Polar Alignment - ASiair vs SharpCap

[pmrid]

I have just done a comparison between the results from a PoleMaster (with SharpCap) and ASiair Plus.

What led me to this was noticing that after a 4 hour imaging run last night (having done an ASiair PA at the start), when the scope returned to the CWD home position, it was actually pointing a noticeable amount to the left(east).

Trying to figure out the reason was not easy. For me anyway.

I was using a SharpStar 94EDPH scope with a .8 reducer/flattener and an ASI120MM Mini guider on a 120mm ZWO scope - all sitting on a HEQ5 Pro.

Throughout last night’s session the guiding was sub-arcsec according to ASiair.

So tonight, I’ve started with a PoleMaster SharpCap PA. It told me that the scope was pointing a good 8 arc-minutes low. So I decided to adjust the height accordingly and followed with an ASiair PA which told me I was out the other way.

So the answer seems to me to be that there is a non-orthogonal error and the scope, sitting in the mount’s saddle needs to be shimmed up a bit.

Any other ideas anyone?

[joshman]

could be one program is not accounting for atmospheric refraction?

[pmrid]

Good point. I’ll check.

[Startrek]

Home position is a function of the mount ( internal electronics and positional encoders)not the software. Home position is the reference starting point for Goto alignment. Even if your are polar aligned with a poor error say 15 arc minutes , the mount should return exactly to the starting Home Position. The only reason why it wouldn’t is maybe the Dec or Alt clutch have slipped slightly during Goto or have been over tightened.
If your Home position is set correctly ie tripod / mount level , Altitude set to local Latitude, counterweight shaft pointing down front tripod leg facing true south and orientation of tripod central axis aligned to true south as best you can get it ( I use solar noon shadow method ) then the mount should return to Home Position no matter what software you use.
In regard to ASI air , I don’t use these toys. From what I’ve seen and read ZWO massage proprietary software like PHD2 to make it function with the Air, doesn’t sound like a reliable system
Ive been a member of the ZWO forum for years and the forum is constantly bombarded with posts about problems with the Air , some cockpit errors mostly device errors
I use EQMOD and Stellarium for Goto and Nav, it’s been rock solid for 6 years
I’ve had varying polar alignment values from 1arc min to 7 arc mins ( it can change night but night due to atmospheric seeing ) but all my mounts return exactly to the Home Position each time for donkeys years.If your using PHD2 with the Air then good luck, I just watched a great presentation by Bruce Waddington (PHD2 support developer) regarding PHD2 Calibration and he wanted nothing to with the Air.

If your getting sub arc sec guiding are the Dec values close to the Ra values, if they are vastly different then on long duration imaging sessions say 3 to 5 hours you will experience a bit of rotation or image shift to one side or the other. In any case your mount if set in the Home Position correctly should return to that position irrespective of guiding performance
To me it seems like either a minor intermediate or spurious mechanical issue in the mount or the Air ?

Good luck with it and hope other folk much smarter than me chime in and offer advice

Clear Skies