Plate Solving misses the Target - can tolerances be set?

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GaryS
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Plate Solving misses the Target - can tolerances be set?

#1

Post by GaryS »

I've been trying to find and image several variable stars and am finding that ASTAP, via Sharpcap, is missing the target when I use it to follow up on my initial 'slew' to the object based on either J2000 or JNow coordinates. In NINA, one can adjust the 'precision' required on a plate solve so I'm hoping there's a similar feature somewhere in Sharpcap Pro. My problem originates mainly, I think, from the small FOV (32'x22') of my ASI178mm. None the less, it seems that a 'successful' plate solve shouldn't miss the target entirely.

Thank you for any help with this problem!
Gary
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Re: Plate Solving misses the Target - can tolerances be set?

#2

Post by admin »

Hi Gary,

there's no option for tolerance of the plate solve, but the first thing I would check if seeing the sort of problem you describe is the co-ordinate system that your ASCOM mount claims to be using. Some mounts claim to be working in J2000 co-ordinates to ASCOM , but then do not convert to/from JNOW for the actual pointing of the telescope - this leads to plate solving taking you to the wrong place. In particular, EQMOD can be set up to do this if you set the co-ordinate system to J2000 in the Eqmod settings.

The correct results should be obtained from...

* Making sure your ASCOM driver reports JNOW as its co-ordinate system
* Going to the target in JNOW co-ordinates from your planetarium program
* Plate solving in SharpCap - SharpCap understands the need to convert from the J2000 reported by the plate solving engine to JNOW

If you are still having trouble, grab a SharpCap log, as the whole plate solve/recenter/co-ordinate system thing is recorded in gory detail in the log :)

cheers,

Robin
GaryS
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Re: Plate Solving misses the Target - can tolerances be set?

#3

Post by GaryS »

Hi Robin
Now I’m confused. I had assumed that whichever coordinates I used, the mount would attempt to go to those coordinates not somewhere else.

I’m mostly entering the custom coordinates manually into iOptron’s Commander software for my CEM70. Are you saying that the mount thinks I’ve entered J2000 and decides it would rather convert those coordinates to Jnow and go there? I guess it could do it the other way as well. Wouldn’t iOptron state that in its manual?

What if I instead connect the mount to Stellarium and then select, say, a variable star in Stellarium’s database and have that application instruct the mount where to go? When I select or search for the star, Stellarium locates and centers it and displays both its J2000 and JNow addresses on the screen. When I execute the goto command, which coordinates does it send to the mount… and, if it sends JNow, might the mount decide to use J2000 or vice versus? This is a real mess… 2 messes really:

Mess 1: if a mount receives specific coordinates to go to and the decides to go elsewhere…..and:

Mess 2: If Stellarium, for example, displays both sets of coordinates, but when you execute a goto from within it, you have no idea which coordinates it sends to a mount.

I never had issues of not having the target onscreen after a slew and platesolve until I began doing photometry with my ASI178mm which has a smaller FOV than my ASI294mc. Now that problem appears to be further complicated by the uncertainty about which coordinates are actually being used by the mount.

I’m not sure where to go with this coordinate problem but, at least I know now that SC doesn’t have a way to specify a plate solve tolerance.
Cheers
Gary
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oopfan
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Re: Plate Solving misses the Target - can tolerances be set?

#4

Post by oopfan »

Hi Gary,

RA and DEC coordinates are always quoted with respect to a specific epoch. Many times you will see "J2000" after published coordinates. When I was a kid we used "1950" coordinates. Why do we need this? Because the earth's axis wobbles like a top, the North Celestial Pole (NCP) doesn't point to the same place in the sky from one day to the next, or even one moment to the next. Robin in his reply talked about J2000 versus JNOW. J2000 refers to the NCP at a specific time in the year 2000. JNOW refers to the NCP at this very moment. Right now we are 21 years, going on 22 years, from the position of the Earth's axis in the year 2000. Therefore, RA and DEC are considerably different between the two, certainly enough for your telescope to notice. Robin's point is that you need to make sure that the Plate Solver and your mount are using the same epoch. If the Plate Solver is yielding J2000 coordinates, then you need to ensure that your mount is interpreting these as J2000 coordinates, not JNOW coordinates.

EDIT: Sorry, meant to write "epoch" not "epic"

Brian
GaryS
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Re: Plate Solving misses the Target - can tolerances be set?

#5

Post by GaryS »

Thank you Brian.

So, with my smaller field of view, wouldn’t I want to send the mount to where the target is now rather than it’s coordinates 22 years ago.

If the mount is referencing J2000, whats the best way to get it to center a faint target with small FOV today? I notice that ASTAP uses JNow coordinates but if my mount will be using J2000 for the original slew, which coordinates do I tell the mount to go to?
Gary
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Re: Plate Solving misses the Target - can tolerances be set?

