Re: [APML] Polar Drift Alignment Article

[Chuck Vaughn <aa6g@aa6g.org>]

Jim,


> http://home.dc.rr.com/jimjanusz/Polar%20Alignment.htm
>
> Comments and constructive criticism welcome.

"(Caution, don’t allow the mount to drift past the meridian, as that 
will reverse the direction.)"

I do not believe this is correct. The direction of the drift when doing 
azimuth does not switch when you go past the meridian. In order for 
this to be true the drift would have to get less and less as the star 
approached the meridian, go to zero when it was on the meridian and 
then switch direction. I've drifted past the meridian countless times 
and it doesn't do that.

IMO, 15 minutes with no drift, although something good to achieve with 
a permanent mount, will eat up most of your night on a one night 
outing, at least for most people. That's why I recommend 5 minutes with 
no drift. You can shoot for 3 hours at +70 dec on 35mm if you really 
had no drift. If it requires no drift at all for 15 minutes to shoot 
4x5 then I'd recommend people stay away from large formats for one or 
two night outings.

It should be pointed out that with good seeing you can detect 2" or 
less of drift in 5 minutes and that requires a very small adjustment, 
on the order of 10". With 15 minutes of drift the smallest adjustment 
may only be a few arcseconds.

On the positive side I like your suggestion of using a star 30 degrees 
up for the altitude adjustment. Even though I have not changed my 
procedure, I do this all the time and it works fine. Actually I've 
found that the altitude adjustment doesn't have to be perfect if you 
align 5 hours from the meridian and shoot within 2-3 hours because the 
altitude contribution to field rotation becomes smaller and smaller as 
you approach the meridian. OTOH, the azimuth is very important as it 
dominates near the meridian. Fortunately seeing allows azimuth to be 
the most precise adjustment.

Chuck <aa6g@aa6g.org>

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