It's time to cut and drill in the garage again. I decided to make a table-mounted mirror holder rather than go through the gyrations to fully utilize my overhead frame (for now). I first made a contraption in wood that was 24" high and had a bit of a lever arm to hold the 8" telescope mirror. Unfortunately that wasn't stable enough so I set about building a new one out of aluminum and steel.
After a trip to Loews I headed back home to the garage where Andres met up with me and we started working on making a serious mess.
While I was working on cutting my aluminum, Andres cut up some hexcel and particle board that we'll be using for 8x6" mirror mounts.
It didn't take too long for the legs of my mirror contraption to take shape. It's got steel flanges and 8" steel sleeves at the base and 3' square aluminum tubes for the legs.
Time to epoxy everything together. There was a bit of the JB-weld left over so I epoxied one piece of Hexcel to some particle board. The two smaller pieces are part of the movable mirror mount (see below).
Tapping holes for the thumbscrews that will be used to keep the mirror in position along the legs.
Jump forward to the finished product on the table. So far it's pretty easy to adjust and appears to be nice and stable (as of this writing I'm still doing some tests to determine stability).
Once everything was epoxied together and curing I took some time to put the second side on my plate holder (I'd been using only one) and cut some more glass to fit.
I did some double exposure tests last night and determined that I needed to expand my holobot program so I could automate that. This new version of the program has an expanded settle range that allows you to go down to 1 minute. It also includes a third step which allows you to set a delay between the main exposure and a second exposure. If the delay is 0, only a single exposure is made. Here's the new program. HoloBot.nqc 15K
It's been a busy and frustrating week. All last week I ran test after test to determine where my instability is. Since I put the hexcel together back in April and the last time I'd run an interferometer I've added the top framework and put the large laser on top of that.  I eventually narrowed it down to the plate holder, or so I thought. I went as far as to re-tape the glass because it appeared that the film was still settling even after 30 minutes and that was with a 2x2" piece. That seemed to make things worse. On Sunday I decided to tear the table down and set up an interferometer along with the video camera to record the results.
I immediately found that the table was not as stable as I recall from April. Unfortunately I don't have any video from then and don't have detailed notes of its behavior other than the blog entry I made which indicated that I could step lightly a few feet away, stand just west of the table (on the same joists) and not see any movement of the fringes.
I started out by setting up an interferometer and camera and recording the behavior over a 30 minute period, changing the pressure (depth) of the inner tubes and repeating.
After I'd found what appeared to be an optimal pressure I changed out the beam splitter for my plate holder and found that the plate holder was moving quite a lot.
During all the tests I could see that the table was ringing, probably through the metal frame. This showed up as an oscillation that stayed in the system for at least a second. I could see the fringes vibrate as though I were watching a pendulum swing. I had attempted to fill the tubes with foam but that was clearly inadequate.
I've spent some time since doing the following: 1) leveling all my in-use bases using epoxy, marble tile and wax paper. 2) Filling the frame tubing with sand and sealing with RCV. 3) Sealing the tubing joints with RCV and putting RCV under the frame bases. 4) putting the magnetic catch on the laser box (it looks like I'll have to put on a manual catch as the magnet isn't strong enough).
The frame is much better now as there's no ring at all when I hit the frame with something metal or flick it with my fingernail instead, I get a nice dull thud.
If the interferometer shows that things are stable again, I know that I'll still have one problem to fix and that is the creep that I've seen in the interferometer. There's a slow movement of the fringes over time that I've attributed to temperature changes but I need to continue testing to make sure that's what's happening as opposed to a component settling continuously.
This morning I set up an interferometer and had just enough time to level the table before going to work. What I saw was initially promising in that the table appeared relatively stable. I could see some movement when I walked in place a few feet away but it settled down quickly and I didn't see anything in the way of component or thermal creep.
After arriving home I set up the video camera, a two-way radio so that I tell the recording what I was doing and started filming.
As the camera was running I walked heavily around the house, opened and closed doors and just generally tried to create vibrations to see what made it to the table. While all this was going on I was keeping a running monologue going into the radio about what I was doing.
After letting the camera run for about half an hour I increased the air pressure in the tubes to an average of 2 1/2 inches (I don't have a gauge that will measure such a low pressure) and let the table settle for ten minutes. At this pressure I found that I could walk lightly a few feet away from the table and see even less movement than before.
Once again I set the camera to rolling and wandered about the house with the radio in my hand, making noise. I didn't have time to review the videos but have noticed that the fringes settle faster and the table is less sensitive to movement nearby.
While downloading the video to my pc I took the interferometer apart and began setting up for a single-beam reflection test. This particular setup is somewhat simplified from the last in that I've removed one transfer mirror and am largely using only 1/2 of the table. Unfortunately work beckons and I'll have to wait for this evening before I can do a new test hologram.
7:00 pm: I did two single-beam reflection tests this evening, the second of which
turned out nice and bright. The first one was dim because of some unintended double
exposure. I set everything up, measured the beam intensities and left the
lab to program R2 for an exposure. I discovered that R2 had somehow
locked up with the display turned on and I had to disconnect the power to reset it.
When I tried to run the shutter program I found that the power reset had cleared
program memory and the shutter was opened briefly instead.
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