Sidereal Technology driven mount for 20 inch telescope

These are pictures of my altazimuth telescope as I'm making the parts and putting it together.  I'll add more pictures as I get around
to making them.




Underside of the rocker box, showing the 23.5 inch lazy susan azimuth bearing.  At right are the motor mount and idler
pulleys.  Each idler has two SR8 bearings (1-1/8 X 1/2 X 5/16 inch) on a 1/2 inch carriage bolt held in a slot so the idlers can
be adjusted to tension the belt.  The large bearing -- I think made by Hua Feng in Taiwan -- can be found at various suppliers
but I got this one from Lee Valley.  The mount uses 16 of the smaller SR8 bearings, so I bought a couple of 10-packs from
a seller on ebay.

http://www.leevalley.com/US/wood/page.aspx?p=44014&cat=1,250,43298,43316




Here the 24 inch ground board has been fastened to the azimuth bearing, and the azimuth motor and drive belt installed.  The
altitude gear motor is displayed on top.  The motors are overly-large, 24 volt Pittman 14204, with 50:1, size 14 Harmonic Drive
reducers.  I replaced the motors' original 500 count encoders with larger encoder housings and 2500 count disks, giving about
14E6 ticks per telescope axis revolution.  Harmonic drives and belts assure zero backlash.

The gear motors were found on ebay.  The upgraded encoder parts are from U.S. Digital:

http://usdigital.com/products/encoders/




Side of the rocker box, with altitude motor mounted, showing the idlers and belt.  The 3/8 inch thick aluminum mounting
plate is recessed into the plywood side board and is painted, so you can't see it.



Here you can see how the idler pulleys are made using two SR8 bearings, a 1/2 inch bolt and some washers.  I got the 3/4
inch wide XL belts from Polybelt, an open-ended piece for the altitude drive and a custom length continuous belt for azimuth.
The custom belt, size 806XL075, cost something like $35.  Polyurethane and steel.

http://www.polytechdesign.com/




Here is one of the four altitude supports.  Each has two 1-1/8 inch bearings, the same as used for the idlers.  The axle
is 1/2 inch aluminum rod, with its ends turned down to 7/16 to hold it in place in the U-shaped piece. This is screwed to a
maple block that is glued to the plywood side board.  On another picture you can see how one of these blocks is shimmed
to center the mirror box and its motion in the rocker box.  Ah the vagaries of woodworking...


.
This is the top side of the driven altitude bearing, which is a 22 inches diameter and 2 inches wide.  I made the altitude bearings
a full circle instead of the traditional Dob half circle trunnion because it looks good and because it's so easy to fasten the steel
rim and to tension the drive belt with turnbuckles, like this.  I epoxied aluminum end pieces to the drive belt and strengthened
them with steel washers.  On the other side of the rocker box is a narrower altitude bearing that has only the steel rim.  A local
machine shop sheared the inch-wide stainless steel rim strips from 16 gauge plate stock.




Looking down into a corner of the mirror box.  You can see one of the edge supports, next to the fan that blows across the
mirror face.  The mirror cell sits on a 3/8 inch thick aluminum bottom plate that also counterbalances the top of the OTA.
This plate weighs 17 pounds and has a lot of thermal inertia but because of the efficient radial airflow between the plate and
the mirror, due to the bottom fan, and because aluminum is so conductive the plate cools to ambient in just a few minutes.

The Papst 120mm fans are rated for 24 volts but I run them at 12 volts for cooldown or 5 volts, via a 7805 regulator, for quiet
observing.

I bought the aluminum plate from a local machine shop and found the fans on ebay.



Here the OTA is angled away so you can see the cell bottom plate, showing the fan and three collimation bolts (with
locking wing nuts), and two edge supports.  These edge support posts are simpler to implement than a sling and have very
good performance.  They contact the mirror at 90 degrees separation, at its front-to-back center of gravity, where I super-glued
short lengths of plastic zip tie to serve as contact points.  I used this handy calculator from Robert Houdart's Cruxis Project
site:

http://www.cruxis.com/scope/mirroredgecalculator.htm

The OTA can swing from one horizon through the zenith to the other horizon because there is another pair of edge supports
on the other side of the primary (and in this picture those are downward and supporting the mirror).  This is handy for casual
visual use when I want to move the eyepiece from one side of the telescope to the other, e.g. to avoid glare from a streetlight.
But under automated SiTech goto control only one altitude quadrant (e.g. from zenith to southern horizon) and one pair of
edge supports is used.

At the lower left you can see the azimuth motor in the corner of the rocker box.

 

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