Coronado PST MOD2 Black RING in the middle of sun!

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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

OK, I'll retest a PST etalon at the next opportunity.
In the meantime some reading matter on etalon tuning.
http://www.astrosurf.com/viladrich/astr ... P-Coro.htm
etalon_tilt_pressure_v4.xls
(293.5 KiB) Downloaded 106 times
Coronado 60 review.pdf
(571.38 KiB) Downloaded 95 times


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi

I have thought it would be neat to colour code doppler shifted images and 3D layer them in Photoshop.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

Astroart can now present a GIF file which shows that animation.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi Ken,

Thanks for the reading matter.

This shows why you need to be on the centre comb I think.

https://www.researchgate.net/figure/Fil ... _282972240

Cheers. Andrew.
Last edited by AndiesHandyHandies on Sat Sep 25, 2021 9:15 am, edited 1 time in total.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi

RodAstro observed the M class flare yesterday with his Quark and was concerned as it looked nearly white and bright if he was safe.

I asked here if anyone had done a spectrum of a Quark but no answer. Concern is long IR really. Whilst you are testing the PST would it be possible to do a full spectrum on a bare quark? I could not find any info on the web before.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

Andrew,
If you send me a Quark, I’d be happy to test it.
I don’t have one, and there are none in use locally.
We’re in lockdown and can only travel 5km.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

OK, Limited opportunity but did collect some data:
1.The limit of rotation for the PST adjuster is approx 160 degrees
2.Looking from the back (black box side) Extreme RHS tuning is well into the blue wing. Extreme LHS is just beyond Ha CWL into the red wing. The Ha CWL is approx LHS minus 10-15 degree.
3. The orange ring is compressed as the tuner is rotated towards the LHS i.e. towards the red wing.
The PST orange ring I tested has only one "foot".


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Merlin66 wrote: Fri Sep 24, 2021 10:24 pm Andrew,
If you send me a Quark, I’d be happy to test it.
I don’t have one, and there are none in use locally.
We’re in lockdown and can only travel 5km.
Hi

Fair enough.

That is a bit limiting.

Thanks. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Merlin66 wrote: Tue Sep 21, 2021 1:24 am OK, here's a copy of the original email:
Hi Ken,
The PST uses compression tuning. Probably the worst way imaginable to tune an etalon. The Teflon ring in the chamber is the main point of pressure. The silicone ring 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 feet at all it is deforming the plates around the feet. That is why the silicone is trimmed to even out etalon while it is crushed. The compression itself is not capable of enough force to compress the feet. And compression the feet since they are fuse silica as well would in fact crack crush or powder the feet to a point of loss of contact. Thus destroying the etalon.
Hope that helps.
Hi Ken

"The PST uses compression tuning."

later

" The compression itself is not capable of enough force to compress the feet. And compression the feet since they are fuse silica as well would in fact crack crush or powder the feet to a point of loss of contact. Thus destroying the etalon. "

How is this possible?
"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."
Not compatible with the various no of feet seen from zero to two. Surely look through the etalon and then stick a foot on based on that.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Merlin66 wrote: Sat Sep 25, 2021 1:37 am OK, Limited opportunity but did collect some data:
1.The limit of rotation for the PST adjuster is approx 160 degrees
2.Looking from the back (black box side) Extreme RHS tuning is well into the blue wing. Extreme LHS is just beyond Ha CWL into the red wing. The Ha CWL is approx LHS minus 10-15 degree.
3. The orange ring is compressed as the tuner is rotated towards the LHS i.e. towards the red wing.
The PST orange ring I tested has only one "foot".
Hi Ken

So if its going towards the blue turning it from the LHS where its red its tilting. Valery reports the reflections off the etalon moving as its tuned. And I have seen pictures of the reflection at the front of a PST doing just that.

So the orange ring with or without feet is holding the etalon tight on the teflon ring at the LHS. As it is tuned towads the RHS the pressure reduces and the uneveness in the foam, aided by a foot/feet where needed, allows the etalon to tilt, off the teflon ring OR just a bit or reduction in the pressure allows the teflon to rebound where the pressure is less, and it is working by compression but of the teflon, which needs less force I presume than compressing spacers. Its not enough to bend the etalon plates or there would be banding issues.

"When you have eliminated the impossible you are only left with the possible".

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi Ken

Cogitating in the bath.

Or its relying on machining tolerance in the adjuster thread? Which would be in a random orientation and depend on how the etalon housing thread seats. So testing has to be done in situ for tuning and the optional feet.

If you take an etalon apart and remove the orange ring it needs to be indexed so its put back in the same place. Has anyone taken the ring out and heated it gently to get it to re-bound?

Try wrapping a bit of PTFE round the adjuster thread. Does it change the rotation needed to tune? There seems quiet a bit of leeway. Is it then more sensitive? Does the sweetspot wobble about less as you tune?

