SBIG ST-4 User manual

SANTA BARBARA INSTRUMENT GROUP
PO Box 50437 •1482 East Valley Road, Suite 33
Santa Barbara, CA 93150
Phone: [phone] •FAX: [phone]
Email: [email protected] •Web: www.sbig.com
Model ST-4 Star Tracker
Imaging Camera
Operating Manual
Table of Contents
Instrument Overview.............................................................................Page 1
Star Tracking Operation.........................................................................Page 4
Imaging Camera Operation...................................................................Page 8
Host Computer Software Overview......................................................Page 10
Using the ST-4 with an IBM PC or Compatible...................................Page 18
Using the ST-4 with a Macintosh...........................................................Page 28
Observing Suggestions with the ST-4 Imaging Camera......................Page 37
Problem Solving with the ST-4.............................................................. Page 38
Appendix A.............................................................................................Page 39

SBIG ST-4/0490 Page 1
Model ST-4 Star Tracker / Imaging Camera
Operating Instructions
INSTRUMENT OVERVIEW
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.

SBIG SANTA BARBARA INSTRUMENT GROUP
Page 2SBIG 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

SBIG SANTA BARBARA INSTRUMENT GROUP
SBIG ST-4/0489 Page 3
still operates normally. If the telescope control is
modified for the ST-4, and the ST-4 is unplugged,
the drive may not run since the normally closed
connection in the ST-4 has now been removed. If
this situation is a problem it is best to build up a
mating connector to replace the ST-4 box that has
the appropriate pins shorted together (usually
just two are required to enable the RA drive to
work).
For joystick type controllers, the four relays
in the ST-4 are used to simulate the joystick being
pushed to its limits. Two relays are used for each
axis or rheostat of the joystick as shown in Figure
3. For the Right Ascension rheostat you would
use the +X and -X relays, and for the Declination
rheostat you would use the +Y and -Y rheostat.
c
nonc
switch
common
normally open
normally closed
A: Standard Hand Controller Switch
B: Modified Hand Controller Switch
normally open
normally closed
c
nonc
relay
common
c
nonc
Figure 2 Pushbutton Interface
rheostat nc
c
no
nc
c
no
relay
A
BC
A
BC
A: Standard Joystick B: Modified Joystick
relay
Figure 3 Joystick Interface

SBIG SANTA BARBARA INSTRUMENT GROUP
Page 4SBIG ST-4/0490
STAR TRACKING OPERATION
The instrument panel is illustrated in Figure 4.
The ST-4 instrument is furnished with a power
cable, a cable to the CCD, and a third cable for
the telescope's hand controller. There is no
power switch; plugging in the power turns the
instrument on. The instrument will come-up in
FIND AND FOCUS mode, which displays the
greatest pixel value found in the readout of the
CCD array, and the location of that pixel. The
greatest pixel reading will drop as the CCD cools
in temperature. After about 2 minutes, the CCD
will have cooled to its optimum temperature.
Note:
The display values range from 0 to 99
corresponding to percentage. Pixel X and Y
values of 50 correspond to a pixel centered
within the CCD, while a brightness value of
99 corresponds to a completely saturated
pixel, i.e., the star is too bright for tracking.
Instrument Startup for Tracking Purposes
1. Insert the CCD head into the eyepiece tube
such that it seats accurately against the tube.
The CCD will not be damaged by light when
powered, so it can be easily handled.
2. Begin calibrating the CCD by completely
blocking the open end of the telescope to
remove all light (the CCD is very sensitive -
it will saturate in very low ambient light
levels).
3. Observe the VALUE reading. This reading
should fall to a reading of around 10 and
stabilize within two minutes of the
instrument being powered-on. Press the
INTERRUPT button to halt the FIND AND
FOCUS mode (or any other mode for that
matter).
4. Push the TAKE DARK FRAME button. The
microcontroller will readout the CCD and put
the data in the dark frame memory.
VALUE XY
89 45 37
SBIG ASTRONOMICAL
INSTRUMENTS
STAR TRACKER/IMAGING CAMERA
TAKE
DARK
FRAME
FIND
AND
FOCUS
CALIBRATE
DRIVE
TRACK
MODE SELECT
INTERRUPT
MENU
ADJUST
Figure 4 Instrument Panel
5. Push the FIND AND FOCUS button and
uncover the telescope to begin the collection
of light by the ST-4. The instrument will
automatically begin taking frames of CCD
data, subtract the dark frame stored in
memory, and display the maximum value.
Direct the telescope to a star and adjust the
telescope's position to approximately center
the star image on the CCD by noting the
reading on the X and Y displays.
6. If the star is too bright (VALUE reading 99)
then either the exposure must be reduced, or
a fainter star chosen for tracking. In order to
correct this condition, press the INTERRUPT
button to stop the collection of data, and

