STAR 2000 is a completely new method of correcting the tracking drives of a telescope, while
taking an image with a CCD camera. Unlike all other autoguiders, or dual CCD self-guiding
cameras, STAR 2000 uses the imaging CCD itself to sense drive errors and generate
correction signals. This is possible, because the Starlight Xpress 'MX' cameras use Sony
'Interline transfer, Interlaced CCDs', which have a special structure not found in 'Full frame',
'Frame transfer' and 'Progressive scan' CCD devices, as used by other manufacturers.

This structure splits the pixels into two, independently readable halves, and so we can read
one half at short intervals for guiding information, while the other half is integrating a long
exposure image. Because the guiding half-pixels see the same field as the integrating
half-pixels, any object within the CCD field may be chosen for guiding and none of the camera
field is lost! To use STAR 2000, you do not need any extra optics or cameras, just a small
interface box to connect the serial port of the computer to the guider port of the telescope
mount.

The principle of S.T.A.R. is based on the special CCD structure of the chips used in our MX5
and MX9 cameras and it cannot be used with ‘full frame’ CCDs, such as the popular Kodak
devices. It is also incompatible with our HX516 camera. The MX5 and MX9 use ‘Interline
Interlaced’ CCD chips, which are constructed with each pixel split into two vertically stacked
halves. Each half can be ‘read out’ independently, so it is possible to integrate a long
exposure on one field of half pixels, while the other field is read out at short intervals and the
data used for guiding the telescope. Half way through the exposure, we can read out the
contents of the integrating field, swap fields, and integrate the rest of the exposure on the
other field. This preserves the best image resolution and eliminates ‘aliasing’ effects.

To give a fast guider update rate, the guiding field is read out as an 80x40 window around the
guide object, while the unused lines are ‘dumped’. Using this technique, it is possible to feed
corrections to the telescope drive as often as one every second when guiding on a fairly bright
star (typically about mag. 11). The sub-pixel guiding accuracy of S.T.A.R. 2000 will give you
tightly defined, round star images, however long the exposure time that you use!

The advantages of S.T.A.R. are very great, but there are two negative aspects, which you
should be aware of. The main loss is in the sensitivity of the CCD, as only half of the pixel
area is integrating an image at any one time. This is slightly offset by the halving of the dark
current, which also results, and can be completely compensated for by increasing the
exposure time – not a big problem when the camera can guide itself! The other problem is that
the CCD amplifier must be switched on momentarily to read out each guider frame. Despite
the short times involved, this results in some contamination of the upper left hand corner of the
image, by light emitted from the amplifier transistors (electroluminescence), and this needs
the subtraction of a special dark frame to remove it. However, the glow is quite faint and may
be ignored on many images, or be removed with the ‘Gradient filter’ in the ‘Filters’ menu.

What you need:
To use S.T.A.R. 2000, you must have the following hardware and software:

A Starlight Xpress ‘MX’ series camera. The monochrome MX512 / 516, colour MX5-C and
MX916 are currently supported.
A telescope with ‘Autoguider’ capability. Most modern SCTs have this facility in the form of a
6 pin ‘RJ11’ telephone style connector on the mounting base or handset. If your ‘scope is ‘ST4
compatible’ it should work with S.T.A.R. 2000. Adaptors for other mounts, such as the Vixen
Skysensor, will become available as the details are resolved. At the time of writing (Feb.
2000), the system has been tested with the LX200, LX50 and Celestron CI700 German
mounts with good results.
A special ‘Serial Interface’ box from Starlight Xpress. This contains a microprocessor and 4
coloured ‘direction indicating' LEDs. It connects between ‘COM1’, or ‘COM2’ on your
computer and the autoguider input of your telescope. Two outputs are provided, one of which
is for use with an LX200, via the ‘RS232’ socket, and the other for use with non ‘GOTO’
mounts. Please note that this unit is for use with modern ‘electronic’ drive systems and
cannot operate motors, or relays, directly. A suitable relay box is likely to be available in due
course. A computer with parallel and serial (RS232) ports. The software will work with most
machines, but we recommend a 100MHz, or faster Pentium, with at least 16Megs of memory.
The current version of S.T.A.R. 2000 is Win 3.11, Win 95 and Win 98 compatible. Future
issues are likely to be 32 bit Win 95/98 (Win 2000 in due course).
The latest issue of either ‘STAR_M5’ (MX5), or ‘STAR_M9’ (MX9) software.