Would lapping the adjuster threads improve tuning then? Danger of upsetting the orientation I suppose.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by marktownley »

AndiesHandyHandies wrote: Sat Sep 25, 2021 10:53 am Try wrapping a bit of PTFE round the adjuster thread. Does it change the rotation needed to tune? There seems quiet a bit of leeway. Is it then more sensitive? Does the sweetspot wobble about less as you tune?
I've done this, there is less movement of the sweet spot.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi

Actually is the centre of the teflon ring smaller than the centre of the orange ring? If so then the simplest explanation is the highest/stiffest part of the Orange ring is applying tilt by it pivoting on its side of the teflon ring. The etalon glass is stiff enough not to be distorted by the force required. Feet have to added if the variations in the stiffness of the orange ring are not enough, or maybe to account for manufacuring tolerances.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi Ken,

When the adjuster is on the LHS the orange ring is under most pressure and the etalon is pressed against the teflon ring. Mark confirmed that putting teflon tape on the adjuster ring the sweet spot moved about less, so I do not think its slop in the adjuster causing any tilt. When it pressed against the teflon ring the etalon should be at right angles to the optical axis, which is needed to be near the centre comb. CV says it needs to be within a degree orthogonal and half a degree is needed for tuning, one degree between combs.

When the pressure on the teflon ring mounted on the adjuster ring decreases then the differential pressure from the orange ring comes into play, accentuated by the extra stiffness, foot, or feet for 'the special one'.

There are two possible mechanisms:

1. Decrompression on one side of the teflon ring allows it to expand, opposite where a stiffer spot or the added feet are, and so the etalon tilts. So its de-compression tuned.

2. The feet or stiffer spot keep pushing that side of the etalon and it tilts down on the other side with the teflon ring under the feet acting as a fulcrum where the pressure is now lower. My best bet at the moment.

In either case the direction the etalon tips in should be orthoganol to the foot in your case, useful you have one with a foot.

How can we tell which one it is? Cogitating with an icepack on my back.

We need to set the PST on tune and then rotate the ajuster ring back to the LHS. We need to integrate the force needed to compress the orange ring. When its tuned back the integrated force is acting on the teflon ring on one side as the other side de-compresses. Stick a bar into one of the tuner holes and press it with a set of scales. I did that on my 16" dobsoinian with the roller bearings on the bottom and normal teflon bearings and plastic tube on the side. Luckily the stiction was 500g and the friction was 300g on both axis, which is why its easy to push at 45deg. We then need CV to calculate if the teflon modulus of elasticity and the net force will produce the right tilt.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

Merlin66 wrote: Sun Sep 26, 2021 6:35 am Andrew,
As the pressure reduces on the orange ring, the CWL tends towards the blue.
When there’s no pressure on the orange ring (and we assume also on the etalon) it is sitting well into the blue wing.
What mechanism would move the CWL under increasing pressure towards the red????


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

Just posted a correction.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

AndiesHandyHandies wrote: Sat Sep 25, 2021 9:22 am
Merlin66 wrote: Tue Sep 21, 2021 1:24 am OK, here's a copy of the original email:
Hi Ken,
The PST uses compression tuning. Probably the worst way imaginable to tune an etalon. The Teflon ring in the chamber is the main point of pressure. The silicone ring 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 feet at all it is deforming the plates around the feet. That is why the silicone is trimmed to even out etalon while it is crushed. The compression itself is not capable of enough force to compress the feet. And compression the feet since they are fuse silica as well would in fact crack crush or powder the feet to a point of loss of contact. Thus destroying the etalon.
Hope that helps.
Hi Ken

"The PST uses compression tuning."

later

" The compression itself is not capable of enough force to compress the feet. And compression the feet since they are fuse silica as well would in fact crack crush or powder the feet to a point of loss of contact. Thus destroying the etalon. "

How is this possible?
"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."
Not compatible with the various no of feet seen from zero to two. Surely look through the etalon and then stick a foot on based on that.

Cheers. Andrew.
Hi Ken,

Have you seen signs of the orange ring being butchered with a knife?

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Merlin66 wrote: Mon Sep 27, 2021 4:42 am
Merlin66 wrote: Sun Sep 26, 2021 6:35 am Andrew,
As the pressure reduces on the orange ring, the CWL tends towards the blue.
When there’s no pressure on the orange ring (and we assume also on the etalon) it is sitting well into the blue wing.
What mechanism would move the CWL under increasing pressure towards the red????
Hi Ken,

Whats the correction?

"As the pressure reduces on the orange ring, the CWL tends towards the blue."
You said with the adjuster ring turned towards the LHS from the back that the foam was fully compressed. I assume that despite the 'feet' all the foam is compressed and the etalon is pressed all round onto the teflon ring. If the compression in the teflon ring is causing the tilt it should be tilted towards the 'foot' a bit, but not enough to be more than half a degree from orthogonal to be on the central band as it then should tilt away from the foot.

"When there’s no pressure on the orange ring (and we assume also on the etalon) it is sitting well into the blue wing."
When is there no pressure on the orange ring then? When the adjuster ring is rotated to the RHS? How can you tell there is no pressure? How can you tell if the 'foot' is not still pressing the etalon onto the teflon ring?

