Wide Integral Field Infrared Spectrograph

The Wide Integral Field Infrared Spectrograph, or WIFIS for short, is a unique near-infrared integral field spectrograph with an exceptionally wide field. When coupled with the Steward 90″ telescope on Kitt Peak, Arizona, it will have a 50″x20″ field-of-view (FOV). Currently existing near-infrared integral field spectrographs (IFS) typically have very narrow FOVs because they are optimized for observations of high redshift galaxies, which are relatively small in size. WIFIS will have the unique capability to observe very extended objects in the near-infrared and has an etendue that is comparable to wide-field optical IFS. Therefore, WIFIS science will be highly complementary to programs carried out by optical IFS, such as CALIFA, SAMI, and the upcoming MaNGA surveys.

Latest News:

All mechanical components have arrived and we are currently beginning tests of individual subsystems and carrying out the alignment of optomechanical systems (updated Nov. 11, 2015).

Expected first light: Jan. 2016 (updated Nov. 11, 2015).

Current Status:

WIFIS is undergoing full integration (Nov. 11, 2015).

Toronto Team:

Prof. Dae-Sik Moon (PI), Dr. Suresh Sivanandam (Project/Instrument Scientist), Dr. Richard Chou, Mark Ma, Elliot Meyer, Miranda Jarvis, Bryn Orth-Lashley, and Max Millar-Blanchaer

Collaborating Institutions:

University of Arizona (Contact: Prof. Josh Eisner), Korean Astronomy and Space Science Institute, University of Florida

Basic Parameters:

  • Telescope: Steward 90″ Bok
  • Seeing: ~1.5″
  • Operating Wavelength: 0.9 – 1.35 μm (ZJ-band mode), 1.5 – 1.7 μm (H-band mode)
  • Integral Field Unit: FISICA Image Slicer
  • Field-of-view: 50″ x 20″
  • Slice Width: 1.1″/slice
  • Slice Length: 50″
  • Spatial Sampling Along Slice: 0.5″/pixel
  • Spectral Resolving Power: ~ 3,000 (~2,200 in H-band)
  • Spectral Sampling: 0.22 nm/pixel
  • Detector System:  2048 x 2048 HAWAII-2RG (1.7 μm-cutoff) with ASIC readout

Science:

Sensitivity:

We have made continuum and line flux sensitivity predictions for WIFIS observations at the Bok 90″ telescope. These predictions are for point sources, and the spectra are binned spatially by 1.5″x1.5″. If you would like to calculate the extended source sensitivity, just divide the binned solid angle from the predicted flux values. The observations are 1 hour in length and include a 50% overhead for observing off-source in order to perform sky subtraction. However, if your source does not completely fill the field, you may be able to sky subtract using background spaxels. In addition, include a 20% overhead for acquisition and calibrations. Note: We have a spectral cube simulator for WIFIS observations. However, it is not yet available for general use. If you need a more careful estimation of integration time for a complex source, please email me.

  • zJ-band (0.9 – 1.35 μm) sensitivity:

lineflux

Figure 1: Unresolved line flux sensitivity for a one-hour WIFIS observation.

 

continuumflux

Figure 2: Continuum flux sensitivity for a one-hour WIFIS observation.

 

The data files for the above plots can be downloaded from these links: line sensitivitycontinuum sensitivity.

  •  H-band (1.5 – 1.7 μm) sensitivity:

The H-band performance is largely dominated by the thermal background of the instrument due to its warm optics. For this reason, two sensitivity predictions representing the best and worst case thermal background fluxes are given. If you decide to use the H-band mode, we recommend you use the more conservative sensitivity estimate.

hbandlinesensitivity

 Figure 3: Unresolved line flux sensitivity for a one-hour WIFIS observation. The blue and green curves represent the best and worst case sensitivities, respectively, due to the instrument thermal background.
hbandcontsensitivity
Figure 4: Continuum flux sensitivity for a one-hour WIFIS observation. The blue and green curves represent the best and worst case sensitivities, respectively, due to the instrument thermal background.

 

The data files for the above plots can be downloaded from these links: best-case line sensitivityworst-case line sensitivitybest-case continuum sensitivityworst-case continuum sensitivity.

Optical Design:

The optical design of the spectrograph consists of the following functional blocks: 1) reimaging optics, 2) image slicer, 3) collimator, 4) grating, and 5) camera. Figure 1 shows the optical layout and the ray trace. The design work was done by Richard Chou .The primary design choices were keep the entire design simple and cost-effective. To accomplish this, we focussed on a spectrograph that only operated in the 0.9-1.35 μm range. This meant that all of the optics could be kept warm and did not need to be housed in a cryogenic dewar. We could also use glass materials for lenses. We do, however, complete designs for a cryogenic version of WIFIS that operates in the full near-infrared band (Chou et al. 2010). Dependent on the success of this instrument, we may build a version with greater wavelength coverage.

bok_wifis_diagram
Figure 5: Optical Layout of WIFIS. Most of the optics are warm with the exception of the cryostat , which consists of a thermal blocking filter, field-flattening lens, and a HAWAII-2RG infrared detector.

 

The reimaging optics consist of off-axis parabolic mirrors (OAPs). They convert the f-ratio of the telescope to the value appropriate for the image slicer. We will be able to modify these optics in order to take the instrument to another telescope. It is our eventual plan to take this instrument to an 8-10 meter-class telescope to carry out more sensitive measurements of distant objects.

The image slicer was constructed and tested by the University of Florida and is on loan to us. The imager slicer slices a rectangular field into 18 different slices that are rearranged to a long slit. The slicer has 22 slices, but only 18 are used in the WIFIS design. An example of the WIFIS field along with its slices is shown in Figure 2. It is worth noting that one is able to resolve spatial features along the long direction of a given slice, but not along the short direction of a slice.

ngc2623v2

Figure 6: A simulated WIFIS observation of a merging galaxy system. The full WIFIS field is shown with the largest red rectangle. The full field is 50″x20″. The smallest rectangles represent each individual slice of the integral field. The width of each slice is 1.1″.

 

The remainder of the spectrograph follows a conventional long-slit spectrograph design consisting of a collimator, a diffraction grating, and a spectrograph camera. Only the last lens of the spectrograph camera, a custom transmission filter, and the H2RG IR array are housed in a cryostat. A custom thermal-blocking filter is required to carry out the most sensitive measurements in the 0.9-1.35 μm band. Because all of the fore-optics and most of the instrument itself is warm, they emit considerable thermal emission. The filter is designed to suppress all out-of-band thermal emission, which is the detector is sensitive to out to 2.5 μm.

References:

Wide Integral Field Infrared Spectrograph Nearby Galaxy Survey – Sivanandam, S. et al. 2013, Invited Talk, Dissecting Galaxies with 2D Wide-field Spectroscopy Conference (Lijiang, China)

The development of WIFIS: a wide integral field infrared spectrograph – Sivanandam, S. et al. 2012, Proc. SPIE

The optical design of wide integral field infrared spectrograph – Chou, R. C. Y. et al. 2010, Proc. SPIE

FISICA: the Florida imager slicer for infrared cosmology and astrophysics – Eikenberry, S. et al. 2006, Proc. SPIE

Integral Field Spectroscopy Wiki

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