Phase II Preparation

This section describes how to prepare IGRINS-2 observations using the Gemini Observing Tool (OT). Basic information on the OT is available here.

We strongly recommend that users begin with the automatically generated OT templates when preparing IGRINS-2 observations. These templates will be pre-filled with the targets and observing conditions from the Phase I proposal. Phase II submissions must include both science observations and calibrations. Additional information on near-infrared observations can be found on the Near-IR Resources page. Below, you will find a more detailed description of how to use the OT for your IGRINS-2 observations.

IGRINS-2 in the Observing Tool

IGRINS-2 offers a single configuration for all observations, covering wavelengths from 1.49 to 1.80 microns (H-band) and 1.96 to 2.46 microns (K-band) with a resolving power of R~45,000. Adding an IGRINS-2 component to an existing observation, or adding 'IGRINS-2 Observation' provides access to the IGRINS-2 instrument component.

The available items are:

• Exposure time: The minimum exposure time is 1.63 seconds, and the maximum exposure time is limited to achieve a S/N of ~250 for a single ABBA sequence for bright targets, to prevent persistence. For faint targets, the maximum recommended exposure time is 1200 seconds. The Fowler Sample (i.e., read mode) is automatically set based on the length of the individual exposures.
• Position Angle: The position angle can be set to 'Fixed' or 'Average parallactic'. In the latter case, the value for the PA will be selected at the time of observation. Selecting the average parallactic angle minimizes slit losses. If a fixed angle is selected, a value of 90 degrees (slit orientation in the E-W direction) is recommended.

Observing Strategies

Guiding

IGRINS-2 observations require the use of the peripheral wavefront sensors (PWFS2) for guiding. The automatic guide star selection in the OT will find a suitable guide star under the given observing conditions. The guide state in the offset iterator should be set to 'guide' for all science dithers, and typically 'freeze' for the sky dithers. Ensure that the PWFS2 probe arm does not cause vignetting in the science field of view (FoV). This can be checked by using the Position Editor to visualize the probe positions.

Acquisitions

There is no need to specify an acquisition sequence in the OT. However, you can optionally request additional Slit Viewing Camera (SVC) images of the field, labeled as 'SVC image (Optional)' in the OT. This sequence will provide an image of the target positioned out of the slit, as well as an image of the target through the slit.

If the target is faint or in a crowded region where there are uncertainties in identifying it, please include a finding chart with K-band images. Note that the SVC image is in the K-short band.

Offsetting (Nodding, Dithering)

Infrared observations typically require background subtraction. To achieve optimal sky subtraction, it is advisable to allocate equal amounts of time to both the objects and their corresponding sky frames. For point sources, the default offsets follow the ABBA nodding pattern, with +/- 1.25" shift along the slit. Extended objects require larger offsets for sky observations. You may need to set the guiding to 'freeze' at sky positions outside the guiding range of your configuration. Dithering the sky positions by a few arcseconds is recommended to facilitate the removal of coincidental point sources.

The Offset iterator can be used to define the sequence of dithers or sky offsets. The coordinate system for offsets is (p, q), where q remains parallel to the slit. Offsets are always specified in arcseconds relative to the base position.

The Repeat iterator loops over the subsequent elements as specified. If there is more than one ABBA nodding, make sure to increase the 'Repeat' value above the Offset component, rather than increasing the 'Observe' value.

Calibration Observations

PIs should specify calibration observations, including 'before' and 'after' telluric standards in the Phase II OT. Given the strength and variability of sky lines in the IR spectrum, it is crucial to observe telluric standards at least every 2 hours, preferably every 1.5 hours of observations. The OT templates include telluric standards, defined similarly to science observations, with the 'Observe Class' set to 'Nighttime Partner Calibration'.

Flats and Sky observations do not need to be defined separately, as they are taken each day/night as part of the partner calibrations.

Overheads

Telescope Overheads

Telescope overheads for setting up each new science target include time for slewing the telescope, starting guiding, and centering the target on the slit. These collectively take ~7 minutes. An acquisition overhead of 7 minutes per observation is automatically accounted for in the OT. Long observations may be split over several nights to better accommodate them in the queue. Please allow for one acquisition per every 2 hours of science observations when calculating the time required for your project. Since IGRINS-2 has on-instrument guiding and flexure compensation, re-acquisitions are not required.

Overheads during taking data

During an observing sequence, overheads are typically dominated by telescope motions and array readout. The overhead for telescope offsetting is typically 7 seconds for along-the-slit offsets. The readout overhead for each frame is ~38 seconds, for a Fowler sample of 16 (exposure time greater than 24 seconds), which provides the lowest read noise.