SM40 damaged etalon -possible repair???

[Merlin66]

I'll share the trials and tribulations of George's SM40 etalon.....
The story so far.....
He was unlucky enough to drop the etalon/scope/ camera onto the concrete patio!
Everything survived....but
When looking through the re-assembled scope, it gave multiple images! Obviously something wrong. The scope and camera checked out OK - that just left the etalon.
He's sent it to me to "look at".
Before attempting to strip it down (Thanks Mark for some great info...) I thought it might be best to at least test it first...
Hmmm in the spectroscope it showed the typical transmission "comb" of an etalon - interesting!
But I found, like George, that it also gave multiple images, you can count around five...I tried the PST BF and a couple of BF10/ BF15 filters with the same visual results....

I'm slightly puzzled by this outcome..The pass bandwidth of the blocking filter is around 8A...does this mean that the etalon is now producing 5 "spikes" in the space of 8A?????
How else could additional images be seen???
The spectroscope "comb" certainly doesn't look much different from that obtained from the fully functional SM60 I use - I'd have thought a 0.7A "spike" every 2A or so would look completely different!!
Any comments/ words of wisdom?
I'll start the strip down later today and give a running commentary to the group....

[swisswalter]

Hi Ken

thank you for the report. Good luck in fixing the etalon. Maybe there is only one spacer decontacted and we have now a wedge type of gap between the plates

[marktownley]

Those are my thoughts too Walter...

[gbealnz]

God on ya Ken, I've already suggested to George via Skype that you simply drop is from a similar height, but on the other side of the SM40, thereby shaking it back into place. He for some reason opted to send it to you instead.
It will be good to see and hear how this goes.
Gary

[Merlin66]

Gary,
I did think of that ;-)
but decided on a more rigorous scientific approach!

[MjrTom]

Glad the info on stripping the SM40 came of use to you

I have now managed to suss out a method of attaching the SM40 to my Solarscope SV50 using a 62-52mm stepdown ring and it works a treat.

Good luck with sorting out the SM40 etalon.
How about maybe using the edge of a sheet of paper as a makeshift gap gauge to see if infact the two halfs of the etalon are no longer parallel? Maybe using differing thickness of paper to get the required spacing and just slide the corner of it in to measure the gap?

Just an idea

Mark

[Merlin66]

Mark,
Yes, your comment's and photos I'm sure will be VERY helpful.
I will take a before and after spectroscopic hi-res spectrum (1800 l/mm grating) of the SM40(also the SM60 as a "benchmark master") before/ after dissassembly.
I have quite a few precision gauges I can use to assist the evaluation (I do build spectroscopes ;-) )
Keep you (and George) posted.....

[colinsk]

Glad the info on stripping the SM40 came of use to you

I have now managed to suss out a method of attaching the SM40 to my Solarscope SV50 using a 62-52mm stepdown ring and it works a treat.

Good luck with sorting out the SM40 etalon.
How about maybe using the edge of a sheet of paper as a makeshift gap gauge to see if infact the two halfs of the etalon are no longer parallel? Maybe using differing thickness of paper to get the required spacing and just slide the corner of it in to measure the gap?

Just an idea

Mark

Even glass is to unstable for spacers. There are made from zero expansion materials or tuned in an oven.

[MjrTom]

I'm not suggesting using the paper as spacers just as a gauge of the spacing

[Merlin66]

The attached photos show the "bare" etalon after the cover is removed....
The dark spots you see at the edges are the dabs of RTV holding the etalon in the housing.
There are six on each plate - it looks like they were injected through small access holes in the housing. The etalon sits on a foam ring.
I'll slowly drill through the holes and remove the RTV....this should allow the etalon to be removed from the housing. I'll note the position and direction of all the parts - in case we get lucky and want to re-assemble!
I assume one or more of the 5(?) small glass spacers is no longer in "optical" contact. I did try the application of some fair pressure in the hope of re-setting the spacers, but this didn't happen.
Before I did any further strip down I checked the spacers with a fluoro lamp/ filter (pseudo monchromatic light) there were two out of the five which showed bands...these are the de-contacted spacers...
Following the previous comments on repairs - I applied a fair amount of thumb pressure on these points but they refused to re-contact..
The six holes and dabs of RTV in the front plate (bottom of holder position) were drilled out with a 2mm drill ( you could see the fringes clearly in the two rogue spacers) and the etalon pushed out....immediately the plates separated!
Checking each internal surface with a very bright light and x30 loupe I found that four of the spacers were still sitting on one plate and the fifth on the other....It's interesting to note that the shape and size of the spacers varies - "ragged squares" comes to mind.
The next step will be to clean the plates with Acetone and attemp to re-contact them.
Also, the centre "spots" - the central area of the etalon plates is clear and non reflective - but I was unable to see/ find and evidence of a central spacer???? (See below ***)
As the RTV "clips" don't exactly apply any pressure to the etalon plates I sincerely hope that the re-contacted etalon becomes more "solid" - getting it back into the housing and re-applying the RTV will be "interesting"
*** After very careful searching I've found a VERY small , 2mm x 0.5mm? spacer sitting in the centre of the etalon plates (under the central spots)
It'll be interesting as to how it will re-contact.
I don't trust the Acetone I have so I'll get some new stock before continuing.
No great news in all this but the story continues.

[swisswalter]

Hi Ken

thanks for sharing the progress on the project. The internals of the SM40 look very similar to the LUNT LS50 except for the central spacer of course.

Very pure solvents, aceton or alcohol is important.

[Merlin66]

Walter,
managed to pick up some pure Acetone.....
Stay tuned....

What about the centre spacer - any comments from the team????

[swisswalter]

Hi Ken

I wouldn't touch it at all if possibel

[Merlin66]

It's so small compared with the others I thought for a moment it must be a broken part from a larger piece - checked, checked and triple checked both surfaces but no other parts found....

[MjrTom]

When I took the housing apart to remove the ERF I did think that the foil disc did seem overly large....