From varsik@sunspot.noao.edu Thu Apr 11 08:28:26 1996

The dust counter is a small portable unit, the Met One model 227B.
It works by drawing air through a tube into the counter, where
photodiodes measure the light produced by an infrared laser
and scattered by the dust particles.  The air pump draws 0.1
cubic foot of air per minute through the counter.  The counter
is automatically run for one minute every 15 minutes.  Therefore
to obtain the number of particles per cubic foot you multiply
the raw counts (the numbers in the weather log) by 10.

The counter is mounted in a weather-tight enclosure about 50 feet
up on the APO weather mast.  This places the instrument in the free
air flow, away from the effects of most human activities.  It also
places the counter at roughly the height above ground for the
proposed CLEAR telescope (see below).

The dust counter experiment has two main purposes:

1. To provide a record of dust concentration for comparison with
   other weather, coronal sky brightness, and mirror scattering
   data.

2. To provide real-time dust concentration data for use by scientists
   and observers at SPO and APO.

My interest in the experiment has to do with the proposed CLEAR telescope
which NSO is working on.  CLEAR is planned to be a large (2 to 4 meter)
reflecting coronagraph, which will be used for both solar and nighttime
astronomy.  In such an instrument, mirror contamination is a big problem,
and dust control is crucial.  One aspect of determining the feasibilty
of CLEAR is therefore to determine just how much dust we have to deal
with.

Regards,

John Varsik
varsik@sunspot.noao.edu