Re: [APML]OT-Drift Alignment

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

Hi Don,

Did you account for film (detector) size in your calculations?  FR can be
more noticeable on larger formats.

Jim

----- Original Message ----- 
From: "westergren" <westergren@netzero.net>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Tuesday, November 25, 2003 8:00 PM
Subject: Re: [APML]OT-Drift Alignment


> Serge,
>
> There is no one formula that calculates the error directly, but
Covington's
> book "Astrophotography for the Amateur" has the formulas for Dec Drift and
> Field Rotation in Appendix B.  Using these, I have calculated that the
> allowable drift during polar alignment is 0.4 arc sec in 10 minutes for a
> scope with fl=530mm.  Longer focal lengths can tolerate more polar drift,
> such as 1.5 arc sec per 10 minutes for fl=1620mm.  You can ratio your
> allowable drift for your focal length.  The criteria I used to calculate
> these values are the worst case field rotation remaining under 1 arc sec.
> That 1 arc sec FR would distort a circular 3 arc sec diameter star into a
> 2x3 or 3x4 arc sec oval and be barely detectable.  Larger, brighter stars
> will show a lot less effect..
>
> Don
>
> ----- Original Message -----
> From: "Serge Theberge" <serge.theberge@wti.on.ca>
> To: "'Discussion of Film Astrophotography'" <astro-photo@seds.org>
> Sent: Tuesday, November 25, 2003 05:01
> Subject: RE: [APML]OT-Drift Alignment
>
>
> > I have been told that for astrophotography, not only you must have a
good,
> > sturdy mount, but you must have good polar alignment.  I have seen
various
> > rules of thumb about what is "good enough" polar alignment such as no
> > visible drift @ 200X for 5 minutes, others state 15 minutes, etc.  My
> guess
> > is that there must be some kind of formula (or rule of thumb) linking
> > "maximum acceptable polar alignment error" to f-speed, magnification,
film
> > grain size, exposure duration, etc.  Can anyone enlighten me on this?
> >
> > Thanks
> >
> > Serge Theberge
> > Toronto Centre
> > RASC
> >
>
>
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