Coronado PST Etalon tuning anti-slip bearing modification

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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Rusted » Sun Mar 29, 2020 6:59 am

Thank you.

A fascinating and well illustrated mod.
For which I have been waiting patiently following my attempts at etalon motor drive.
My removing the original O-rings freed up the etalon for easy driving but now it is too floppy.

What we really need now are accurate dimensions of this black packing/bearing ring of yours.
This will help to identify an everyday source. Is it hard plastic? Rubbery? Flexible? Rigid?
Do you know its original purpose? Any sign of a manufacturing serial number or maker?

Thanks again. :bow


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Merlin66 » Sun Mar 29, 2020 7:30 am

Hmmm,
Mark produced “anti tilt” spacers for the PST.
To tune the PST etalon in must be allowed to tilt.....


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by AndiesHandyHandies » Sun Mar 29, 2020 8:27 am

Hi

What is actually in the PST body?

And how does turning the ring tilt the PST?

What is the orange bit?

Andrew.



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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Merlin66 » Sun Mar 29, 2020 8:49 am

Ok,
The orange “washer” also has a additional small block or piece attached.....you can see it if you look at a ring.
As the tilt ring is rotated the etalon is pushed against this ring and the small block causes the etalon to tilt and change the tuning.
Basic tilt tuning of the etalon.


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Rusted » Sun Mar 29, 2020 11:50 am

I'm not arguing here: Honestly! :mrgreen:

But Andrew's packing mod is taking place on top of the etalon.
So he has not changed the orange packing ring. Nor its purpose.

I'm off to have another look at my own PST etalon.
From [increasingly fuzzy] memory it has no extra tilting block on the orange ring.
That doesn't stop it from tuning.

I also have black, white and orange plumbing washers/seals in assorted sizes and materials to play with. ;)


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Rusted » Sun Mar 29, 2020 1:40 pm

Sorry, I thought I had discovered something interesting. But can't connect the looseness of my outer shell [tuning] ring with the etalon's own behaviour hidden inside its housing.

I have measured the depth of every recess in the etalon's housing components and can find no sloping surfaces which might press the etalon by one edge alone to tilt it.

Without a [raised] tilting block the sponge can apply no asymmetrical pressure to tilt the etalon. The sponge ring is symmetrical and of equal thickness all around. The machined bed on which it rests seems to be orthogonal.

How much etalon tilt is actually involved in practice? Perhaps the tilt required is too small to allow easy measurements even with a Mitutoyo vernier caliper?


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Merlin66 » Sun Mar 29, 2020 10:41 pm

The Lunt patent on the PST is readily available...
https://patents.google.com/patent/US7149377
The attached extract when read along with Fig 13 and Fig 14 of the patent may give an insight to the tuning method.
Extract_Lunt_Patent.pdf
(35.3 KiB) Downloaded 97 times
Patent_Fig13.JPG
Patent_Fig13.JPG (38.3 KiB) Viewed 3552 times
Patent_Fig14.JPG
Patent_Fig14.JPG (25.34 KiB) Viewed 3552 times


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Rusted » Mon Mar 30, 2020 6:31 am

Many thanks for that. I had no idea Lunt was responsible for the basic Coronado PST design.

The reason for my confusion was trying to square your mention of a tilting block on the sponge ring with my complete lack of one. :shock:

I had already guessed that only perpendicular compression was needed to tune my etalon.
Then discovered that my etalon was completely slack and rattled freely over half of its normal tuning range.
This was when the etalon was set on band by centring the original tuning hole in the housing cut-out.

None of this would be obvious unless the etalon was dismantled to allow it to be tuned with an external rod in the edge drilled, tuning disk.

Say I apply more pressure from the tuning disk, simply to maintain enough compression for mechanical, location reasons.
Then it should go off-band and remain out of reach until comprpession is re-applied by reversing in the opposite direction.
Which makes absolutely no sense at all. Perhaps there is another H-a band nearby if I apply far more pressure?
This seems highly unlikely and would make the tuning even stiffer at the other end of the tuning range.

Yesterday, we had freezing gales so I couldn't make any useful tuning trials on the sun to confirm or deny my findings.


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Merlin66 » Mon Mar 30, 2020 6:54 am

The tuning range is only from about 2A above H alpha to down in the blue towards the continuum.


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by AndiesHandyHandies » Mon Mar 30, 2020 7:15 am

Hi

So when I tune the PST etalon and a reflection wobbles about does that imply there is something lose inside the housing or is it normal?

Andrew.



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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by marktownley » Mon Mar 30, 2020 7:21 am

AndiesHandyHandies wrote:
Mon Mar 30, 2020 7:15 am
Hi

So when I tune the PST etalon and a reflection wobbles about does that imply there is something lose inside the housing or is it normal?

