A question about guidescope alignment for PA routines

[dannybgoode]

Up till now I’ve been doing a quick dirty PA just using the scope and my guided subs have proven to be good to at least 5 minutes PHD2 shows guiding to be within about 1”. 
 

However since I have SharpCap I thought I may as well give it a try for its PA routine but that got me thinking.

 The PoleMaster makes sense to me - it sits in the exit hole of the PA alignment scope so I can see how that the aliens the mount. 
 

With a guidescope however does the alignment of the scope itself matter at all? Mine isn’t particularly aligned with my main scope as it doesn’t particularly need to be - I don’t use it for finding, only guiding. It’s pointing roughly in the same direction of course but it’s not nailed on and even if it were then the main scope might be out compared to the mount itself. 
 

So my question is - does it matter or will the alignment routine still align the mount itself rather than the guidescope? 

[vlaiv]

I don't think it particularly matters, but can't be sure.

Probably depends on method of PA alignment used. Method exists that does not require precise alignment of scope to RA axis - it only examines field rotation in a given scope - after all its about where scope is pointing along the rotation of RA axis and what is expected if RA axis is properly aligned to NCP.

On the other hand, different polar alignment routine requires alignment of optics with RA axis - same as with PoleMaster and such - these observe single field around Polaris and how it rotates when the RA rotates.

[Ouroboros]

This question came up a week or so back. The consensus of opinion was that it didn't matter. 

I use the polar alignment routine in KStars/EKOS using the guidescope. I normally do an approximate polar alignment first using the polar scope and I just make sure the guidescope can see the pole star before starting the PA alignment routine.  

 

[dannybgoode]

Thanks guys and apologies-must have missed it first time round. 
 

I’ll give the SharpCap routine a whirl then and see where I get to   

[GraemeH]

The way that the SharpCap polar alignment works is by finding the centre of rotation between 2 platesolved images in terms of RA/Dec coordinates after moving only the RA axis. In theory, there is no restriction on where the scope points - it's just that I believe SharpCap only has solving indexes for near the polar regions. There is definitely no requirement for your guide scope to be aligned either with the polar axis of the mount, or with the main scope. 

[dannybgoode]

5 minutes ago, GraemeH said:

The way that the SharpCap polar alignment works is by finding the centre of rotation between 2 platesolved images in terms of RA/Dec coordinates after moving only the RA axis. In theory, there is no restriction on where the scope points - it's just that I believe SharpCap only has solving indexes for near the polar regions. There is definitely no requirement for your guide scope to be aligned either with the polar axis of the mount, or with the main scope. 

Excellent, thanks Graeme - that makes perfect sense and given that it works like this means that PA at new locations should be relatively trivial so have one less excuse to load the car up and get to if not a dark site, certainly a better one compared to my back garden

[Ouroboros]

1 hour ago, dannybgoode said:

Thanks guys and apologies-must have missed it first time round. 
 

I’ll give the SharpCap routine a whirl then and see where I get to   

No problem. Topics very often come up again. 🙂 

I was thinking your 1" polar alignment accuracy was very good for "quick and dirty".  I suppose it depends on how quick and how dirty. But I usually find it's more like some fraction of a minute of arc when aligning by eye and polar scope.  

[dannybgoode]

34 minutes ago, Ouroboros said:

No problem. Topics very often come up again. 🙂 

I was thinking your 1" polar alignment accuracy was very good for "quick and dirty".  I suppose it depends on how quick and how dirty. But I usually find it's more like some fraction of a minute of arc when aligning by eye and polar scope.  

I could of course be reading the graph wrong - I am new to all this guiding malarkey!  Still perfect stars on 5 minute subs so must be guiding pretty well.  As for quick and dirty - if I am spending more than a couple of minutes trying to see through the damned PA scope then that's too long as far as I am concerned!

[Ouroboros]

@dannybgoode   Yes .... visual PA is a pain in the neck - literally! 🙂  Your might find your guiding error is a tad better after PA with SharpCap and probably more stable. 

Edited November 23, 2019 by Ouroboros