#6

Post by oopfan »

Hi Gary,

The short answer is that J2000 is just as good as JNOW, but make sure that all subsystems use the same. In other words, look at how your mount is configured and then look at the Plate Solver. They must use the same epoch.

Brian
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Re: Plate Solving misses the Target - can tolerances be set?

#7

Post by admin »

Hi,

as Brian says, if everything agrees then it all works. Or, if everything knows how to talk to other things in the right way then it all works. I can explain what SharpCap does, and how it should cope as long as other things tell the truth about their capabilities.

1) SharpCap knows that all plate solve results from Astap and Astrotortilla/Ansvr/ASPS come in J2000 co-ordinates.

2) SharpCap takes any co-ordinates used in sequencer target selection to be J2000

3) SharpCap asks the ASCOM mount what co-ordinate system it uses via ASCOM's ITelescopeV3.EquatorialSystem property (https://ascom-standards.org/Help/Platfo ... System.htm). Correct behaviour will happen in two possible cases here

a) The mount says J2000 and is telling the truth - that is all co-ordinates going via ASCOM are J2000 and the mount converts internally to JNOW before moving the telescope to JNOW equivalents

b) The mount/ASCOM driver really work in JNOW and the mount says anything *except* J2000 in response to the EquatorialSystem request.

In case B, SharpCap handles the conversion to/from JNOW as required. In case A, the mount should be doing it.

If the mount reports JNOW but actually uses J2000 or the other way around then it won't work properly (and there is a bug in the mount ASCOM driver that the manufacturers should fix).

I honestly don't know if other software like Stellarium does the same as SharpCap in trying to deal with the J2000/JNOW situation as well as possible :(

If you are interested in digging further, you can see which co-ordinate Stellarium sends by watching the mount co-ordinate values after a GOTO from stellarium - do they change to the JNOW or J2000 co-ordinates?

If you do a plate solve/sync in SharpCap then the log will tell you whether your mount claims to be JNOW or J2000 - if it is JNOW you will see info like this in the log

Code: Select all

Info   	01:39:53.135519	#1 	Mount reports JNOW, converting results position from RA=05:35:20,Dec=-05:18:33 (J2000)									in async Task<bool> SharpCap.ViewModels.PlateSolveAndResync.SolveImpl(bool gotFrame, string tmpFile, CancellationTokenSource tokenSource, IMount model, SizeF? imageSize)
Info   	01:39:53.372518	#1 	Conversion result is RA=05:36:25,Dec=-05:17:39 (JNOW)																	in async Task<bool> SharpCap.ViewModels.PlateSolveAndResync.SolveImpl(bool gotFrame, string tmpFile, CancellationTokenSource tokenSource, IMount model, SizeF? imageSize)
Debug  	01:39:53.373515	#1 	Notification (Status=OK): Plate solve succeeded, position found to be RA=05:36:25,Dec=-05:17:39 (JNOW, offset of 0,15 degrees)				in void SharpCap.UI.NotificationViewModel.DisplayMessage(NotificationMessage message)
If the mount reports J2000 then the first two lines will be missing, but you will still see the info about the notification being shown.

cheers,

Robin
GaryS
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Re: Plate Solving misses the Target - can tolerances be set?

#8

Post by GaryS »

Thank you so much for this Robin.

Before I dig into my logs( if I can find them), I can confirm that I see in the ‘green’ SC text that comes up at the top of the SC screen after a successful plate solve, a phrase that includes the final coordinates and word “JNow”. I am using ASTAP which you mention above uses J2000. So, is the mention of “JNow” somehow referencing the mount or has ASTAP been changed to JNow?

Apologies if I’m being ‘thick’ but it’s a total pain having to manually find my target when Plate solving ‘succeeds’ but the target is not on screen.
Regards
Gary
GaryS
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Re: Plate Solving misses the Target - can tolerances be set?

#9

Post by GaryS »

oopfan wrote: Sun Dec 05, 2021 8:50 am Hi Gary,

The short answer is that J2000 is just as good as JNOW, but make sure that all subsystems use the same. In other words, look at how your mount is configured and then look at the Plate Solver. They must use the same epoch.

Brian

Thank Brian. So, are you saying that if my mount uses JNow and ASTAP uses J2000, I cannot use ASTAP? … or, just that the plate solving by ASTAP will be done based on reading the mount’s JNow coordinates as though they were J2000?

It’s surprising that I’ve been doing EAA with Sharpcap and iOptron mounts for 3+ years and never ran into this issue. I guess I’ve been oblivious to the mismatch potential in how equipment and sw handle coordinates.
Regards,
Gary
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Re: Plate Solving misses the Target - can tolerances be set?

#10

Post by oopfan »

Hi Gary,

You asked:
So, are you saying that if my mount uses JNow and ASTAP uses J2000, I cannot use ASTAP?
No, not at all. You just need to dig into the setup of your mount to tell it to accept J2000 coordinates. Either that or find some way to tell ASTAP to emit JNOW coordinates.

Brian
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