"What mechanism would move the CWL under increasing pressure towards the red????"
The etalon is not being tuned by compression of the mica feet I presume. As its clearly being tilted as the adjuster ring is rotated to the RHS. And the CWL is moving towards the blue meaning it must be tilt tuned.
As you tune towards the RHS the reflection off the front of the etalon should move away from the 'foot'.
When you start to move towards the RHS from the LHS the back pressure from the orange foam decreases. Because there is more pressure on one side, in the plain foam ring or aided by foot or feet (foot), as the adjuster is rotated one side of the etalon opposite the 'foot' experiences less pressure. At some point that can reduce enough for the differential compression across the teflon ring to tilt tune the etalon, but with pressure still on the other side. Thats the differential pressure we need to try and measure to see what the 'differential rebound' of the teflon ring on that side would be and if thats enough across that diameter of the teflon ring from the 'foot' to tilt tune the etalon.
If its not enough the tilting from the orange ring being bigger? than the teflon ring can also tilt it in the same direction away from the 'foot'.

As there are two possible ways the same tilt can be imparted it hard to tell which is predominant or whther both are in play.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

Andrew,
""
How can you tell if the 'foot' is not still pressing the etalon onto the teflon ring?
""
You can go to the extreme and see the orange ring "go loose" within the assembly. As you continue to rotate the tuner the etalon appears to turn but the orange ring remains stationary.
I don't see any evidence of tilting. The rear of the etalon appears in complete contact with the white teflon ring at all times.
If the etalon tilted would it not show in the contact with the teflon ring?


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi Ken,

"You can go to the extreme and see the orange ring "go loose" within the assembly. As you continue to rotate the tuner the etalon appears to turn but the orange ring remains stationary."
Do you mean turning it to the far RHS? If the orange ring de-contacts with the etalon what holds the etalon against the teflon ring? In Marks diss-assembly video the etalon is loose in the cell and covered in clear grease. Clearence round the edge of the etalon. A depression in the orange ring a bit off centre.

"I don't see any evidence of tilting. The rear of the etalon appears in complete contact with the white teflon ring at all times.
If the etalon tilted would it not show in the contact with the teflon ring?"
If compression in the teflon ring is doing the tilting it can be in contact all the time. If the etalon spacers were being compressed to tune the CWL would shift to the red? This would mean the teflon is much more compressible than the mica feet?

If each etalon is checked and for some a foot or feet ( or bits carved off the foam? ), are added in order for it tune properly then they must be applying a off-axis additional force for a reason?

I assume the teflon ring is attached to the underside of the tuning ring. As it appears to be in the video.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi

I have read the patent and the text talks about using pressure actuators to pressure tune an etalon!
And also speculates on springs or other elastic materials for the spacers.

The drawings do look like a PST though, which we know has no pressure actuators.

Ken has shown the CWL of the PST shifts towards the blue as its tuned which is the Hallmark of tilt tuning.

https://patents.google.com/patent/US7054518

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

Andrew,
I've never seen an orange ring "notched out" only the added foot.
"""If compression in the teflon ring is doing the tilting.."" I don't believe the teflon ring is being compressed/ distorted. I think it's just a mechanism to apply uniform pressure on the etalon.

"" the CWL of the PST shifts towards the blue as its tuned""" Yeah, but............I equally could have said the CWL moves towards the red wing as it's tuned. What's the definition of "tuning"
In the "rest" position - no movement of the tuner, and minimal/ non pressure on the etalon, sitting on the RH stop the CWL is in the blue wing. Then "tuning" does take you to the red wing.......


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

AndiesHandyHandies wrote: Mon Sep 27, 2021 10:08 pm Hi

I have read the patent and the text talks about using pressure actuators to pressure tune an etalon!
And also speculates on springs or other elastic materials for the spacers.

The drawings do look like a PST though, which we know has no pressure actuators.

Ken has shown the CWL of the PST shifts towards the blue as its tuned which is the Hallmark of tilt tuning.

https://patents.google.com/patent/US7054518

Cheers. Andrew.
Hi

Looks like the PST drawings were used to describe old ways with a noddy wedge added for non uniform pressure. And a child drew the last diagram with springy bits. Flying a flag to try and get priority on 'novel use' for solar filters for mainstream technology elsewhere.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Merlin66 wrote: Mon Sep 27, 2021 10:49 pm Andrew,
I've never seen an orange ring "notched out" only the added foot.
"""If compression in the teflon ring is doing the tilting.."" I don't believe the teflon ring is being compressed/ distorted. I think it's just a mechanism to apply uniform pressure on the etalon.

"" the CWL of the PST shifts towards the blue as its tuned""" Yeah, but............I equally could have said the CWL moves towards the red wing as it's tuned. What's the definition of "tuning"
In the "rest" position - no movement of the tuner, and minimal/ non pressure on the etalon, sitting on the RH stop the CWL is in the blue wing. Then "tuning" does take you to the red wing.......
Hi Ken,

Before the tuning starts the etalon needs to be within 1 degree of orthogonal. I tune my PST etalon afresh by turning to the LHS and then towards the RHS and the small sweet spot soon appears and as I keep moving becomes an expanding circle.