Setting up the system:
For best results, it is important to set up your telescope mounting as accurately as possible.
Alt-Az mounts can be used with S.T.A.R. 2000, but will cause field rotation during exposures
and are often rather poor at smooth tracking. We recommend that you always use an
equatorially mounted ‘scope, with its polar axis accurately aligned on the pole. If your mount
is prone to sudden shifts of aim point during tracking, usually due to poorly made gears or
bearings, then S.T.A.R. will be unable to fully compensate and some trailing will result. Slow
and smooth errors in position are well corrected and so this is what you should aim for. Check
that all your drive gears are firmly fixed to their shafts (loose gears are surprisingly common!)
and that they engage closely, without binding.

Connect up the computer and camera in the usual way and install the camera onto the
telescope. Now plug the serial port 9 pin cable (provided with the S.T.A.R. serial interface) into
either COM1 or COM2 of the computer and connect the other end into the 9 pin socket on the
serial interface box. Next, take the 6 way RJ11 telephone lead and plug one end into the
appropriate output of the serial interface. Which output you use will depend on your telescope,
but the Meade LX200 and its clones are compatible with that labeled ‘LX200’. All other mounts
will use the ‘Standard’ output. Connect the other end of the RJ11 lead to the ‘Autoguider’ or
‘Auxiliary’ input on your telescope, but use the ‘RS232’ input, if it is an LX200.
Once the above connections have been made, you are ready to start up the system.


Starting to guide:
Power up the camera and computer, and start the telescope drive. If you have not installed the
S.T.A.R. software, do so now and then start up the software control panel.
Click on ‘Set Program Defaults’ and examine the current settings. In the ‘Telescope Guiding’
box, set the interface selection to ‘LX200’, or ‘Via Control box’, as appropriate. Also select
‘COM1’ or ‘COM2’ according to availability - ‘COM1’ is the 9 pin port provided on the rear
panel of most portable PC computers.

Now adjust the other values as follows:

Set ‘RA Pixels per sec’ and ‘Dec Pixels per sec’ to 5.
Set all backlash and hysteresis values to 0.
Set ‘Ignore drift < 1 pixel’ to OFF.
Set ‘70mS delay between direction changes’ to ON.
Set ‘Default guide training time’ to 2 secs.
These values will be refined later, but should work reasonably well with most telescopes.

You can now test communications between the computer and telescope mount, as follows:

Press ‘Set slew values’, followed by ‘Save Program Defaults’.
Select ‘Telescope control’ in the ‘File’ menu.
Press each button in turn and observe the LED lamps on the serial interface box. As each is
pressed, the appropriate lamp should light, and the telescope will begin to slew in the correct
direction. This slew will be slow (unless you have deliberately selected a fast slew mode via
the telescope controls) and so you will need to observe it through the telescope eyepiece, by
watching stars drift by. If the LX200 output is in use, the lamps will light in a chaotic fashion,
but the ‘scope will still slew correctly.
If all is well, you can proceed to testing the guiding system itself, otherwise, check your setup
and confirm that the correct COM port etc. is selected.

Refit the CCD camera to the telescope and rotate it so that the mounting bracket is aligned
approximately East-West. Focus in the usual way on a star and then point the ‘scope towards
the object you have selected for imaging. I recommend something near the celestial equator
and with a reasonable number of useful guide stars around it, to make life a little easier!

Take an image of the field, using an exposure time of 1 second and selecting ‘Auto contrast
stretch’ in the ‘View’ menu. Several stars should be easily visible in the displayed image, but,
if not, try increasing the exposure time a little. Remember that the ideal guide star is one
which is bright enough to be easily detected in a 1 second exposure, or less.

Once you have the test image and can see an appropriate guide star, go to the ‘File’ menu
and click on ‘Select guide star’. Now click the mouse on the image, with the pointer on your
selected star. The pointer will jump onto the star and a message box will show its
co-ordinates in a message window – press ‘OK’ to confirm the selection. You have now
selected the guide object and are ready to start an exposure.

