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  #21  
Old 21-01-2019, 10:08 PM
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mynameiscd (Andy)
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Location: Langkoop, Victoria
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Thanks to Ken, Paul, Chris, Andy, Martin, Karl, Darrell, Gary, Raymo, Russell, and Markus.
Been flat out at work to even get my scope out and we're in the middle of another heatwave.
You have all given me heaps of info to digest.
First to align my polescope with my mount.
Second to align my telescope with my mount. Ive noticed there are adjustment screws on my dovetail so now to work out the best way to do it.
I think if the polescope though the mount is spot on and RA and DEC are zeroed when parked, then align my scope with the same spot in the sky as the polescope ( not that easy ) I should be close.
Then do a drift and then zero and check again and adjust the dovetail.
I do like the sound of a Polemaster but i think i need to do my 'apprenticeship' and get Drift and Darv totally under my belt.
I bought a second hand Heq5 with the PEC error already set. I still wondering if i should redo the PEC as well after i get everything square and aligned?
I leave my mount in the same place and cover it and put my scope on and off. I put some marks on the dovetail so it goes in the same place each time so when i get my mount polar aligned it should be the same every time.
Thanks for all your help, its started a bit of a debate on how to do what and whats beter etc.
After reading a bit i think its down to what works for you and how much time you have etc.
Thanks Gary for pointing out that there is no real perfect alignment so thats my excuse anyway.....
Cheers all
Andy
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  #22  
Old 21-01-2019, 10:25 PM
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Stonius (Markus)
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Quote:
Originally Posted by gary View Post
Hinks addressed this very problem in a paper to the Royal Society in
1898.
Dang! So I was pipped at the post with that one by a mere 121 years. Oh well, I'll just keep working on my pet theory that energy and mass are different forms of the same thing.

It's interesting what you say about the refraction of the pole itself. I hadn't considered that, and it makes sense that those further north would have very different levels of refraction as the pole sinks towards the horizon.

I also didn't realise that king tracking was a thing of the past, more or less. I use it on my Losmandy GII, more because I liked the sound of what it did, but maybe I shouldn't bother.

Thanks for sharing your extensive knowledge.

Cheers

Markus
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  #23  
Old 11-02-2019, 12:22 PM
tvandoore (Tim)
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Fascinating stuff.

Shows how spoilt/lazy we all are with tools like sharpcap's polar align and astrotortilla.
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  #24  
Old 23-02-2019, 08:15 AM
Sunfish (Ray)
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Location: Wollongong
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I have been using drift alignment to align my mount since starting and always rely on it. Not because I am so clever but because it is all I know and can’t see the pole. But I have got pretty good and quick at it to check after I have bumped the pier plate.. Thanks to

http://canburytech.net/DriftAlign/DriftAlign_1.html

I notice when I checked this with the PhD drift tool it seemed to agree with my visual set up which I did not expect. The HeQ5 however has not enough resolution in its encoders to give as good an alignment answer as the old Argo Navis 10000 step encoders on a fork . It is all over the shop and perhaps you have pointed one of the reasons.
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  #25  
Old 23-02-2019, 11:15 AM
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RobF (Rob)
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Quote:
Originally Posted by gary View Post
Hi Raymo,

Not quite.

It is important for practitioners to appreciate that there is no such
thing as a 'perfect' polar alignment.

It does not exist and cannot be achieved.

At best, one can find an optimal compromise depending on where you
are imaging in the sky.

One problem is that for any given elevation in the sky, the amount of
“lifting” to a star caused by refraction is different when compared to a
star at some different elevation.

What’s more, as the star advances across the sky in elevation, the
amount of “lift” is continually varying.

Plus within the FOV, points in the sky that are at lower elevations are
“lifted” more than those at higher elevations. The wider the FOV,
the more the “compression” within the image.

That also means there will still be some field rotation within
the FOV on an equatorial telescope.

So for any given point in the sky you wish to image, the optimal polar
axis will be slightly different and unfortunately continually changes with time.

So the fact that an equatorial mount moves around its polar axis in a
perfect circle can, in some ways, be considered a mechanical
compromise, as the stars don’t circle the sky in perfect circles.

What's more, since the amount of “lifting” to a star caused by refraction
is different when compared to a star at some different elevation,
that also means the tracking rate will continually vary.

So in the case of a long drift test, one is simply averaging the misalignment
error in the part of the sky the test was performed.

The result will differ at some different part of the sky.

Best Regards

Gary Kopff
Mt Kuring-Gai NSW

I wonder about the top end mount users that wax lyrical about "10 mins unguided with my SuperBrand XYZ with triple axis encoders". Shirley they must be pointing in an area of the sky where their current PA has "lucky" correction for refraction then?
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