Re: [APML] Re:Drift Align -- some care required

[Aplanatic@aol.com]

Jack wrote:

> To do a proper polar alignment you need an object on the meridian and
>  another one on or near either east or west horizon. 

I don't know why, but I feel compelled to once more bring this up.  
The last couple of times I discussed this I don't think the message 
got through.  I would certainly appreciate it if someone with a 
mathematical and/or geometric bent took a close look at this issue.

To make a long story short, don't use stars for drift alignment 
that are close to the east or west horizon.  Refraction can have 
a significant effect on the alignment accuracy for such stars.  Since 
drift due to polar misalignment is *independent* of the declination 
of the star, you can (and should) use stars that are far from the 
horizon for north-south polar alignment.

Here's what I recommend:  Do the east-west polar alignment 
using a star close to the meridian with an elevation of at least 
30 degrees.  Refraction will have almost no effect on such a star 
because it is close to transit (not moving either higher or lower).

Do the north-south alignment using a star that is due east or west 
of Polaris and relatively close to the pole (say 20 or 30 degrees).  
This will make the effect of refraction very small.  Stay well away 
from the east or west horizon.

Here's an old post of mine that goes into more detail on the effect 
of refraction on drift alignment:

http://www.system.missouri.edu/ics/staff/andy/APML/ARCHIVES/APR99/msg00649.htm
l
  
Most of the mail I have received about this goes something like:  
Well, I've been doing it my way for XX years and it seems to work 
OK.  That's fine, of course, and I won't press it.  But you should 
at least know that there are small, but potentially noticeable 
errors induced by refraction for long exposure photography when 
you use a star close to the east or west horizon to drift align.

Dave Rowe

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