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One thing I did notice that surprised me is stars hopped around a lot whilst they were being merely tracked.
So to provide more detail on what occurred.
Calibration – appeared fine
I set on the SkySensor2000-PC the auto-guide rates down from 0.5 to 0.4 sidereal for both RA and DEC and I left the backlash for RA and DEC set to the default 100 settings (there seems to be a lot of slop in the DEC backlash).
I checked with GPINTCheck pulses were working fine in +/- RA and DEC – all okay.
With the pulse guide set to 400 ms during calibrate – it took around 40 pulses to move the targeted star 24.5 pixels in RA and about 46 pulses in DEC to do the same (angle calculated as close to 0.0 and the rate 0.88), but there were about 20 pulses in DEC to clear backlash.
On selecting a star and refreshing every 1.0 versus 1.5 versus 2.0 versus 4.0 seconds I observed quite a bit of movement before initiating guiding, for example the X, Y co-ordinates might cycle like:
RA / DEC
204.8, 391.4
205.0, 392.0
204.2, 391.2
205.0, 392.0
204.2, 392.0
204.0, 391.4
205.1, 392.0
So with the tightest focus I could achieve using a 127mm aperture MAK with a 1500 mm focal length – cut to 750mm by a focal reducer into a Meade DSI, selecting faint stars that didn’t overload the light well of the DSI’s pixels – I was seeing stars jump by about +/- 0.3 pixels per every few seconds.
Now I knew that a 900 second, unguided shot at 2300 mm focal length (side saddle) was producing beautifully tight, circular stars, so I intuited it was seeing that was moving these guide stars so wildly.
So I set minimum pixel before a pulse guide is triggered up from 0.2 to first 0.4, then 0.5, then 0.6 then 0.9 then 1.2 pixels – before finally giving up on guiding!
Guiding – weird results within 30 seconds – huge over corrections
What eventuated was massive over corrections – causing the guide star to be lost within 30 seconds. After maybe 20 seconds a pulse would be sent (that I guess was way too big). The star would move out of the 16 pixel search region, and I guess PHD then sent another over large pulse in the same direction). The result was a huge drift.
I turned DEC guiding off – same thing still happened in RA, it just took slightly longer.
So then I set RA aggressiveness down from 100% to 90% to 50% to 20% - all to no avail. In each instant the corrections were huge and lost the star way too fast!.
I didn’t try expanding the search area from 16 pixels to say 50 pixels – not that that would address way to large pulses being sent.
I’ll ping these details to Craig at Stark labs and see what he advises – stay tuned!
Last edited by g__day; 03-01-2008 at 07:34 PM.
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