Re: very strange problem with AP 900 tracking

Dick Locke

Howard and Rolando,

Thanks very much for the quick replies. Fortunately it's the simplest
of problems, somehow tracking was set to LUN. Duh! I should have
thought of that! I guess it's easy to bump the 8 accidentally and
change the rate, and the rate stays set even when cycling power
(unlike my old mount!) I guess staying up all night impairs one's
thinking ability, and I'm still getting used to the new mount.

Now if I can only get some clear skies for the remainder of my
astro-vaction we'll see how much my guiding improves when running at
the correct rate!

Thanks again,

-Dick Locke

--- In ap-gto@..., chris1011@... wrote:

In a message dated 10/9/2007 5:44:50 PM Central Daylight Time,
dtlnew@... writes:

I'm seeking advice for what seems to be a fairly large RA tracking
rate error with my AP 900GTO. I'm using an STV, and during drift
alignment using the classic star in east & star south, I'm seeing what
seems to be a RA error of around 60 arc seconds per minute. The dec
graph is very flat, indicating at least a fair alignment. I also took
some (accidentally) 4 minute unguided photos with my DSLR the other
night and they had long trails. So, this seems to confirm something
is amiss.
1) Polar alignment will affect the RA drift rate. Try this: Aim your
scope to
the Dec zero due south. If you have no Dec drift, the azimuth axis
is set
correctly. If you have no RA drift, the altitude axis is set
correctly. If drift
in RA is slow, lower the altitude axis. If fast, raise it.

2) the tracking rate on the keypad should be set to SID (sidereal)
in the
Main Menu. If set to LUN or SOL it will be slow.

3) make sure that your clutch knobs are tight enough to prevent

4) if none of this works, take the motor cover off and look at the spur
gears. You may see the spur gears turning just fine, but if the
final drive gear on
the end of the worm shaft is loose (i.e. the set screw has backed
off) then
the spur gear might be turning, but slipping slowly on the worm
shaft, and thus
causing the worm itself to drive slow. In that case, remove the
large spur
gear with a screw driver and tighten the little setscrew on the
final spur -
make sure it is centered on the flat on the worm gear shaft.

I am beginning to understand what may cause this set screw to slip
in some
cases. What tends to happen is people want to eliminate all backlash
and so set
their worm mesh so tight that the worm cannot move easily against
the worm
wheel. The motor is strong enough to turn the spur and break the set
screw loose
against all that friction, and thus the little spur goes round and
round but
the worm shaft stands still or moves only slowly. When tightening
the mesh,
make sure that you can physically still move the spur gears easily
by hand before
turning the power on. If you can't easily spin the large spur, if it
considerable effort to turn this gear, then you have set the mesh
too hard, and
you will end up with problems.


See what's new at

[Non-text portions of this message have been removed]

Join to automatically receive all group messages.