PART 1: Preparing for a flight and how infrared objects are located


By Ben Burress, Tracker Operator

There can be a great deal of preflight preparation work for a
Tracker Operator, or there can be very little, depending on the
nature of the objects the astronomers want to observe.

The easiest research flights are those where the objects that
the astronomers are interested in are bright, starlike objects
(stars, planets, bright asteroids and the like).  These objects
are easy to find using an acquisition chart, and once spotted
the only thing to do is to instruct the Video Star Tracker to
grab it, move it to the astronomers' "boresight" (the spot in the
camera field where their instrument is looking), and tell them
to start taking data.  

Most often, however, it's not quite that simple, as with the flight
tonight.  Tonight (September 27, 1995) we will be flying what I 
judge to be a "typical" KAO flight plan.  On board is astronomer
and instrument owner Dr. Al Harper of Yerkes Observatory, with guest 
investigators Drs. Jackie Davidson and Harold Butner.  Their list of 
objects include IRC +10420, L 1455, Saturn, L 1031B, IRAS 23568+6706,
TMC 1A, GG Tau, and M 42.  IRC +10420 and Saturn are objects of
known infrared brightness, and will be used by the investigators
to calibrate their detection system, so that they will
know how much signal from their system corresponds to how much
brightness from the rest of the objects on the flight.

Preparation for most of the objects on tonight's flight involves
several activities on my part, the first of which was to obtain
a list of the objects and their precise celestial coordinates from
the astronomers.  I have several optical and infrared catalogs
in my office which I can use to find coordinates and possible past
measurements of infrared brightness.  However, quite often there are
a number of points of interest to an infrared astronomer within
a single object, and, not being able to read their minds, I must make
sure that I prepare my charts based on their own list of pointing

Once I have the coordinates, I consult our files of finding charts
and materials which have been developed over the years to see if
charts for the objects exist, and if so, I pull them out.  There are
two basic charts I bring along for every object:  an "acquisition"
chart and a "tracker" chart.  The acquisition chart covers a region
of the sky about 8 by 12 degrees and contains stars of 9th
magnitude and brighter.  This chart I use for initially finding
(acquiring) the star field of the object.  The tracker chart's
dimensions are about 25 by 18 arcminutes (the full moon is about
30 arcminutes in diameter as seen from the Earth).  This chart
contains stars as faint as 14th magnitude, which is about the
faintest magnitude that is visible through the
tracker telescope/camera.  The tracker chart also contains plotted
points for all infrared positions that the investigators will

If a particular object on the flight's list has never been observed
on the KAO, then I must generate a new acquistion and tracker
chart set.  Prior to 1990, these charts were made by hand, plotting
on graph paper positions of stars and objects.  It was time consuming 
work, often taking as much as an hour to create one set.  Today, however, 
we use a computer program called "Pickles", which reads the Hubble 
Space Telescope Guide Star Catalog from a CD ROM and plots the stars it 
reads.  The HST GSC is a catalog of precision star coordinates created for
use with the Hubble Space Telescope, and contains over 20 million
stars.  This catalog is well suited for use by Tracker Operators on
the KAO.  The star coordinates are very accurate, and the catalog
contains stars as faint as 16th magnitude, much fainter than our
cameras can see.

Once I have a complete set of charts, I must go about the most
time consuming part of preparation:  generating a list of offset
guide star coordinates for the Yerkes D.O.G.  The D.O.G. is a
Digital Offset Guider; a motorized X-Y stage that moves the main
telescope's focal plane camera around within the focal plane (actually
there is a fiberoptic bundle connecting the D.O.G. and the focal
plane camera, but the effect of moving the bundle is the same as
if the camera itself were being moved around).  The purpose of the
D.O.G. is analogous to steering a boat to an unseen destination by
using other visible landmarks.  Say you know what direction you want
to sail your boat, even though the island you are bound for is
beneath the horizon.  You might use another island which is in view
and calculate how far off your bow (or stern) you need to keep
that island in order to maintain your desired course.  With the
D.O.G., the known coordinates (in Right Ascension and Declination)
of the invisible infrared source and of a nearby visible star are
fed into a computer, which calculates exactly how far the D.O.G.'s
motors must be driven, both horizontally and vertically, to place
that star precisely on the guiding mark in the focal plane picture
(in other words, once the D.O.G. is offset to the calculated position,
by placing the Guide Star on the focal plane camera's guiding mark
you are actually placing the infrared source position on the
investigator's boresight (sensitive spot).  Thus for each object
which must be "offset tracked" I must find one or more suitable
stars (bright enough to see in the focal plane camera image, and
close enough to the infrared source position so that the D.O.G.
can reach it).  To meet the requirements of a "suitable star", the
star must be at least 12th magnitude in brightness and reside no
more than 10 arcminutes from the infrared source.  In most
cases, finding a guide star is not difficult; in most areas of the
sky there are enough stars of suitable brightness and close enough
together to be "dogable".  Once a complete list of object and guide
star coordinates is gathered, the list is typed into the computer
for reading by the D.O.G. program.

The last thing that I do in preparation for a flight is to generate
tracker overlays.  Though the D.O.G. can offset to stars in the
focal plane and allow fine guiding, the focal plane field of view
is small (only a few arcminutes), and recognizing a star field
in that field of view is difficult without the use of the tracker 
telescope.  The tracker telescope/camera system is a small 
cassegrain reflector with a sensitive video camera at the
focus.  This scope is mounted on the frame of the main telescope, and
is coaxial with the main mirror's optical axis.  As I said, the
field of view of the tracker telescope is a little smaller than the
area of the full moon.  On the video monitor at my station on the
plane, the tracker telescope image has a scale of one arcminute
per centimeter, which is exactly the scale at which we create our
tracker charts.  I create a tracker overlay by placing a transparent
sheet of plastic over my tracker chart and tracing the positions of
stars and infrared source positions with a grease pencil.  In flight
I will use this overlay by holding it up to the video monitor and
lining up all of my grease pencil marked stars with the real stars
on the screen.  Although I can't see the optically invisible infrared
objects on the video monitor (the tracker camera being sensitive to
optical light only), by overlaying my grease-penciled map on the
real stars, I can see where on the screen the infrared objects
reside.  Some day, perhaps on SOFIA, the old plastic tracker overlays
will be replaced by computer generated star positions.

So, with this arsenol of charts and coordinates, I walk onto the
plane and hope very hard that I haven't forgotten anything....

| LFS Home | Give Us Feedback! | LFS Overview | Search Passport to Knowledge |Passport Home |