Dave Sage wrote:

> The question becomes then - how do you tell non-parallellism of the forks
> (with the RA axis) versus a simple tube that is not exactly perpendicular to
> RA (ie setting circle is off). The symptom of the light cone moving when you
> rotate the RA axis would be the same for both faults wouldn't it?

This is what happens. In an equatorial mount there are three mechanical
misalignments that cause predominantly east-west errors and which are
hard to tell apart. (I'm leaving out polar axis misalignment because this
has a characteristic signature and is relatively easy to determine.) The
three misalignments are:

IH Zero-point error in HA

CH Nonperpendicularity between the OTA and the Dec axis

NP Nonperpendicularity between the Dec axis and the polar axis

(The names IH, CH and NP are what the TPOINT analysis tool calls these
effects.)

In terms of what you see through an eyepiece as you move north-south:

* IH causes east-west errors that diminish as you approach the pole (in
proportion to cosine Dec). So a 200 arcsec zero point error puts you
200 arcsec out on the celestial equator, but at the pole the pointing
is unaffected - all you get is a 200 arcsec field rotation.

* CH causes an east-west error that is constant as you move north-south.
If it's 100 arcsec on the equator, you'll miss the pole by 100 arcsec.

* NP causes no pointing error on the equator (just a tiny field
rotation), but builds up as you approach the pole (in proportion to
sine Dec).

That's how you tell them apart. In principle you could do this by
observing just three stars at different Decs, noting the east-west error,
and solving manually, but in practice this wouldn't work very well. A
better approach is to solve for these three terms as part of a more
complete, all-sky, pointing model.


Patrick Wallace http://www.tpsoft.demon.co.uk

p.s. Since the "TPoint doesn't work" correspondence a couple of weeks
ago, I've been able to establish some of the things that have been going
wrong, and in due course more information will appear about this. For the
time being, TheSky/TPoint/LX200 users should make sure that during a
mapping run they do *not* hit the sync button each time they set on a
star, and that they remember to disable TPoint's refraction corrections by
setting the pressure to zero.

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