SBIG SANTA BARBARA INSTRUMENT GROUP
SBIG ST-4/0489 Page 5
return control of the ST-4 instrument to the
keyboard.
Push the MENU button. The brightness
display will then read "EA" (exposure adjust)
and a "1" will appear on the X pixel display
(indicating a default exposure time of one
second).
Repeatedly pushing the ADJUST key will
scroll through a list of exposure times, from
0.1 second, to 20 seconds. Adjusting the
exposure time to shorter times will reduce the
star brightness, while choosing a longer
exposure time will increase star brightness.
When the desired exposure time has been
chosen, press the MENU button again (the
ST-4 will display "CA" (Calibration Adjust),
and then press it several more times until you
see a bA displayed in the value box. bA
stands for brightness adjust; two modes are
available, A for average, or F for faint. F
increases sensitivity by 9 times. You should
set this parameter to A for initial
familiarization. Press the MENU button
again until you see the boost (b) parameter
displayed. This is a boost factor, where
greater values mean greater gain. Initially set
this value to 1. Press the MENU button again
to return to the normal operating mode (after
being interrupted "HELLO" appears on the
display).
Note:
You must take a new dark frame if you
change the exposure time, the brightness
adjust, or the boost factor.
7. When the brightness level has been adjusted
to an acceptable level, focus the telescope by
turning the focus knob and observing the
VALUE display. At best focus, this number
is maximum. Be careful to take your hand off
the telescope between adjustments or the
telescope vibration will smear the star image
over multiple pixels within the CCD,
reducing the brightness. Atmospheric
turbulence will also tend to smear the image,
so it may be helpful to watch several
sequential exposures when critically
focussing the image.
A table of typical VALUE readings for
different magnitude stars is shown below in
Table 1. This assumes a 1 second exposure
and a typical response of the CCD. Use this
as a reference for determining whether the
system is properly focussed.
Star ST-4 VALUE Reading
Magnitude 60mm Refractor 8 inch SCT
4 99 99
5 60 99
6 24 99
7 9*80
8 4*32
9 1*13
10 -5*
Table 1 Typical VALUE readings
8. When the focus is adjusted, remove the CCD
and insert an eyepiece into the tube, sliding
the eyepiece in the tube until the image is in
sharp visual focus. With a knife or other
sharp object, scribe the eyepiece on its side at
the end of the tube. This eyepiece can then be
used to quickly center and focus the CCD in
the future Place the CCD back into the tube
so that it seats against the tube as before.
9. Position the star image approximately in the
center of the CCD (X=50, Y=50) using the
telescope controls. Push the UP, DOWN,
LEFT, and RIGHT buttons for a few seconds
to make sure that each relay control is
correctly interfaced to the telescope hand-
held controller unit. If the buttons are
working correctly, the star image will move
in four different directions (but not
necessarily up, down, left, and right).
10. Push the CALIBRATE button. The ST-4 will
automatically drive the telescope in each
direction, determine which direction
corresponds to +X and +Y, and calculate the
correction speed of your drive in all four
directions (in pixels moved per second). This
process takes about 30 seconds. This is a
four-step process with the ST-4 exercising the
four relays, and after each step the ST-4 will
momentarily display the location and
brightness of the brightest object in the field
of view. If the image moves too little (less
*Use an increased boost factor and the faint star
mode when working at these levels.

