Geoscience Reference
In-Depth Information
collect 20-30 GB of data in a clear night, a nearly real-time data pipeline
is needed for processing.
The typical readout time of our 2-channel CCD camera is around 2 s. To
use a CCD for a 5 Hz rate survey mission, a special technique is required.
The strategy we use is called shutterless “zipper” mode operation.
17
,
18
This
technique takes advantage of the standard CCD readout process, where pho-
toelectrons have to be read out sequentially in each channel. The readout
time for each row (2048 pixels) is approximately 1 ms. With some modifica-
tion in both its firmware and our control software, the readout process can
be “held” for, say, 200 ms (the “hold-time”) after a few rows were read out.
These few rows (for instance, 64 rows) of data constitutes a fundamental
unit of TAOS raw data, namely, a “rowblock”. All stars and sky patches
in a 2048
64)
at the final readout. However, a star image in each rowblock might be an
exposure in one of the, say, 32 cycles (32 = 2048
/
64). A different star in the
same rowblock may come from an exposure in a different cycle. Figure 1
is a simplified version which illustrates some special features of this oper-
ation. The rowblock readout reaches a steady state after a few cycles. (It
takes four cycles in Fig. 1.) In reality, stars spread across the whole field
randomly as shown in Fig. 1(e). Without an original stared image, there
would be no way to recover the coordinates of a star from a rowblock. This
hold-readout cycle can be continued indefinitely with the shutter left open
and the telescope tracking a target field. Usually, tens of rowblocks will be
stored as a single file for the convenience of data processing.
Through remote control, two predicted asteroid occultation events were
recorded successfully in our zipper mode images as shown in Figs. 2 and 3.
In the first event, a bright star HIP 079407 (
m
V
=8
.
80) was occulted
by an asteroid (51) Nemausa (
m
V
=11
.
9, diameter = 150 km) at around
18:55 21 February 2004 (UTC). The hold time of each rowblock is about
0.5 s in Fig. 2. A magnitude drop of about 3.5 magnitude for approximately
10 s was detected by the TAOS B telescope. In the second event, a bright
star HIP 050535 (
m
V
=8
.
6) was occulted by (1723) Klemola (
m
V
=15
.
7,
diameter = 31 km) at around 12:10 5 June 2004 (UTC). The hold time is
0.25 s in Fig. 3. An occultation lasted for approximately 1 s was recorded
by two telescopes simultaneously.
There are a few disadvantages to shutterless zipper mode which must
be addressed. For fast data acquisition the cameras are run in shutterless
zipper mode. The shutter remains open, small rowblocks (e.g., 64 rows) of
the CCD are read out (readout time less than 200 ms). The CCD counts
×
2048 field will be squeezed into each rowblock (say, 2048
×
