Coronado PST MOD2 Black RING in the middle of sun!

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Coronado PST MOD2 Black RING in the middle of sun!

Post by OlegLviv »

Hello everyone my name is Oleg am from west Ukraine ;)
A few monts ago bought Coronado Pst 40mm 9xxxx series with main golden lens this coronado without ITF bloking filter but etalon consruction same 10xxxx series and 11xxxx that create PST MOD 2.
I have Refractor 127/1140mm+Coronado BF-15+ZWO 174MM+Daystar Interference Eliminator+Telewue Powermate 2.5x3x.
First of all knew that my refractor F9 not F10 and bought Baader Glaspath Corrector 1.25x for Mark V ZEISS Baader Binocular lens to become more f10, but than cut my tube for 6 centimeters found my focus with Baader Glaspath Corrector 1.25x.
But when I saw on the monitor screen a black ring in the middle of the frame I was surprised what it is ..... a segment of 200mm is observed, nothing should be cut and so on.....In direct focus without Barlow lenses, this ring is very visible, if you rotate the standard, you can see how the difference in the standard changes, but it is impossible to pick up this ring in any way.:oops:
I noticed that half the field was a photosphere and half a chromosphere!
When I put the barlow lens 2.5 the situation changed a little for the better but there are still these differences and if you bring this part of the ring to the edge of the sun it can be removed but it's inconvenient ..... that's when I put the barlow lens 3x the focus is bigger and of course the rings are invisible but all the same it is if you insert the eyepiece and look.
If you need more photos of my PST MOD2 I can show :bow
Please HELP ME :bow
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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by marktownley »

Hi There Welcome to the forum!

More images would be useful, but looks like an out of focus sweet spot to me.


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

Post by OlegLviv »

Please look a few photos in direct focus with Baader Glaspath Corrector 1.25x!
Baader Glaspath Corrector 1.25x is attached in front of the etalon.
In the photos I showed different settings of the etalon and the look of PST MOD2
What do you think this might be?
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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by OlegLviv »

A few photos too
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Re: Coronado PST MOD2 Black RING in the middle of sun!

Post by marktownley »

It's the sweet spot. It's perfectly normal. You need to tune the etalon so instead of a dark ring it forms a dark circle in the centre. This is the onband region, you should be concentrating on, use ROI if you need to.


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

Post by OlegLviv »

Im Understand it is normal for more pst coronado :bow
Soon i will be have Lunt60 I can stacking for my refractor 127/1140mm him? I read Lunt60 have 35mm etalon? Sweet spot in this method will disappear with Lunt 60 etalon?
Thank you!


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

Post by marktownley »

Hi There,

The sweet spot won't disappear, it will just be larger as the etalon diameter is larger.

With a 127mm frac and a PST you will be needing to be zoomed right in to quite a small fov to not get the sweet spot. I know, i've been there!

Mark


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

Post by Rusted »

I have been struggling with the PST sweet spot [ring] myself.
Starting with a 150/8 and then a 150/10 refractor.
The 1.25x GPC was only used in front of the etalon on the 150/8.
Recently I moved this 1.25x GPC close behind the etalon on the 150/10.
This helped to spread the sweet spot ring slightly.

I always use a T-S binoviewer GPC on the camera nose. [ZWOASI174MM.]
The choice of GPC [1.6x, 2x and 2.6x] depends on the seeing conditions. Normally I use a 2x GPC.
Only very rarely can I use the 2.6x. This is only useful for a single AR [sunspot] in fine seeing conditions.
Because the field of view is extremely small. A 2x Shorty Barlow is always too high a power.

I use different frame sizes in SharpCap to limit the H-a off-band area around the sweet spot ring.
Normally I can use 912x912 with perfect tuning but not usually a larger frame size.
The corners of the image are off-band, brighter and relatively featureless.
Sometimes I must use 800x800 if I can't tune well enough.
This small frame size does not tolerate much cropping in final, image processing.

