Re: Re: [APML]OT-Drift Alignment

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

Hi Serge,

When I moved up to medium format film, I had a lot of trouble with polar
alignment because I was using the various schemes found on websites. These
are not very quantitative and usually just give a cook book method such as
"use a high magnification eyepiece and watch the drift for 10 minutes". In
my case, I still had trailing. I use an off-axis guider on the medium format
film which gives an angular separation (A in the formula) of 3.5 degrees,
for 60 minute shots at 1268 mm focal length. If I want 6 microns or less for
objects up to 60 degrees, I need to be aligned to within 1 arcminute. My OTA
can produce 20 micron stars so 6 microns seems a reasonable amount of
allowed trails. Also, it turns out to be about 1 arcsecond which is
absolutely best case seeing. Anyway, this turns out to be pretty challenging
but using the STV it can be done consistently.

You can find the article on-line at NASA's repository here
http://adc.gsfc.nasa.gov/

Steve...

----- Original Message ----- 
From: "Serge Theberge" <serge.theberge@wti.on.ca>
To: "'Discussion of Film Astrophotography'" <astro-photo@seds.org>
Sent: Wednesday, November 26, 2003 3:55 PM
Subject: 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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