Professional observatories often employ a monitor to determine the quality of seeing
each night. This can be useful in helping to decide whether to take certain kinds of images,
or whether to image at all. If you happen to be at the observatory you can sometimes just
look through an eyepiece and see whether the night “looks” good or not. But more and
more of our customers are mimicking professional observatory installations with remote
observing sites and fully automated observing systems. Remote can mean anything from
a few meters to a few thousand kilometers. No matter where one observes, it is often a time consuming matter to get ready for a nights imaging session. It would be convenient to know before hand what kind of results one could expect. Even if the expectation was that the night’s seeing would be no better than several arc seconds, the type of imaging one decided to set up for could be changed to make the best use of the conditions available for that evening.
SBIG has developed an automated unit for monitoring and
logging the seeing throughout a night. The Seeing Monitor,
pictured on the right, uses an ST-402ME camera with custom
optics and software. The Seeing Monitor is intended to be set
up once and left outdoors for an indefinite period.
The Seeing Monitor lens is a
150 mm f/5.3 lens mounted
to the ST-402, all contained
inside a weatherproof box.
The box also contains a
USB extender, and a 12
VDC power supply for the
camera. The window in
the top of the box is clear.
The window is heated to
prevent condensation on the outside. The USB extender
allows operation up to 150 feet (50 meters) from the
controlling PC. The lens and box is permanently pointed at
Polaris by the user. It is assumed the user will mount posts in
the ground outside his observatory or home for this purpose.
Roof mounting is not recommended because small vibrations
from the building may affect the monitor’s measurements.
When properly aligned, one will get an image of Polaris. Of course, Polaris is not exactly at the
pole. The field of view is just large enough that the entire
orbit of Polaris about the north celestial pole can be captured
no matter what time of
night the measurements
are taken with the camera
set up on a fixed mount.
There is no need to take
dark frames for such short exposures. This system is used
to measure the seeing by measuring the horizontal jitter in
the position of Polaris at high speed. A set of equations then
can be used to calculate the zenith Full Width Half Maximum
(FWHM) that one will obtain in a long exposure image from
the rms jitter. The jitter is
measured by reading out
the CCD while it is being
exposed by the light from
Polaris in Time Delay and
Integration (TDI) mode.