I made most progress by removing the textured rubber, tuning band around the etalon housing.
Then I removed the outer metal shell. [The thin metal tube which supports the rubber tuning ring.]
I screwed a metal rod into the inner etalon tuning ring in place of the small screw.
This allows me to tilt the etalon itself within its original PST housing.
NOT tilting the glass alone but the metal housing in which the etalon sits.

Even this is an endless battle to get both the tilt and tuning correctly on band at the same time.
[i.e. Simultaneously] When the PST etalon is on band I get amazing images with fine detail.
So it is well worth the constant fiddling and frustration. I use a large [27"] hi-res monitor to fine tune and centre the image.

A Lunt etalon remains a distant dream at monopoly money, European, monopoly importer prices.


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

Post by AndiesHandyHandies »

Hi

Why are you tilting the etalon in addition to using the provided tuning method, compressing the orange foam and so the small spacers to alter the etalon spacing?

Cheers. Andrew.
Last edited by AndiesHandyHandies on Wed Sep 29, 2021 4:22 pm, edited 1 time in total.


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

Post by Valery »

PST etalon has UNCOMPRESSABLE spacers. It is a tilt tuning etalon. When rotate it's tuning ring, the etalon tilts. I checked this with reflections movements (sun and lazer).


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

Post by AndiesHandyHandies »

Hi Valery,

Nothing is un-compressable.

How does it tilt then?

Cheers. Andrew.


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

Post by Rusted »

Perhaps the tilting is a result of non-linear compression of the rubber ring.
Though I add to this by tilting the etalon housing within the outer shell.


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

Post by AndiesHandyHandies »

Rusted wrote: Sat Sep 18, 2021 1:27 pm Perhaps the tilting is a result of non-linear compression of the rubber ring.
Though I add to this by tilting the etalon housing within the outer shell.
Hi

Is the rubber ring just pushing the etalon onto the teflon ring? How can it tip, and how in a linear and re-produceable way?

You measured the force being applied as I recall, and CV worked out it would compress the spacers enough to produce 0.5A shift in the CWL.

Cheers. Andrew.


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

Post by AndiesHandyHandies »

Hi

Looked at this video again and it the other way up to what I understood. And the orange foam has two feet.

www.youtube.com/watch?v=llSJy99Om9E


So that explains why the sweet spot moves about in the field as you change the pressure as not a sophisticated method.

Pre-sumably Rusted is tipping the etalon in another way in order to centralise the sweet spot???

Using a tilter is another way, but I assume the foam and repeated tuning is not very reproducable.

Make a better engineered replacement?

Cheers. Andrew.
Last edited by AndiesHandyHandies on Tue Sep 28, 2021 11:36 am, edited 1 time in total.


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

Post by Rusted »

Andrew,

Making a tilter is not easy when only the etalon itself must tilt. [Along with its housing.]
It is vital that no force is applied to the glass components of the etalon.
That could easily break the molecular bond.

Tilting the outer housing would tilt the entire optical train after the etalon.

I am tilting the internal etalon housing independent of the outer housing.
Normally, the screw which connects the inner etalon housing to the outer shell can apply no tilt.
I have removed the knurled rubber band and thin tubular shell to allow access to the inner hole plate.

My rubber ring has no feet.


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

Post by AndiesHandyHandies »

Rusted wrote: Sun Sep 19, 2021 6:04 pm Andrew,

Making a tilter is not easy when only the etalon itself must tilt. [Along with its housing.]
It is vital that no force is applied to the glass components of the etalon.
That could easily break the molecular bond.

Tilting the outer housing would tilt the entire optical train after the etalon.

I am tilting the internal etalon housing independent of the outer housing.
Normally, the screw which connects the inner etalon housing to the outer shell can apply no tilt.
I have removed the knurled rubber band and thin tubular shell to allow access to the inner hole plate.

My rubber ring has no feet.
Hi

How do you think the original tuning works then?

David G in CN says that the tuned area is only a band unless you can tilt it in x and y correctly.

If the foam ring has only 2 feet then it could only tilt in one direction.

Is there a foam ring between the tuning ring and the etalon as well? If not its hard to see how it can tilt at all.

