Argo Navis dob alignment stars question.

[gb_astro]

When using a dob the AN manual recommends selecting a pair of alignment stars that give you a good separation in both azimuth and altitude, without getting too close to zenith.

However "good separation in altitude" seems to place one star pretty high.

So the question is just how close to zenith can you go before you have a problem?

Thanks,
gb.

[erick]

Just guessing here. The zenith problem would come about through the azimuth encoder. Obviously, if you pick a star at the zenith, you can rotate the dob around 360 deg on the azimuth and still have it centred. I guess I don't go much above 70 deg elevation - for convenience, if nothing else. Also I don't drop much below 30 deg elevation for alignment stars.

[Sentinel]

Well...it's not really going to matter too much, just that it's inconvenient to point into Zenith.

Just stick to stars between 20 and 60 degrees Alt, and make sure you space the Azimuth stars at 120 degrees +. Go a little higher if you must.

Even better, do a quick TPAS run as well (6+ stars) and you'll be sweet.

[gary]

Hi Geoff,

Thanks for the post.

As a rule of thumb, two alignment stars with around 30 degrees angular separation
or more tends to provide good alignment on most mounts. The difference in the
altitude of the two stars might be only 10 or 20 degrees or so, so you don't need
to be picking one star down in the grass and the other up near the zenith.

It is often wise to avoid the pole of the scope when choosing alignment
stars. The pole of the scope is at the zenith on a Dob and at the celestial
pole on a polar-aligned equatorial mount.

The reason for this is that any non-perpendicularity between the Az/Alt or
RA/Dec axes will result in a pointing error residual which reaches its
maximum value at the pole of the scope. If no TPAS pointing model is
in place, then an alignment star near the scope's pole might have a larger
pointing error residual than one chosen away from the pole.

Practically, this tends to only be something northern hemisphere observers
with polar aligned equatorial mounts have to be conscious of, namely with using
Polaris and for Dob owners who at some given location on Earth and at some
moment in time might consider using a star at their zenith. On a Dob, the
awkwardness of 'Dobson's hole' makes using the scope at this position somewhat
awkward, so most observers naturally avoid it.

Polaris by the way is a perfectly good choice for northern hemisphere observers
with non-polar aligned mounts such as Dobs.

Argo Navis handles all orientations of the scope and all sky positions in a
trigonometrically rigorous manner, so if it were not for the finite geometric fabrications
errors within the mount, using alignment stars near the pole of the mount would
be as good a choice as any.

As a footnote, when a mount does have Alz/Alt or Alt/optical axes non-perpendicularity
(i.e. what TPAS calls NP on a eq mount and NPAE on a Dob and CH on an
eq mount and CA on a Dob), there is an area around the pole of the scope
to which it can never mechanically point. This area of exclusion means that at
some level of eyepiece magnification, that part of the sky is denied to the observer.

Thanks again for the post.

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

[gb_astro]

Gary, (Paul, Eric) thanks for the info.

gb.