be added and removed at will. Rows (iteration steps) may be rearranged using the arrow buttons.

Visualisation in the Position Editor and selection of guide stars

The Position Editor displays not only the primary science beam (Nod A Chop A, equivalent to Nod B Chop B) but also the two chop beams (Nod A Chop B and Nod B Chop A). Display of the chop beams is toggled using the Position Editor View...Display Chop Beams menu, as shown right. The main beam is shown with solid light blue line, the chop beams in dashed light blue.

As an alternative to setting the chop angle and throw using the Michelle component described above, these parameters can be set interactively using the Position Editor in draw mode, by clicking on either of the central blue square "handles". (Note the caution given above about not making the chop throw larger than 15 arcseconds). As usual, you can also set the instrument position angle interactively by dragging the blue triangular handle located next to the main beam. (The chop beams rotate too, of course, although their alignment relative to the main beams is unaffected).

A peripheral wavefront sensor (PWFS) guide star must be defined for all Michelle observations. You can find guide stars by clicking on the "guide stars" icon at the top of the position editor or the same button at the bottom of the target component. Alternatively, the catalogue navigator, accessed via the "catalogs" icon at the top of the position editor, can be used to show names, locations, colours and magnitudes of potential guide stars.

Using the Position Editor View...PWFS FOV Display...Display at menu you can display the vignetting pattern of the peripheral wavefront sensors as they appear at all of the chop positions. (The WFSs move too slowly to follow the chop and so remains fixed at the same x,y position in the focal plane, corresponding to a different apparent position on the sky when the secondary mirror is tilted). The magnified view at the upper right should be used to check for double guide stars (potato-shaped stars and >1 star of similar brightness within the PWFS FOV); the SIMBAD catalogue can also be used for this purpose.