Re: [APML] Polar Drift Alignment Article

["Jim Janusz" <jjanusz@dc.rr.com>]

Hi Chuck, thanks for your comments.  Please see below for my comments or
questions.  Also, if you would like, since I used a good bit of your work,
feel free to use any of this document on your web site with my permission.


>
> "(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.

I can't comment on this based on the math but I have had the occasion of
drifting past the meridian.  It seemed to me that when this happened all of
my adjustments went haywire, adjustments made things worse.  I had assumed
that the reason was reversal of the direction of corrections, as if I were
now drifting on a star west of the meridian.  Any ideas why the drift would
change?  If I get a chance I will experiment and confirm or refute my
findings.

> 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.

I have been working from experience on this point.  Brad Wallace recommends
5 to 15 minutes here: http://voltaire.csun.edu/polar.html

In AMOCP he recommends  "When a telescope is aligned for long-exposure
astrophotography a guide star should drift less than 10 arc sec during a 2
hour exposure (and preferably less than 5 arc sec.)"

I shoot 6x7 and 4x5 and need the 15 minutes.  I have learned to acquire this
level of accuracy in less than an hour.  I'm usually finished drifting
before the end of astronomical twilight.  I guess everyone should determine
what works for them.  Obviously your excellent work confirms your findings.
Although I think you are now permanently mounted, right?  Lucky dog! (G)

>
> 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.

Agreed......

>
> 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.

Agreed and thanks, this concept came from an APML discussion some time ago.

Jim.


>
> Chuck <aa6g@aa6g.org>
>
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