The modulus of elasticity for glass is 50 and for PTFE is 0.5, in modern units, so the PTFE is 100 times more compressible.

At the LHS the orange foam is firmly pressing the etalon on to the PTFE ring then onto the adjuster body, which should be whithin 1 degree of the etalon housing axis I would hope. On the far RHS if there is no contact between the orange ring then the etalon can be anywhere, as its not a sliding fit and loose in the housing.

If the manual testing indicates a foot needs adding then a larger off axis force is needed.

Where is the refection off the front of the PST when on the LHS? And how does it move as you start turning the tuner to the right?

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi

As you tune the PST from the LHS past the sweet spot the spot expands into a ring.

I would expect the tilt and compression tuned etalons have the rings go the opposite way?

So in the DS Ken tested the he should see that difference, and he knows the back etalon is tilt tuned and the front etalon has Richview compression tuning.

Cheers. Andrew.
Last edited by AndiesHandyHandies on Tue Sep 28, 2021 9:40 pm, edited 2 times in total.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi

And the reason the feet need to be added is the press the etalon onto the PTFE ring so the CWL is in the red prior to tuning. As the cleaved or manufacture spacers will have a tolerance and the rest position might not be on the right side of the centre comb.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by marktownley »

Interesting thread chaps! Enjoying reading the discussion.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Bob Yoesle »

A few things I wanted to emphasize. There are three ways to tune an etalon:

1. Change refractive index of the etalon gap - usually via air pressure changes.
2. Change the etalon gap spacing - piezo electric, mechanical compression; heating solid etalons.
3. Tilt the etalon.

There appear to be no other implementations used in commercially made solar etalons, and compression tuning is pretty much as described in the applicable patents.

The spacers used in commercially available air-spaced etalons are made from polished fused silica or other ULE materials, not mica - which would have an unsuitable CTE to maintain on-band tuning with temperature variation.

An air-spaced etalon that is air-pressure tuned has a native CWL lower (i.e. blueward) of the H alpha emission. Increasing the air pressure increases the refractive index of the etalon gap, and shifts the CWL redward to get the etalon on-band. See the etalon equation.

Any etalon that is tilt-tuned has a native CWL that is higher (i.e. redward) of the H alpha emission - including the RichView etalons which are also tilt-tuned.

Tilting the etalon shifts the etalon blueward to get the etalon on band. Inferring that a tilt-tuned etalon has an increased etalon gap path, and therefore the increased gap spacing shifts the etalon blueward appears erroneous -- along with the corollary that compressing the etalon spacers and narrowing the gap shifts the etalon CWL redward. If you again examine the etalon equation, you will note that decreasing the refractive index (vacuum (decreased air pressure) tuning) of the etalon gap, or decreasing the etalon gap spacing, and tilting the etalon - will all decrease the CWL.

This is exactly what is seen when observing mechanical pressure applied to a tilt-tuned etalon with a Hydrogen spectrum tube - the etalon CWL is shifted blueward to be on-band. If the CWL shifted redward with spacer compression and the resultant smaller etalon gap spacing, the etalon would only go further off-band and demonstrate a larger central interference ring.

Using the Light Machinery Etalon Designer will show that slightly decreasing the air-spaced etalon gap will shift the CWL blueward:


Gap spacing 0.50000 mm:
native spacing.jpg
native spacing.jpg (249.15 KiB) Viewed 4957 times


Gap spacing 0.49999 mm:
Compressed.jpg
Compressed.jpg (231.42 KiB) Viewed 4949 times

Note that decreasing the etalon gap has shifted all the etalon peak(s) blueward. It seems that if you compress the etalon enough to shift the etalon peaks sufficiently blueward, the adjacent peak will be shifted enough to make it appear the etalon has shifted redward.

Compressing the etalon spacers will not crush or de-contact them, or otherwise damage the etalon, and has been used from the early days of Coronado to tune air-spaced etalons, including the PST:
The glass feet have a Youngs modulus [modulus of elasticity] and are actually very compressible at the optical level. By physically applying pressure directly to both sides of the etalon plates you squeeze the internal feet [spacers]. In fact, you can squeeze the feet enough to move the etalon through a significant CW range with a reasonable force...The physical compression of these systems was used early on as a way of tuning the etalon to the desired CWL. It was abandoned after a few years due to the inability to effectively compress the center foot and the outer feet uniformly. The product was the MaxScope 70. The compression system was re-introduced for the PST. A system that did not have a center obstruction to the etalon to deal with.
Note that if the compression of the spacer(s) is not sufficiently equal, as occurs with only central spacer pressure applied in the RichView etalons with very high native CWLS (i.e. the periphery of the etalon is effectively not tuned), results in the etalon gap differential becoming too great, and the etalon bandpass widens significantly.

I hope this helps to clear some of the issues up.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi

From two posts Ken made doing actual tests with a spectrograph.

Bob,
I think part of your logic is wrong.
As the effective gap increases (i.e, due to tilt etc.) the CWL moves to the blue.
The Richview tuning decreases the gap and moves the CWL to the red.
I use a double stack of SM60 filters; the rear is tilt tuned (T-max) and the front one with Richview tuning and can demonstrate the above with the spectroscope.

