GoTo Mount azimuth error

[hefyjefy]

I am trying to get a really good alignment, the mount is the Orion Skview Pro Equatorial. Scope is ED80.  I have pretty much tied down the polar alignment, also checked the cone error and everything seems good.  After doing several 3 star alignments however the reported azimuth error is always more than 1 degree.  (Elevation error looks OK at about 18secs)

I am not ideally situated here due to buildings etc so stars low on the eastern and western horizons are difficult.  Could this be the problem?  Or could it be a problem with the mount? 

Thanks for your thoughts,

Geoff

[jambouk]

I don’t know the mount.

How good is the Subsequent GOTO when you use it?

Are you using a high power eyepiece to centre the alignment stars?

I suspect this error is a calculated error in your PA, based on the relationship of the three stars in your star alignment, and is open to a number of errors itself when the handset calculates it. I would try just doing a two star alignment with both stars on the same side of the meridian and se what the reported error is then.

[hefyjefy]

I am using a reticle eyepiece 12.5mm, scope is (ED80T CF 480mm)

The subsequent GOTO is poor (sometimes outside the view of the low power eyepiece) and that's the problem.  Since the object I am looking for is not visible (by me) either in binoculars or the scope I need to be sure I am pointing in the right direction before taking images. 

OK thanks, I will try the 2 star on the same side of the meridian.

Geoff

[johninderby]

Bit of info on the mount.

The Orion SkyView Pro mount:

This robust mount for serious amateur astronomers can support OTAs of up to 9kg. It sets a new standard in precision and is also excellent value for money. With a total weight (including counterweights) of 15 kg, the mount remains portable but at the same time it is also very sturdy.

[jambouk]

I would definately try using two stars on the same side of the meridian and see how that goes. Also make sure you end you alignment steps with the same two direction keys on the keypad, so up and right say, to the insides are always meshed in the same way. If you want to tighten things even more, once you've centred the star with the 12mm, put a 2x Barlow in and re-centre at 6mm before pressening enter. The other thing is to make sure the Polar Alignment is ball park OK before you start - this isn't normally an issue with star alignment errors, but if you are totally off (RA axis pointing south) it won't help.

James

[hefyjefy]

OK folks, thank you!  The clouds moved away, I tried a two star alignment and got an immediate improvement.  I spent some time on the polar alignment first.  The pointing and tracking are both much improved.  I guess I don't really understand how the alignment algorithm works, you would assume that 3 stars would give a better result than 2 but that seems not to be the case?

[jambouk]

I think as you get slicker, and more used to the processes it all becomes much easier and probably more accurate. There is also the issue of "luck" - sometimes it just dones't work and other times it does, with no obvious differences. You may well find that the next time you do a 3 star alignment the GOTO is good.

For me, redcing the amount of cone error I had allowed me to go back to a three star alignment with good pointing accuracy. The book says the three star routine is the best for compensarting for cone error, but maybe my cone error was too great even for that. But if you've found a process that works, stick with it. You must remember though, that when you undertake a two star alignment on one sid eof teh meridian, the subsequent GOTO accuracy on the other side of the meridian may be less than perfect.

Pleased you have it sorted.

James

 

[hefyjefy]

So this is starting to make some sense.  I recently discovered that the apparent backlash on the mount is different depending on which side of the Meridian I am on.  Clearly something related to the mount - I am investigating...

Thanks again!

Geoff