FOCAS Integral Field Unit (IFU)


The IFU for FOCAS was developed by a team in the National Astronomical Observatory of Japan (PI: Dr. Shinobu Ozaki, Advanced Technology Center, NAOJ) to improve the capability of FOCAS. It is an image slicer type IFU with the field of view of 13".5 x 10".0 and 0".435 slice width.
The definition of position angle of the IFU is written here.
Please check the FOCAS IFU web page by the developer (external link) for the details of the design and performance.

The layout of the IFU field of view

Important Notice to Proposers

The IFU is moved to/from the observing position by using the existing mask exchange system of FOCAS.
Although the mask exchange system is confirmed to mechanically work fine with the IFU, there are several limitations to its operation.

  • Only 1 regular mask can be installed at the same time with the IFU because the IFU occupies 9 of 10 slots of the Mask Stocker Unit.
  • The IFU can be moved only when the telescope is at the zenith position. So, the telescope must move once to the zenith position if you need a direct imaging before/after an IFU observation.
  • A daytime work is required to switch from IFU observation to another observation which requires two or more regular masks. This poses strong limits on the scheduling of IFU observations and may affect the allocations.

Available Grisms and Sensitivity

Any of the FOCAS grisms can be combined with the IFU.
However, the spectral resolution (λ/dλ) will be ~ 1.1 times smaller and the wavelength coverage becomes narrower than the 0".4 long-slit case.
Please note that the IFU has been tested only with 5 grisms on the sky. There might be unexpected problems especially with the grisms which have not been teseted on the sky.

The table below lists the expected sensitivity with the following conditions.

  • extended source (flux is given in per square arcsec)
  • CCD binning 2 (spatial) by 1 (spectral)
  • total 1 hour integration (3 exposures of 20 minutes each)
  • extraction aperture is 0".43 (slit width) x 0".4 (2 binned pixels along the slit)
  • signal to noise ratio is 5 per spectral pixel
  • for emission line sensitivity, it is assumed that the intrinsic line width is negligible compared to the instrumental spectral resolution

For more general cases, please follow the instructions in "Exposure time calculator" in this web page.

Please note that the following calculation is made for the case without the moon in the sky. If there is the moon, the signal to noise ratio can be largely reduced especially at the blue side.

Grism Blaze Filter Order Wavelength(*)
Dispersion λ/Δλ Wavelength for
S/N calculation
Å ÅÅ/pixel Å Continuum
(Unresolved) Emission Line
300B 5500 SY47 1 4700 - 7600 -1.34 900 5500 2.7e-18 1.4e-17
L600 1 3700 - 6000
300R 7500 SO58 1 5800 - 9550 -1.34 900 7500 2.4e-18 1.3e-17
L600 2 3700 - 4900 -0.64 1800
VPH450 4000 L550 1 3870 - 5060 -0.37 2700 4500 1.6e-172.4e-17
VPH520 5200 none 1 4540 - 5830 -0.39 2700 5200 8.1e-181.3e-17
VPH650 6500 SY47 1 5600 - 7200 -0.60 2300 6500 5.3e-181.3e-17
VPH850 8000 SO58 1 6050 - 9750 -1.17 1350 8200 2.4e-181.1e-17
VPH900 9000 SO58 1 7850 - 9950 -0.74 2700 9200 2.8e-188.3e-18

(*) This is the wavelength range which is covered in the whole field of view.

In general, the coverage is wider around the center of the FoV. Please see the plots in this page for details.

Observation Procedures

The following is a typical sequence of FOCAS IFU observation.

  1. If the IFU is not in the position, move the telescope to zenith and install the IFU to the observing position. (3min.)
  2. Move the telescope to the target position and start guiding. (3min.)
  3. Take short exposures through the IFU and adjust pointing. For each exposure, a reconstructed IFU image is automatically created to check the target position in the field of view. The expected overhead for acquisition is 5-10min. for bright targets.
  4. Change the grism and filters, and take science exposures.
  5. Take a comparison (ThAr) and a continuum (halogen lamp) frame. (3-4 min.)
  6. If necessary, move the telescope to zenith and remove the IFU.

For a faint target (> 20 AB mag. (R or I-band)), it is strongly recommended to prepare a nearby (within about 1 arcminute radius) bright object (< 20 AB mag.) for acquisition and go to the faint target by using the coordinate offset from the bright object.

Notice to Observers

  • There is an absorption feature around 4750Å with ~30% depth and ~10Å width which originates from the mirrors in the IFU.
    The central wavelengh of the feature varies with the position in the field of view (4720-4775Å).
  • Central part of the slicer channel #10, which is 10th of the 23 slices and located near the field center, has 10-20% lower efficiency than the surrounding region.
  • Plaese confirm if a bright source is not at the sky slit position. There is one way to do so in this web page.

Please check "Instrument descriptions" in this web page (external link) for the details.

20 July 2021

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