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All Weather
SEEING MONITOR
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,
Figure One: Seeing Monitor
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
Figure Two: 10 second image of
Celestial North Pole and Polaris
throughout a
night. The
Seeing Monitor, pictured on the right, uses the same ST-402ME camera board
and weatherproof box as the Meteor Camera with some different optics and
different software. The Seeing Monitor is intended to be set up once and left
outdoors for an indefinite period.
The Seeing Monitor uses an uncooled, shutterless version of the ST-402ME
mated to a 150 mm focal length F5.3 lens inside the 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 as shown in Figure
Two. Of course, Polaris is not eactly 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.
Figure Three shows a sequence of images over a night superimposed. The
position of the pole is now quite apparent.
Figure Three: Sequence of 10 second Images Superimposed
The streaks below Polaris in these images are due to the fact that the camera is
shutterless and Polaris is exposing the CCD while it is being read out. This
has no effect on the calculations for this application. Also, there is no need to
take dark frmaes for such short exposures. This system is used to measure the
seeing by measuring the hoizontal 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. An
example of the resulting image is shown in Figure Four.
Figure Four: TDI Readout Image of Polaris
Polaris leaves the bright streak on the right. The CCD is binned vertically by 4
pixels, which causes the start region to be compressed into the top third of the
frame shown here. The data below Polaris is fluctuating wildly in brightness
due to scintillation, the same effect that causes stars to “twinkle”. What is not
obvious here is that the line is being deviated left and right as Polaris’s position
is perturbed by seeing. The software measures this perturbation, and
automatically calculates FWHM at the zenith. The readout is fast, so a new
measurement of Polaris’ position is being obtained every 5 milliseconds. This
is important, since too slow a rate will underestimate the seeing jitter due to
exposure averaging.
The results can be very revealing. For example, at the test site, we have two
kinds of clear nights. The first, most common, clear night is a two to three hour
period between sunset and the fog coming in from the ocean like a wall. The
second is when we get “Santa Ana” winds off the mountains behind Santa
Barbara, which is a hot wind characterized by really clear, but highly turbulent
air. It can get pretty good after midnight. In Figure Five, below, a graph shows
how good it got one night while monitoring with the seeing monitor. What
started out as a night with rather poor seeing turned into a very good night after
about 1:00 AM.
Figure Five: Excel Spreadsheet showing Seeing over a “Santa Ana” Night
This kind of information can be very helpful for remote imagers or anyone who
must decide whether it is worth it to begin a nights imaging session, and if so,
when. It can also signal when a night is degrading to the point that it is not
longer worth the effort of continuing the next hour long series of exposures.
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