SBIG ST-4/0490
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Model ST-4 Star Tracker / Imaging Camera
Operating Instructions
I
NSTRUMENT
O
VERVIEW
The Model ST-4 Star Tracker / Imaging Camera is a multipurpose instrument. It can be used as
an automatic star tracker to take long guided exposures of the night sky, or, in conjunction with
a personal computer (PC), as a highly sensitive imaging camera. This manual describes the
technical concept of the Model ST-4, the interfaces to the telescope and computer, and general
operating instructions.
Tracking Overview
The Model ST-4 uses a charge coupled device
(CCD) detector to detect star images. The
detector can be understood by examining Figure
1. An array of 192 by 165 detector elements,
called pixels, is arranged as shown in the figure.
We refer to the horizontal axis as X, and the
vertical axis as Y.
X
Y
165
Pixels
192
Pixels
Enlarged View
Showing Pixels
Figure 1
CCD Configuration
In operation the pixels convert photons into
electrons, and store them until read out by the
ST-4's microcontroller (the ST-4 has memory for
two images, one for light frames, and one for
dark frames, as explained further below). For
example, if a star's image is present at pixel
position 121,132 the computer will determine its
position by noting the increased signal from that
pixel. If the star is drifting due to guiding errors,
it will appear at a different position in the next
exposure, say at pixel position 123,132. The
computer then calculates how far the star has
drifted from the original exposure, and toggles
the telescope drive accordingly.
The microcontroller (in the ST-4) can take an
exposure (called integration), read out all the
pixel values, and calculate the necessary
telescope correction in less than a second. The
tremendous sensitivity of the CCD enables guide
stars as faint as 8th magnitude to be tracked with
a 1 second exposure and a 60 mm guide
telescope.
The calculating power of the ST-4 enables the
star's location to be determined to a fraction of a
pixel accuracy, enabling very accurate tracking.
Imaging Overview
The Microcontroller referred to in the tracking
section is built into the ST-4. This microcontroller
can communicate to an XT or AT compatible PC,
or a Macintosh
*
, over the ST-4's RS-232 serial
link (the serial interface of the ST-4 is also
compatible with the RS-422 ports used on the
Macintosh).
A full image can be transmitted at 19.2K
baud within 18 seconds and over a distance of
100 feet. Data transfer rates as high as 57.6K
*
Macintosh is a registered trademark of Apple
Computer, Inc.
Содержание ST-4
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