Instrument descriptions

Optical layout

FOCAS IFU refers the Advanced Image Slicer layout developped by Content, R. (1997, Proc. SPIE, 2871, 1295). The layout can cooperate with the other observation modes such as an imaging mode and a slit spectroscopy mode in an imaging spectrograph.

An image is magnified by a foctor of ~5.3 by the magnification optics consisting of one doublet lens and one concave spherical mirror. The slicer divides the magnified image into 23 slices. Then the pupil mirrors reimage the sliced images, and make the F ratio same as that of the telescope. The slit mirrors locates the exit pupil at infinity.


In the top view, only chief lays are shown because of clarity and circles with radius of 100 mm and 200 mm are also shown for size reference. The orange circle shows the FOCAS field of view (6 arcmin). In the side view, only 3 channels (center and outermost 2 channels) are shown.

IFU in FOCAS

FOCAS IFU is installed in the FOCAS mask storage. The storage can hold 10 mask plates, and the IFU occupies a 9-mask-plate space in it. One mask plate can be installed along with the IFU. The existing mask exchenger system puts the IFU on the telescope focal plane.


One mask plate is inserted in the top slot of the mask stacker (Left). The right picture shows the installation of the mask stacker to FOCAS.

Data

 FOCAS has two 4k x 2k CCDs. Spectra for Ch01-12 and Ch13-24 are in Chip1 (right in the below figure) and Chip2 (left), respectively. Ch24 is the sky spectrum. Obtained data for each CCD is stored in separated fits files.

The ch10 slice mirror has chips at the edge around the center. Due to the defect, the efficiency is about 14% smaller around the center of the Ch10 spectrum with respect to the surrounding region.

Data cube

A data cube created by the pipeline software has the following format.
NAXIS1: slice length direction
NAXIS2: another spatial direction perpendicular to NAXIS1
NAXIS3: wavelength direction

Throughput

 Average throughput of the IFU is ~85%. The throughput depends on the field position. The lowest throughput is at the bottom-right courner in a reconstructed image.




Efficiencies shown in the above plot were calculated from the IFU throughput and the previously measured efficiencies including atmosphere, Subaru and FOCAS.

Absorption feature originated form the coating

 There is an absorption feature around 4750Å with ~30% depth and ~10Å width which originates from the mirrors in the IFU. The central wavelength of the feature varies with the position in the field of view (4720-4775Å). This feature is not seen in the sky spectrum.




Sky subtraction

 FOACS IFU has a slit to obtain a sky spectrum 5.7 arcmin away from a target. Below figures show examples for the sky subtraction. The object is a star in the globular cluster, NGC4147. Total exposure time is 1800 sec (300 sec x 6 exposures). Please note that this data was taken under very poor weather condition.