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!

Post by AndiesHandyHandies »



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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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