Quote:
Originally Posted by rogerg
For this part in your procedure, when pointing at the teTV aerial (or whatever) does the telescope not have to be pointing at 0 degrees altitude? If it can be at about 20 degrees this might be possible for me.
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Hi Roger,
The following background may be helpful.
For the following, let us assume the mount is in an Alt/Az configuration
and is level.
Alt to Optical Axis non-perpendicularity (CA) provides a constant pointing
error residual in Az for all elevations (i.e. all values of Alt).
Az to Alt non-perpendicularity (NPAE) provides an error residual in Az that is zero
at zero Alt and its maximum value when the OTA is pointing at 90 degrees
in Alt (i.e. zero zenith distance).
In surveying on the original optical theodolites, surveyors speak in terms of
"left-face" and "right-face" readings. When you take a sighting, then rotate the
mount 180 degrees in Az and then swing the OTA back through the forks to
invert it and then re-acquire the target, you are taking a "left-face" and
"right-face" reading. Both CA and NPAE errors reverse their apparent
directions when you do this procedure.
However, to null out CA, pick a target with zero elevation. That is because
at any other elevation your error will be a function of both CA and NPAE.
Once you have nulled out CA, choose a target at a higher elevation
and attempt to null out NPAE.
If you study the last two animated graphics and the accompanying text on this web
page on our web site, it will give you further appreciation.
See
http://www.wildcard-innovations.com....nt_errors.html
The material on this page forms part of some of the presentations I have given
at star parties on telescope pointing errors and analysis.