SBIG SANTA BARBARA INSTRUMENT GROUP
Page 6SBIG ST-4/0490
than 2 units) or too far (greater than 30 units)
the relay closure time should be adjusted by
pressing the MENU key twice to get to the
"CA" option, and then repeatedly pressing
the ADJUST key to scroll through the time
adjustment values (1 to 20 seconds).
11. If no errors are displayed at the end of the
CALIBRATE procedure (E1 or E2 in the
Value display), then position the guide star
on the center of the CCD, adjust the exposure
time if necessary, and then press the TRACK
button. The ST-4 will automatically guide the
telescope until the INTERRUPT button is
pressed.
Note:
The ST-4 constantly corrects drive
adjustment times to try to improve tracking.
The visual X and Y displays show the
tracking error seen during each exposure in
units of 0.2 pixels (i.e. a displayed Y value of
-3 indicates that the star moved 0.6 pixels in
the -Y direction during the exposure). The
number displayed after the A in the value
location is the average error for the last 16
correction periods.
If the star is completely lost during an exposure,
the unit will stop tracking and the display will
display the brightness and location of the
brightest object in the field of view (like the FIND
AND FOCUS mode), and the track lost relay will
be activated for one second after 5 consecutive
misses. The telescope will not be corrected again
until the star reappears.
Calibration steps 10 and 11 should be
repeated whenever the telescope is substantially
re-positioned in Declination (to correct for longer
RA adjustments near the poles). Calibration
should also be repeated if the telescope mount is
of German Equatorial design and the telescope
tube flips from one side of the mount to the other
(reversing adjustment directions). Care should
be taken to orient the CCD head so the RA and
Declination axes line-up with the sides of the
CCD head (you can tell by looking through the
glass window at the CCD or by noting the
orientation of the Serial Number tag on the rear
of the head which is oriented like the CCD). This
adjustment is not critical. The tracker will work
in any orientation. It just makes it easier to make
sense out of the x and y readings and to use the
pushbuttons if the axes are lined up to the RA
and Declination axes.
Explanation of Menu Items
The menu choices which can be adjusted to a
particular telescope's configuration can be
viewed by pressing the MENU button
repeatedly. The different menu items have the
following effect:
Note:
For each menu item, pressing the ADJUST
button repeatedly will scroll through the
choices, finally jumping back to the lowest
value choice. There is no way to back up.
EA: Exposure Adjust
The exposure (integration time) used by the
ST-4 in the tracking mode can be set to be
from 0.1 to 20 seconds. The readout time of
the array is 0.14 seconds; the smearing
produced by the readout time for short
integration times is not significant in the
tracking mode.
CA: Calibration Adjust
The amount of time (in seconds) the drive is
left on during each move when the
CALIBRATE mode is executed. If the time is
too short the move will not be accurately
determined. If it is too long the star will
move off the array. Set this parameter such
that a move of 5 to 30 units result.
SA: Scintillation Adjust
The ST-4 modifies the correction factors
determined in the CALIBRATE mode, if
necessary, to improve the tracking. This
modification is performed only if errors
greater than the SA factor are seen (the SA
factor is in pixels). Telescopes with extremely
long focal lengths (>10 feet) may find the
tracking is improved if this value is increased.
Also, its setting should be increased if the ST-
4 shows any tendency to "run away" during
tracking.

SBIG SANTA BARBARA INSTRUMENT GROUP
SBIG ST-4/0489 Page 7
bA: Brightness Adjustment
Setting this to A is the normal mode. If this
parameter is set to F the ST-4 will set each
pixel equal to the sum of the 3 x 3 pixel box
centered on that pixel prior to performing the
tracking calculation. Since star images are
often smeared over several pixels, this
collects more light from a faint star. A new
dark frame should be taken after altering this
value.
H1: Hysteresis (backlash) Adjustment, X-axis
This parameter sets the amount of extra time
that the drive is operated when the direction
of adjustment is changed. Many telescopes
have severe backlash, and the star image may
not move for several seconds after a
correction is made. The parameter is the
number of tenth second increments added to
the calculated move when the move direction
is reversed from the previous move.
H2: Hysteresis Adjustment, Y-axis
This parameter is identical to the H1
parameter above except for the Y-axis.
b: Boost factor
This parameter boosts the internal gain of the
ST-4, enabling fainter stars to be tracked with
short exposures. Care must be taken that
thermal variations of the CCD head do not
cause the star to be lost when working with
very faint stars. If this value is altered a new
dark frame must be captured.
Please review the problem section at the end of
this manual if problems are encountered during
tracking.