Andrew.
It's normal, it would just be better if it didn't wobble at all :D


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Rusted » Mon Mar 30, 2020 3:06 pm

Since the tuning point is fixed you could add a tubular spacer to contain the etalon assembly.
Something soft, like PVC, rather than hard metal. You'd probably need to bore and turn this tube to exact size in a lathe.
What about taping the etalon to stop it moving laterally if it still does so at best tune? Unlikely as it is under compression.
Beyond best tune, in either direction, it hardly matters. In my case there is no rattle at the best tuning point.
My etalon tunes identically for both proms and surface.

There are fixed, mechanical limits on tuning range for the PST etalon.
I had the etalon arc open to allow maximum tuning range with a thin rod.
As the tuning disk rises, on its coarse thread, it is stopped by the plate above it which carries the negative lens.
As the tuning plate sinks down, with tuning, it cannot compress the solid mass of the etalon beyond a certain point.
The poor mechanical advantage of the coarse thread probably protects the etalon from being crushed by over-torquing.

The factory chosen screw hole isn't quite centred on mine but is close enough.
I have discovered that my PST etalon works slightly better in reverse than the original, factory orientation.
This is [arguably] purely psychological and I was just lucky with the seeing this morning after the etalon reversing procedure.


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Merlin66 » Mon Mar 30, 2020 9:38 pm

OK guys...here's the definitive answer on how the PST etalon is actually tuned!
After almost 15 years of guessing/ discussion/ debate etc. I managed to get a reply from my contact who has worked with assembly and testing of the PST in production....
I asked the question:

The options I think are:

1. The PST etalon tuning is achieved by tilting of the etalon
2. The PST etalon is tuned by pressure applied to the spacers in the etalon
3. The PST etalon is tuned by increasing/ decreasing the atmospheric pressure around the etalon
4. Another method????

The answer.....drum roll..... #4 another method!!!

The PST uses compression tuning. The Teflon ring in the chamber is the main point of pressure. The silicone ring ( the orange foam) is designed to smash the etalon until it comes into tune. Since it never compresses evenly the silicone ring is trimmed and cut so that it applies pressure as evenly as possible. This can only be done while applying pressure with the rotational compression ring and trimming parts of the silicone while looking through the chamber at a hydrogen spectrum light. It is not compressing the spacers at all, it is deforming the plates around the spacers. That is why the silicone is trimmed to even out etalon while it is crushed.

I don't think any of us figured on that answer.......


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Rusted » Tue Mar 31, 2020 5:59 am

Thank you kind sir. :bow

You really are incredibly good value to have around! A veritable, walking encyclopedia. :D

So, this raises the spectre that:
A: Rotating the Teflon ring.
B: Dismantling the etalon housing.
C: Rotating the etalon sandwich.
D: Inverting the Teflon ring.
Will all produce unknown changes in performance.

Do any of these changes simply adjust the centre point in the tuning disk holes?
Or might it adjust the bandwidth?

Remember that the compression force is manually adjustable.
We could even end up with gurus offering "snake oil" etalon, tuning services.
Like they do with a certain, infamous, Scottish, HiFi turntable. :mrgreen:


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by marktownley » Tue Mar 31, 2020 6:49 am

Good info Ken


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Wah » Wed May 20, 2020 6:47 am

Merlin66 wrote:
Mon Mar 30, 2020 9:38 pm
The PST uses compression tuning. The Teflon ring in the chamber is the main point of pressure. The silicone ring ( the orange foam) is designed to smash the etalon until it comes into tune. Since it never compresses evenly the silicone ring is trimmed and cut so that it applies pressure as evenly as possible. This can only be done while applying pressure with the rotational compression ring and trimming parts of the silicone while looking through the chamber at a hydrogen spectrum light. It is not compressing the spacers at all, it is deforming the plates around the spacers. That is why the silicone is trimmed to even out etalon while it is crushed.
Hi Merlin, does it mean it works like a lever, while the spacers work as fulcrums, the orange foam works as the effort and the plate works as the load?



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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Merlin66 » Wed May 20, 2020 6:54 am

Wah,
Basically yes. The pressure/ counter pressure close to the edge seems to distort the plates to effectively increase the gap between them.


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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Wah » Thu May 21, 2020 5:32 am

:seesaw :seesaw :seesaw :seesaw :seesaw
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Re: Coronado PST Etalon tuning anti-slip bearing modification

Post by Merlin66 » Thu May 21, 2020 5:46 am

Wah,
According to the description given....yes, something like that!
I was told it definitely was not tilt tuning, or that the spacers were flexible enough to be squeezed.....


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