Sunday, September 3, 2023

"Measure Thrice; Mark Twice; Cut Once"

I think this comes out of carpentry but it is valuable in every craft.

I have been wanting rotating rings for my 8" reflector for some time. On an equatorial mount they make it easier to get the eyepiece in a more comfortable position.  I had a set made by Cave Optical which I used with this scope on a Cave Optical mount, which was a fine mount when designed in the 1950s.  Unfortunately it added a lot of weight where the telescope was already a little heavy for the mount (vibrations took too long to end).  The rings held the tube with a bunch of screws through the tube into nuts on the inside; way more than needed.  They were also far inside the 64" tube that getting to them tells me my arms were much longer 25 years ago than they are now.

I ordered a rotating ring assembly from Parallax Instruments.  These are about $640.  They hold the tube in place with 1/4"-20 plastic thumbscrews. If these do not give enough grip, I can use steel versions that just cut a slight depression in the fiberglass.

Of course, you need a pretty precise measurement of the OD of the tube.  I measured ONCE (this is called foreshadowing in literature) the OD as 9 11/16". 

Not thinking carefully because the first ring seemed to sort of slide onto the bottom of the tube, I did not bother measuring the ring ID or tube OD. 

I have repaired the fiberglass tube in the past.  Once because my daughter was just learning to drive and misjudged where the Mitsubishi's bumper ended relatively to the telescope and repairing various holes when I need to relocate the mirror cell a few inches forward to get a focal point inside an SLR (film, not digital).  Also repairing all the holes from the Cave Optical rotating rings fiasco.

The results of those repairs were functional, but not pretty.  Tube diameter was not consistent because I did not sand the surfaces adequately after each repair.  So I just sanded off lumps, while tapping the rings up the tube with a plastic-headed hammer.  An interference fit this should not have been.

After a hour moving an inch or two a minute, I slowly removed the ring.  The ID of the ring was 9 3/4" and the OD of the tube had miraculously grown to 9 3/4".  Yes an interference fit.

Fortunately Parallax Instruments seems to have caring amateur astronomers working there.  They offered to turn the inside rings to a size that should allow the thumbscrews no interference fit to hold the tube in place. My lsthe is too small to turn a ring this size.  No charge.

While removing the inner rings from the outer rings in which the inner rings turn, I was a bit surprised at the cleverness of the design.  I should not have expected needle bearings.  The outer ring has a raceway in which the inner ring turns.  Six bolts hold the inner ring in the raceway, pressing down on Teflon bushings, and what seems like lithium grease.  You tighten these bolts to adjust how easily the rings turn.  There is also a locking thumbscrew to prevent rotation. 

Just a few parts needed and a small amount of machining to make an awesome product.  I am looking forward to putting these in place in a week or two.

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