Quote:
Originally Posted by MattT
Hi Gary,
A question on East-West side for the tube in the initial alignment with a GEM.
I read a few threads on Yahoo group but still am confused on this 
Tried it all out for the first time last night  and aligned on Acrux with the tube, that is myself and the focuser end was on the East side of the mount... is that right?
Got a warp factor for Acrux of 0.0 when it was in the crosshairs so I guess so far so good. Couldn't remember what to do next and stopped there, but hope to get out a few times next week to work it out.
Cheers,
Matt |
Hi Matt,
GEMs have the attribute that you can look at many specific parts of the sky
using two distinct mechanical orientations of the scope.
When the OTA flips over the meridian, the direction sense of the Dec encoder
reverses.
This is an attribute of three dimensional space rather than because of some
caveat of the Argo Navis.
Let us assume that your mount is polar aligned and you have set the SETUP MOUNT
type to GEM EXACT ALIGN.
The only thing you need to remember is that the SETUP ALT STEPS encoder
direction sense sign (the + or -) has to be set to a value that is consistent
with which of the two alternate orientations the mount is in when you perform
the star alignment.
Thus you will hear the terminology such as the tube on the East side of the
mount pointing West and vice versa.
Having the OTA on one side of the meridian and pointing to the other then
eliminates circumpolar stars. Circumpolar stars can otherwise bring about an
ambiguity depending on if they are above or below the pole.
So avoid using Acrux at your latitude because it is circumpolar. Choose
something more northerly.
What we recommend these days is to consider using the GEM EXACT ALIGN setting in
conjunction with what we call a short TPAS run. TPAS stands for Telescope
Pointing Analysis System, which is an in-built feature of Argo Navis that
appears under the SETUP MNT ERROR functionality.
To use this technique, set the GEM EXACT ALIGN setting in SETUP MOUNT and set
your latitude and longitude accurately in SETUP LOCATION. Also set your time
accurately either from SETUP DATE/TIME or better still via your PC running an
NTPS client and using Argonaut Version 2 (see page 188 of the Edition 11 of the
Argo Navis User's Manual).
Align on one star and then use TPAS to sample the position of at least four
stars (see pp 122-149 for a description of how to use TPAS in the section of
SETUP MNT ERRORS).
With the press of a few buttons, one then fits a mount error model that includes
the polar misalignment terms in azimuth and elevation and and what are called
the index error terms for the RA and Dec axes. The polar misalignment terms are
abbreviated as MA & ME. The Index Error terms are abbreviated as ID & IH.
In practice this takes probably less than five minutes. It then typically
results in excellent pointing performance because Argo Navis is mopping up any
residual polar misalignment in its pointing model. Dropping the mount down
within a degree of the pole, using the GEM EXACT ALIGN setting and combining it
with a short TPAS sampling run will typically ensure everything lands in the FOV
without having to do a FIX ALT REF step.
What is also good about this approach is that it is extensible. All mounts have
a certain amount of geometric and gravitational flexure errors that can be a
detriment to their overall pointing performance.
The TPAS system allows you to analyze and potentially compensate for many of the
common geometric and flexure errors within the mount/OTA. For example, you might
devote one night, perhaps when the Moon is out. to sample say 20 to 50 stars and
to use the in-built TPAS to analyze the data for any likely persistent mount
errors. You can then store these parameters in the unit as a mount error model.
On a subsequent observing night you can re-synchronise this model by sampling as
few as four stars and your pointing performance will typically return to as good
as the night you did the long sampling run.
That the RA and Dec axis on your mount are pretty square (NP error) and that it
is dwarfed by the optical to Dec axis non-perpendicularity (CH error) is pretty
common on GEM's. What we recommend is that rather than saving the CH term for
use as a fixed TPAS term for use on a subsequent night, just leave it to COMPUTE
and sample a few more stars on the night to then peg it.
So you can think of TPAS a "super N-star alignment" but really it is much more
than that because of its complex modelling.
Otherwise there is a little bit to take in here and I don't want to make your
eyes glaze over with it all at once, but I cannot over emphasise how quick and
convenient it is to use the EXACT align setting in conjunction with a short TPAS
run is in practice.
Should you have additional questions or require clarification on any of the
above, please do not hesitate to contact me.
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