Re: Precise polar alignment of the azimuth axis

Steven Panish

10 min. unguided at 2800mm FL is pretty darn good!  Before I used SharpCap's method, I tried various manual drift techniques and only occasionally got it that good.  (C11??)  Now I understand that refraction at different altitudes is an issue - perhaps that was why the PA was never sufficiently reliable for longer unguided exposures.  Additionally my permanent pier is 10" iron pipe and quite tall, so thermally induced dimensional changes are likely an issue.

But although I have not done a new PA since using ConeSharp a few months ago, I earlier found all the procedures to give different results.  Not 4 minutes though.  That's pretty big.  Even when using the SharpCap procedure, redoing it after the 90deg rotation yields a difference  of typically 1minute.  So I split the difference of the two positions and am off about .5min on both.  It is a very quick method, which makes me prefer it over drift. 

Periodically I run the PHD2 guiding assistant, which gives a measure of PA.  It seems very sensitive to sky location, and also varies widely over time, not necessarily settling reliably.  Odd.  After letting it chug along for about 15 minutes it told me the error was 2'.  Sigh.  And with the drift techniques, including semi-automated ones such as in PemPro and PHD2, I found too much interaction when alternating between the Alt and Az settings.  I'd get it perfect on one, do the other, and going back to the first it would be off and/or would never improve with iterations.  That might be better now that the orthogonality is much better.  

At any rate, I gave up on unguided, it's too difficult for a relative newbie like me running at C11 and C9.25 focal lengths.  And I'm doing a lot of 20 min exposures on Sh2 objects.  PHD2 Guiding makes it easy, so I'm just sticking with that until there is some reason not to.  Now my gripe is that I can't get rid of the field curvature with the reducer/correctors.  Well, it's all a learning experience!


On Sun, Dec 27, 2020 at 1:07 PM John <obee11@...> wrote:
Aloha Steven and Deonb,

  Yes, I have been using the Sharpcap polar align tool and really like it.  And I understand what all is going on now with the az and alt (ie 2 star)
drift alignment.  I'm going to try the PHD2 drift align next and then try another drift align with just a camera (no other software).  And then compare
these various techniques (as what Deonb is doing)...

  I'm kind of curious to see how PHD2 and SharpCap compare (as to what each one is reporting in the way of polar align error).  And then try Roland's method.
I'm sure either method is fine, especially if a little modeling is thrown in afterwards.  Just curious at trying a few different techniques more than anything else.  And I
still need to look into that SharpCap cone error business!

  I was able to get 8 mins unguided exposures the other night in the Pleiades region and ran out of time to try 10 mins.  I did get 10 mins on either Deneb or Sadr,
(on a different night) but you could tell some oblong stars were creeping in to the mix.  One 12 min exposure showed definite oblong stars.  But all without modeling!


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