SBIG SANTA BARBARA INSTRUMENT GROUP
Page 8SBIG ST-4/0490
IMAGING CAMERA OPERATION
The ST-4 works quite well as an electronic
imaging camera when connected to either an IBM
PC or compatible or an Apple Macintosh. Some
details concerning the operation of the CCD in
this mode are necessary to understand all the
controls and features available in this mode.
CCD Pixel Dimensions
The CCD pixels are not square. Their dimensions
are 0.01375 millimeters wide in the X-direction
and 0.016 millimeters tall in the Y-direction.
Note that the CCD active area is therefore 2.64 by
2.64 millimeters, or exactly square (about one
tenth inch on a side).
Caveats of CCD Readout
The CCD is read out electronically by shifting
each row of pixels into a readout register at the
Y=0 position of the CCD, and then shifting the
row out through an amplifier at the X=0 position.
The entire array shifts down one row when a row
is shifted into the readout register, and a blank
row is inserted at the Y=165 position. Note that
the CCD elements are still collecting light as they
step down to the readout register. Most
commercial CCD cameras use a more expensive
CCD which has what is known as a frame
transfer readout mode, where all active pixels are
shifted very quickly into a pixel array screened
from the light by a metal layer, and then read out
slowly. The SBIG ST-4 CCD minimizes the effect
of not having a frame transfer buffer by reading
out the array very quickly, in about 0.14 seconds.
As long as the CCD exposure is greater than
about one second this technique will reduce
streaking of the stars to acceptable levels.
Planets pose a particular problem to the CCD
since they are so bright that exposures of 1
second at f/10 are badly overexposed. The ST-4
has a Half Frame mode for planets and bright
stars to solve this problem. In the Half Frame
mode the upper half of the CCD is used as a
frame buffer for a bright image positioned in the
lower half of the CCD. A short exposure can be
taken (down to 0.01 seconds) and the bottom
half of the array shifted rapidly up to the upper
half. The 82 lines of short exposure data can then
be readout at the normal rate. This method
works quite well, and uses enough pixels such
that 0.5 arcsecond per pixel scale factors can be
achieved while viewing an entire planet.
Unfortunately, this technique does not work
for the moon, since the moon's image typically
fills the CCD. The only way to image the moon is
to use neutral density filters to attenuate the light
down to where the CCD can be used for a 1
second exposure without saturating. The same
holds true for images of terrestrial scenes during
daylight.
When a long exposure is taken, a glow will be
noticed in the upper left corner of the image, near
pixel (1,1). This is apparently due to heating of
the array by the readout electronics increasing
the dark current. This glow can saturate the
array in the corner in exposures several minutes
long and cause a blank region to appear in the
subtracted data. Using exposures short enough
that saturation does not occur in the corner will
reduce this cosmetic problem to acceptable levels.
Dark Current
The CCD can not take an unlimited exposure.
During an exposure in the dark the pixels will
slowly fill up due to an effect known as the dark
current of the device. This dark current is
reduced by cooling, and this is why the CCD is
mounted on a single stage thermoelectric cooler
in the CCD head. The window is used over the
CCD to keep moisture from the CCD while it is
powered. If the window is removed the CCD
will frost rapidly (in seconds). In a room
temperature environment the cooled CCD pixels
take about 5 minutes to fill up due to dark
current. At lower ambient temperatures the CCD
will take longer (maximum exposure time
doubles for roughly every 20 °F drop in
temperature) until around 32 °F, where the cooler
power is reduced to avoid damaging the CCD
with excessive cold. The dark current can be
subtracted from images as described in a later
section, but the effects of the dark current filling
the CCD can not be avoided.
CCD vs Film
The CCD is very good at the most difficult
astronomical imaging problem; imaging small,