[Shiraz (Ray)]

don't know if any of this will help Greg - it's almost impossible to glean anything useful from the published data.

The SX device uses 4 stepper motor drives and has 5 ms steps - but they don't tell you how big the step is, so there is no way to work out the maximum slew rate. You should be able to make the SX unit work with any set of cameras. The device is sold as a corrector for mount errors, with no mention of seeing correction. I have used an earlier model SX. It worked as specified and integrated seamlessly with phd2. It was capable of correcting errors (whatever the source) of up to about 5hz for normal excursions at 1"/pixel.


The SBIG website is similarly uninformative, but the AOX uses what appears to be a voice-coil-like magnetic drive and it can work at up to a 10hz update rate (for an unspecified correction amount etc.). Claims are made for it being able to correct seeing errors, but the supporting data provided on the website are very old simulation results, based on unspecified assumptions plus some (old?) imagery, again taken under unspecified conditions - not tests from the sort of system that you would use, so not of much benefit to your decision making.

FWIW, both devices probably do basically the same thing, so it looks to me like the SX is the best option for your setup, purely on the basis of ease of getting it to work with your cameras. If you had an SBIG camera, the AOX would be the clear choice. The SBIG device likely has an edge in correction speed, but that is conjecture and not based on any real data. In any event, the ability to run an AO at high speed will depend on the availability of a bright guidestar and it would be interesting to hear from others what rates they can actually guide at in the real world. My understanding is that others see benefit from an AO at a fairly low guide rates in good seeing. You can find an informative discussion on guide rate considerations in the old AO8 manual on the SBIG website - the general ideas for stepper drive (like the SX) and direct drive (like the AOX) are presented.
"The AO-8 is somewhat slower than an AO-7, since the motor and
gearbox can only tip the motor at 18.75 degrees per second, or 75
pixels per second. In this design moves are slew-rate limited. What
this means to the user is that the user should use a focal length that
does not produce too large of a star image. For example, if the seeing
is bad and the star is 9 pixels across, it is probably jumping around by
4.5 pixels or so with each look, which would take 60 milliseconds to
move. If under such conditions you reduced your focal length to
where the star was only three pixels across, only 20 milliseconds are
spent making the move, and you can achieve a higher update rate with
the AO-8. The total overhead in the software for an AO-8 move is 45
milliseconds. If, for example, you are taking 50 millisecond
exposures with an optimized focal length (~3 pixel Full Width Half
Maximum stars), the total cycle time is 45 + 50 + 20 ms, or 115 ms (8
updates per second). The AO-7 advantage was that a move of any
length is 20 milliseconds, but the exposure and overhead times were
similar. Based on our experience and knowledge, the corrections
made by the AO-8 help with reducing ground layer atmospheric
turbulence, and are quite effective in reducing guide errors and wind
buffeting. The smallest increment of move for the AO-8 is 0.075
degrees per axis, or 2.7 microns, sbout a third of a pixel."