I don't know if anyone has ever tried to time it in practice, but Celestron programmed the hand control to issue the corrections every 30 seconds. The processor in the hand control would likely delay the interval if a higher priority task occurs (user accessing the menu, a flood of commands via the port on the bottom of the hand control, etc.). Any delay should be very brief but would be totally unpredictable.
All that said, I've always recommended exposure duration of no longer than 20-25 seconds, take a good number of them, and throw out the bad ones where the tracking correction occurs during an exposure (any image with a little 'jump' in it). Since you will not exactly know when the start of the update sequence occurs (basically it is right after the last movement was sent to the MC), you won't benefit by trying to create a sequence of:
- 25s exposure
- 5s wait
- 25s exposure
- 5s wait
it is more likely you will experience nearly 100% bad exposures as the tracking update will occur during the 25s exposures, not the 5s wait periods.
Author of "The NexStar Users Guide II"
Author of "The NexStar Users Guide"
Author of "NexStar Observer List"
Thanks for the input, Mike. Good point about other operations coming in, potentially shifting the 30 second period. I suppose it would depend on how the timer was implemented (poll/sleep + execute, versus a true timer).
If it were a strict 30 seconds, it should be fairly trivial to land the 5s window through a couple trial and errors (e.g. if star trails are seen, stop and offset another few seconds; rinse, repeat); and assuming the intervalometer was also accurate, one would then simply set the repeat for an hour or so and hope everything lines up. I'm not too confident both systems would be this accurate together, however.
Followup question, coincidentally I found another one of your posts that suggested in wedge mode (EQ tracking), this track correction is omitted:
>> When on a wedge (even with less than perfect polar alignment), the situation is different. As long as you perform an EQ Alignment, or manually set Tracking Mode to EQ North/South, tracking is handled differently. In that case, the motor control makes no movement in the dec (alt) axis and tracks at a set sidereal (default) rate in RA (azm). There are no rate updates from the hand control and thus no minuscule jumps.
(EDIT: link https://www.cloudyni.../#entry7760751)
In this case, if I'm doing polar alignment something else is up with my star trails entirely.
Either way, once rain clears up in a couple days I'll try it out and report back.
Thanks again for the input!
Edited by TrustyChords, 14 February 2019 - 07:20 PM.