Parts I and II are part of my original blogging back before I started using blog software.
This is a continuation of the some of the discussion from this thread.
I’ve done a ton of testing over the last few days, changing air in my inner tubes, swapping lasers and optics, moving my big laser off the table, wandering around the house doing various housely things and recording what happens to the fringes in an interferometer.
Here’s what I’ve found.
1) My new plateholder isn’t stable enough. When it’s used as the beamsplitter holder (with plate glass as the BS) there is a noticeable decrease in stability. Rattling of the window screen outside my office (caused by the wind) will show up a moment later as a small 1/8 fringe jitter. Walking around upstairs will show up as full wavelenth (WL) shudders and opening and closing some doors in the house does the same. Swapping the holder out for a dedicated beamsplitter results in a lack of the aformentioned fringe behavior. In fact, I can walk in-place lightly about 10′ from the table and only see about 1/20th WL movement with the dedicated BS and would see full WL shudders with the plateholder.
2) The Air conditioning must not be on. I thought I could get away with leaving it on auto during a session because I’d done it many times and hadn’t had any apparent problems. I have a canopy around the table to keep any currents out but the real problem is changes in pressure causing the inner tubes to change shape and my table to twist (see #3). This shows up in the fringes as creeping or what I call breathing where the fringes move back and forth somewhat rhythmically or sometime move suddenly and back into place as if the table is gasping and then letting out air slowly.
3) My table could benefit from being stiffer. I’d originally designed it with 4 layers of hexcel but had good results when leaving it as two layers resting together. IE, I’d effectively created to tables, one just resting on top of the other. For more information see this thread. At this point I either need to bond the two halves together or just remember to keep the AC off while shooting. Or both.
4) Count-weights are a good thing. Some time ago I built scaffolding over my table the the dual intention of putting my big laser on it and hanging components from it. I’ve done the former but not the latter and during my recent testing I’ve found that the weight imbalance from the big laser was increasing the movement triggered by pressure changes in the lab. Adding a few small sand bags at the other end of the table took care of that problem.
5) 2lb of pressure in each inner tube is the best for this table, half loaded. I’ll have to do more testing with all the mounts on the table (my table fully loaded looks like a forest of mounts) but at least I’ve verified that pressure still is the ideal for my table.
I’m going to fix my plate holder today and run another video test but it looks like I may be good to go for a while longer once that’s done.