Do you have good pictures of all the component parts?

There is a paucity of information on how the PST etalon works on the net it appears.

Cheers. Andrew.


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

Post by Rusted »

I must be repeating myself endlessly by now:

The firm rubber ring sits at one end, inside the threaded inner housing.
A PTFE [?] O-ring sits at the other end. Both compress onto the periphery of the glass, etalon sandwich.
The rubber ring acts as a brake on the etalon as well as a compliant bed.

When rotated, the coarse thread of the inner housing squashes the glass etalon sandwich between both rings.
The etalon glass has clearance within its housing unless packing is used.
I tried plumbers PTFE tape around the glass etalon to try and maintain alignment with the optical axis.
This ploy didn't really help to improve the sweet spot ring.

So I now tilt the entire inner housing with a rod screwed in one of the tuning, screw holes.
This allows rotation and tilt of the etalon while leaving the glass unstressed, within its inner housing.

Blocking Filter rotation, relative to the etalon, seems to help spread the sweet spot ring.
I have "clocked" my Lunt B1200 so that Lunt's huge name dimple is aligned with the focuser drive shaft.
All of my fittings and extensions have three thumbscrews fitted for security and alignment.


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

Post by Merlin66 »

Guys, I can only remind you of the input from Brian at Lunt....
The PST etalon assembly is distorted during tuning.
See the previous messages.......


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

Post by AndiesHandyHandies »

Rusted wrote: Mon Sep 20, 2021 6:07 am I must be repeating myself endlessly by now:

The firm rubber ring sits at one end, inside the threaded inner housing.
A PTFE [?] O-ring sits at the other end. Both compress onto the periphery of the glass, etalon sandwich.
The rubber ring acts as a brake on the etalon as well as a compliant bed.

When rotated, the coarse thread of the inner housing squashes the glass etalon sandwich between both rings.
The etalon glass has clearance within its housing unless packing is used.
I tried plumbers PTFE tape around the glass etalon to try and maintain alignment with the optical axis.
This ploy didn't really help to improve the sweet spot ring.

So I now tilt the entire inner housing with a rod screwed in one of the tuning, screw holes.
This allows rotation and tilt of the etalon while leaving the glass unstressed, within its inner housing.

Blocking Filter rotation, relative to the etalon, seems to help spread the sweet spot ring.
I have "clocked" my Lunt B1200 so that Lunt's huge name dimple is aligned with the focuser drive shaft.
All of my fittings and extensions have three thumbscrews fitted for security and alignment.
Hi

In Marks video of taking the etalon assembly apart there is a white ring in the adjuster ring and an orange foam ring at the back end which he said had feet underneath, and there appear to be two quiet close together. Is that the same in yours? So the adjuster ring is pushing the etalon onto the foam ring?

How does the original arrangement achieve the tilting?

Why does the foam ring not just push the etalon against the white ring?

Cheers. Andrew.


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

Post by AndiesHandyHandies »

Merlin66 wrote: Mon Sep 20, 2021 6:14 am Guys, I can only remind you of the input from Brian at Lunt....
The PST etalon assembly is distorted during tuning.
See the previous messages.......
Hi

When you say 'distorted' does that mean

Its compressed so the mica feet are compressed, ie the actual etalon is physically compressed and the air gap changes, CV did caculate from Rusted's pressure measurements that would give 0.5A of CWL change

or

the etalon just tips, but if the orange foam ring is pushing the etalon back to the white ring how is it tipped? The two feet do suggest it is tipping.

Rusted could have gone from changing the air gap to tilting the etalon as a tuning method?

Valery says the reflections off the etalon move so that indicates it does tilt. But may not be the tuning method?

We are talking about small movements.

Cheers. Andrew.


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Re: 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 »

Andrew,
I can’t seem to be able to search for the previous message.
The action of the two rings caused a distortion of the etalon where the outer edge “drops” relative to the main body.
The “feet” on the orange ring were to even up the pressure on the edges.
NO tilting was envisaged.
I will find the original reply from Brian and republish.


