setting up for polar alignment.

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setting up for polar alignment.

Post by dean_kos » Thu Aug 16, 2012 6:48 pm

May I suggest the possibility of a workshop on polar alignment, its the only thing I struggle with and never seem to get rite !! No matter how long I take to set the mount up or how presise I think I am I still get considerable star trails even at 1 min exposures or anything above 30 seconds to be honest and its grinding me down now !! I'm not sure if it's the mount that has a flaw somewhere, i.e. polar scope not calibrated properly or something similar or if it's just me ! but, whatever the problem, its driving me insane !!! I feel that my images would improve dramatically if this was sorted so some sort of a practical session on this would be gratefully accepted.....
All this said, am I trying to be to ambitious on a portable setup? I do appreciate that I would be a lot better off with a fixed pier or observatory, but not quite ready to build this yet, hopefully soon. Any suggestions ??

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Re: setting up for polar alignment.

Post by rwilkinson » Thu Aug 16, 2012 7:31 pm

Well, Carl and I did actually cover this in the first two tutorials this summer, but we could certainly revisit it in the next series.

In the meantime it's worth re-checking the alignment of your polar-scope (by pointing it down at a distant terrestrial object, and ensuring that it stays dead-centre in the cross-hairs as you rotate the RA axis through a complete circle).

However the surest (but not the quickest) polar alignment method is the drift technique - this is the most concise explanation I've seen: ... ift#drift
(you may need to navigate to TECHNIQUES.. Polar Alignment.. Drift Technique from the side-frame).
I've never used this for my portable system, but those with fixed piers will probably have spent an entire evening refining it as far as they can.

But even if the polar axis is spot-on, there will still be mechanical inaccuracies in the tolerances of the gears in your drive system, which will cause back-and-forth trailing (periodic errors).
The best way to eliminate all these errors is an auto-guiding system - ask Dave or Gerald, or have a look around the StarGazers' Lounge area down at Kelling Heath.
However (in my opinion) this is great if you have an observatory, but OTT for a rapid-deployment portable system like mine - so I'm limited to sub-minute expsoures (depending on my focal length).


Re: setting up for polar alignment.

Post by dean_kos » Thu Aug 16, 2012 8:04 pm

Yes I know this was covered, several times, in depth and I do understand fully how it all works and how to set it up, alas, after following all the procedures pointed out I still seem to be miles away ...... I feel that a practical session where we all get 'hands on' with the mounts could actually tell me whats wrong, be it me or the mount and how to rectify it would help me immensley, I have a sneaking suspicion that its the polar scope that needs calibrating i.e.the cross is not staying perfectly central when the head is turned but help in sorting this would be very gratefully recieved as I have tried and failed several times. Also a general 'once over' of the mount. an m.o.t. if you like wouldnt go a miss by one of our 'experts'.
One idea I have played with is asking brian to manufacture a small ring that would hold the polar scope reticule perfectly central whithin the tube as apposed to adjusting with the three grub screws, maybe this would be a fool proof way of knowing the polar scope was dead center, what do you think ?


Re: setting up for polar alignment.

Post by imartin » Thu Aug 16, 2012 8:19 pm

Hi Dean,

Once you have callibrated the polar scope you should not need to perform the operation again - unless you remove it or severely knock it.
The thing that surprised me most about callibrating the polar scope is how small an adjustment of each of the three alignment screws needs to be made. In some cases it seems as if you are not moving the screw at all. Sometimes these really small adjustments can be achieved by just tightening your grip on the allen key.

I am more than happy to pop down to yours and assist in collimating your polar scope. I think you have my number or just reply on here.



Re: setting up for polar alignment.

Post by dean_kos » Thu Aug 16, 2012 8:44 pm

Hiya Ian, yeah that would be great thankyou, my 'sausage fingers' dont really do small adjustments very well. seen as you have done this before you must be more qualified than me to get it somewhere near. I will drop you a text or email, thanks again.

To add to this, I am going to try and master the drift method as Ross mentioned earlier as this seems to be a bit more imperfection friendly and will make a good back up method when all else fails. The link he posted above explains this really well and it does seem to be whithin the realms of my capabilities :?

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