Accurate Polar alignment technique

[peter_4059 (Peter)]

Originally Posted by rally

Peter,

Sounds good.
Why dont you look at the Tpoint documentation and have a look at the maths they use behind their Terms - pretty sure its mostly published
Go to Patrick Wallaces (original author) official Tpoint site not the Sb site for this information

OK thanks - will take a look.

[Merlin66 (Ken)]

I used The Sky when I "got serious" I still have the V6 disks....
As time and my needs changed I found CdC and EQMod as well as my trusty AstroArt met 99.9% of my needs. All the additional catalogues are there UCAC4, NEO, asteroids/ comets etc.

For other areas (solar imaging) I went for FireCapture and SharpCap.
My plate solving (when needed) is AllSky Plate Solver

[gregbradley]

Sounds a lot like the accurate polar alignment tool in Sky X which works off a TPoint model and then works out how far off you are then you align the mount to a particular suggested star and viola! You are there

It works like a dream and is not that slow once you work out how to do plate solving and is extremely accurate.

Greg.

[gary]

Originally Posted by Wavytone

It is feasible... the polar alignment error is a constant offset in altitude and an offset in azimuth.

...

Incorrect, study the spherical trig in say Smart's book. The the polar alignment error is a constant offset in altitude and an offset in azimuth, and is not a constant error in dec.

Sorry, this is not correct.

Polar alignment error is not constant in altitude but in fact a function of altitude.

The effect of refraction is to "lift" the stars position in altitude.

The amount of lift is highest at the horizon and zero at the zenith.

Declination for a very distant object is essentially always constant.
In fact, these days, RA and Declination are fixed to a reference frame
pegged to very distant compact radio sources.

These days, the origin of the RA/Dec co-ordinate system is in fact at the solar system's barycentre.

Originally Posted by Wavytone

Yup, atmospheric refraction produces a vertical shift (ie in the apparent altitude) of a star - it appears higher in altitude than it really is. This produces an offset in its hour angle (ie RA) AND its declination - and worse, these offsets vary as the star moves across the sky.

So wherever you aim your polar axis, its really only an approximation and you DO need to guide in RA and dec to account for the changing offsets as the stars altitude changes.

You start on the right track in saying "atmospheric refraction produces a vertical shift (ie in the apparent altitude) of a star - it appears higher
in altitude than it really is"., but unfortunately seem to run amiss from there.

The RA/Dec of a star is essentially fixed (ignoring longer term effects such
as proper motion, etc.)

As it moves across the sky, its RA/Dec is immutable.

One would need to lift the mount in its altitude (aka elevation) axis
to compensate.

The UK Schmidt does this.

As I discuss in a talk I give now and then called "The Myth of the
Perfect Polar Alignment", it comes as a surprise to some enthusiasts that
these issues with polar alignment and field rotation in astrophotography
were studied and published as far back as 1893.

Arthur R. Hinks in the Monthly Notices to the Royal Society in 1898
derived a nice table that showed for various declinations and
hour angles in which you propose to image suitable offsets by which
to raise the scope's polar axis.

A good compromise is to attempt to align the mount's polar
axis not with the true pole but the refracted pole.

Hope this helps.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au