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

Post by AndiesHandyHandies »

Hi

If CV calculation is right could you convert the PST etalon from tilt tuning to pressure tuning by replacing the sponge ring with a more solid ring and put something soft under the top white ring? Can locate the etalon better sideways then and no moving about of sweet spot when tuning or reflections. Also if better located its better then for auto-collimating the etalon with the telescope optical axis. My 127mm seems a bit out of alignment but a auto-collimation with a tilter improved the sweet spot.

Cheers. Andrew.
Last edited by AndiesHandyHandies on Tue Sep 28, 2021 11:39 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

Found an old thread.


Collin,
As you may know I'm a research engineer and chemist and I have been at for 28 years now. I was playing "dumb" because I've also done the calculation as well and came up with the same results. The math shows that if the spacers are Quartz you can't compress them using the mounting method used in the PST. On top of that I called a good friend who has 50 year of experinece in designing and fabricating optics including many Etalon for scientific research and ask him the same question about the "pressure" tuning using Quartz spacer against the foam ring. He didn't believe it either. If the Etalon was mounted in a metal cell and then maybe you could get enough pressure but your approaching the breaking point of the Quartz and you would need to put a very uniform pressure on the plate. The PST method won't do that. The fact that there is play in the threads shows that and also shows that the force being applied is maybe a few PSI at best. He also knew of no other spacer material except for the typical materials of Zeroder, Quartz, and Cervit. Polymers won't work because they have a huge Coeff of thermal expansion, and the air gap won't be stable. You also can't polish polymers to the needed uniformity in thickness and make optical contact with them.
What he believe is going on it that most Etalons are made so the outer surfaces are very slightly wedged and these are being used in the PST. There are a couple of reasons for it, one is to throw any ghost reflections out of the field of view. The wedge angle is usually small, only a few arc minutes so it would be difficult to measure by looking at the thickness around the rim of each plate. There are usually alignment marks on the edge of the Etalon show were the high spot is located. The wedge also is used for tuning because it slightly bends the light coming out of the inner parallel face. This change the optical path length through the air gap so you can rotate one plate agianst other to tune it. In the case of the optically contacted Etalon and how it is mounted in the PST against a foam ring and then pushed on by a Teflon O-ring what we both believe is happening is that the Teflon O-ring will first comes in contact with the high spot of the wedge of the front surface of the Etalon. As the tuning ring is advanced it pushes on the high spot and cause the Etalon to tip backward into the foam. The more you turn the ring the more tilt you it until the face of the Etalon is in complete contact with the Teflon O-ring. It is a simple method to tilt tune the Etalon. It doesn't require some exotic spacer material and it doesn't break the laws of physics to generate huge forces to compress Quartz.

All the Best,
- Dave

So with Mark showing two feet on the under side of the orange foam ring there must be enough slack in the tuning ring thread for it to tip as the two feet restrain it from going further on their side. The etalon has enough coated surface so wherever it is in the cell it accomodates the beam.

So there we go. Crude.

To get round banding issues you would have to re-manufacture the etalon cell to be on a three point tilter instead.

Any wedge on the outer faces of the etalon will be anti-reflection measures.

Like all the pictures of un-touched prisms show them at a slight angle to the beam for the same reason and the penta-prism is used as it can be off set but the beam still goes 90 degrees out with respect to the incoming optical axis. Small amount of astigmatism clearly does not matter.

Cheers. Andrew.


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

Post by Rusted »

An internal etalon tilter could be made using a suitable tube and two sets of spaced, radial screws.
A central O-ring could provide a fulcrum to avoid moving too far from the optical axis.
Much just like "proper" telescope finder rings. I don't know why it hasn't occurred to me before now. I was fixated on tilter plates.

An alternative would be one set of three radial screws and a compliant ring at a suitable distance.
A standard O-ring seems obvious. Much like the awful "Skywatcher" finder brackets, but much better designed.

A third alternative might be one, or two, eccentrically bored rings. To achieve tilt by independent rotation.

