You are in: Instruments > bHROS > Performance and Use > Components > Fibers and Slicers

Fibers and Slicers

Fiber Input

The fiber input of bHROS is at the Cassegrain focus of the telescope, where the f/16 beam results in an image scale of 621 µm/arcsec. The fibers, with microlenses, are mounted in a cassette which can be placed into the focal plane of the telescope by GMOS-South (as if it were a MOS mask or IFU). This permits guiding with the GMOS-S OIWFS. The fiber microlenses are positioned to within 20 µm (0".032, or 1/20 of the median seeing) with respect to a frame of reference defined by the GMOS-S OIWFS.

Fiber Modes

There are two modes of operation with bHROS, outlined below:

Object-only Mode: In this mode, a single fiber with a projected diameter of 0.9 arcsec feeds its own dedicated image slicer. This mode is envisaged for work in median-to-relatively poor seeing, where the large fiber aperture with maximize throughput, or work on bright targets where the sky background is negligible.
Object/Sky Mode: In this mode, two fibers, each with a projected diameter of 0.7 arcsec are provided. The fibers are separated by 20 arcsec, one each for object and sky. Each of these two fibers are fed to its own image slicer, such that the slits produced by the slicers place object and sky spectra adjacent to each other in the camera focal plane.

Image Slicers

Three image slicers of a Bowen-Walraven design (giving high throughput) are used to produce slices 0.14 arcsec wide in the spectral direction to achieve a spectral resolution of R=150,000. Two image slicers are used for the object/sky mode and produce five slices from their 0.7 arcsec fibers, while the object-only image slicer produces seven slices from its 0.9 arcsec diameter fiber. There is sufficient space between orders, and between the object and sky fibers in object/sky mode, to allow up to 8X binning in the spatial direction. An unprocessed image of the order produced in the single fiber mode is shown below along with an example cut across the order.

The slit lengths produced by the image slicers in the two modes of operation, projected to the camera focal plane are 1.09 mm (2.7 arcsec) and 2.59 mm (6.5 arcsec) for the dual and single-fiber modes, respectively. In the object/sky mode, the slits produced by the two slicers are separated by 0.41 mm (1.0 arcsec) in the camera focal plane.

Owing to the wavelength dependent slit rotation introduced by pre-cross dispersing the echelle, a goniometer (slicer rotation mechanism) is used to ensure a vertical slit at each user-specified central wavelength. This is especially important given the very long (up to 220 pixels in object-only mode) slit produced by the slicer.

Last update September 1, 2005; Steven Margheim