Call up the camera control panel and select a 1 second (or the length necessary for your
guide star) exposure time. Click on the check box ‘Self guide next exposure’ and ‘1 second’
should appear in the self guide exposure box. Now set the exposure for the image (try 5
minutes as a good starting point) and click on ‘Take picture’.

The control panel will disappear and be replaced by the guider window, with a set of number
boxes above it. The guider software will now attempt to centre an 80 x 40 pixel box around the
guide star and this will appear in the guider window after a few seconds delay. The number
boxes will also begin to show the star co-ordinates and the differential guiding error as each
new guide field is displayed.

If all is well, the error numbers will remain at generally less than 1 pixel and the star image will
sit steadily at the same location in the guide box. If, however, the star begins to drift away, it
is possible that your drive guide directions are reversed in either RA or DEC (or both). Press
‘ESC’ to stop the exposure and go back to the ‘Set Program Defaults’ menu, where you can
use the ‘Swap n/s direction’ and ‘Swap e/w direction’ check boxes to reverse the appropriate
drive. Another problem, which can arise, is that the star may remain close to the correct
position, but will oscillate around it by two or three pixels, rather than holding nearly steady.
This can be due to several different faults, such as poor drive accuracy, windy conditions, or
bad seeing, but may also be caused by an excessive ‘Loop gain’ in the self-guider feedback
loop. Try increasing the ‘Pixels per second’ values in the appropriate default setting box, until
stable following is achieved. Once the guider is set up correctly, it should hold a guide star
within 1 pixel of its correct position indefinitely, and you can try taking test exposures of
various objects to confirm its accuracy.

After your first test of the self guider, there is a good chance that the set-up will still need
some optimising. You have the option of manually setting test values of backlash and
hysteresis, or using the ‘Training’ routine in the software. Backlash is the time delay between
a reversal of a drive motor and the start of a positional correction of the telescope. Hysteresis
is the time it takes for a motor to stop, after the power is removed. Many mounts have internal
software to compensate for these errors, but you may need to set values other than zero to
get the best performance from less sophisticated ones.

The ‘Training’ option can be used as follows:

Select a star to guide on, and then the drive training period, using the ‘radio buttons’. The best
training time will vary with the response speed of the mount, but use as long a time as
possible, consistent with not losing the guide star out of the guide window. Start the training
cycle and the software will then sequentially drive the telescope in all four directions, while
recording the displacement of the reference star. After the cycle has completed, the optimum
correction speeds, backlash correction and hysteresis values will have been calculated and
inserted into the default boxes. Note that the pixels per second rate for the R.A. correction will
vary with the cosine of the declination angle, so you may need to alter this value, or re-train
the system when changing between objects at a widely different declinations.

Exposure times and image calibration

S.T.A.R. 2000 will enable you to greatly extend the exposure times of astronomical images
and so detect much fainter objects. The limiting factor is set largely by the increasing effect of
light pollution on these extended exposures, but dark sky sites will permit 30 minutes, or
more, to be used. You will also begin to see signs of amplifier luminescence at the upper
left-hand corner of your images and this will make dark frame subtraction more important than
usual.

The software includes an option to generate properly matched dark frames, by recording the
number of guiding downloads executed during the taking of the image and using this to add
the correct amount of amplifier glow to the resulting dark frame. To use this option, you must
record the dark frame immediately after the ‘light frame’, or the download count will be lost.
Simply cover the ‘scope aperture, select ‘Take matching dark frame’ in the self guider part of
the camera control interface and press ‘Take Photo’. Application of the ‘Gradient filter’ (under
the Filter Menu) can be used as a final ‘tweak’, if necessary.

Flat field frames can be taken in the usual way, and are not affected by the use of S.T.A.R.
2000.

This completes our first look at S.T.A.R. 2000. No doubt more information will be added as
time goes by.

Good Imaging!

Terry Platt;

February 6th, 2000

Price - £139.50

MX5 + S.T.A.R 2000 Package - £634.50 inclusive of UK mainland delivery.
MX5C + S.T.A.R 2000 Package - £759.50 inclusive of UK mainland delivery.
MX516 + S.T.A.R 2000 Package - £734.50 inclusive of UK mainland delivery.
MX916 + S.T.A.R 2000 Package - £1534.50 inclusive of UK mainland delivery

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