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SBIG ST-4/0490
P
ROBLEM
S
OLVING WITH THE
ST-4
This section discusses common problems you
may experience while using the ST-4 and
discusses possible solutions to those problems.
PROBLEM
: Can't find the star!
This problem is usually due to the telescope
being badly out of focus. Remove the CCD and
put a piece of ground glass or strip of Scotch
Tape over the end of the telescope eyepiece tube.
Point the telescope at a bright object such as the
moon or a planet and focus the image on the
diffuse screen (this is easiest if you look at the
screen as if you were sighting down the eyepiece,
but from 12 inches away. (The CCD optical
position is actually about 1/8th inch in from the
end of the tube). Move to a bright star. Center
the image in the tube and then insert a high
power eyepiece.
Without changing the focus, adjust the
telescope to center the image in the eyepiece.
Then insert the CCD and the star should be
readily apparent in the display values. It is very
handy to boresight another telescope to the guide
telescope during initial use of the CCD. This
allows the CCD to be rapidly moved to a variety
of stars of different brightness.
PROBLEM
: Error message appears after the
drive calibration step.
MESSAGE: E1 1 (or 2, 3 or 4)
This message appears when the instrument
did not 'see' the star move at all in one or
more of the four directions (+X, -X, +Y, -Y)
during calibration. Re-check the drive
controls using the four drive adjustment
buttons on the box.
MESSAGE: E2 1 to 4
This message appears when the ST-4 did not
see an adequate correction in one of the four
correction directions. A correction of at least
5 pixels (2 units) must be seen in all four
directions. If this is not the case, the
instrument assumes these is no correction
equipment for that direction and ignores
corrections in that axis in the future. This
feature is helpful, for example, when no
Declination motor is installed on the
telescope mount.
PROBLEM
: Star images in photos are not round!
First, check to see in what direction the
images are smeared. If it is in Right Ascension,
which is most likely, consider the following
possible problems.
1. The guide telescope focal length is so short
that adequate tracking is not possible. The
guide telescope should have a focal length of
at least 1/4 that of the primary telescope
doing the photography.
2. The telescope drive is very poor quality or
ST-4 exposure times are too long. Some
telescope drives produce rather abrupt hops
in their position which create motion too
rapid for the ST-4 to follow. Also, some
drives drift so badly that they can not go
uncorrected for even five seconds. In these
cases, the tracker exposure time (which
controls the time between corrections) must
be reduced. This may limit the ability of the
ST-4 to track faint guide stars.
If the error is not only in Right Ascension, it is
possible that differential deflection is occurring
between the guide telescope and the primary
telescope. The simplest solution in this case is to
use an off-axis guider, or the on-axis guider
offered by SBIG.
Note that the ST-4 tracker is capable of better
correction than a human because it averages the
position of the star over a period of seconds,
while a human sees the star dance about an
ambiguous central position due to atmospheric
turbulence. Also the ST-4 makes precise drive
adjustments in response to a guiding error, while
a human stabs away at the pushbuttons until the
star is again centered.
Another error can occur when guiding on a
star some distance off-axis with an improperly
adjusted mount. The guide star is tracked
accurately, but stars within the camera field of
view will rotate slightly about the guide star.
This effect is small, but significant in careful
work. A one half degree error in correctly
positioning Polaris could produce an image
rotation in the telescope of 5 minutes of arc per
hour. If you were to take a 0.5 hour exposure
