SharpCap cannot GoTo near the meridian

[Borodog]

Bizarre behavior tonight I've never seen before. I set up shortly before Saturn transited, and SharpCap could not complete a GoTo to it, no matter what. Plate solution was fine. SharpCap just kept reporting that the slew stopped in the wrong place. It took me about half an hour to figure out a workaround, which was to simply use the handcontroller. It placed Saturn dead center in the electronic finder, and then I simply panned it over to the offset point for the main camera.

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[admin]

Hi Mike,

that's very likely the ASCOM driver (Celestron behind the Device Hub?) doing something odd. The message you are seeing is triggered at the end of a GOTO movement if the reported mount co-ordinates differ from the GOTO target by more than 1 degree. I haven't actually had it happen to me, but when I was digging into some logs it was clear that this sort of thing happened sometimes and then caused all sorts of issues with the following actions, so it seems better to fail straight away in this situation rather than carrying on.

You are most likely right that this is somehow connected with the meridian being nearby. Are you in AltAz or EQ mode?

Since you are using the device hub, if it happens again, you can turn on logging in the device hub and track what commands the ascom driver is receiving and what position that it is reporting, to see if my analysis that it is failing to complete the slew is accurate.

cheers,

Robin

[Borodog]

Robin,

EQ mode; the mount is a CGEM.

My theory is that it is SharpCap's backlash compensation. When SharpCap issues a correction to the pointing, it starts with a large slew to get beyond any backlash upon return, often much larger than the actual change that's required. I theorize this large pre-slew is hitting the mount's rotation limit on the other side of the meridian. When you GoTo a target from the hand controller, this does not occur; it just slews directly and does not seem to worry about backlash. So the workaround (now that I understand it) is straightforward; if it happens again, just make sure the mount is plate solved and synched, and then slew to the target from the hand controller, then manually pan to the offset point. It only took so long because I didn't understand what was going on. I thought perhaps the mount was experiencing some sort of mechanical or electronic failure, or had developed a balance issue or something.

I need to review SharpCap's hardware settings; perhaps I have the backlash comp set overly large or something.

Mike

[admin]

Hi Mike,

backlash correction for mount GOTO movements is only in a couple of places in SharpCap

* Solar/Lunar mosaics, where it is adjuatable.

* The 'slew the mount to solar system target...' step in the sequencer.

The latter also has an adjustment via the 'Set solar system backlash allowance...' step.

If you aren't using either of those (and the second one has to be used explicitly via an edited sequence) then any backlash compensation is probably coming from the ASCOM driver instead. Not sure where that leaves you...

cheers,

Robin

[Borodog]

Thanks Robin. I think you must be right that it is coming from the ASCOM driver then, because it definitely happens for a GoTo commanded by SharpCap, and definitely not for a GoTo commanded from the hand controller. In any event, it seems rare (target very near meridian), and I have a simple workaround, so I think all is good.

[Borodog]

Just want to update this in case you or anyone else have any ideas what's going on.

I ran into this problem again last night, once again with Saturn near the meridian. Except this time it was worse, in that, even though I had done a plate solve and resync, so the mount knew where it was pointed, when I tried to slew to Saturn using the hand controller, which worked last time, it would inevitably slew about 6° east. No errors, just no Saturn. If I slewed from SharpCap, I would get the error where it says that the slew stopped in the wrong place, which it had. If I slewed from the hand controller, it would just slew to the wrong place. Furthermore, I needed to go west, but the mount just would not let me, using either the on-screen controls or the hand controller, once I got to within about 5° of the target. I am not sure what is going on. I still think this is somehow the rotation limit, but I could find no settings for that in the ASCOM hub or driver. I think I will have to delve into the settings on the hand controller. The other thing I'm wondering is if there is a Daylight Savings Time mismatch, but that doesn't seem right, as I was able to slew to Saturn earlier, before it was close to the meridian. Not really sure what is going on.

Eventually I parked the mount, disconnected SharpCap, power cycled the mount, did not wake from hibernation, but instead did a solar system alignment to Saturn, which I was then able to offset plate solve and resync, which worked. If this happens again, I will just go straight to that procedure.