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.
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.
|(Unresolved) Emission Line
||4700 - 7600
||3700 - 6000
||5800 - 9550
||3700 - 4900
||3870 - 5060
||4540 - 5830
||5600 - 7200
||6050 - 9750
||7850 - 9950
(*) 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.
The following is a typical sequence of FOCAS IFU observation.
- If the IFU is not in the position, move the telescope to zenith and install
the IFU to the observing position. (3min.)
- Move the telescope to the target position and start guiding. (3min.)
- 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.
- Change the grism and filters, and take science exposures.
- Take a comparison (ThAr) and a continuum (halogen lamp) frame. (3-4 min.)
- 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