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Old 22-08-2019, 10:05 AM
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Merlin66 (Ken)
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EQMod initial GOTO constant error?

I've raised this question a couple of time on the EQmod group...
Chris has given me some assistance with the use of the alignment interface..but need some better weather to investigate and trial.

Just wondering if other have had the same experience.

Observatory mounted NEQ6, very good PA alignment, CdC and using a 60mm electronic finder.
Upon unparking, start sidereal tracking, select target star (CdC) and then initiate GOTO...every time on this first GOTO the centre of the FOV is about 2 degrees away from the target star but ALWAYS in the same direction (and ALWAYS by the same amount.)
By moving the scope and re-syncing I can usually carry on....
After parking and re-starting the same "error" appears....

You'd think if the initial park position is good, and the modelling of the number of pulses/ arc min was good then the GOTO should not behave like this??????
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Old 22-08-2019, 10:24 AM
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The_bluester (Paul)
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I was finding something like that when I was using APT as my capture software, a blind plate solve and sync resolved it each time but I found I needed to build a pointing model. Using sequence generator I have found the initial goto to be a lot more accurate but I have no idea why. I park the mount in the same way as I used to. The only difference is that SGP uses a different sync model in EQMod where is syncs to the curren location each time rather than building a pointing model, but by my understanding it should not make any difference to the next start up.

Do you move the mount at all manually while it is powered off? I assume you park it via EQMod before shutdown.

I might try an experiment next time and move my mount well off the normal park position before powering it up and see where it parks to after it has been synchronised. If it returns to the offset park position and is close to the target before sync the next time you may be able to just offset your park position a little. It depends on if parking the mount is returning it to the same point in the sky or the same point on the worm, the PEC routine suggests it might be the same point on the worm as that demands the mount be parked before switching off but it can be moved by releasing the clutches later without upsetting the PEC.
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Old 22-08-2019, 11:43 AM
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Merlin66 (Ken)
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Paul,
The mount is parked and unparked to EXACTLY the same point in EQMOD.
Part of my problem is understanding why movements should be so inaccurate.
X pulses in RA = Y arc min in RA.
Why should the number of required “pulses” differ??
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Old 22-08-2019, 12:28 PM
Startrek (Martin)
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I use EQMOD with StellariumScope and Stellarium
I don’t have a permanent pier but I leave my scope out for 2 weeks at a time
Each time I initiate the Goto to my first star it’s always out by a decent amount. Once that first star centred and sync’d , the next target is usually very close and so. If I triangulate the target with sync’d stars it’s perfectly centred
My mount always goes back to the same parked position, although the icon on Stellarium seems to end up 5mm past the SCP mark
On each power up I have go through the same procedure, in my case the pointing model is never saved
Could your issue be CdC ?
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Old 22-08-2019, 01:05 PM
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Merlin66 (Ken)
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Martin,
As you've said:
""Each time I initiate the Goto to my first star it’s always out by a decent amount. ""

Why??????

I don't think it's anything to do with the planetarium software used....
Something in the movement modelling???
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Old 22-08-2019, 02:11 PM
Startrek (Martin)
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Ken
I have the EQMOD project manual and just read pages 55 to 62 on Alignment
Basically the first Goto Star is a calibration point for the pointing model so once it is sync’d the Algorithms designed into the software have more control over pointing as more calibration points are sync’d
The two co ordinate transformation Algorithms, nearest point and 3 point plus nearest improve accuracy once your past the first calibration point or sync’d point , and therefore building a data based pointing model across the night sky through your planetarium
The manual describes centering your first alignment Star in a few areas in this Alignment section so I guess they didn’t design sophisticated Algorithms to slew precisely to your first star plus there are many factors that could affect the accuracy like setting your home position perfectly ( near impossible) , levelling your mount perfectly ( near impossible ) , setting perfect latitude ( near impossible ) and others
So I guess we will have to grin and bear it . I’ve been centering my first alignment Star since day one , so it doesn’t really bother me at all
Hope the above helps and makes some sense
Cheers
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Old 22-08-2019, 02:24 PM
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Martin,
Yes I've seen all the stuff in the manual.....
I'm working my way through "Alignment Interface", "Append on sync" v's "Dialog" and auto loading the model upon unpark - doesn't seem to make any difference so far, but in fairness to Chris S. more tests to be done.

""many factors that could affect the accuracy like setting your home position perfectly ( near impossible) , levelling your mount perfectly ( near impossible ) , setting perfect latitude ( near impossible ) and others ""

I would have thought using a well aligned observatory mounted system, using excellent parking accuracy that an initial GOTO, (forgetting the refinement of "co ordinate transformation Algorithms") would have been able to achieve better than the constant 2 degree difference I see every time.

Maybe I expect tooo much - but would like to understand the drive mechanical limitations.
(Shift Offsets (DxSA and DxSB) have been mentioned as a possible avenue to follow....)
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Old 22-08-2019, 02:36 PM
Startrek (Martin)
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Yeah agreed
Although it’s been around for a while , EQMOD is still a great astro tool to use and simplistic in its operation
It’s also open sourced ( Free ) to us amateurs which is helpful
Cheers
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Old 22-08-2019, 02:41 PM
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Merlin66 (Ken)
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I'd definitely second that and highly recommend EQMod to all amateurs with suitable SW mounts.
Chris S and his team provide outstanding support.
(They modified the guiding models to include support for our spectroscopy slit guiding and added AirMass to the position screen at my request! You can't ask for much more than that.)
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Old 22-08-2019, 07:01 PM
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I know you are doing spectroscopy but have you got any prospect of a direct camera attachment to plate solve and compare the parked position to what EQMOD is reporting?
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Old 22-08-2019, 09:04 PM
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billdan (Bill)
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EQMOD isn't outputting Jnow co-ordinates instead of J2000 by any chance?

You could you mask the problem by slightly modifying your computer clock.

If sidereal moves at 4° per minute in RA and your error is 2° in the same direction (E or W), you could fast forward or slow down your EQMOD time by 30 secs and catch up or reverse that 2° error, wouldn't that fix it?

If the error is in DEC you could modify your latitude setting to compensate.

Last edited by billdan; 22-08-2019 at 11:00 PM.
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Old 23-08-2019, 09:29 AM
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Merlin66 (Ken)
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Bill,
It's not a J2000/ J now type problem. The offset (which I'm about to accurately measure) is in both RA and Dec.
I think the EQMod DxSA and DxSB settings should (!!??) provide a shift offset which may help....we'll see.

Paul,
The FOV of the spectroscope slit plate is only 9.8 x 7.4 arc min and double images caused by internal reflection from the plate make it unsuitable for plate solving.
I have used the 60mm electronic finder (105 x 80 arc min) on some difficult targets to plate solve the field and then superimpose the solved image on CdC (using UCAC4) to positively identify faint targets .

The custom park position is set by the motor counters (in Alt Az) rather than RA/ Dec but appears to give the same numbers each time.....

Last edited by Merlin66; 23-08-2019 at 09:31 AM. Reason: added info
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Old 23-08-2019, 09:59 AM
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The_bluester (Paul)
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It would be interesting to see if the EQMOD reported park position agreed with a plate solved position.
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Old 23-08-2019, 10:33 AM
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Merlin66 (Ken)
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Paul,
Thinking about it...my custom park position leaves the OTA horizontal within the observatory - below the walls.
No sky visible to take a plate solve image....
If pain persists, I could set up another park position to try the plate solve just another PITA!
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Old 23-08-2019, 12:20 PM
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Yeah, that makes it harder. The whole point obviously being to try to see if the parked position agrees with what EQMOD reports it as.
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