Re: APCC & NINA Platesolver "sync" question

Jeff B

Roland, I really like your explanations of the differences between Syns and Recal.

Just one tiny question for clarification:  "Sync: you send the mount via manual move to a spot in the sky that you are absolutely certain is the star."  Does "manual move" mean by hand, not using the paddle buttons or could (should?) it include the paddle buttons?  No before hand slewing ?

Ok, that was two questions. 😁


On Wed, Dec 30, 2020 at 10:58 AM Roland Christen via <> wrote:

When I use SkyX, I do a “Closed Loop Slew” to a star, which will slew to the star, take a picture, plate solve and center the star. I then “Synch” on the star in SkyX. Is this the same as your fourth paragraph?
Yes, this is exactly correct.


-----Original Message-----
From: davidcfinch9 via <>
Sent: Tue, Dec 29, 2020 7:15 pm
Subject: Re: [ap-gto] APCC & NINA Platesolver "sync" question

When I use SkyX, I do a “Closed Loop Slew” to a star, which will slew to the star, take a picture, plate solve and center the star. I then “Synch” on the star in SkyX. Is this the same as your fourth paragraph?
David C. Finch

Sent from the all new Aol app for iOS

On Tuesday, December 29, 2020, 5:41 PM, Roland Christen via <> wrote:
Sync and Recal were two commands that we used from the beginning on our very first controllers.

Recal:  When you send the scope to a star, the planetarium software sends the co-ordinates to the mount where it is stored in memory. The mount moves there via the GoTo command. For example, you send (GoTo slew) the mount to a star named Accrumulous, which is at exactly 10hrs00min00sec and Dec of 45deg00min00sec. That number is now in memory in the CP controller. However, you wish to center this star in your CCDchip/eyepiece, so you move it around until it lands on the crosshairs. The new number is where the scope thinks it is pointed, but you wish for that to be the original number. So by issuing a Recal, that new number is replaced by the stored number of 10hrs00min00sec and Dec of 45deg00min00sec.

Sync: you send the mount via manual move to a spot in the sky that you are absolutely certain is the star
Accrumulous. The mount's internal co-ordinates could be anything, but you wish to replace them with
10hrs00min00sec and Dec of 45deg00min00sec. Remember, that co-ordinate is NOT in the mount's memory because you never issued a co-ordinate GoTo command to go there. So, in order to sync on that co-ordinate, your external planetarium program will send the numbers 10hrs00min00sec and Dec of 45deg00min00sec along with a Sync command. The mount controller will dutifully replace the internal co-ordinate with the new one which you sent in your sync command. A co-ordinate and sync command does NOT cause the mount to move, it is not a move command. It is only a re-alignment command.

When i use SkyX with the ASCOM driver, I will click and slew to an object in SkyX, then image and center the object via my keypad or ASCOM driver or APCC buttons. I then do a sync on the object in SkyX. Works flawlessly every time. SkyX knows the object co-ordinate, asks you if that's the one you want to sync on and then does the sync when you say YES. Bingo, done.

I don't know how else to explain it better. Maybe it's still mush?


-----Original Message-----
From: Dale Ghent <daleg@...>
Sent: Tue, Dec 29, 2020 3:54 pm
Subject: Re: [ap-gto] APCC & NINA Platesolver "sync" question

> On Dec 29, 2020, at 16:14, Xentex <michael@...> wrote:
> I just started using APCC and a Mach2, and my usual imaging program is NINA with ASTAP doing platesolving.
> My other mount is a Celestron CGEM and there is a pretty simple process to sync it to the sky when started up.  You pick a star in the mount driver's pseudo-planetarium software (CWPI), go to, center it in eyepiece, then click sync.  I generally did the centering with platesolving, and in NINA always had the "reslew to target" and "Sync" options turned on.  I never knew exactly what was happening, but didn't really care.  It just worked.

"Reslew to target" makes NINA issue another slew to the desired coordinates after a plate solve is done and the mount is updated (synced) with the results of the solve. This process repeats until the results of the successive plate solves indicate that your mount is pointing within the Pointing Tolerance of the the desired coordinates. The Pointing Tolerance is a setting found under Options > Plate Solving.  Given the wide variety and qualit^H^H^H^Hcapabilities of mounts out there, this tolerance can be large or small.

> With the Mach2, there's both a "Sync" and a "ReCal" button in APCC, and the manual explains you only want to "Sync" once per session, and you want to be pretty careful about where you're pointing when you're doing it.  I can't see my mount from where I control it, so that got me wondering whether the NINA "Sync" option is doing a "sync" or a "recal".  And then I started wondering what exactly is happening.

The NINA sync option does a classic sync through the ASCOM driver. There is no provision in the ASCOM telescope driver standard for issuing it in the form a ReCal, which is a command and semantic that is unique to A-P mounts. If set, the A-P ASCOM driver (or APPC) will transparently convert Syncs to ReCals as they are received from applications such as NINA and normally this option should be On.

> So first question, is it useful and safe to have the "Sync" option turned on in NINA's platesolve section?

Is it useful? In general, yes. In the context of the Mach2, it might be superfluous, but not deleterious, after the first time the mount is synced since the Mach2's encoders allow it to know and regulate where it is pointing in the sky without the aid of a plate solve, given a proper polar alignment.

Is it safe? It's generally safe when it's done with a non-encoder-enabled mount in the proper orientation, with counterweights down. Syncing with the mount in a cw-up orientation can end up being dangerous when the mount is told to slew to something later, and the mount dutifully does so, but drives the telescope under the mount and into the pier/tripod legs. There might be safeguards against this that I'm not aware of in the CP or APCC/ASCOM driver, though.

This brings up a good question, though - what if you're running a pointing model and the driver gets a sync? Will the sync perturb the pointing model? This, I don't know because there have been scant clear nights since I personally started using APPM back in mid-November, so maybe Ray can chime in on this. I know that Bisque's ASCOM driver for TheSkyX and T-Point contain an option to silently discard syncs issued by applications when a pointing model is active as, in their system, a sync can throw off the active pointing model.

> Second question, how am I supposed to actually use the ReCal thing in APCC?  For example, last night I wanted to center a particular star as part of an alignment routine.  I used Stellarium to command the mount to the star.  It wasn't quite centered, so I platesolved with the "reslew" and "sync" options on.  To my surprise, it wouldn't center.  And to my even greater surprise, when I tried to figure out why I noticed that APCC, Stellarium, and the plate solver all showed different RA/Dec for where the center of the frame was.  There was about an arcmin difference between them.

You were probably within the aforementioned Pointing Tolerance and NINA determined that there was no need to reslew. Coincidentally, the default value for NINA's Pointing Tolerance setting is 1 arcmin. Reduce this tolerance to force the centering process to be tighter. Not many mounts are capable of such accurate pointing, hence NINA's generally conservative default of 1 arcmin.


Roland Christen

Roland Christen

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