ETX on a DS mount
Inspired by similar conversions on Weasner's
Mighty ETX Site, I recently bought a cheap Meade DS-80EC with a 497 AutoStar at Costco and adapted the mount
to fit my old ETX-90RA. I'd always loved the optics and hated the mount on the scope, particularly because the
azimuth "bearing" allowed a lot of altitude wobble. The converted mount is much more stable, works better
as a spotting scope, permits easy removal of the ETX, and works with other small scopes (I have an old 60mm spotting
scope that fits nicely). It also turns out to be a great platform for panoramic photos with a regular camera, because
it's easy to level & center.
Other conversions I'd seen all involved mounting a small aluminum plate between the forks, using an angle iron
on each side bolted to the fork bearings with the nuts supplied in the original mount. I went one better by machining
a long plate with a center groove. This way the scope can slide for balance (e.g. move it forward when there's
a camera on the back) and it's easy to accomodate things with different bolt spacings. The plate is actually a
piece of door threshold from Home Depot, which is nice and thick and has reinforcing ribs on the bottom. I used
a table saw with a carbide blade to rip a little off each edge so it would fit, and to cut the center slot. The
latter required a dicey plunge cut, which is not especially safe, so imitate at your own risk. In the photos you'll
notice a skid mark where the plate got away from me, in spite of a small jig I'd made. Don't even think of doing
this without push sticks and eye & ear protection.
The hardest part of the conversion was getting the nuts off the bearing shafts on the original mount so that
they could be passed through the side angles. On the motor side, there's some kind of adhesive on the threads making
it very difficult to remove the nut. Each shaft has a screwdriver-type slot in an end, but I found it impossible
to get enough torque on those safely. My solution was to slip a short piece of 1/2" copper pipe over the bearing
surfaces so that they could be clamped with a pair of vise grips (exercising extreme caution to be sure the grooved
jaws didn't cut all the way through the pipe and damage the bearing surface). Then it was fairly easy to spin the
nuts off with a 10" crescent wrench.
Happy to provide additional details to anyone who's interested.
- Tom
tomfid @ earthlink . net
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The finished product. Seems to work really well, though it's been too cloudy since I finished to give the GOTO
a real workout. There's still a little wiggle in the mount, but not enough to be annoying, and the field recenters
itself after a nudge (which the ETX mount did not necessarily do). The main sources seem to be play in the fork
bearings (tolerances are quite loose) and flex of the plastic motor housings.
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Rear view of the finished product. Note how the OTA sits quite high, which makes it a little top-heavy so that
you have to be a little careful during non-motorized operation (altitude unlocked). I'd hoped to get the tube centered
in the forks for better balance, but the DS-80 arms are too narrow. Perhaps a DS-114 would work better in that
respect.
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Detail of the slotted deck. Note the interruption in the middle to keep the two halves tied together for rigidity.
Keeping the deck as rigid as possible couples the two bearings, mitigating the effects of their sloppy tolerances.
The side angles are 1" x 1" x 1/16" aluminum angle stock. I would have preferred to use something
thicker, but then threads on the shafts are too short. On the motor side, I had to add a toothed lockwasher to
keep the shaft from slipping against the aluminum angle.
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Another view of the deck.
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Closeup of the motor side. I hadn't discovered the need for a lockwasher at this point, so you don't see it,
but you can already tell that the thread depth is barely adequate. I added the washer between the nut and the aluminum
angle. The aluminum butts directly against the shoulder on the shaft, which lets it rest against the brass bushing,
but I seriously doubt I'll put enough miles on it for wear to be an issue.
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End view of the deck. Note lots of metal dust around - should have cleaned the deck more thoroughly before assembly.
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