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Gordon Talbot, Maarten Blanken, Romano Corradi, Begoña García
(ING), Johan Pragt (ASTRON)
AutoFib2,
the prime focus, multi-object spectrograph of the WHT, was recently upgraded
with the installation of the Small Fibre Module. This allows to reach a fainter
limiting magnitude and to observe a larger number of objects than with the
previous large fibres.
With the new module, the image of each fibre (1.6 arcsec diameter in the
sky) projected onto the CCD is presently undersampled with the present camera
of WYFFOS, the Nasmyth spectrograph fed by AutoFib2. The projected full-width
at half maximum of each fibre is in fact ~1.4 pixels both in the spectral
and spatial direction.
As part of the upgrade of the instrument, a new camera for WYFFOS with a
longer focal length (293mm instead of 132mm of the present camera) has therefore
been designed. The new WYFFOS Long Camera will provide an adequate sampling
of the fibres of AutoFib2, increasing the spectral resolving power up to
9500 with the Echelle grating presently available with WYFFOS.
The Long Camera, in combination with a large format CCD, will also allow
further developments, such as introducing a larger number of fibres, or significantly
increasing the spectral resolution by adopting smaller fibres in multi-object
or integral-field spectrographs.
Schematic view of WYFFOS new camera. [ JPEG | TIFF ]
Beginning in the early nineties proposals for a new camera for WYFFOS were
made by the RGO. The camera continued to be developed there until its closure
in 1998. ING then took over project leadership and continued developing the
concept culminating in 2001 in a definitive design.
In 2001 the optical design and construction of the WYFFOS Long Camera was
contracted to ASTRON, based at Dwingeloo, The Netherlands. Just before last
Christmas the design passed the Preliminary Design Review (PDR) held at ASTRON.
Some of the ASTRON team come originally from the Kapteyn Institute of Roden
and some are relative young designers who together have recently worked on
the instruments VISIR and MIDI for the VLT and VLTI. Those from Roden have
a long relationship with ING, which is renewed with this project.
The preliminary design was made from ZEMAX optical data and tolerance calculations,
to 3 dimensional file of optical lines generated by ZEMAX, copied into the
drawing package pro-Engineer, into 3D dimensional principal sketches of the
structure in pro-E. Also 3D strength and mostly stiffness calculations where
done to prove the quality.
The accompanying illustration shows the camera (with the WYFFOS covers removed)
looking from the cryostat end. The existing Hartmann shutter from the original
camera will be reused in a new position. Light passes through a meniscus
lens and is reflected from a folding flat onto a spherical mirror before
passing to the detector through a cut-out in the flat. A field flattener
lens is positioned just before the cryostat window.
The meniscus lens will be made out of N-BK7 material, the diametre will be
200mm and the original optical design has been changed to use spherical surfaces,
rather than the aspherical earlier design. The folding flat is made of Zerodur
about 230mm wide with a square hole in the middle.
What catches the eye is the large spherical mirror, again Zerodur, with a
diameter of about 670mm and a weight including structure of about 200kg.
This heavy mirror is the optical element that is the most sensitive for tolerances
of the whole camera and is also very sensitive for temperature changes in
position in respect to the flat mirror and detector. So the heaviest part
will be adjustable in all its directions and will move in respect to the
table to compensate for temperature expansion (see the thin, long,
low-expansion rod) while the rest of the camera will be mostly non-adjustable
optics.
Hereafter the light goes through the square hole in the middle of the flat
and via a field flattener lens it will be projected into a standard ING cryostat.
The camera is optimised to use as a detector two MIT Lincoln Labs low-fringing,
high-QE CCDs butted together as a two chip mosaic of 4K by 4K 15micron pixels.
These are being purchased by ING as part of a consortium of observatories
and will be mounted in a standard cryostat and integrated with ING’s Data
Acquisition System (UltraDAS) using an SDSU-2 controller. This work will
be carried out by ING on La Palma.
The final opto-mechanical design started early in January and is in full
progress with commissioning planned for semester
2003A.
