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SBIG ST-4/0490
baud are supported, and work very well over
shorter cable runs. A partial image transfer
mode is also supported which sends 1/4 as many
pixels (one value for each group of four adjacent
CCD pixels) in 5 seconds. This mode of
operation is very handy for focussing the
telescope and finding objects. Finding faint
objects is easy using this method; the outline of
the Ring Nebula is clearly seen in exposures as
short as 10 seconds with an 8" Schmidt
Cassegrain Telescope (SCT) operating at f/10.
In imaging mode, the microcontroller in the
ST-4 is told to take an exposure by the PC. It
does so, and stores the resulting data in memory
within the instrument. This data is then relayed
over the serial link to the host computer. The
data is retained in the ST-4 until the next
exposure is captured or power is turned off. The
host does all of the extensive data manipulation
required by the user, such as contrast
enhancement, and also can store data on disk for
later study. This is a very attractive feature; the
computer allows a quick look to be taken at the
image, within seconds of the event, and a more
detailed look at any later time, such as a rainy
night or during the day.
The CCD can be exposed for integration
times longer than five minutes. The pixels slowly
fill-up in the dark due to a phenomena called
dark current, and they saturate at about the five
minute point when the CCD is cooled to a
temperature near -30 °C using the single stage
thermoelectric cooler used in the ST-4. The waste
heat of the thermoelectric cooler (about two
watts) is dissipated into the air by convection
around the CCD head. The CCD has a sensitivity
comparable to ASA 20,000 film, if such a film
speed were available. The CCD has a limited
resolution due to the small number of pixels;
much greater resolution would be degraded by
the limitations of most computer graphics
screens.
Note:
With the CCD running at lower than ambient
temperatures, you will wonder why dew and
frosting aren't a problem. First of all, the
chamber containing the CCD is small, and
only a small volume of air surrounds the
CCD. The small volume minimizes the total
amount of water vapor in the air, which will
frost onto the coldest surface inside the head
(which is the bottom of the CCD). Although
frost may initially form on the top of the
CCD, in a matter of minutes it will migrate
and be trapped at the back of the CCD.
System Interfaces
The following equipment is a prerequisite to
running the ST-4 CCD Star Tracker / Imaging
Camera.
1. A Telescope with pushbutton or joystick
slow motions in at least Right Ascension
and and hopefully also Declination.
2. A guide telescope, 50mm aperture or
larger, or an off axis guider arrangement.
For pushbutton type controllers the ST-4 tracker
controls the telescope the same way you do;
through the RA and Declination slow-motion
adjustment switches, the interface to which is
shown in Figure 2. Four relays in the ST-4 are
used to operate the switches. Most telescope
drives have two Declination motor adjustment
switches which are normally in the 'open'
position. Pushing the button or closing the relay
both apply voltage to the motor. Study Figure 2
carefully, along with your pushbutton control, to
determine the correct configuration.
Most telescopes have one Right Ascension
switch which is normally closed; opening this
switch slows down the drive. The other Right
Ascension switch is normally open; closing this
switch speeds up the drive. It is apparent that
the relay contacts which are brought out on the
cable, 4 groups of three (normally open, normally
closed, and common) are all that is necessary to
control the telescope. The cable pinouts are
described in Appendix A.
When the ST-4 tracker is connected to the
telescope, the hand controller is not disabled, and