You'll have to do the math for the spacing. To ensure a suitable degree of tilt. Assuming the desired tilt angle is known. ;)


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

Post by marktownley »

Here's an animation I made a few years ago that illustrates how I thought the PST etalon tilts...

Notice the wedge shaped etalon as per Dave Gs email.

Imageetalon-animation by Mark Townley, on Flickr


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

Post by AndiesHandyHandies »

Hi

I doubt its ecomomic to make high quality wedges like that.

The fact there were two feet on the bottom of the orange foam ring points to a simple tipping method.

Cheers. Andrew.


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

Post by Merlin66 »

If the spacers were squeezed during tuning the effective gap would be reduced and the CWL would move towards the red wing.
This doesn’t happen.
Tuning of the PST moves from the red wing to the blue wing. The effective gap must therefore be increasing.
This can be caused by tilt or distortion.
It has be confirmed by designer that it is based on distortion tuning.
Why not just accept what the designer says????
The feet on the orange ring were to balance the distorting forces and give a better outcome.


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

Post by Merlin66 »

Looking at the suggested animation, the effective gap would reduce during tuning.
This doesn’t happen.


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

Post by Merlin66 »

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.


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

Post by marktownley »

Merlin66 wrote: Mon Sep 20, 2021 10:34 pm Looking at the suggested animation, the effective gap would reduce during tuning.
This doesn’t happen.
It was only my 'surmising' 8 years ago now, just throwing it back in for new comers who may have not seen it, :)


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

Post by AndiesHandyHandies »

Rusted wrote: Mon Sep 20, 2021 4:22 pm An internal etalon tilter could be made using a suitable tube and two sets of spaced, radial screws.
A central O-ring could provide a fulcrum to avoid moving too far from the optical axis.
Much just like "proper" telescope finder rings. I don't know why it hasn't occurred to me before now. I was fixated on tilter plates.

An alternative would be one set of three radial screws and a compliant ring at a suitable distance.
A standard O-ring seems obvious. Much like the awful "Skywatcher" finder brackets, but much better designed.

A third alternative might be one, or two, eccentrically bored rings. To achieve tilt by independent rotation.

You'll have to do the math for the spacing. To ensure a suitable degree of tilt. Assuming the desired tilt angle is known. ;)
Hi

Fran Hourigan who was an optical designer at Wise Optics in the UK gave me a tip on adjusters which was to use 4 instead of 3. Its steadier and easier to intuitavely adjust. As metal is ductile it take up small strains, which also damp out vibrations. He made me a lovely 14" F4 on 1" plate glass for my society.

I have a 16" Dobsonian. I put 5 wind down bolts with tommy bars on the base board, so I could compare 4 feet to 3 feet. With the tommy bars I could roughly torque the feet up to bear a similar weight. And the base board will bend a bit. When moving the OTA about there was less tipping and it was steadier using 4 feet. I also had nylons rollers on plain brass bearings made to use instead of teflon pads which just wore out. Similar to HC2 focusser. Thes reduces sticktion and the differential across the wheel in diameter provides the friction. Now I can with the focusser easily push the OTA at 45 deg to both axis.

You could copy the HC2 design for your nested rotaing rings? Just move and they self lock. You have movement in x-y then.

Cheers. Andrew.


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

Post by Rusted »

I have been advocating four legged "tripods" for years.
True tripods are only good for stools on cobbled floors.
A four legged pier is deformable enough to allow rigid support without rock.
The radius of support for four feet is vast compared to a tripod.
So the tipping [hinge] line between any two feet [of four] is a huge improvement in safety and stability.
Having five feet is even better. Because it approximates even more closely to a circle. Many office chairs have five feet.
Now what was the question again? :D

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

Post by AndiesHandyHandies »

Hi Rusted

When you have sorted out a 'trick' etalon tilter this is what I would do.

Assemble the set up with no glass in it and with a tilter. Put a Cheshire or suitable stop in the focusser. Auto-collimate the set up so the focusser is centred on the optical axis. Does not matter then if lens is off and we are only using a small field. I use a Xmas card with a hole puched in and a 1/4" thick cross on one side and stand 20 feet in front use the objective as an optical lever.

