Geoscience Reference
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are rapidly shifted with respect to the star by the size of the rowblock.
By the time a rowblock has traversed from the top of the CCD to the
bottom where it is read out, it will accumulate all the stars that were in
the original 2048 row image into a rowblock. Depending on sky and seeing
conditions, a different size of the rowblock can be used. One downside of
this technique is that sky background is always collected especially during
each hold-time. The sky background could be 5 or 6 times (
2048
/
64)
brighter than that of a stared image. Because it takes a finite amount of
time to read in each row of the rowblock, a small percent of each star's flux
is imprinted in each row creating streaks in the image for stars brighter
than 10th magnitude. This will further decrease the number of usable stars
for detections. Closing the shutter during readout can remove the streak if a
fast and robust shutter is available. To reduce the sky background, one will
need something different. For instance, the use of a mask, a fiber system
or a novel electronic design such as a multi-channel frame-transfer CCD or
CMOS should have an improvement in the sky background level by one or
two magnitudes.
Nevertheless, the system capability was demonstrated successfully.
TAOS is supposed to conduct a “blind” occultation survey. That does not
prevent us from observing a rare predicted event whichever might come
close to the TAOS site. A local network is organized for such an event
as well. Direct measurements of known Centaurs (e.g., Pholus
20
)orbig
TNOs
21
are possible. The light curves derived can also be used to study
a variable star which varies on a larger time scale.
22
Similar techniques
have been developed elsewhere independently to detect a lunar occultation
event
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and to study speckle imaging of binary stars.
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3. A Simulator for TNO Occultation Survey
Synchronous mode operation with three TAOS telescopes began in the win-
ter of 2004. Reliable light curves, free from bad weather, poor seeing, lost
tracking and other non-ideal conditions, are expected to be produced in
large amounts. An immediate question would be, “What do the occulta-
tion survey results mean?” First of all, it is straightforward to compare the
event number counts with a model prediction. The simplest model should
be a geometric model where a certain solid angle is occupied by the fore-
ground TNOs (with some kind of size and orbital distribution assumed) for
a given time duration. A certain probability that a background star will
be occulted can be calculated.
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,
14
,
25
,
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However, it has also been pointed
