NIRWALS

The near infrared integral field spectrograph NIRWALS extends SALT’s capabilities into the near infrared, providing medium resolution spectroscopy at R = 2000-5000 over the wavelength range of 800 to 1700 nm.  Its integral field unit (IFU) is an elongated hexagonal bundle of 212 fibers, each of which subtends 1.3 arcsec on the sky, approximately matching the median site seeing.  The IFU has on-sky dimensions of 29 x 18 arcsec, ideally suited for resolving nearby galaxies.  A separate 38-fiber bundle simultaneously samples the sky.  It can be adjusted to distances ranging from 48.6 to 159.2 arcsec from the object IFU with a gimbaled jaw in the Fiber Instrument Feed (FIF) that maintains telecentricity and common field angles for the object and sky bundles.  Sky fibers are interleaved with object fibers along the 8-arcmin long spectrograph slit for optimizing sky subtraction.

More information is available from the following document.

• Updated: SALT NIRWALS Description and Performance (Updated June 2024)
• SALT NIR Integral Field Spectrograph: Description and Performance Predictions

Setting Up Observations

Observation Settings

• Please tick the “Do not Flip PA by 180 degrees” if it is critical that the Position Angle is not flipped by 180 degrees.  If this option is ticked, the Position Angle will be limited in a similar manner to our MOS observations due to tracker rho limitations. See below:
• 
• The instrument currently only operates using Up the Ramp Group sampling and Faint Gain. These are the only settings that have been tested.
• The FIF acquisition camera is not designed for faint (>~16mag) or diffuse sources.  Therefore we recommend using reference stars for the acquisition of faint or extended targets.  These can be set up by providing a finder chart and identifying the star in the field that should be used as a reference and then determining the offsets required to move from the star to the target.
• Standard observations require “Science/Target” observations followed by a “Sky” observation.  These are labeled by the “Science” and “Sky” exposure types in the PIPT.  The “Sky” exposure is one where the object fibre bundle is offset from the target to an area with no bright sources in order to measure the sky.  This is typically done with offsets of ~60″,  but it will depend on the field.  This is the only mode of sky subtraction that has been tested, and the data reduction pipeline assumes this observational setup.  The “Sky” exposure’s exposure time must be the same as the science exposures. That one sky exposure will be subtracted from the science exposures by the science data reduction pipeline.
  • PIs can test out sky subtraction where the object bundle is only moved small increments (of the order of a few fibres), , but we have not yet tested the effectiveness of this and it is not yet supported by our pipeline.
  •  PIs should be aware that due to the detector degradation there are many more bad pixels for fibres that are not in the middle of the IFU field of view, which might affect the data produced from these small dithers.
• There are two long wavelength cut-off filters available: the 1.5 micron and 1.7 micron filter, the former is typically used for all setups with Grating Angle 37 and lower .
• The commissioning and testing focused on 4 main setups:
  • Grating Angle =25, Camera Angle=50.
  • Grating Angle =29.5, Camera Angle=59.
  • Grating Angle =37, Camera Angle=74.
  • Grating Angle =50, Camera Angle=100.
  • All setups between these are currently considered non-standard, but we are continuing to test and take observations at intermediate angles.
• We strongly advise PIs to limit individual exposures to less than 600s. (see NIRWALS documentation for further information).

Exposure and Offset/Dither steps

• Observing blocks are divided into steps. Each step consists of an instrument configuration, exposure time and an offset. The offset directions are the on-telescope (i.e. with the field rotated by the position angle) positions with x being horizontal and y being vertical.
  For PA=0: +x will offset the tracker/FIF to the left, and -x will offset the tracker/FIF to the right. +y will offset the tracker/FIF to the bottom and -y will offset the tracker/FIF to the top.
  For each step the offset is done before the exposure is performed. The instrument configuration is kept constant for each of the steps.
• There are two types of offsets that can are be used:
  • Tracker Guided offsets (these are used for offsets that are >~6″).
  • FIF offsets are the most accurate but are only recommended for use
• Either one of these needs to be selected when defining non-zero offsets in the PIPT.

Calibrations

• Darks will automatically be taken for each exposure time used, and these will be provided with the data.  Currently separate darks are used for each unique exposure time.   They are taken with the same detector settings, but the
• Arcs and flats can be taken right after on-sky observations or uncharged calibrations can be taken at the end of the night. If the latter case is requested, please add a note to the block and proposal notes that this is requested. This functionality will be added to the PIPT soon.
• The calibrations for the 4 main setups have been tested; but other setups are currently being observed and added to the list.  If the setup you are using does not have a set of established calibration settings,  Ar, Kr, and Ne lamp arcs will initially be taken to determine the optimal setup.  And this will be used in subsequent semesters.
• Please note that using calibration lamps for setups with Grating Angle =40 or higher does not yet produce robust wavelength calibration.  PIs are welcome to attempt to obtain good calibration solutions using the combination of all three calibration lamps, but we currently recommend that sky lines be used for more robust wavelength calibration.   The pipeline does not currently have an option to use sky lines for wavelength calibration, this will be implemented in the future.
• If telluric observations are required, PIs will need to identify which stars to observe and then create separate blocks for them. The typical exposure times for J~6mag stars are 60s, but this depends on the type and seeing.   The telluric observations are typically observed after the target observation. They need to have observing windows after the main target is visible and they need to have a “Science” and “Sky” observation.

Fibre Overlay:

• Please note that the central fibres are not all placed in the same positions on the detector.  The fibres that have traces in the middle of the detector are on the outer parts of the object IFU’s field of view, these fibres also the most affected by the bad detector pixels.

The above image shows the fibre overlay and relative positions of each fibre in the object bundle.  The fibres are number according to their position along the slit, this corresponds to their position on the extracted 2D fits image, counting from the bottom of the detector/image.

The image below shows a number system for images where all the sky bundle fibres have been removed, these images will be the ones used in the science data reduction.

Fibre-layout file for object-only images: NIRWALS Object-only fibre layout

Available NIRWALS Data Products

The following data products will be made available for each observing block:

1. Raw Data : data that has undergone pre-processed (bias-subtracted, non-linearity corrected, fitted) to convert multi-frame data to a single fits file per exposure with the counts/s units.
   • Science, Arcs, Flat fields and Darks
   • Filenames: NCCYY#####.NRamp.fits ; where #### is the file number and NRamp is the ramp number.
   • File headers (see  https://nirwals.readthedocs.io/en/latest/index.html  for more information on processing software by R. Kotulla)
     • PRIMARY : header information
     • SCI  :  processed image in counts/second
     • MEDIAN  : TBD
     • NOISE : TBD
     • NPAIRS  : number of pairs used in fitting
       
     • MAX_T_EXP : The maximum exposure time before saturation
     • PROVENANCE : information on what files and software was used for processing
     • FLAGS :   0: fitting went okay,  >0: various fitting errors
2. Product Data : data that has been processed by the primary and secondary (science) data pipelines.
   • Individual Dark exposures
   • Combined Darks
   • Wavelength fits and zero point fit plots
   • dphN*.fits : dark subtracted files
   • dphN*.a.fits : dark subtracted, fibre extracted and wavelength calibrated data
   • dphN*.cf.fits : Continuum fit of the target and sky exposures (only object fibres)
   • dphN*.cs.fits : Continuum subtracted target and sky exposures (only object fibres)
   • dphN*.sf.fits : Spectral fit of the target and sky exposures (only object fibres)
   • dphN*.ss.fits : Sky subtracted target exposure (only object fibres)
   • dphN*.ssc.fits : Sky subtracted target exposure with continuum added back (only object fibres)