mx_spec --  description  of  field  parameters  for  the  MX Spectrometer

MX FIELD SPECIFICATIONS

The MX Spectrometer on the Steward Observatory 2.3m telescope has
a  45  arcminute  diameter  circular field of view.  There are 32
fiber optic probes which can  move  about  in  this  field  under
computer control.  Each probe has an object fiber on its tip with
a sky fiber 40 arcseconds farther back on the probe.   The  fiber
core  diameters  are  200  microns  in  diameter corresponding to
spectroscopic entrance apertures of 2.0 arcseconds on the sky.  A
set  of  4  arcsecond  fibers was added in September 1987.  The 4
arcsecond fibers are  located  20  arcseconds  back  from  the  2
arcsecond  object  fibers (halfway between object and sky).  Four
of the probes have 4 arcsecond  sky  fibers.   Additional  fibers
might  be  added  to  each probe in the future.  The width of the
aluminum probe tips prevents two neighboring probes  from  coming
closer  than  25 arcseconds.  Every other probe around the circle
is adjusted to be in focus on either the inner or  outer  portion
of  the  curved RC focal plane.  Odd numbered probes move tangent
to  the  inner  part  of  the  focal  plane.   These  probes  are
preferentially  assigned  to  objects  in  the center part of the
field-of-view during the target selection process.

Remote acquisition and guiding  are  accomplished  by  placing  a
guide star at the exact center of each field to be observed.  The
brightness of this center star should be chosen so that it can be
easily  and  safely  seen  with  the intensified television.  The
intensified television has a field-of-view of 20 x 30 arcseconds.
This  expanded  scale  allows  for better guiding, but limits the
sensitivity of the television.  The optimum magnitude  range  for
center stars is 10 > v > 16, with the ideal magnitude being those
stars which just barely show diffraction spikes  on  the  Palomar
Sky  Survey  plates.   Try  to  avoid choosing galaxies or binary
stars or high proper motion stars for  the  guide  objects.  (SAO
reference  stars used for measuring coordinates on the Sky Survey
often show significant proper motion  since  1955.)   Because  of
this  proper  motion problem, it is preferable to measure Quick V
coordinates with GASP.  Be careful not to measure saturated  star
images  with GASP.  Guiding is sometimes also verified by using a
bialkali photomultiplier which can view the slit output.


Observing Hint: Make sure you have  a  finder  chart  which  will
allow  you  to  verify  that the acquisition TV is looking at the
correct  center  star  in  the  one  arcminute  field  of   view.
Otherwise  you  might  get  many  spectra of blank sky.  A finder
chart of the full 45 arcminute field at roughly  the  2.3m  scale
(10"/mm) is nice but not essential for observing.  Coordinates of
nearby SAO stars to which we can offset the  telescope  are  also
useful.  Offsetting the telescope from one center star to another
is a useful technique to verify the proper star has been found.

Corollary: If you've seen one center star, you've seen 'em all.

SEE ALSO mxcoord.mx_format, mxcoord.coordinates