Re: [APML]: Periodic Error Problems

[Brad Wallis <wallis@pepe.jpl.nasa.gov>]

>      Using the ST-4, I have found the periodic error to have an 8 minute
> period and to vary about 3.5 - 4 arc minutes along the RA axis and 1.5 - 2.0
> in Declination. 

Obviously this is a very poor gear system ...  but ... you probably had the ST-4
not aligned in N-S-E-W coordinates ... my guess to account for a 'periodic error
in DEC' ...  polar alignment problems do not account for a periodic DEC drift ...
...  you *might* ( doubtful if the DEC period equals the RA period ) be seeing
the effects of overshoots in DEC corrections but this would not really be
periodic ...

If you are going to continue to use this mount and the errors are really this
large, then I'd strongly urge keeping the ST-4 guiding exposure to real short
( like 1 second ) integration times  ( or less than 1 second !! )...

FYI sveral G-11 mounts that I know of have periodic errors of 5-10 arc seconds ...
as a point of referecne for you and as a suggestion for a solution ...

Make sure your testing measurements are accurate ...  but this is actually
semi-typical from what I have heard for some common commercial mounts ...

Brad Wallis