Starlight Xpress SXV-M7 - Manual (Page 20)

Handbook for SXV-M7 Issue 1 June 2004
20
Using ‘STAR 2000’ self-guiding with the SXV-M7
How S.T.A.R. 2000 works:
S.T.A.R. 2000 is a unique and patented method of automatically controlling your
telescope drives, while capturing a long exposure image with your Starlight Xpress
CCD camera. Unlike off-axis guiding devices and dual-CCD self-guiding cameras,
S.T.A.R. 2000 can guide on almost any object, which is visible within the CCD
frame! You have the entire CCD field of view to choose from and can even select a
slowly moving target, such as a comet nucleus, or asteroid, to guide on during the
exposure.
The principle of S.T.A.R. is based on the special CCD structure of the chips used in
our SXV_M5, M7 and M9 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 M5, M7 and M9 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 (electro luminescence), 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 it can be removed with the ‘Gradient filter’ in the ‘Filters’ menu.
What you need:
To use S.T.A.R. 2000, you must have a telescope with ‘Auto guider’ capability. Most
modern SCTs have this facility in the form of a 6 pin ‘RJ11’ telephone style