Then the etalon is on centre and you can make it orthogonal to the optical axis to start with by auto-collimating the reflection off the front.

Cheers. Andrew.


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

Post by AndiesHandyHandies »

Hi

Out of interest I did this search

https://www.google.com/search?q=pst+eta ... CA0&uact=5

Not many results and this may be the first thread to mention the orange foam feet or spacers meaning it must be tilt tuned.

Cheers. Andrew.


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

Post by AndiesHandyHandies »

Hi

Also talk of manually tuning the Orange foam ring is 'marketing urban myth', like widefield Televues being quality assured at F4 to impy they are coma correcting for Newtonians, to make you think its not just been thrown together.

Can a time and motion expert say how long it would take to make the etalon part up?

Cheers. Andrew.


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

Post by AndiesHandyHandies »

Hi

Tuning methods for a Fabry-Perot etalon are

1. Tilting - PST

2. Heating the mica spacer - Quark

3. Changing the refractive index of the air between the plates by increasing its pressure - Lunt

4. Patented RichView tuning which uses novel spacing technology and actuators to then compress the etalon and so the spacers. Coronada 90mm DS.

Cheers. Andrew.


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

Post by AndiesHandyHandies »

Hi

I have discovered via Bob that the early PSTs were tilt tuned and the later ones compression tuned.

Is there a way of telling? Serial no or change from yellow to blue coating on the front lens? Or the tilt ones have the orange foam?

Cheers. Andrew.


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

Post by Merlin66 »

Andrew, I do not believe that the information given by a senior and well respected Lunt engineer can be called an “Urban myth”.
If Bob has some evidence of PST tilt tuning, he should put it forward.
Until then my money is on Brian’s explanation.


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

Post by Merlin66 »

I have been involved in modding PST’s since 2007 and I have never come across an example which did not have the orange ring.


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

Post by Bob Yoesle »

If Bob has some evidence of PST tilt tuning, he should put it forward.
Hi Ken,

What I actually said was that "some early PST's may have been tilt-tuned." Emphasis added. I have no evidence of this tilting methodology, it is just read from posts in past amateur forums. David G. and Valery D. frequently state this was (and is) the PST tuning method:
I have taken a number of PST apart and recontacted the Etalons. The tuning mechnanism is that the Etalon is sitting against a foam ring and as the foam is compressed the Etalon tilts to tune it. It is that simple You can see the Etalon tilt by a number of degrees when you look down into the scope thru the objective . The Etalons plates are about 1/2" thick with Quartz spacers so the force needed to change the shape would be very high. The foam that Etalon sets against would have to full compressed first before any pressure would be applied to the Etalon, with a force need to change the spacing of the plates. The patent may states one thing but mechanism is tilt tuning. - Dave
On the other hand we have Spectral Joe, a physicist/engineer, and solar observer:
I don't know where the persistent belief that the PST etalon is tilt tuned comes from. The relevant patents, US 7,054,518 B2 and US 7,149,377 B2, make it clear that the etalon is tuned by compressing the spacers, the foam ring is to make the compression force symmetrical. Dismantling a PST etalon confirms this, it's built exactly as the patents show. People find it hard to believe that the spacers can be compressed, but on the spatial scale (sub-micron) involved it works just fine. You might convince yourself that they patented one thing and built another, but take one apart and see. Joe
If the spacers were squeezed during tuning the effective gap would be reduced and the CWL would move towards the red wing.
With regard to the effect of compression, it appears to me that compression and decreased etalon gap spacing would decrease the CWL:

λp = 2 n t cos θ

etalon parameteres.jpg
etalon parameteres.jpg (22.64 KiB) Viewed 3595 times

n = gap refractive index
t = gap thickness
θ = angle of incidence (tilt)

Therefore I infer from simple dimensional analysis:

1. As refractive index of the gap increases, the wavelength will increase (shift red-ward) - air pressure tuning;
2. As the gap thickness decreases, the wavelength will decrease (shift blue-ward) - "Richview" tuning;
3. As the tilt angle increases, the cosine decreases, and the wavelength decreases (shift blue-ward) - tilt tuning.

