The MOS Model Assumptions

The assign software contains a model of the construction and configuration of the MOS and Shane telescope.

MOS Hardware Assumptions

The focal plane mechanism of the MOS houses of a steel plate with a Cartesian coordinate system defined by the micropositioner system. At the input end of each fiber is a magnetic ``button'' which can be moved to various points on the MOS plate. Within the buttons the tip of each fiber has a prism that directs light into it. The input end of each MOS fiber is protected within a cylindrical ``hypodermic'' tube. Each of these hypodermics is threaded through its own pivot.

The buttons have fixed diameter and must not collide with each other. The hypodermics have a fixed length and must not be pulled out of their pivots. The hypodermics have a fixed diameter and must not collide with each other or with a button. Each pivot has a position fixed at the time of manufacture, and its neighbors constrain the motions of the fibers across the focal plane. The button at the input end of each fiber has a default ``parked'' position at which it is left when not in use.

The pivots are arranged in ``banks'' of 10 along arcs of a circle (nearly) centered on the telescope axis. There are 10 banks of pivots plus one large coherent guide fiber bundle. There is a detent in the MOS plate at the park locations. A button must not be moved to any location farther from the center than the parking ring, and it must not be within its own diameter of the park location of any button.

The output ends of the MOS fibers are also housed in hypodermics which are aligned in parallel to form a ``slit'' of light as input to the spectrograph. Due to the length of the spectrograph slit and the minimum possible separation of these hypodermics there are currently more pivots than there are installed fibers. Each of the fibers has been individually identified and named so that its condition can be tracked.

Individual fibers can become broken during observing runs. Between observing runs when the MOS is serviced the fibers can be rearranged at both the input and output ends.

Configurable Parameters

The fiber configuration data file documents the associations between fibers, pivots, and slit.

Hard Coded Parameters

Hard coded MOS parameters are found in the file mos.par. These include all of the button and fiber diameters, and the length of hypodermic which must remain behind the pivot as leeway.

Shane Telescope Assumptions

There are a number of subtle points about the Shane telescope that are needed by the model.


The current MOS software does its astrometry using geocentric coordinates. There are no topocentric corrections for the diurnal motion of the observatory. Thus the geodetic position of the telescope is not needed. The MOS software does do refraction calculations, so the astronomical position of the telescope is needed.

In the absence of an astronomical position the code is currently using the catalog position from the Astronomical Almanac. This position is now known to be based on erroneous inputs; it should be modified to match the position as determined by NGS. It is presumed that the uncertainties in the refraction model are greater than the errors induced by not using an accurate location.

Atmospheric Refraction

The atmospheric refraction above the Shane is calculated using the model from a 1980s version of Pat Wallace's Starlink Libary for Astrometry (slalib). The code was translated from the original VAX-Fortran to a more standard variant of Fortran 77. All refraction parameters in use by the program are currently hardcoded in the file shane.par.

Axis Alignment

The location of the polar axis of the Shane telescope contributes to the rotation of the field of view. The position of the pole is currently hardcoded in the file shane.par. Note the commentary in that file which questions the validity of this model and its parameters.

World Coordinate System Assumptions

The WCS is currently hardcoded using parameters stored in mosshane.par and code in center.F and refract.F. The expression of the WCS does not match that used by any of the FITS WCS draft language. The parameters include:
Back to the MOS home page.
Steve Allen <>