John,

I used to have GOTO problems with our 16"lx200 (in an
observatory). The problem was solved with a method
described by Bruce Johnston in his page:

http://members.aol.com/ccdastro/goto.htm
This takes a couple of hours but requires no tools of
any kind. And the results were improovment of some
500% in GOTO accuracy and even more in the tracking!
In case you didnt try it already I think you better
give it a shot.
Goodluck!
Assaf B.

> Doc,
>
> Thanks again for the suggestions. Actually,
> aligning the two axises to be
> orthogonal was the first thing that I did and that
> reduced my goto error from
> 1-2 degrees to what I'm seeing now. I spent alot of
> time tonight trying to get
> a better idea exactly how bad the goto accuracy is
> and what helps/hurts it. As
> I mentioned before reducing the slew speed to about
> 4 helped alot and I was
> able to get about 5-10 arc-mins. However, that was
> only on a few stars and
> tonight when I tried a wide assortment of
> stars/objects over the entire sky I
> get alot of variation in the accuracy. Anywhere
> from about 1-60 arc-mins with
> a slew speed of 4! For example, tonight I did my
> 2-star alignment using
> Regulus and Vega. I could then GOTO these stars
> from absolutely anywhere in
> the sky and they would be almost right on the
> crosshairs of the reticle every
> single time! However, I could then GOTO a star
> maybe 90 degrees away and it
> might be off by up to 60 arc-min (a little RA and a
> little Dec). But I could
> always go back to the alignment stars with perfect
> precision. To me that says
> the encoders and drives are working perfectly;
> however, I don't understand why
> the accuracy is so good for the alignment stars (or
> any star I sync on) and
> others be so far off. Any ideas on this one?
>
> This really gets to be quite frustrating and if a
> goto doesn't hit the target
> in the FOV then I might as well just star hop
> manually. I'm beginning to just
> think I'll have to sell this one for a lower price
> and just hope my new one
> will be better. I know one thing for sure: I will
> thoroughly test the GOTO
> accuracy on the new one and if it's not as the Meade
> spec says then I'll return
> it for one that does. There just really seems to be
> alot of variability in
> pointning accuracy from scope to scope.
>
> So what's your opinion on the 12" vs 10"? Do you
> think that the extra 2" of
> aperture is worth the extra mechanical problems
> (i.e. decreased pointing
> accuracy) let along the extra $1300? I assume so
> since you own the 12" but I
> thought maybe you purchased the 12" with the
> intention to correct these flaws
> with modifications.
>
> So exactly what do you feel that a new 12" that is
> working good can
> realistically achieve as far as pointing accuracy
> AND mirror shift? In other
> words what would you expect out of the box in order
> for you to not return the
> scope?
>
> Thanks again!
>
> John
>
> >
> >
> > John Teel wrote:
> > >
> > > Doc,
> > >
> > > Thanks for the info. I just finished checking
> out the encoder signals and
> > I
> > > see no problems at higher slew rates. I used a
> dual-channel scope and
> > looked
> > > at the testpoint that goes to the photodiode to
> the input of the
> > comparator. I
> > > then looked at the output signal of the same
> comparator just to verify that
> > no
> > > encoder pulses were being missed by the
> comparator because of some type of
> > > level shifting occuring at higher frequencies.
> That appears to be working
> > > correctly and the comp is catching all of the
> encoder pulses and I saw no
> > dc
> > > shifting as the slew rate was increased.
> However, I didn't check the phase
> > > between the two bi-quad signals. What would
> happen if the signals weren't
> > > exactly 90 degrees out-of-phase? I'm a little
> rusty on a system like this
> > (I
> > > design analog circuits but only ICs so I haven't
> dealt with encoders in a
> > > while). I did notice quite a bit of jittering
> in the waveform on the
> > photocell
> > > test points. Almost looked like a bad ground
> connection but I'm not sure.
> > Of
> > > course I would think any jitter would create
> false triggers and cause the
> > scope
> > > to undershoot and not overshoot.
> >
> > Within limits, the exact phase is not highly
> important. It must just
> > be the case that the leading edges get to the
> computer in the right
> > order. The logic circuits decode the bi-quad
> signals into a series of
> > pulses (the rate) and a DC level which indicates
> forward or reverse.
> >
> > >
> > > Any ideas on what else to check? I've seen
> other posts of people having
> > the
> > > same problem on really new scopes (mine is about
> 4 years old) so I wonder
> > if
> > > this 30 arc-min of overshoot is typical. I
> don't know a whole lot about
> > the
> > > mechanics of the system (just what I've read of
> your articles) but it seems
> > to
> > > me that having so much of the control path
> outside the feedback loop is
> > bound
> > > to have accuracy problems such as this unless
> the machanical side is highly
> > > precise.
> >
> > This is really the big flaw in the Meade system.
> The computer knows
> > exactly where the motor shaft is to one encoder
> pulse. There are 90
> > on the encoder. But as you note, the rotten
> little gear train is out
> > of the loop as is the worm and spur gear.
> >
> > Still 30 arc minutes is a lot. With the drives
> tuned up, good balance
> > and good PEC training you should get about 3 to 5
> arc minutes.
> > >
> > > Personally, I can live with simply running at a
> slew speed of 4-5 but I'm
> > > wanting to sell this scope and upgrade to a 12"
> and I would prefer to not
> > try
> > > and sell a scope with a problem. If this is
> actually a problem with the
> > scope
> > > then of course I would tell any potential buyers
> and that would probably
> > lower
> > > the price. It already has about 15-20 arc-min
> of mirror shift which I've
> > been
> > > unable to eliminate (tried regreasing the mirror
> baffle) so I would prefer
> > to
> > > not also have goto problems.
> > >
> > Unfortunately you will find the 12" the worst of
> the bunch. Too much
> > OTA on too little mount. It is the same mount for
> all the LXs you
> > know. You will have the same mirror shift
> problems, only more. Same
> > everything. If you want a precision instrument go
> to the RC scope or
> > mount the optics on a better mount.
> >
> > I would suggest a thorough mechanical tweaking
> first. The two axes
> > might not be orthogonal. That really screws up
> the pointing. See the
> > declination bearing rebuild for hints about
> getting the forks
> > orthogonal. People do get 3 to 5 arc minutes
> regularly from these
> > doggies. :-)
> >
> > Good luck -- Dick
>
>
=== message truncated ===


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