It appears from the Richview SM90II etalon I have examined that they (like most tilt-tuned etalons) have a native high CWL, and applying a compressive force to the center of the etalon (i.e. etalon gap narrows) brings the center on-band. This appears to comport with the above dimensional analysis whereby the CWL would be blue-shifted.

From what I recently observed with the LS35 etalon using a Hydrogen spectrum tube and applying compression around the periphery of the etalon face - thereby assumed to compress the spacers - the etalon also went from having a high CWL to a lower CWL with the compression applied at the periphery.

The quote you posted from Brian states with regard to the compression "It is not compressing the feet at all it is deforming the plates around the feet... The compression itself is not capable of enough force to compress the feet." I am not of want to contradict Brian, but I'm having a hard time understanding how "deforming the plates around the feet" would work in the way it appears to in my limited experiences. I have found no patent describing this deformation. There is only the patent describing compression of the spacers themselves. Then again Brian sure knows his stuff, and I'm not anything other than a curious amateur trying to understand these devices as best I can.

PS - I do note that Brian states the PST uses a Teflon ring to apply pressure to the PST etalon face, which would apparently be the bearing surface to interface with the rest of the etalon assembly in order to provide axial compression pressure.


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

Post by Merlin66 »

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.


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

Post by AndiesHandyHandies »

Hi

So if all the PSTs have the orange foam ring then they should all be tilt tuned.

If Marks video showing two little feet on one side of the orange ring is right then why is there a need to trim the foam?

Cheers. Andrew.


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

Post by Rusted »

My own, fuzzy logic suggests that the PTFE/Teflon O-ring forces alignment on the PST etalon.
The etalon element cannot tilt out of alignment because the O-ring is so hard.

The orange rubber ring ensures resistance and compliance but cannot, of itself, apply any tilt.
The reference axis for the etalon must always be orthogonal [perpendicular] to the Teflon O-ring's [three point] contact surface.

Which further suggests that if tilt is desired then the O-ring's groove must be machined at a slight tilt.
Which might be deliberately achieved with an eccentric work holder in a CNC lathe.
Or, far more likely, accidentally by typically poor [random] CNC machining using [typically] unskilled, machine minding, labour.
The coarse "tuning" thread is another possible means of applying random tilt. Accidentally or otherwise.

Note that the inner etalon housing and etalon element rotates.
Providing tilt, on demand, if the etalon's housing has tilt machined in.
Which [to the cynical] is a possible form of tuning.

Managed by changes [clocking?] at the factory under a suitable H-a test beam.
The complete randomness of the machining is overcome by optical testing [rotating] to get the etalon on band.
The multiple holes in the inner ring scream GUILTY in this respect!
If precision machining was employed then only one or three tuning holes are needed.

The PST etalon is tuned [somewhat haphazardly] because it must be. Simply to have a saleable product.
It certainly explains the [known] wide variation in PST H-alpha image quality.
"Tuning" the PST etalon is no more than production repair [i.e. fixing] of whatever the machining throws up completely randomly.
PST quality is a lottery. Like all lotteries, the many losers pay the winner. With a nice, fat profit to the lottery holder.

Any tilt I deliberately apply is external to the etalon element and applied to its inner metal housing.
All and repeatedly, in a desperate attempt to spread the inadequate sweet spot ring in my own set-up.
If I can overcome the sweet spot problem with my clumsy, tilt and turn efforts then I can achieve quite reasonable images.
Given the right seeing conditions.

I have added images to show the effect of tilting the PST etalon and "clocking" the Lunt BF against the PST etalon.

22.09.2021 11.28 1.25x 2x gpcs.jpg
22.09.2021 11.28 1.25x 2x gpcs.jpg (148.68 KiB) Viewed 3567 times
22.09.2021 14.33 1.25x 2x gpcs noise 4 ap 24.jpg
22.09.2021 14.33 1.25x 2x gpcs noise 4 ap 24.jpg (147.24 KiB) Viewed 3567 times


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

Post by Bob Yoesle »

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.
Last edited by Bob Yoesle on Wed Sep 22, 2021 5:04 pm, edited 1 time in total.