OK, Limited opportunity but did collect some data:
1.The limit of rotation for the PST adjuster is approx 160 degrees
2.Looking from the back (black box side) Extreme RHS tuning is well into the blue wing. Extreme LHS is just beyond Ha CWL into the red wing. The Ha CWL is approx LHS minus 10-15 degree.
3. The orange ring is compressed as the tuner is rotated towards the LHS i.e. towards the red wing.
The PST orange ring I tested has only one "foot".

When I re-tune I turn the ring to the LHS from the back view and it only needs turning a bit to show the sweet spot as Ken says above.

And the test on the DS with known tuning methods shows that the tilt tuned one goes to the blue as its tuned and the PST also shows the blue shift so the PST must be tilting.

It seems to me.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Bob Yoesle wrote: Wed Sep 22, 2021 12:48 pm
As the effective gap increases (i.e, due to tilt etc.) the CWL moves to the blue.
The Richview tuning decreases the gap and moves the CWL to the red.
I use a double stack of SM60 filters; the rear is tilt tuned (T-max) and the front one with Richview tuning and can demonstrate the above with the spectroscope.
Hi Ken,

This does not make sense to me.

1. An air-spaced etalon is an etalon which follows the same laws of physics as a mica etalon, and with mica etalons, the etalon is tuned via heating, which increases the etalon gap via thermal expansion, which thereby increases the CWL. DayStar Whitepaper pg 13:
A Fabry‐Pérot etalon changes peak transmission wavelength with changing distance between the reflective sides. This can occur by a change in temperature through thermal expansion of the mica wafer. In order to maintain the center wavelength (CWL) of the Fabry‐Pérot etalon in conditions of varying environmental temperature, a consideration for tuning must be made. An increase in temperature causes an increase of the CWL of transmission peaks. The etalon varies 1.0Å per 17.0 °F (9.4 °C). The Quantum uses a temperature regulated oven to effect tuning and control of wavelength.
So why would an air-spaced etalon perform the exact opposite (decreased gap = increased CWL)?

2. I have used Hydrogen spectrum tubes to observe the change in etalon fringes as compression is applied to the center of a Richview etalon (and the periphery of a LS35), and the CWL is lowered by observing the central donut becoming smaller with compression. Christian has also evaluated a Richview etalon, and notes the native CWL is high, and the etalon is tuned by applying Richview compression. If decreasing the gap via compression increased the CWL, the etalon would never come on-band, and indeed would go farther off-band.

The issue of tilt seeming to increasing the gap distance and resulting in blue-shifting the CWL is indeed a factor which originally had me confused. However, I was assured that tilt blue-shifted the etalon per the etalon equation cosine theta figure, not the presumed increased distance traversed through the gap. Therefore the equation is absolutely correct, and not my original "common sense" interpretation and conclusions. I have yet to have a conversation with anyone fluent enough in etalon physics to resolve the seeming contradiction. It seems paradoxical to me still, but that's the reality of etalon physics.
Hi

" This can occur by a change in temperature through thermal expansion of the mica wafer. In order to maintain the center wavelength (CWL) of the Fabry‐Pérot etalon in conditions of varying environmental temperature, a consideration for tuning must be made. An increase in temperature causes an increase of the CWL of transmission peaks. "
The change in temperature can be up expanding the wafer or down contracting the wafer. Its confusing where it says 'thermal expansion' only.

" In order to maintain the center wavelength (CWL) of the Fabry‐Pérot etalon in conditions of varying environmental temperature, a consideration for tuning must be made "
This just means that the FP is tuned using the heater to be on band. When its on band how the CWL changes out of band is not stated.

" notes the native CWL is high, and the etalon is tuned by applying Richview compression. If decreasing the gap via compression increased the CWL "
What does 'high' mean here? To the blue end of the H-alpha line? And what dows 'increased the CWL' mean? It needs to state if its blue wards or redwards of the h-alpha line for clarity.

I have read that whilst tilting has the repeated reflected beam moving sideways as long as its near enough to the original beam the phase relationship still works. One reason for tilting to be less good than compression or change by heat control of a solid etalon.

My thought experiment thinking about how the integral no of waves needing to fit in a slightly changed gap agrees with Kens observations of the DS with a spectroscope.

Has anyone got a compression tuned air spaced etalon and as you go through the tuning range do the rings collapse onto the sweet spot?

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Bob Yoesle »

Hi Andrew,
What does 'high' mean here? To the blue end of the H-alpha line? And what dows 'increased the CWL' mean? It needs to state if its blue wards or redwards of the h-alpha line for clarity...
I apologize - these are the commonly used terms with solar etalons, which I assume most people on this forum are familiar with. CWL (central wavelength) is the native wavelength to which the etalon is tuned when manufactured. Generally this CWL is below or above the H alpha emission to allow for tuning the etalon to the H alpha emission at 656.28 nm. Tuning "high" generally means above (greater/longer than/towards the red) the H alpha emission at 656.26 nm. "Low" means below/less than 656.28 nm towards the blue. A mica etalon CWL is tuned low, and heating a mica (or any other solid etalon such as fused quartz - despite its low CTE) causes thermal expansion of the etalon gap, and shifts the etalon CWL higher to be on-band at 656.28 nm at well above ambient temperatures in order to have CWL stability at 656.28 nm.

