AAOmega: a Multi-purpose Fiber-fed Spectrograph for the AAT

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2 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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AAOmega: a Multi
-
purpose Fiber
-
fed Spectrograph for the AAT

Greg Smith
1
(gas@aaoepp.aao.gov.au)
, Will Saunders
1
Terry Bridges
2
, Vladimir Churilov
1
, Allan Lankshear
1
, Peter Gillingham
1
, Roger Haynes
1
, John Dawson
1
,


David Correll
1
, Lew Waller
1
, Roger Haynes
1
, Gabriella Frost
1


1.
Anglo
-
Australian Observatory. 2
.
Department of Physics, Queen's University, Kingston, Ontario.

.

Figure 1

AAOmega optical and mechanical system diagram

The opportunity will be taken to make improvements to the positioner and to
replace the old Quantex guide camera with a more modern one.

FIBERS

AAOmega MOS fibers will be Polymicro FBP 140 168 198 ( replacing 2dF’s
‘wet’ FVP fibers). The better red transmission allows us to use the 38 meter
length required to implement a bench
-
mounted spectrograph.
A circuitous
route, down the Serrurier truss and through the Coudé optical tunnel in the
horseshoe has been found, between the top end and the
Coud
é West room.
This involves much fiber handling when 2dF is removed and installed at
instrument changes as the MOS fiber bundles will remain attached to the 2dF
top end when it is removed from the telescope.


The number of guide fibre bundles on each field plate will be doubled to eight
per plate.

The SPIRAL IFU uses FVP fibers and we accept that these (~18 metres long)
will have virtually no throughput at 950 nm.

SPECTROGRAPH

The main system parameters for the spectrograph are:



MOS Slits

Two 145mm long, 392 fibres/slit



IFU Slit


145 mm long, 512 fibres



Collimator

F/3.15 dual beam Schmidt



Beamsplitter

Dichroic, 570nm cut



Gratings


VPH



Pupil


190mm diameter



Cameras

F/1.3 Schmidt



Detectors

2048 pixels spectral x 4098 pixels spatial, 15 um pixels


Each camera and its E2V CCD44
-
82 detector are optimised for their
wavelength range.

Blue camera


370 to 580nm


Red camera
-

560 to 950nm

INTRODUCTION

AAOmega is a dual beam bench
-
mounted spectrograph to replace the
two existing 2dF spectrographs which are mounted on the top end ring
of the Anglo
-
Australian Telescope. The 2dF fiber positioner will be
retained and new multi
-
object spectroscopy (MOS) fibers will be run
from 2dF to the spectrograph which will be located in the Coud
é West
focus room. These new fibers will be 38 metres long (compared with
the original 7.5 metre 2dF fibres).

The existing SPIRAL 512 fibre IFU, at Cassegrain focus will also feed
the AAOmega spectrograph. This will replace some of the functionality
of the recently decommissioned RGO Spectrograph. The existing
SPIRAL slit will be replaced to fit AAOmega slitlets. The full optical
and mechanical system is shown in Figure 1, below.

A view of the assembled spectrograph is shown in Figure 2. The base for
construction is a 2400 x 1500mm optical table. The detector controller
electronics boxes which sit on top of their cameras are not shown. The
construction is modular for ease of assembly, alignment and maintenance.
The modules are: the slit exchange mechanism, collimator module, two
dispersers (including mounts and rotators), and two cameras and the two
camera articulation assemblies.

Slits and slit exchange mechanism

MOS slits are assembled from a 40 slitlets, each containing ten fibers,which
are located on chords of the nominal slit arc. (The IFU slit contains 32 slitlets
of sixteen fibres.) A field lens is contacted with the fibre ends using optical
grease. The slits are removable from the spectrograph with their fibres
attached.

The slit exchange mechanism is based on a wheel with four stations, on for
each of the two MOS slits, one IFU slit and one illuminated calibration slit.
These slits are rather loosely attached to the wheel. When a slit is in the
observation position in the collimator it is clamped via a set of kinematic
mounts to accurately position it. A single blade type system shutter may be
deployed in front of the observing slit.

Back
-
illumination of the MOS fibers is required so the 2dF positioner may
see the fiber buttons at the field plate.
The two MOS slits, which each
correspond to one of the 2dF field plates, are mounted at 180
°

from one
another so they may be serviced by a common back
-
illumination unit. This
unit incorporates a clamshell that engages with the slit at the back
-
illumination position and closes over it to prevent stray light escaping into the
spectrograph. LEDs are used with a ‘bent’ cylindrical condenser lens to
illuminate the slit. The IFU slit does not require back
-
illumination.

Collimator

The double beam Schmidt collimator is an open frame construction. It incorporates a
mirror, a dichroic beamsplitter and two corrector plates, on for the red beam and one
for the blue. The correctors differ only in the design wavelengths for their anti
-
reflection coatings. The mirror is 500mm in diameter but only a vertical strip 190mm
wide is illuminated by the slit so the non
-
working portion of the mirror surface is
masked to suppress stray light. Hartmann shutters for spectrograph focusing are
mounted immediately in front of the mirror.

All optical components are mounted in ‘billboard’ style mounts.

Dispersers

A VPH grating is located in each beam. A full suite of gratings is provided for the
spectrograph, to cover low dispersion (R~1500), medium dispersion (R~3500) and
high dispersion (R~8000). These are manually exchangeable and each grating holder
is encoded so the grating in place may be identified by software. (Gratings will not be
exchanged during night.) To facilitate this each grating is mounted in its own housing
which incorporates kinematic seats.

Grating holders are masked to suppress stray light which falls outside the beam
footprint.

Gratings are mounted on DC motor driven rotary tables which incorporate incremental
encoders so grating angle may be changed by the spectrograph control system in the
course of the night.

Cameras

The red and blue F/1.3 Schmidt cameras are similar: they incorporate identical (except
for coatings) mirrors and and correctors but the curvature of the field flattening lenses
and the spacing of the optics differ slightly. See Figure 3.

Detectors are cooled using liquid nitrogen, so the cameras are evacuated. The 250mm
diameter corrector plates are doublets and function as vacuum vessel windows. To
resist air pressure loads the doublet must act as a composite structure so cementing of
the doublet elements is critical.

A mechanism is included to allow adjustment of axial focus and tip and tilt of the
detectors with their field flattener lenses. This design is based on one used in the 2dF
spectrograph spectrograph cameras wherein a stiff ring carrying the detector assembly
via vanes is attached to a thin flex ring at two points. The flex ring is attached to the
camera structure at two points at 90
°

to the previous points. Three motorised, encoded
micrometer actuators are attached to the stiff ring and the combination of the strokes of
these determines the translation and tip and tilt of the ring. See Figure 4.

Camera Articulation

Camera articulation is provided by Newport RV350 HAHLT rotation stages, which
incorporate an incremental encoder into a geared DC motor driven worm drive. The
overhanging moment load that would otherwise be imposed on the rotary stages is
relieved using recirculating ball bearing carriages on curved rails under the rear of the
camera..

Environment

The
Coud
é West room exhibits temperature stability of order 0.1
°
C over four hours. To
assist in maintaining this level of stability it is intended to restrict access to the room to
daytime only. Beneath the room is a large concrete block mounted on air bags for
vibration isolation. The AAOmega spectrograph will be mounted on this using legs
passing through clearance holes in the floor.

Figure 3

Exploded view of camera

Figure 2

General view of AAOmega spectrograph

Figure 4

Detector focus mechanism


exploded view