Skywatcher 12" Dob with Goto .. 2 star alignment accuracy VERY poor.

[Starpaws]

OK. We have the Synscan goto, and the 2 star alignment is not working, in the sense that the accuracy is VERY poor. The scope is brand new, only a few days old. It had this problem right from the day 1.

To cut a very very long story as short as I can make it ...

I complete the 2 star alignment up to where I enter the name of the second star. It tells me where it thinks the second star is, and it goes there.

But it is out by several degrees.

Using the controller, I manually point it to the second star and tell it what it is looking at. It tells me Alignment Complete and then says CAUTION..... THE PREVIOUS VALUE FOR NPE IS APPLIED ....

But ... it is not working. As far as I can tell, it is insisting that the second star is where it thought it was in the first place, and not where I tell it it is. All of the pointing from there on is based on where it thinks the second star is.

Maybe an example.

I find Arcturus as the first star.
I tell it Alpha Crucis is the second star. It goes say 4 degrees East of it. I manually move it to Alpha Crucis and it says it is aligned.

If I tell it to find Arcturus, it goes reasonably close to it.
If I tell if to find Alpha Crucis, it goes to where it initially thought is was, 4 degrees East. So it is as if it hasnt accepted my manual movement

If I tell it to find say Antares, roughly half way between them, then it goes (somewhat predictably) 2 degrees to the east.

So the star sphere as it THINKS it is compared to what it really is, seems to be rotated by several degrees around where my first alignment star was.

If I tell it to go to Arcturus, the first star, it goes very very close to it.

The software version on the controller is 4.39.04

[Rainmaker]

Have you set the handset in Alt Az mode?
Have you set the time and location correctly including DST-off

If all these are correct then: instead of choosing '2 star alignment' and pressing Enter, continue to scroll to 'Cone Error' and zero all digits, then press Enter, then continue to NP error and again zero the digits, press Enter again. Then scroll to 2star alignment and the NPE message should not come up again....

Selecting alignment stars separated by at least 90 degrees in azimuth and about 30 degrees in altitude works best with Synscan. Centre them well using a reticle eyepiece if you have one, if you don't have a reticle then defocus till the star image fills about a third of the FOV, it will be easier to centre it.

[Malcolm]

The mount must be accurately leveled. If it's not level, the alignment will be off. Fit leveling feet to the ground board. First fit three 19mm hardwood blocks to accept the threads of the leveling feet, which you can get from Bunnings. If the mount is leveled, the second star is always found very close to centre, at least it is for me, and tracking all night is very good on multiple targets.

Next, ditch the handset and use a WiFi dongle and the SynScan app. After a two star alignment is complete connect SkySafari Pro and navigate direct from the star map. The handset is a PITA.

[Rainmaker]

The NPE is 'Non-Perpendicularity Error', to do with the alt and azimuth not operating perpendicular to each other. Sometimes this error is induced when the mount base is set on a slope, it is not the slope itself that is the problem, rather the fact that it can cause the OTA to be not seated correctly..... usually these errors are very small.....

[Startrek]

Checklist

Az Alt Mode ( not EQ )
Ensure scope is perfectly level on all 4 sides of base
Set scope horizontally level in home position ( facing true south or true north )
Ensure Longitude and Latitude are correct
Ensure time is correct ( Daylight saving No )
Ensure time zone is correct
Ensure star filter is off for alignment stars in handcontroller
Use an illuminated reticle eye piece for centering alignment stars ( eg Orion 20mm 70 deg centering eye piece )

Note: My second alignment Star is always slightly off but mostly in the FOV of the centering eye piece

Cheers

[Rainmaker]

It (4.39.04) will continue to apply the previous NPE value until cleared...

P.S. You can also do a 'Factory Reset' which will clear the CE and NPE error data.... for a fresh start :thumbsup:

[Allan_L]

What Martin said + (I use a 12mm Orion illuminated reticle ep)

Also I used a great little App called SynScanInit which gives you all the info to type into the handset during initialising in the correct order. great!

BTW: the Levelling is very important.
Instead of attaching levelling feet to the base of the mount, I used to make a levelling platform. So you could easily level the platform before sitting the mount onto it.
Also benefit is that it keeps the base of the mount out of the wet grass.

I used "form ply" from Bunnings. (as below)cheap and weather resistant
Cut it into an appropriate triangle shape.
Cut a round hole in the middle to accommodate the central bolt on the base and holes for the mount legs and have levelling feet from Bunnings at each of the three corners.
Works for me. And leaves the mount unaltered.

Disclaimer:
I do have a financial interest in Bunnings, in that my wife works there. ;)

[Allan_L]

Rules from Skywatcher for choosing alignment stars:

1. Same side of meridian (N S line)

2. Altitude difference: Between 15 and 60 degrees - aim for about 30

3.Azimuth difference of 45 to 135 degrees - Closer to 90 degrees is best.

4. approach the second alignment star using the same approach as a normal goto (I think Right and Up is default but it is modifiable)

Good Luck.
If problem persists there may be a fault. It has been reported before on IIS.

[gaseous]

On top of everything else mentioned, make sure the two stars you choose for your alignment stars aren't more than about 60° apart in azimuth. I'd been using Arcturus and Rigel Kentaurus recently, and the alignment and subsequent go-to/tracking was pretty ordinary. Last night I used Arcturus and Antares (which are about 68° apart), and the go-to and tracking was excellent. The Skywatcher documentation has a recommended min-max altitude and azimuth distance separation for your alignment stars - at the moment, Arcturus and Alpha Crucis are virtually on opposite sides of the sky, which probably isn't helping your efforts.

[gaseous]

Sorry, didn't see Allan's post :D

[Startrek]

I use both EQ mode for my 6” and 8” imaging newts and AzAlt mode for my 12” Goto visual newt and never bothered finishing the centering using the Up and Right buttons on the handcontroller. For alignment I just centre the Star using any button and my alignments are fine.Finishing with the Up and Right button centering I think was to reduce backlash in EQ mode but it really doesn’t matter in Az Alt mode