Near-IR Integral Field Spectrograph (NIFS) in the Observing Tool

Like all Gemini facility instruments, NIFS is operated through the Gemini observing tool (OT). A proper NIFS phase II proposal should specifiy the observations to be taken in detail, just as if they were being executed at the telescope. The best way to create a phase II proposal is to start with the NIFS phase II OT-Library of predefined observing sequences. This will ensure that many of the details are automatically taken care of. Once familiar with these default sequences, the observer can modify them as desired.

NIFS Component

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

Selecting a grating, filter, and focal plane mask

Typically, the grating for the observation is chosen first. Then the default central wavelength will automatically be entered into the NIFS component. NIFS covers essentially all of the associated photometric band in one shot, and the default settings are designed for this case. If the filter is chosen as "Same as Disperser," then the appropriate default blocking filter is selected at the time of observation.

The focal plane mask should be "clear" for regular observations. Several focal plane occulting masks are available for observations of bright targets with faint companions. An "imaging mirror" may be selected for acquistion; chose "in" for this case, otherwise "out.".

NIFS has four gratings physically mounted in the dewar. Each default configuration of grating choice uses a dual blocking filter, one of Z-J, J-H, or H-K for Z and J, H, and K gratings, respectively. The following table explains the proper combinations:

Controlling the exposure

The exposure time is set by clicking in the dialog box 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 resulting in a single image written to disk. The value of N is set by typing an integer in the "coadds" window. The total integration time in each output image will be the "Exposure Time" times the number of coadds.

Setting the position angle

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

Array readout mode

Choose either bright or faint object mode. Objects fainter than about K~10 should use the faint object mode. The array chacteristics given in green at the bottom of the OT screen will get updated when a different readmode is selected.

Altair AO Component

The component allows the observer to choose 0.85 um and 1 um dichroics for Altair. The 1 um dichroic should be used for JHK-band observations  and 0.85mu dichroic should be selected for Z-band observations.  Observers should choose the default "following" for the Cass Rotator to take out field rotation and keep objects fixed on the NIFS detector.

Future releases of the OT should include a selection for the Altair field lens. For now, observers should specify use of the field lens in an OT "Note" component. Normally, the field lens would not be needed with NIFS since it is intended to increase the field over which the Altair correction is effective (high Strehl). Since the NIFS field is small, there is no need to use the field lens. However, the corrected field will impact the use of the OIWFS if the OIWFS guide star is far off axis (>25 arcsec), and depending on the expected observing conditions (seeing). In this case, it may be necessary to use the field lens (see below).

The use of the laser guide star (LGS) is not yet specified in the OT.

NIFS Iterator Component

The NIFS Sequence  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. Use this component to change the instrument configuration during the observation, for example, to repeat the same basic observation at two grating settings. For observations that take a significant amount of time (approximately greater than 1 hr) iterating the instrument configuration between multiple gratings is not a good idea. This is because observing conditions may change, calibrations need to be taken, or the queue operator will only have limited time on any given night to complete part of the observation. In this case, break up NIFS observation into a separate "Observation" and best practice is to cut and paste your observations and change the instrument configuration for each separate observation. This allows for more flexibility at the telescope. Short observations of bright targets are good candidates for an instrument configuration iterator.

Below we see a few of the iterator features in the right panel of the OT tool (the left panel shows multiple observations in the NIFS Library with the details of observation 1 shown):

An iteration sequence ("NIFS Sequence" in the above image) is set by choosing "NIFS Sequence" from the "iterator" menu. The table columns in the "NIFS Sequence Component" are items over which to iterate. In this example we are iterating over gratings (and hence filters and central wavelengths which are not set automatically in this component) so there are three 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 four NIFS configuration steps in this observation and an offset iterator below (in this case with a five-point dither) an "observe" element would produce an observe command for each of four grating setups times the number of offsets (five), using the specified integration time, focal plane mask, etc. specified in the main NIFS component:

In the above figure, the iteration table shows parts of observe 4, 5, and 6 (of 20 total). The first five exposures are taken with the step 1 configuration (in this case the K grating) with the telescope dithering before each exposure (p, q offsets). After step 5, the instrument is re-configured with the H grating for the next five offset exposures.

Rows or columns may be added and removed at will. Rows (iteration steps) may be rearranged using the arrow buttons. Rows will be added or removed by clicking "Ad Step" or "Delete Step" buttons, and columns can be deleted by clicking "Delete Item". Adding column can be done by selecting one from the "Available Items" box.

Specifying AOWFS Stars and Guide Stars for NIFS

Information for the base position (i.e. the science object) and guide star(s) may be editied by clicking the appropriate line in the target table at the bottom and then editing the various fields through the text boxes above.  Changing the type of target may be done using the "tag" pull down menu at the upper left.

Details on how to add guide stars (of any type) can be found in the OT Tutorial. Guide stars can be added (click "new") or deleted (click "remove") manually. A better way to define guide stars is to use the position editor to query an image of the field and add stars interactively from the on-line catalogs. The USNO catalog and 2MASS catalog give accurate positions for visible and near infrared sources.

An example image field is shown below (from the position editor) with the science field (small green box) and WFS fields overlayed (blue and yellow circles). The NIFS pickoff probe is outlined as well in red. The yellow circle is the OIWFS field and the blue one is the useful Altair field for wavefront correction. Since the AO fold mirror in the Cassegrain instrument cluster first sends the telescope beam to Altair, the Altair field is unvignetted. The NIFS science field pick off will vignette the OIWFS field as shown. The Cassegrain rotator may be adjusted (position angle) to access OIWFS guide stars which would otherwise be vignetted.

Things to remember or consider for choosing suitable NIFS guides stars:

• All observations must have a guide star. This will usually include at least an AOWFS star.
  
• AOWFS stars should be as bright and close to the target as possible, and no further than 25'' away. See the Altair Perfomance and Use page. Full correction requires a WFS star with R<11; partial correction can be achieved with stars as faint as R ~ 14.5 (on-axis).
• Always make sure to check that the selected observing conditions for cloud cover (CC) are consistent with the guide star brightness.
  
• The OIWFS can be used in combination with the AOWFS star for long observations where flexure between NIFS and Altair might be important. This mode may also help with precise re-centering of the science field following telescope motions (e.g. after a large offset and when a focal plane mask is being used). Performance of the OIWFS will depend on the distance to the guide star from the AOWFS star since the entire field is corrected by Altair and the effects of anisoplanatism will degrade the image quality at large distances. OIWFS stars should be roughly K<14, and commissioning experience shows that OIWFS stars can be as far away as 48 arcsec (without resorting to the field lens).
  
• It is possible to use NIFS without Altair. In excellent seeing, if no guide star is available for Altair, one could consider using the PWFS2. In this case, the "Altair Component" in the main observation should be deleted.
  

Last update: 2006 February 7, B. Miller
Previous version: 2005 November 22; R.Blum, T. Beck, I. Song
Original page created, 2005 November 18;  R. Blum