During the second week of August, I experienced some setback on the 1200 project and was able to recover.
Early in the week, I had the chance to perform the test to find the infinity focus point for the lens. It was a convenient 30 inches past the flange. How nice!
Unfortunately, when I reached out to Parallax for the quote on a custom aluminum tube, Joe advised that they would not be able to meet the design requirements. Bummer. Back to thinking about alternate ways to provide a light-tight back to the lens.
When I was taking the setup down the following day, I found a few things that were less than pleasing.
First, I saw that the position of the rear standard was significantly off from where I'd expected it to be. Turns out that it had drooped due to a loose screw. Easy enough to fix once I found an insert to bring the 3/8-16 thread socket to another 1/4-20 thread. With two mounting screws, it's unlikely that it will move in the future.
Second, I saw that the rotating film back that I'd picked up on ebay was not very tight. The film holding elements were loose and allowed rotation or movement with just a finger touch. As this was part of the early Toyo design, there didn't seem to be a way to lock down the rotation. Luckily, by the time I searched on ebay again, I was able to find a more modern back both domestically and in JP. Ordered them both and between the three I should be able to cobble together a precision back with ground glass.
At this same time, I decided to reach out to Michael Herman for his network to find a Losmandy Titan. This started a long email exchange, talking about the methods to improve the Losmandy mount design. Suffice to say, the Titan is a great mount, but it suffers from being based on the lesser choices from the family. Specifically, the choice of pin bearing instead of a centering bearing can give the RA axis a chance to flop when the meridian is flipped.
The amount of movement is low, yet enough to show up in pointing model from one side to another. This is probably the source of the polar alignment error from east to west.
One way to eliminate this flop error is to check the polar alignment WITH the flop in mind - understand the cone error and measure the polar alignment on both sides of the meridian. Split the difference and accept the error on both sides.
Another way would be to replace the bearings.
Michael shared a document that he'd been making that covers much of the methods that can be used to improve the mount. I ahve done many of these steps - including the McLennan gear boxes and some of the RA4 bearings. I had found less than optimal use of the belleville washers - mostly because I had installed incorrect ones incorrectly. I also may have installed the thick and thin thrust bearing races in the wrong order.
With some back and forth, additional research and shopping, I have ordered the following parts:
From McMaster:
RA4 belleville washers
RA4 high precision bearings
Caged versions of the RA and DEC shaft pin bearings, longer than standard.
From Amazon:
Bearing puller and press
Also from Amazon but unreleated to the rebuild - the guide scope that will go inside the rectangular tube.
From Misumi:
Thrust bearings with races - it's likely that by using the strap wrench to tighten the clutches that I've damaged the thrust bearings.
From Michael:
His high friction clutch pads so that I don't have to use the strap wrench.
From Newark (aka Fastenal):
4 more McLennan gear boxes to upgrade the G11 and to have a spare.
The parts will slowly drift in over the coming weeks. When Michael is back in town at the end of the month I may have the chance to rebuild one of the G11 with the newer parts.
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