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

Post by AndiesHandyHandies »

Merlin66 wrote: Wed Sep 22, 2021 5:33 am 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.
Hi Ken,

When you took the PST etalons apart were the orange rings all like Marks with two little feet on one side. Were they part of a moulding or stuck on?

Cheers. Andrew.


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

Post by AndiesHandyHandies »

AndiesHandyHandies wrote: Wed Sep 22, 2021 4:12 pm
Merlin66 wrote: Wed Sep 22, 2021 5:33 am 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.
Hi Ken,

When you took the PST etalons apart were the orange rings all like Marks with two little feet on one side. Were they part of a moulding or stuck on?

Cheers. Andrew.
Hi Ken,

If we have a tuned etalon at a certain wavelength where a whole no of wavelengths fit in then if the space increases slightly for whole wavelengths to fit in the wavelength has to be smaller and one more wave fit in. So thats why it shifts to the blue. And vice-versa. Where the wavelength increases and one less wave fits in.

Counter intuative.

Cheers. Andrew.


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

Post by AndiesHandyHandies »

Hi

This continues for a bit and then we get to a point where a longer wavelength starts to fit in again and we get back to the original wavelength which is how the combs are created.

We did an etalon experiment at Birmingham. We had one mirror on a micrometer and 3 light sources. Two were given. We had to move the mirror a certain amount, the same for each wavelength and count the no of fringes there were for each. We were given a clue as to what it was. Everyone else spent 2 weeks with a mechanical calculator going through guesses until by co-incidence they found the right answer by co-incidences.

I was the first under-graduate to ask to use their brand new IBM360, with 1MB of free memory as it was 18months late. I got given a Fortran 70 manual which is in my sons loft.

So when I ran the program I could do lots of calculations. It was interesting there were lots of co-incidences within an iterference pattern envelope. The centre loop was biggest and just one co-incidence at the centre with the highest point on the graph. I think this means you need to auto-collimate the front of the etalon first to be nearest to the brightest peak in the comb which I would think will be when its at exactly 90deg to the optical axis.

I ran a beuraue service after that and saved people 2 weeks in the lab. Unfortunately the lab supervisor was the second year chief tutor so when I complained about my supervision groups useless tutor I did not get much change.

Cheers. Andrew.


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

Post by Merlin66 »

Andrew,
From memory, there were some orange rings with one block and some with none. I can't remember seeing a ring with two feet.


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

Post by AndiesHandyHandies »

Merlin66 wrote: Thu Sep 23, 2021 5:22 am Andrew,
From memory, there were some orange rings with one block and some with none. I can't remember seeing a ring with two feet.
Hi Ken,

Thanks for the feedback.

So that seems to indicate the etalons are built and fergled in some way as said.

I assume the pads are stuck on then.

RodAstro says the PST is well built as it needs to be. The prism holder is machine out of a solid block of alloy for instance.
He says my PST is from the first year of production, his mate bought it.

When Mark took his apart the etalon just dropped out. Not mechanically indexed in any way and 'rattling' about in the holder. So that implies where it sits is not critcal for its functioning. And you can see the orange foam has sunk with age and pressure.

RodAstro said Stuart eventually had to have a solid alloy tube machined out for his triple stack as things screwed together was too floppy.

The patent which comes up associated with the PST when I search, Bob linked to, clearly states it uses compression of the etalon.

As Rusted says you would expect the foam to push the bare etalon back onto the teflon ring, so when the sweet spot wobbles about as you tune it must be play in the adjuster thread. So one assumes the pads are put in to take out play in the thread??? Valery says the reflections off his PST etalon move about as its tuned.

Your experiment with the DS shows that the tilted etalon CWL goes towards the blue and the Richfield compression one towards the red.

Can you do the same experiment on a PST? If its made to the patent the CWL should move to the red.

Thanks. Andrew.


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