Building optics, Part 47 - condensed and completed.

Having been convinced that this optics stuff is boring, I'll condense and try to wrap it up.

If you go back to the post a couple of days ago, "Building optics - the optics, Part 1," the areas pointed to by '3' show the edges of the glass. From the inner mandrel to the intermediate mandrel, the optic was in sextants, which means 6 pieces of glass comprised the circumference on each side. The left side is the top of the optic and the right side is the bottom. At the intermediate mandrel, the optic transitioned to dodecants, or 12-tants, meaning 12 pieces of glass form the circumference on each side. Of course, with every layer the circumference changes, so the glass changes, too.

This is one end of FM1 at about 100 layers. The lines are the ends of the spacers. The double lines occur at the edges of each piece of glass, with a spacer on the edge of each. You can see that the double lines double about 3/5 of the distance from the center, the intermediate mandrel. Where the spacers change from single to double there are two layers of a special, wide spacer. The two new rows of spacers are initially mounted on glass only, with no spacers underneath, which requires some special processing in the grinds.

When Tom and Iliya arrive each morning, the optics look like this. The spacers have already been ground to the correct height. They vacuum and then wipe the optic to remove graphite dust, then check for problems. Then they initiate scans of the spacers followed by scans of the glass segments. The scans are closely monitored by engineers to ensure that assembly is within specifications.

This is the measuring device, called an LVDT, that performs the scans. It is turned toward the optic and travels back and forth on the carriage. It is calibrated by touching it to a standard location on the lathe, and then by grinding and checking a special piece of graphite called a sacrificial.

The scans take a couple of hours. Tom and Iliya then return to the lab. They apply epoxy to the exposed spacers and then set new sections of glass upon it. The epoxy is special, made to endure numerous heat cycles and vibration. It is mixed every day and the amount used and linear density on each spacer is recorded. After new glass is placed, new spacers are epoxied and placed on the glass. Those are the spacers that will be ground to the correct height after the epoxy has hardened.

This is a single strongback, which is used to mount two spacers to the glass. One side is covered with Teflon tape. The other side will be covered, but is not in this photo. The Teflon is used to hold the spacer in place, and also to prevent epoxy from getting on the strongback.

Looking at a strongback from the end.

A double strongback, which holds the spacers that are mounted to the edges of the glass sections.

The new spacers are already loaded into the strongbacks. The techs apply epoxy and then slide the strongbacks into position using the guide wheels on each end of the optic.

This is a strongback in position, holding a spacer against the glass.

After the strongback is in place, a device called a load bar is slid into the guide wheels above the strongback and then pinned to the guide wheels. When air pressure is applied through the tubing, pistons move down to hold the strongback and spacers firmly against the glass. The illustrated load bar is for a double strongback.

This is a closeup of an optic at the center. The white lines are threads of Teflon that were caught in the epoxy. They are removed manually or during the grinds.

When I arrive at the lab at some unfrogly hour, this is what I see. Double load bar to double load bar marks one piece of glass, with three single load bars in between. I begin by removing the load bars.

There are 48 load bar/strongback pairs on each optic - 12 doubles and 36 singles. Because of their weight, they have to be removed in balanced groups of 12. This shows 12 strongbacks after the load bars have been removed.

After removing a set of load bars I remove the strongbacks (numbered because they go in the same place each layer) and put them in trays. When all are unloaded I remove the guide wheels, do a few preparatory tasks, check the computer settings, and then begin the grinds.

This shows the beginning of a grind. The optic is spinning on the right. The carriage is behind it, carrying the spinning grindstone along the length of the optic. It's kind of scary and amazing, but it works!

Once the grinds are started I inspect the strongbacks that were removed. The Teflon tape typically was pulled off of several. I inspect the rest to verify the integrity of the tape. Each strip of tape is replaced at least once a week. I install new tape as necessary, then install new spacers in each strongback. I have a few other tasks, like changing chart recorder paper and reprogramming a status sign, and THAT'S ALL, FOLKS! (I can't remember the ditty that's supposed to play here.)