Development of an integral field unit (IFU) with an image slicer
Shinobu Ozaki, Satoshi Miyazaki, Takuya Yamashita, Takasi Hattori, Norio Okada, Kenji Mitsui (NAOJ)
We are developing an integral field unit (IFU) with an image slicer for TMT. The prototype IFU will be installed into the optical imaging spectrograph
FOCAS on the Subaru telescope to verify the performance and to carry out some science projects. We have finished the basic investigations, and theparameters of the prototype IFU for FOCAS are as follows; Field of view is 13.5 x 10.0 arcsec2. Slice width is 0.4 arcsec. Number of slices is 25. We areplanning to make test fabrications for the slicer and the pupil mirror array. First light is expected in 2012. Based on the results, we are keen to develop theIFU for WFOS/MOBIE to enhance its versatility. In this poster, we will show the IFU layout, the parameters, and the current status.

5, Slicer

1, Our plan

Because slicer surface has curvature, it is very difficult to be fabricated with high precision machining.
Polishing method is adopted for slicer fabrication.

Subaru/FOCAS

TMT/WFOSFabrication plan:

First, we develop the IFU for the optical imaging spectrograph, FOCAS, on the Subaru telescope.
Then, we will start to investigate the IFU for WFOS on TMT.

Polishing stacked glass plates.

Realigning the plates.

We are planning to prototype a slicer with 5 or 6 slice mirrors.

2, Why image slicer type IFU
An image slicer type IFU can use most efficiently detector imaging area than other type IFUs.
We can cover wider field of view or wider wavelength range.

6, Pupil mirrors
Spherical surface causes large aberration due to large angle in the edge channels.
Ellipsoidal surface is adopted for pupil mirrors because of smaller aberration.
Pupil mirror

Fiber type

Image slicer type

Pseudo slit

Comparison of the spot diagrams. Slicer and pupil mirror are
only considered. The other optical components are assumed
to be ideal lens.

Lenslet array typeFigures are from IFS wiki (http://ifs.wikidot.com/).

3, Optical layout

0.1 arcsec FWHM ~ 20.6µm rms radius

We referred and modified the optical layout of the GNIRS IFU (Allington-Smith et al., 2006,
MNRAS, 371, 380) at the Gemini telescope.

SlicerSpherical surface

Ellipsoidal surface

Top view
Pupil mirrors

Side view
Pickupmirror

From a telescope

To a spectrograph

Pseudo slits
Slit mirrors

Enlarger

Telescope focal plane

SlicerWe are planning to make test fabrications with both high precision machining and polishing methods.
After comparing between two methods, we decided a fabrication method.

1, Light from the telescope is reflected by the pickup mirror.
2, Then, the enlarger creates the magnified image on the slicer.
3, The slicer slices the image and delivers the light to each pupil mirror.
4, Pupil mirrors create pseudo slits with the same F ratio as the telescope.
5, Slit mirrors reflect the light to the spectrograph and make the IFU exit pupils at infinity.

4, Parameters
Slice width (arcsec) *FOCAS / Subaru
0.4

MOBIE / TMT†
0.4

FOV (arcsec2)**

13.5 x 10

17 x 12

Number of slices
Fore-optics magnification

25
3.8

30
3.8

Slice dimension (mm x mm)
Pupil mirror size (mm)Slit mirror size (mm)24.9 x 0.737~6~7140.9 x 3.32
TBD
TBD* : Fixed parameter

Residual from the circle of curvature (left), and expected fringe pattern observed with an interferometer (right).
We can measure these ellipsoidal surfaces.

7, Coating for the mirrors
The IFU has many mirror surfaces, so high reflectivity is required for the mirrors.
We will apply broadband multi-layer dielectric mirror coatings.
These coatings are usually made on glass base plates. We have to check the performances of thecoating on metal surface (e.g. durability).

**: Aspect ratio of FOV is fixed to be about 1.4.
†： The values for MOBIE are preliminary.

5, Possible fabrication methods
MethodHigh precision machining

PolishingMaterial

MetalGlass

Fabrication speed○△

Aspheric surface○△

Monolithic fabrication○

×Surface roughness△○

Reflectivity of the metal coatings.

Example of the broadband multi-layer dielectric
mirror coatings. (From Sigma Koki web page
http://www.sigma-koki.com/)

8, Future works

Diamond turning

Specular light
Specular + scattered light Detailed optical design and tolerance analysis. (in progress)
 Test fabrications (2011)
 Slicer with polishing Pupil mirror with both high precision machining and polishing Dielectric coating on metal surface
 Mechanical designs, fabrications, and performance verifications of the IFU for FOCAS. (2011-2012) Installation to FOCAS and commissioning observations on the Subaru telescope. (2012)
RMS surface roughness (nm)Surface roughness : Ra 0.7nm

TOSHIBA MACHINE CO.,LTDFigure error : 46nm P-V

 Start investigations for the IFU for WFOS/MOBIE. (from 2013)