I too once thought the tilting the etalon increased the etalon gap and this was how tilting tuned an etalon, but I was mistaken. It may be time for you to throw out your thought experiment, as I had to do, and pay attention to both etalon theory, knowing we are dealing with the wave nature of light, and the proven mathematics of etalons:

λp = 2 n t cos θ

I encourage you to enter any values for n (refractive index), t (etalon gap distance), or cos θ (tilt) and see what happens to λp (wavelength). Use any arbitrary values you like.

Etalon physics do not change for an air-spaced etalon, and an increased air-spaced etalon gap also increases the CWL "towards the red." Decreasing the gap (as with "RichView" spacer compression) decreases the CWL lower "towards the blue." However, the assumption that tilting the etalon increases the etalon gap which decreases the CWL, and therefore etalon gap decreases would increase the CWL, is erroneous, and has been addressed in the previous post(s) and links therein.

The reference to a "collapsed onto the sweet spot" is mixing apples and oranges. The "sweet spot" is the Jacquinot spot, and refers to the size of the on-band portion of the field of view of an etalon. Typically for a 0.7A air-spaced etalon this is 1 degree. It is not the "collapse" of the central interference fringe ring (not all the rings) into a small central dot when the H alpha etalon CWL is tuned at 656.28 nm.
Has anyone got a compression tuned air spaced etalon and as you go through the tuning range do the rings collapse onto the sweet spot?
Yes. I previously showed (and linked to a Solar Chat forum post) concerning a SM90II DS RichView compression-tuned etalon, whereby compressing the central spacer decreased the etalon gap in the center of the etalon and brought the etalon gap on-band in the center. This compression behavior has also been demonstrated with the LS35 and LS50 mods in another series of posts. These all show conclusively that compressing an etalon spacer - and thereby decreasing the etalon gap - decreases (blue shifts) the CWL to be on-band, or blue-shifted the CWL away from 656.28 nm if the etalon was already on-band with no tilt.

No further demonstration of the facts should really be needed, except to the point of being redundant. But I'm an Board Certified member of the:

Department-of-redundancy-department.gif
Department-of-redundancy-department.gif (60.37 KiB) Viewed 4911 times



Using the Light Machinery Etalon Designer, I here again show a 20 mm diameter air-spaced etalon where I have chosen a CWL of 656.38 nm, and a spacing of 0.50000 mm:

Etalon native spacing.jpg
Etalon native spacing.jpg (105.29 KiB) Viewed 4906 times

Changing the mirror spacing to 0.49995 mm brings the etalon on-band for H alpha at 656.28 nm:

Etalon compressed spacing.jpg
Etalon compressed spacing.jpg (101.88 KiB) Viewed 4906 times


Here's the animation of the previous two images which makes the blueward shift with a decreased etalon gap spacing more obvious:

Etalon spacing animation.gif
Etalon spacing animation.gif (491.34 KiB) Viewed 4905 times
Click for larger view.


It has also been demonstrated on multiple occasions with the Hydrogen spectrum tube and solar imaging whereby an etalon which is tuned "high" to allow for tilt tuning (LS35, LS50(s), SM90II) - and which when the etalon spacers are physically compressed - decreases the etalon gap and therefore the CWL to be on-band. Therefore it seems rather obvious: If a tilt-tuned etalon is manufactured to have a high CWL to allow for tilt tuning in order that the CWL is decreased by tilting to be on-band, and applying mechanical pressure either to the center or the edge spacers of the etalon also brings it on-band, then mechanical pressure - which reduces the etalon gap spacing, also decreases the CWL of the etalon. It does not move the CWL to the red. IF IT DID, THE ETALON - WHICH IS ALREADY TUNED ABOVE THE HA EMISSION LINE - WOULD NOT COME ON-BAND.

Therefore, one can use either tilt tuning or spacer compression to tune the PST etalon (or as I have shown via LS35 compression tuning, almost any etalon with a high CWL). Andy Lunt - who should know - states compression tuning - as originally patented by him and other Coronado principles - is the method of PST tuning; not tilt tuning. Rich view etalons are both tilt tuned and compression tuned due to this reality. Both methods will decrease the etalon CWL.

Ken's original postulate regarding tilt-tuning, gap increases, and the CWL shift conclusions are lacking support; etalon gap increases do not decrease the CWL, and therefore etalon gap compression does not red-shift the etalon. My assumption is that Ken's spectroscope showed the next higher etalon peak being lowered with compression (as shown in the above Etalon Designer example), but which remained above where the original etalon peak started, and led to the conclusion the CWL went higher instead of lower.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi Bob,

I found this PDF last night where a diagram shows what you are saying, after some effort. I find it hard to find references to how compression tuning works.

