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OT Details for NIRI

The instrument non-specific aspects of the Observing Tool (OT) are described elsewhere. Detailed information on NIRI Component of the OT and the NIRI Iterator is provided below.

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

Selecting a camera, dispersion and focal plane mask

Choice of camera, beam splitter, filter, dispersing element and focal plane mask (slit) is made by clicking on the pull down lists (i.e. the down-pointing arrows) and selecting the desired item. For example, there are three cameras f/32, f/14 and f/6, which are normally used with the "same as camera" beam splitter. Note how the window displaying the science field of view changes automatically to reflect the choice of camera, focal plane mask, and beam splitter. For direct imaging no dispersing element is required. Several grisms are available for spectroscopy. When a grism is selected, be sure to select the proper order-sorting filter as well. The desired focal plane mask (e.g. a slit of specified width) should also be selected. Again, the science field of view will reflect the slit chosen.

If you display a view of the field with the position editor and have selected the "science area" button, then this too will reflect the choices of NIRI camera and focal plane mask.

Selecting a filter

For imaging observations, the filter is chosen by selecting one from the list (by clicking on it). The central wavelength is given for each filter to assist with identification. You can move the vertical slider bar or click on the arrow buttons to browse the list. Note that the proper order-sorting filter should be selected for grism spectroscopic observations.

Controlling the exposure

The exposure time is set by clicking in the relevant window and typing the required number of seconds. Each occurrence of the observe element will cause N exposures to be taken and coadded in the instrument control system. The value of N is set by typing an integer in the "coadds" window. The total exposure in each output image will be the exposure times the number of coadds.

The total exposure time (single exposure x coadds x nods) in a single observation should not exceed 4 hours for imaging or ~ 1-1.5 hr for spectroscopy. The limit for spectroscopy is because of flexure between the PWFS guider and NIRI, which can lead to the science target drifting out of the slit.

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" 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.

Array readout mode

The NIRI array is read out in different modes for different kinds of observations. Select the button corresponding to the desired mode. Note that the read noise, recommended minimum exposure time, and suggested background regime are indicated in green to the left and below; they change as the different read modes are selected. The array bias voltage can also be set for "deep well" and "shallow well." The choice of bias voltage affects the well depth, but not the minimum integration time or read noise. Note that the array read mode can be changed in the NIRI iterator, but the bias voltage cannot.

Regions of interest

A smaller sub-array can be read out more quickly than the whole NIRI array. By clicking on the "Regions of Interest" tab, one sees the options available: full frame readout (1024x1024 pixels), central 768x768, central 512x512, or central 256x256. As implied by their description, each subarray is centered on the detector. The sub-array readout should be used to reduce the minimum integration time. Note that the subarray size can be changed in the NIRI iterator by selecting "Builtin ROI" from the Available Items.

Saving changes

The save button accepts the latest changes and stores the program to the local database, 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).

NIRI Iterator

The NIRI 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 we see a few of its features:

You set up an iteration sequence by building an Iteration Table. The table columns are items over which to iterate. In this example we are iterating over filters and exposure time 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 three steps in this table, an observe element nested inside the NIRI iterator would produce an observe command for each of three filters, using the specified integration times:

The items that are available for inclusion in the iterator table are shown in the box in the upper right-hand corner. 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 integration 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.

Viewing the NIRI On-Instrument Wavefront Sensor Field

NOTE: At present the NIRI OIWFS is not available for use.

The mechanical layout of NIRI includes a set of 45 degree pick-off (beam splitter) mirrors which feed the (unobscured) science light path and pass the rest of the ~3 arcmin field to the on-instrument wavefront sensor. Each pick-off mirror is sized appropriately for the FOV of the corresponding camera. The obscuration pattern and region accessible to OIWFS guide stars can be viewed with the position editor by using the view...OIWFS display item in the position editor menu bar.

There are two aspects of note to this display. Firstly the obscured region can be viewed in outline or blocked out. The former is useful for determining if there are potential guide stars hidden by the mirror support struts. In cases where the science orientation is not critical, it may be possible to use such stars with the OIWFS by rotating the instrument position angle. Secondly, the mirrors and support struts are not located at a focal plane and so there is progressive vignetting of the beam near their edges. The obscuration pattern may be viewed with and without this partial vignetting. It might be possible to use a guide star in the partially vignetted region if it is bright (although usually it will be preferable to rotate the instrument slightly).

The obscuration pattern changes depending on the beam splitter selected.