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RE: Re: [APML]OT-Drift Alignment
Steve,
That's a very interesting equation as it quantifes the impact of polar
alignment error on the star trail length on the photograph for guided
photography! I'll have a look at H. Nook's article next time I go to U of
Toronto.
Regards,
Serge Theberge
Toronto Centre
RASC
> -----Original Message-----
> From: astro-photo-bounces@seds.org
> [mailto:astro-photo-bounces@seds.org] On Behalf Of Steve Walters
> Sent: Wednesday, November 26, 2003 11:27 AM
> To: Discussion of Film Astrophotography
> Subject: Re: Re: [APML]OT-Drift Alignment
>
>
> Here's an equation for the accuracy required for polar alignment:
>
> E = 750 * S * cos (D) / ( T * F * A)
>
> Where:
>
> S = worst case length of trailing allowed in microns
> D = declination of the object being imaged in degrees
> T = time duration of the exposure time in minutes
> F = focal length of the system in mm
> A = sky angle between the guide point and furthest part of
> image in degrees E = required accuracy of polar alignment in degrees
>
> A second useful equation is:
>
> R = 0.262 * E
>
> where:
>
> R = Drift rate of a star in arcseconds per minute (without
> guiding) E = polar alignment error in degrees
>
> The first equation lets you determine how close you must be
> when drift aligning and the second lets you know if you've
> achieved that level of accuracy. Using my STV, I simply set
> the dec axis agressiveness to zero and use a timer to figure
> the approximate drift rate. The STV lets you detect drift
> very rapidly and also measure it since it reads out in
> arcseconds. This can be done with a reticle eyepiece but it
> takes longer and is less accurate.
>
> There is an excellent paper by R. N. Hook in the British
> Astronomical Association Journal 2/89 from which I derived
> these equations. Hook presented results for 30 micron trails
> but this was easily generalized.
>
> Steve...
>
>
> ----- Original Message -----
> From: "Franklin A. Holub" <lists@my-spot.com>
> To: <astro-photo@seds.org>
> Sent: Wednesday, November 26, 2003 10:32 AM
> Subject: RE: Re: [APML]OT-Drift Alignment
>
>
> > I remember seeing a java based web page that calculated the
> effect of
> polar axis misalignment. It's on Michael Covington's website...
> >
> > http://www.covingtoninnovations.com/astro/Polar.html
> >
> > Frank
> >
> >
> >
> > ----- Original Message -----
> > Serge, I probably should have said "tongue slightly in cheek". I
> > saw
> some
> > really clear and expert replies - absolutely depends on a lot of
> > factors like film format, focal length, direction in the sky your
> > target will be
> and
> > exposure length. For what it is worth, Rob Gendler said in
> one of his
> > S&T articles that he only aligns using his polar scope and
> keeps his
> > exposures to 10 minutes. I don't think anyone will argue with his
> > results.
> >
> > My observatory mount is very carefully drift aligned for
> sure. My SP
> > mount is very difficult to adjust because the center bolt
> one needs to
> > loosen to adjust azimuth causes the mount to slump in
> altitude. Very
> > annoying so I
> use
> > the polar scope which my good fortune makes very accurate.
> >
> > Stuart
> > ----- Original Message -----
> > From: "Serge Theberge"
> > To: "'Discussion of Film Astrophotography'"
> > Sent: Tuesday, November 25, 2003 9:30 PM
> > Subject: RE: [APML]OT-Drift Alignment
> >
> >
> > > Hi Stuart,
> > >
> > > I agree that boredom does put an upper limit to how much time you
> > > spend
> on
> > > alignment! However, I would think that there is a fairly big
> > > difference between "enough alignment" for a 5 minute vs a
> 45 minute
> > > photo exposure
> > for
> > > example.
> > >
> > > Serge
> > >
> > > > -----Original Message-----
> > > > From: astro-photo-bounces@seds.org
> > > > [mailto:astro-photo-bounces@seds.org] On Behalf Of Stuart Heggie
> > > > Sent: Tuesday, November 25, 2003 8:09 PM
> > > > To: Discussion of Film Astrophotography
> > > > Subject: Re: [APML]OT-Drift Alignment
> > > >
> > > >
> > > > Serge - knowing you'll get some very expert opinion I'll offer
> > > > this - drift align till you can't stand the boredom. My
> attention
> > > > span is not that long. I polar align my SuperPolaris mount with
> > > > the polar bore scope and run with it but guide
> manually. I do end
> > > > up making some adjustments in Dec but they are not much
> and I've
> > > > not noticed any objectionable field rotation on lenses up to
> > > > 180mm. I only use the SP for piggyback. I drift aligned my G-11
> > > > for the observatory.
> > > >
> > > > Stuart
> > > > ----- Original Message -----
> > > > From: "Serge Theberge"
> > > > To: "'Discussion of Film Astrophotography'"
> > > > Sent: Tuesday, November 25, 2003 8:01 PM
> > > > 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
> > > > >
> > > > > > -----Original Message-----
> > > > > > From: astro-photo-bounces@seds.org
> > > > > > [mailto:astro-photo-bounces@seds.org] On Behalf Of
> > > > Franklin A. Holub
> > > > > > Sent: Monday, November 24, 2003 4:25 PM
> > > > > > To: Discussion of Film Astrophotography
> > > > > > Subject: Re: [APML]OT-Drift Alignment
> > > > > >
> > > > > >
> > > > > > I haven't posted in a while but lets see if I can really
> > > > mess things
> > > > > > up... I'm going to make a statement that is going to fly
> > > > in the face
> > > > > > of years of experience and is going to contradict
> what we have
> > > > > > always been told about drift alignment...
> > > > > >
> > > > > > But first, a little background...
> > > > > >
> > > > > > After a couple of years of being happy with "close enough"
> > > > > > polar alignment, I installed a perminant pier and I
> figured it
> > > > was time to
> > > > > > "nail" my polar alignment.
> >
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