Search on: Micromechanical tunable
Fabry-P ́erot interferometers
with membrane Bragg mirrors
based on silicon/silicon carbonitride

One interesting thing is that etalon diagrams show some with parallel glass and some with the outer faces tilted presumably for anit-reflection purposes. Presumably better ones have the tilt face.

Why do some PSTs have feet added to the orange ring, and is the orientation critical?

Why do reflections off the PST face move about whilst its tuned? Has anyone measured the angle the etalon moves through from the LHS position until it comes onto tune. And is it along a line or just random?

When I say sweet spot I mean when the grey bit in tune is a spot and when you tune more it expands into a circle.

You cannot assume 'most people on here know things' when making explanations.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi

Doing a review:

thesmiths said

"Is there typically room for improvement with a PST etalon tuning? My sense is the fairly crude tuning mechanism gives a
combination of pressure and tilt tuning that leads to the often unpredictable results."

As the tuning CWL shift is the same for tilt tuning and pressure tuning its hard to tell how much each method is
contributing to the PST tuning then.

thesmiths said

"Having taken apart the PST, it seems to me that it would be impossible for the foam ring to compress exactly uniformly
(which would lead to some tilting)."

Bob said

"There are three general ways to alter the CWL (central wavelength) of an etalon; tilting blue-shifts the etalon,
changing the etalon gap spacing increases or decreases the CWL, and changing the refractive index of the etalon gap
does the same thing (air pressure tuning). The key with changing the gap spacing is that it must be uniformly changed
so that all parts of the etalon have the same CWL."

How does the CWL change for changing the air gap due to increasing or decreasing the air gap, goes towards the blue when compressed
and red when the gap is increased?

Presumably pressure tuning is always done increasing the pressure and it goes towards the red?


Concerning patents and PST:

https://patents.google.com/patent/US7054518B2/en

The patent refers to actuators over each spacer which is not used in the PST. The diagrams look like a PST
apart from the last one with the sqiggly springs in the etalon and applying force from the outside to the etalon.

Valery said
"
Bob Yoesle wrote: ↑Tue Sep 21, 2021 6:01 pm

However, in 2017 Meade Coronado also refers to the PST as being "rich-view" tuned, which only involves
peripheral spacers.

Hi Bob,

They referred to the Double Stack 40mm external etalon being RichView tuned. Not to internal tilt tuning etalon.


Valery"

So he believes the PST is tilt tuned. And reports the reflections off the PST reflective face moving as the PST is tuned.

Bob says that he was told the PST is pressure tuned by Andy Lunt. But its certainly not using three separate actuators.

The problem is that tilt tuning and compression tuning both move the CWL towards the blue
so its hard to tell from the outside which method is used
or whether the observed tilting of the etalon whilst tuning affects the CWL as well if its compression tuned.

Only by going in and making changes inside the etalon housing could you tell it seems.
Perhaps mark the orange ring so it can be replaced in the same orientation if its needed to get the un-tuned position,
towards the LHS rotated, on the red side of the centre highest comb. And then replace it with a hard rubber washer
and see if the tuning remains the same?

People wanting to 'improve' the PST would have to know what is going on to be able to make changes
likely to improve the image quality. Like Rusted tilting the whole etalon housing as well.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by marktownley »

My take on this - and only my theory not science! There is so much play in the coarse threads on the tuning mechanism that it is possible to tilt the etalon, even if not predictably. Then there is compression happening too. Both of which can contribute to uneven illumination eg. Chris (rusted) experience with his PST mod...


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Bob Yoesle »

How does the CWL change for changing the air gap due to increasing or decreasing the air gap, goes towards the blue when compressed
and red when the gap is increased?
λp = 2 n t cos θ

λp = primary etalon peak (CWL)
n = gap refractive index
t = gap thickness
θ = angle of incidence (tilt)

Decreased etalon gap distance t > blue shifted CWL. Increased / larger gap distance t > red shifted CWL.

Note: this applies to any etalon whether the gap is air-spaced or a solid material (mica, fused quartz, etc.)
Presumably pressure tuning is always done increasing the pressure and it goes towards the red?
I presume you are referencing air pressure tuning. Depends on the CWL of the sealed etalon. If the CWL is low as done by Lunt: Increased pressure > increased refractive index n > increased CWL.

If one converts a high CWL tilt-tuned etalon for pressure tuning: decreased pressure > decreased refractive index n > decreased CWL.

Note: this is why being able to tune an etalon in the Earth's atmosphere is important: pressure changes due to altitude and barometric weather changes can alter the CWL because of the changed refractive index of the etalon gap. Air-spaced etalons exposed to the atmosphere are therefore tilt (or compression) tuned with a high CWL to be tunable through a large variation in altitude or barometric pressure.

