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Fast Microphotometer Scanner (FMS) at NSO/SP (1Dec94)
(from NSO, NOAO Newsletter No. 40, 1 December 1994)

The Fast Microphotometer Scanner (FMS) is a precision microphotometer
which is used to digitize rectangular photographic images. The FMS consists
of a bright Gaussian-distributed laser spot that is mounted so as to scan back
and forth in the Y-axis and to step the photographic image in the X- axis.
The FMS compares favorably with conventional microphotometers in terms
of photometric resolution (žD[SUB rms] = 0.003 at all densities D), and
linear accuracy (<  2.5 žm).

Intrinsic FMS limitations include the minimum spot size (33 žm) and the
range of the densities that can be encoded (5 density units). Also fringes are
introduced across the field. In thin film bases at low densities, fringes are
introduced across the field by multiple-reflection interference of the coherent
laser light within the mylar base. The fringe amplitude is <  žD >  = 0.015
rms for densities between 0.3 and 0.9. Above D = 0.9 the fringes become
undetectable at the <  žD >  = 0.006 level. To avoid fringing, a wet gate is
available to contain the film while scanning.

To make the FMS a more reliable instrument for the user community,
software and hardware maintenance is being turned over to the computer
and electronics group.

NSO will now allocate time on the FMS through the NSO/SP Telescope
Allocation Committee (TAC). We ask all potential FMS users to submit
proposals to the TAC (NSO/SP TAC, P.O. Box 62, Sunspot, NM 88349, or
sp@sunspot.noao.edu). Some advance notice will allow us to do preventative
maintenance on the instrument and associated software before the user's
arrival.

As part of this change a new user manual will be produced and will be
available on our WWW server.

Robert McGraw

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