29.03.2020 PST etalon components.

*

Sunday 29th March: Today I read the familiar description of how the PST etalon works:

The tuning disk [or visible arc] is rotated via the hidden drive screw. Causing the etalon to tilt. The tilt is what causes the change in tuning band around the nominal H-alpha frequency. 

There was the additional information about a block being added to the soft, orange, foam ring on which the etalon unit rests. So that pressure from above automatically cause the additional block to tilt the etalon against the soft base. 

My problem as that my orange ring has no additional tilting block. So how did turning the tuning ring cause the etalon to tilt? It was very obvious, in use, that rotation of the disk did change the tune of the etalon. But how was it doing so in my case? 

 I proceeded to completely dismantle my PST etalon unit. I was thinking that the etalon tilt must be applied somewhere else.

So, out came one of my well used Mitutoyo, vernier calipers and I proceeded to measure the depth of every recess in the housing looking for variations which might tilt the etalon.

I even removed the white Teflon O-ring in the compression/tuning plate and measured the thickness of that. It was perfectly symmetrical in thickness [minor diameter] all the way around.

So then I measured the depth of the recess on top of the re-inserted Teflon ring. No variation there.

The etalon glass sandwich certainly doesn't rotate. Though there is a coarse thread on the compression plate which presses onto it.

I measured the sponge ring thickness and it was identical all the way around. There is no obvious variation in density.

Could the sponge ring be sitting on a sloping bed? That would have a similar effect to a tilting block. It would not be impossible to machine a tilted bed using an advanced CNC machine but it wouldn't be easy.

How much etalon tilt is actually required? Could the angle be so small as to defeat my clumsy measurements? The wind has dropped overnight so I hope to do some real life trials on the sun. With the etalon open and available for continuous tuning in either direction using a suitable rod in the tuning disk holes. I suspect the tuning is a result of linear compression of the etalon gap. But this is pure guesswork.

 Extract_Lunt_Patent.pdf

 https://patents.google.com/patent/US7149377

*