Posted by Herb Chuck wrote: Andy wrote: On the PE 'tweakers'..there are 2 methods I'vb used successfully: - for straight-thru motor/worm shaft, the coupligs can be offset from each - for worm/motor couplings that have a spur gear reducer, the gear on the Tweaking the PE using these methods is *much* easier if the system I can supply the gears with either type of PE adjustment coupling for One key factor we found is that the worm gear should not be in the Andy
on August 14, 2002, 10:18 pm
(information post)
>>I used 2400:1 HDPE worm gears hobbed by Andy Saulietis. The GOTO was
>>astoundingly accurate. A slew of up to 180 deg would place any target in my
>>CB245's 9x12 arcminute FOV 100% of the time (not always perfectly centered,
>>but
>>in the FOV). VERY impressive!
>
>>However, the periodic error was about 100 arcsec, and due to the 2400:1 gear
>>ratio, the period was VERY short when that axis was providing the prime
>>motion
This article was written by Chuck Shaw, a long time astrobuddy and ATM
friend of mine.
These particular gears were experimental prototypes, the idea was to
be able to have a Bartels stepper drive that could have the motor directly
drive the worm shaft, instead of the more complex setup that has drive
rollers and/or reduction gears between the worm shaft & the motor, and
still keep
the microsteps small enough to get smooth tracking. Unfortunately, it
didn't work
as well as we hoped..the very fine (32 tpi) pitch gears were easily damaged
and
the tolorances for end-play/runout on the worm were very tight. In the
meantime
Mel had increased the microstepping capability from 10 to 40
microsteps/full step
so the 2400:1 rediction was no longer needed. The other hears I make are
10 tpi which results in a stronger more accurate gear, the PE on these
is much lower. I have re-hobbed Chuck's gears & he will be testing them
on his 14.5" scope shortly. The 2400t gears will no longer be produced.
other both in amplitude and rotational phase. The eccentric motion
approximates the usual
sinusoidal periodic error and this can be reduced by adjustment until the
PE is minimized..we did this method on the Danciger 32" alt az scope,
reducing the measured pr from ~50 arc sec to < ~5. See attached .jpg
worm shaft is mounted on a hub which incorporates set-screw adjustable
radial offset in any rotational direction..the other gear is on a spring
mountd bracket
that keeps it in mesh with the worm shaft gear. Chuck used this on one of my
older 360t HDPE gears & was able to reduce the PE from ~50 to ~5 arc sec.
has an autoguider that can show the corrections (=P) in near real time.
I have an very accurate tangent drive that can test gears to ~2 arc sec for
PE & I have been able to 'tweak' gears this way as well, but not
as accurately as with an autoguide on an actual mount/scope.
a nominal cost of $20.
high-speed part
of the gear train, ie where even a *small* amount of PE can be a problem if
the
period is very short, ie < 1-2 minutes, and that end play in the worm is
usually the biggest source of PE. My current worm moounts incororate an
end mounted ball pressed against a steel surface..if kept in contact the
end play is negligible.
--Previous Message--
: Hi Herb,
: For CCD work, the last I heard was that the
: gears exhibited too much PE. The testing
: was on a 24" gear IIRC. Andy had an
: experimental fix for that, but I am unsure
: as to the results.
: I'll ask the end user of the gear to see what
: is up and report back.
: Backlash is easily compensated for in the
: software. The compensation is in arc
: minutes with 5 decimal places after that,
: for a precise correction.
: Lenord
:
: --Previous Message--
: Thanks and yes I was planning on using Andy's
: gears. Is there a problem with them and
: CCD? (hope not)
: I was thinking about the backlash the spur
: gear would introduce? It may be better to
: go with a bigger worm gear instead, you
: think?
: Herb
: --Previous Message--
: Hi Herb,
: Somewhat correct. Back when that was written,
: Mel had 10 microsteps per revolution in
: his software. He now has 40. If you do the
: math: gear ratio X microsteps per
: revolution X
: stepper microsteps per revolution= XXXXX
: and then
: XXXX divided into 1296000 you get the
: resolution of your system in arc seconds.
: Mel recommends .5 arc seconds per microstep
: minimum for visual use. For CCD work, some
: get into the .0XXXXX arc seconds per ms.
: Your 720:1 ratio on the RA, with a 400
: step stepper, would yield .1125 arc
: seconds per ms, plenty good for CCD work
: from what I am told. The Dec. on a fork
: mount at 480 might require some added spur
: gearing to the worm shaft to get excellent
: images. Not on a GEM though....
: As far as moving that kind of weight around,
: you can do that wiht the 720 and quite
: possible with the 480:1 Dec. If it were me
: though, I'd add in some spur gears to the
: worm shaft on the Dec. to get a good
: ratio. You can purchase good steel spur
: gears for cheap these days. A couple of
: bushings and a couple of set screws and
: you are in business.
: Are you going to try using Andy's gears for
: CCD work ?
: HTH
: Lenord
:
: --Previous Message--
: Lenord. I was wondering about the gear
: ratios.
: On Mels site he talks about having 1200:1
: to 2400:1 ratios. Is this because the
: stepper motors he was using were 1.8 deg
: per step @ 200 steps per revolution and
: The ones you are using are .9 deg. per
: step @ 400 steps per revolution?
: My fork mount weighs in at about 150+lbs.
: There's a lot of mass swinging around and
: slew rates are not a great factor since I
: won't be swinging from east to west or
: north to south. More than likely I'll be
: moving around in small portions of the
: sky. The initial slew will be the only
: major move I make per night and there's
: always something to do while waiting.
: I have a 720:1 ratio (22") on the RA.
: Should I worry about reducing this further
: or will this be adequate for good tracking
: and CCD?
: For Dec. I was wanting to keep the gear size
: somewhat smaller since it sits at the side
: of the fork and will be more succeptible
: to bumping. A 480:1 (15") would be a
: good size but would I want to reduce this
: somewhat to match more closely the RA
: gearing?
: What are your thoughts on all of this?
: Herb
: P.S. I will get with you about making up what
: I need to have a complete GoTo drive from
: gear train on in 2 to 3 weeks (wallet
: permitting).
:
: --Previous Message--
: Herb,
: Send a message to Andy Saulietis. His e-mail
: address is: iss@pvtnetworks.net
: He can fabricate gears/worms/clutches for you
: and will have an idea for you.
: Take Care
: Lenord Stage
:
:
: --Previous Message--
: I'm attempting to finish a fork mounted
: 16" Newtonian and need some help. I
: have 1" of shaft for RA and can't
: figure out how to get a worm gear and
: clutch system in such a short space. The
: shaft is 2" dia. for RA and 1.5"
: for Dec. The Dec shaft has 2.5" of
: shaft and isn't a problem.
: I should probably have something made for me
: rather than spend money on something that
: will end up in the (bad idea) extra parts
: box.
:
: : :
: :
:
:
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