Posts Made By: Alan Thometz

I've seen the soft foam material used for mouse pad wrist rests as a good material to put between motor mounts and scope.

link here might help...

http://p206.ezboard.com/fbentrideronlinefrm8.showMessage?topicID=98.topic

Sometimes trying something entirely different works. Have you tried a star lap on the full sized tool? You don't need to mutilate the lap. You heat the pitch, for example with a blow drier, not much but enough for a star made of paper to imprint a little of its shape when pressed with the tool. If the back of the mirror is clear you can see the effect when you figure MOT. It disappears fairly quickly but you need to re-heat the lap and re-press well before going on to other figuring steps, to eliminate the bias from your pitch. When it works, it tends to be a little more predictable from center to edge. You can try changing the shape of the star, for example, the tapering of the tips at the edges, to vary the center vs the edge.
A reverse star lap can reduce the overcorrection without having to go back to a sphere.
To parabolize, since you are deepening the pitch with paper you actually want press a "mirror image" of the star on the lap by cutting a star out of a piece of 10" circular paper, putting the star itself aside, and centering what's left, the "reverse outer star" on the lap. This deepens the pitch more on the edge than in the center (loss of contact on the edge). If you want to bring an overcorrected figure back uphill to a sphere, do the opposite and press the star itself on the heated lap. This gives you better edge contact in a graduated way. You use a standard spherical polishing stroke MOT but the effect spread out more evenly between center and edge. I've moved a litte side to side with the stroke but don't know if that's better.
The concept is in Ingalls old Amateur Telescope Making book (updated version) chapter called "Working Uphill" page 91.
The "accidental/unpredicatable" frustrations I've experienced in the past improved by letting the mirror sit longer on the stand before retesting, and it usually makes an appearance when trying to work better than 1/4 wave. Try measuring twice with the sitting period you are using now, compared to a good 12 hours. They should be close.
Also, once you get down to 1/4 wave tolerances try shorter figuring sessions or use zirconium oxide instead of cerium. Once this precise it may be better to time the session based on number of strokes in each position and working only for a number of complete turns of the mirror on tool to avoid astigmatism. I've also overworked elevated zones, wondering why they won't disappear, only to find it took some more time (and ?cooling) for the real result to show up..usually overdone. Somewhere in there I had it right but just kept on going.

I have a big opti-craft, 3.5" shafts in an observatory permanently mounted. It does not have the reputation carried by some other mounts. Appearance is very "utilitarian", put nicely. Extremely heavy. However, it is smooth feeling and acting, and when balanced I can lightly launch the big scope and watch it move a smooth and easy 180 degrees (clutches disengaged), not slowing much, before I have to stop it myself. It never binds, have never been able to appreciate any uneven action/friction at highest powers.
It was originally supplied with tiny synchronous motors (before goto) and small 1/4" shafts on the reduction units, with a smallish 12" worm gear on RA and only 9" in dec. I questioned the small sizes of the recommended gears and reduction units/motors with such a big mount, but he did not take that opportunity to sell larger ones. These little motors did push the 16" scope around without trouble for years, never overheating or quitting, never sounding like they were straining, which I always kind of marvelled at.
I upgraded to new stepper motors a few years ago for a goto conversion, then again just recently with new gear trains/motors to do CCD work. With the newest system, the wildly-erratic periodic error disappeared, confirming my suspician it was not in the big pillow blocks of the mount itself, but in the previous reduction unit/gear train, probably in the gearhead motor itself. There is a smaller, different and smoother periodic error now, but I haven't had enough time to measure it or get PEC going. I've just done track and accumulate up to 30sec so far, but once I learn to cancel drift on the software, I think it will be able to make decent track and accumumlate times...I'm still learning the Bartels software, and new at CCD. Without cancelling drift or engaging PEC, I can track and accumulate for 30sec exposures before oval RA error appears. Admit I forgot to balance well. I don't think the limiting factor is the mount.
I'm not a machinist, but the people I've talked with do not have the highest opinion of it. Attention to detail seems the weak link, a little in design and a little in construction. It's an older machine meant to carry a heavy scope, and I bought it with upgrades to larger Newts in mind, 10 yrs ago. On the other hand, the worm gears are apparently outsourced, and are well-regarded by those who worked with them on this retrofit.
It's carrying roughly 200lbs(?) now, (rated for 600), with about 160 lbs of counterweights. It's being slewed by a belt-driven gear reduction unit made by Lenord Stage, worm itself by Aeroquest. It uses toothed timing pulleys and belts (small parts.com) which are not heavy duty. The motor delivers 50 in-oz torque, pretty strong, and does fine. The mount needs some torque to slew, but it moves smoothly and predictably.
I've attached a photo of the mount/scope, and on the next message a photo of M17 through it, low in the south with crummy seeing, track and accumulate 30sec exposures about 5min total LRGB, with an ST-8.
Basically: not pretty and not state of the art, but probably usable for CCD if you need real carrying capacity. Too early to say of the autoguider will work...but think it will. I'll know more in a few months. Would make sure you get relatively large worm gears and beefy reduction units, no gearheads. Don't know a thing about his bigger reduction units, but they "look good" in the photos. Get stainless shafts. He doesn't seem to have done much with go-to: not sure what he supplies for motors. Mount is likely to be heavy. If you have a smaller scope I'd go for something lighter and more advanced, like an HGM Titan or equivalent. If your scope is small enough to fit on a G-11 or equivalent, I wouldn't go opti-craft. But I'm not letting this one go. Judging by what is written of it (although little is out there) I think it's a bit underrated.

Bought their red laptop shades. No problem, nice product.