FHiRE and FHiREBot are identical, bench-mounted spectrographs, fed by optical fibers.
The Fiber Feed
At the 3.5-m WIYN telescope, FHiRE will be fed with the beam from the WIYN Tip-Tilt Module (WTTM). WTTM will reduce the already small WIYN median seeing disk by a further 0.12 to 0.15 arc seconds, yielding a median seeing image of about 0.6 arc seconds. The use of WTTM to feed an optical spectrograph takes advantage of the superb image quality offered by WIYN to achieve good throughput and spectral resolution with a modest (and less expensive) collimated beam diameter.
Optical fibers with a diameter of 50 microns provide a good compromise between the seeing disk and the desired spectral resolution. While a larger fiber would improve throughput, it would also require a larger beam diameter to achieve the same resolution (or, alternatively, provide lower resolution at the same beam diameter). With a 50 micron fiber, the collimated beam footprint requires only a single echelle grating, so that a grating mosaic is not needed. At 50 microns, the fiber will still capture much of the stellar light at median seeing, and do substantially better in good seeing. A lenslet at the input end of the fiber reduces the WIYN WTTM beam to f/4, giving about 60 microns per arc-sec, so that the 50 micron fiber aperture corresponds to 0.8 arc-sec, or 1.4 median seeing image diameters.
The WIYN fiber interface unit is being designed by the Instrument Development Group at Johns Hopkins.
On the 1.25-m telescope, a lenslet will reduce the original f/8 beam to f/4, giving a "plate scale" of about 40" per mm. A 50 micron fiber provides a good match to the typical seeing diameter of 2".
An off-axis paraboloidal mirror will be used in double pass in a white pupil design, which places a pupil both on the echelle grating and at the location of the cross dispersing grating and camera. The mirror is approximately 0.5 meters in diameter.
The primary dispersing element is an R4 echelle from the Richardson Grating Laboratory. The echelle, sized to accommodate a 125 mm beam, is used at a 74 degree angle. The physical dimensions of the grating are 204x408 mm (it saves money to get the full sized replica!). A VPH cross disperser with about 425 lines/mm, and a diameter of 125 mm, incorporates an order-sorting filter as one of the elements. The format will provide about 75 orders on the detector, with orders spaced about 40 pixels apart, giving room for the object fiber plus two sky fibers, and to allow for scattered light correction.
FHiRE and FHiREBot will make use of 2K x 4K CCDs with 12-micron pixels obtained through WIYN foundry runs producing detectors for the QUOTA and ODI imagers. A mosaic of two devices are needed to provide the full spectral range covered by the FHiRE design. Both spectrographs will utilize MONSOON controllers from NOAO.
FHiRE will be housed in the auxiliary spectrograph room at the 3.5-m WIYN observatory, adjacent to the well-used Hydra multi-object spectrograph. The auxiliary spectrograph room is designed to be thermally stable, and is close to the telescope base cone, where the fiber cable emerges from the telescope. FHiREBot will be housed adjacent to the IU 1.25-m telescope in the Morgan-Monroe State Forest north of Bloomington.
FHiRE is designed to provide fixed spectral coverage and resolution, with "no moving parts" during normal operation. Operations are expected to be highly automated to permit rapid survey operation.
|Expected Median Seeing with WTTM||0.6 arc sec|
|Fiber Diameter||50 microns|
|Input plate scale with lenslet||60 microns per arc-sec|
|Collimator||White pupil, 125 x 500 mm paraboloid,
Radius of curvature, ~0.8 m
|Echelle Grating||31.6 l/mm, 76 deg., 118 x 408 mm|
|VPH cross disperser||425 l/mm|
|Camera||All transmission, f/5|
|Detector||4k x 4k, 12 micron pixels|