Tilt angle θ always decreases the CWL: cosine of 0 degrees is 1, cosine of any other angle is always less than 1 and becomes much less with increased tilt angle:

excel-functions27-4.png
excel-functions27-4.png (41.93 KiB) Viewed 4874 times

Mark is probably spot-on. The PST etalon is compression tuned; Andy Lunt didn't tell me - he told the world in the Lunt Blog which I cited/linked to. But from what I've been reading there may be a lot of slop involved (certainly not the precision actuators that are referred as one of many embodiments of the much wider patents' coverage). The PST is an entry level product, and one usually does not expect any kind of high precision in the mechanical execution. Since I have not owned an H alpha PST, I have no idea about the actual thread specifications, etc.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by marktownley »

Bob Yoesle wrote: Thu Sep 30, 2021 5:45 pm Since I have not owned an H alpha PST, I have no idea about the actual thread specifications, etc.
They're like T-Rex teeth :lol:


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

marktownley wrote: Thu Sep 30, 2021 2:48 pm My take on this - and only my theory not science! There is so much play in the coarse threads on the tuning mechanism that it is possible to tilt the etalon, even if not predictably. Then there is compression happening too. Both of which can contribute to uneven illumination eg. Chris (rusted) experience with his PST mod...
As you said when I asked about wrapping the adjuster with PTFE tape it reduces the in tune spot wobbling about, as it averages out the sloppiness.

Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

I don't have access to my home computer or all my files.
This discussion/ debate is going in circles.

Based on the LightMachinery example the etalon should be around 0.2mm air thickness to give a suitable FSR around 10.7A
As the thickness of the etalon gap decreases the model shows a shift to the blue.
Gert's spreadsheet (see earlier message) set to the same parameters shows a tilt of 0.95 deg would be required to tune a 0.2mm and 636.38nm etalon to H alpha at 636.28nm.
Gert_tilt to Ha.JPG
Gert_tilt to Ha.JPG (59.69 KiB) Viewed 4848 times
My only issue....with a front double stacked Richview SM60 etalon we usually "clock" the etalon and tilt it to remove/ control the ghost.
The front DS etalon is already sitting on a rear etalon which is itself tilted to reach CWL, so it already has some tilt.
I was convinced that the RV tuning allowed the tilt to be somewhat compensated and bring the front etalon onto band with the rear etalon.
(Otherwise the only tuning available is further into the blue and further from the rear etalon CWL????)
Weather permitting I'll re-assess this as soon as I can. (Other users of the DS should be able to confirm this tuning with their system)


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi

Over-night index re-build.

Ken reports that at the LHS position of the PST tuning ring the orange foam is fully compressed. If the etalon is tuned by changing the gap then the gap would increase as the pressure is released and the CWL would move towards the red? Being under pressure at the LHS could explain why feet are added in some cases,to set the initial tilt to the red side of the ighest centre comb for tilt tuning?

Ken reports that moving the Richview lever moves the CWL to the red, is that holding the etalon under pressure and then moving the lever reduces the pressure?

Cheers. Andrew.
Last edited by AndiesHandyHandies on Fri Oct 01, 2021 9:03 am, edited 1 time in total.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

To achieve the 0.95 deg tilt, the 20mm diameter PST etalon would have tilt 0.33mm.
How does that compare with the orange ring compression???


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Merlin66 »

We have seen examples of the RichView central pressure screw actually breaking the supporting disk!!!
There's obviously some serious pressure involved…..


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Merlin66 wrote: Fri Oct 01, 2021 8:10 am To achieve the 0.95 deg tilt, the 20mm diameter PST etalon would have tilt 0.33mm.
How does that compare with the orange ring compression???
Hi Ken,

How would you assess the orange ring compression?

Tune the etalon and mark the starting LHS position and the ending RHS direction.

Shine a laser onto the front, Valery reports doing this, do the tuning range and measure the tilt?

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by AndiesHandyHandies »

Hi

Cogitating in the bath.

It would be sensible for the Richview and the PST to hold the etalon under pressure as a default. So its not free to accelerate and de-cellerate when thrown into a couriers van. And that explains why Ken saw the Richview tune go to the red. And that the PST is likely to be tilt tuned?

When new PSTs are sent out is the tuning ring to the LHS and the prism adjuster wound tight down? As the PST etalon is floating loose if it was sent out with the tuning ring to the RHS and was thrown about it could de-contact. Has anyone received a new PST with de-contacted etalon? And same with a Richview etalon?

When parking the PST perhaps turn to the RHS a bit to reduce pressure on the orange foam. Not all the way as in Marks video the orange ring had an imprint of the etalon and you would not want the etalon to move and get trapped in a different position. Whatever the tuning method.

Cheers. Andrew.


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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by Rusted »

Methinks thou doth complain too much. ;)

Given the vast number of PSTs released into the wild:
One would expect a large number of "walking wounded."
There patently are not these unhappy numbers from this pestilence:
Or t'Internet would be awash with complainants, so.

Apart from that Golden Oldie: Coronado's; "Rust Never Sleeps."
Of which we do here, often tell, as if by Heavy Metal legend.
Albeit from afar and wide, as they do touch verily.
But! Only on those poor vanished souls, of yesteryear. :D

With all due apologies:
To those, for whom, English is not their first.
Nor even second, language.
And [probably] everybody else too. :mrgreen:


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Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
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