Fixing the Cassegrain Mount


Well, after much work on the optics, it's time to fix the mount for the Cassegrain.  The old declination bearings were just plain worn out.  Here's a picture of the tube cradle, tube, and the old declination bearings after disassembly.  A picture of the massive right ascention shaft is shown next.

Before Polar shaft

Mike Conron took the tube cradle and the fork to his garage for cleaning, sanding, and a new navy blue paint job.  The cleanliness of his hands was sacrificed so that the wooden scope parts would be protected from the elements, as shown in the next photo.  Then the "movers" pictured brought the fork into my basement for further work.

Hands Movers

I obtained two 14 3/4" diameter by 1" thick aluminum disks from S&S Machine in California for a very reasonable price.  They are machined round, and the faces are polished, as shown in the next picture.  (No, the reflection did not break the disks.)  These were then bolted to the newly-painted tube cradle, which sits on rollers in the fork mount (next photo).  Great care was taken to make sure the disks were concentric and as parallel as I could get them.

Reflect Fork and cradle

The next two pictures show the rollers and caster which constrain the movement of the aluminum disks.  The rollers are mounted to 1.5" square steel tubing (3/16" thick wall) on the inside of each of the fork arms.  The rollers bear the vertical load, and the casters support the lateral load which is at a maximum when the scope is pointed to the east or west.  The second picture is a closeup of the disks and rollers together.

rollers rollers and disk

After this, I made the two "keeper" assemblies that make sure the disks stay seated on the two lower rollers.  These were simply two bearing assemblies that attached to the center of each disk.  One utilized a 5/8" bearing with a 5/8" aluminum cylinder inside the bearing that was bolted to one of the disks.  The other bearing was a 1" diameter aluminum cylinder, bolted to the other disk, with a piece of nylon, cut out to match the 1" cylinder, which constrains movement.  Both bearings were attached to the fork using pieces of plywood (Baltic Birch).  This permits a slight amount of "give" if the center holes in the disks are off slightly.  I also added a device to vary the friction of the declination motion.

Here's a closeup of one of the bearings and the friction adjustment.

Friction adjust Friction Adj

The result is that the declination axis motion is very smooth, and most of the physical construction on the telescope is done.  Now we need to drill the new mounting holes in the tube for the mirror cell and one fan, paint the new parts and touch up the fork and cradle, install fans, and reassemble.

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