Re: Re: [APML]OT-Drift Alignment

["Steve Walters" <s-walters@comcast.net>]

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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