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


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