Small Etalon Improvement For Only $15 USD (13 Euros)

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Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

I am fortunate enough to have obtained early 2000's era Tucson Coronado filters that have excellent performance. This is not only due to their etalon plate uniformity and gap spacing, etc., but also a central wavelength that is pretty close to being on-band with just a little tilt needed to get exactly on-band.

In my limited experience with etalons, I have found most external air-spaced etalons manufactured these days to have high CWL's. This causes a decrease in performance by requiring excess tilt that creates "banding" - a variation in contrast across the full disc view or image of the Sun when tilted on-band. Coronado's implementation of RV tuning attempts to correct the high CWL with mechanical pressure tuning. Unfortunately pressure is applied to the center spacer only, and results in a non-uniform etalon gap and a widened band-pass when the center of the etalon is brought on-band. This makes (in my opinion) these etalons unsuitable for primary etalon use, although somewhat useful for secondary etalon double stacking.

I recently obtained a pre-owned LS35 "grab and go" telescope to balance out my collection, and it offered pretty-not-bad performance superior to the PST's I have viewed through. And like most of Lunt's etalons, there are only peripheral spacer "feet."

Mike & LS35 crop.jpg
Mike & LS35 crop.jpg (561.78 KiB) Viewed 8250 times
Fellow amateur astronomer Mike McKeag checks out the LS35.

But there is a tell-tail amount of banding contrast non-uniformity with this tilt-tuned only front etalon, and the H alpha spectrum tube showed a somewhat enlarged off-band high central interference ring. Knowing the theory behind mechanical pressure tuning, I decided to try a little experiment.

If I could apply a small amount of mechanical pressure uniformly to the periphery of the etalon, I could theoretically lower the CWL for less tilt, or even get the etalon exactly on-band as is done with the PST tuning. The issue would be how to implement this scheme with uniform pressure, and using readily available materials with little or no modification of the etalon.

The first thing would be to do some measurements to see what was possible:

LS35 mod measurements.jpg
LS35 mod measurements.jpg (159.27 KiB) Viewed 8250 times

As seen above, this showed I had about 5 mm worth of movement available between the etalon cover and the etalon tilt-tune base and internal etalon "pot" when fully seated, which could be used to apply pressure by turning the etalon cover on its internal threads. It also showed when fully screwed down the etalon cover had about 6 mm of clearance between the ERF cell and the etalon face. So it seemed I would have sufficient room to "play with."

Next was deciding how to apply uniform pressure, and I struck upon the idea to use a very thin needle trust bearing with even thinner thrust races that would total only 4 mm thick when combined, which also had the appropriate inside and outside diameters, and leaving plenty of room for a thin (~ 2 mm) rubber gasket to apply uniform pressure to the etalon periphery. This would therefore translate rotational movement in to a uniform axial movement and pressure without any torque applied to the rubber gasket or etalon face.

Cleaning out the excessive silicone used to secure the ERF cell retaining ring, etc., left a nice flat surface to place the first bearing race on. When the Etalon cover was screwed on, it came to within about 1 mm on being fully seated when the etalon surface was reached. Since the cover threads are fine, there is a good deal of fine adjustment possible in the pressure applied, and I discovered I could get the etalon completely on band with room to spare:

LS35 mod components.jpg
LS35 mod components.jpg (172.24 KiB) Viewed 8250 times
LS35 mod needle thrust bearing.jpg
LS35 mod needle thrust bearing.jpg (159.55 KiB) Viewed 8250 times
LS35 mod rubber pressure pad.jpg
LS35 mod rubber pressure pad.jpg (146.19 KiB) Viewed 8250 times

Etalon with no pressure:

LS35 mod no pressure.jpg
LS35 mod no pressure.jpg (94.98 KiB) Viewed 8250 times

Etalon cover at point of contact with rubber gasket - note that about 1 mm of axial movement (compression) is possible:

LS35 mod adjust clearance.jpg
LS35 mod adjust clearance.jpg (151.89 KiB) Viewed 8250 times

Etalon with less than one full etalon cover turn of pressure to come on-band:

LS35 mod pressure on-band.jpg
LS35 mod pressure on-band.jpg (89.31 KiB) Viewed 8250 times

This means the ideal situation of needing little to no tilt, which optimizes etalon performance and hopefully results in little if any banding. I will post my visual impression when I can next observe. Given the apparent success, and the wide availability of various thrust bearing sizes, it should be possible to apply a similar strategy to larger etalons without much complexity if room allows, or if relatively easy etalon cover / ERF spacings modifications can be made.

Cheers!

Bob
Last edited by Bob Yoesle on Fri Sep 03, 2021 2:28 pm, edited 1 time in total.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

This is very cunning Bob! Impressive!

A method that could be applied to other external etalons...

Looking forward to reading more the results you get from this method.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by torsinadoc »

Dang that's impressive!


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by MalVeauX »

Excellent work Bob, I recently used these same materials on a Twilight 2 mount to smooth out the axis and those steel roller bearings are great. It makes a lot of sense to have a cylinder that holds the etalon with spacers and something to thread down, on the roller, to apply pressure this way. Much better than tilt and much better than Solarmax methods (which are horrible, all of them except very very few rare good ones, as you mentioned, are tuned way off and require tons of pressure and tilt and none of it is uniform, they're awful).

I wonder if a pretty specific 3D printed housing could make this work. I also wonder if a large PVC cap with threads could be used for the threaded part, and that a 3D printed unit could be placed inside that PVC cap to hold the etalon exactly. This would be possible I think, and very inexpensive. And then much easier to put onto any donor OTA with a nicer focuser and stuff.

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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

Those are all eminently doable Marty - especially if the existing housings or etalon cover don't have the needed dimensions (width/depth) for a particular thrust bearing. The essential issue is the application of uniform pressure without stressing the etalon rotationally (risk of de-contacting?) and introducing additional etalon gap non-uniformity.

I had a chance to get out this morning, even though transparency was not the greatest. To make the test even more rigid, I used an Andover 1.7 A 10 mm filter added ahead of the BG38 filter which fit perfectly into the BG38's threaded cell, and it is kept secured by the 1.25 inch nose piece. I added a 1.25 inch Baader 7 nm Ha filter to the B600 nosepiece to act as an insurance sub-diameter "ERF" to protect the Andover filter.

LS35 Andover - Baader.jpg
LS35 Andover - Baader.jpg (158.69 KiB) Viewed 8194 times
LS35 Baader extension.jpg
LS35 Baader extension.jpg (158.05 KiB) Viewed 8194 times

Assuming the etalon has a FWHM of 0.75 A, the Andover DS reduces this to 0.65, but more importantly reduces the parasitic continuum form the profile "tails." In this case the double limb from the photosphere was eliminated. Additionally, a reduced FWHM is more revealing of any contrast non-uniformity, and in this case the disc non-uniformity - even with the reduced band-pass, was eliminated! Probably the best $15 I've spent in a long time ;-)


LS35 double stack Andover.jpg
LS35 double stack Andover.jpg (24.09 KiB) Viewed 8194 times
LS35 Sun test.jpg
LS35 Sun test.jpg (158.15 KiB) Viewed 8194 times

So while this is no hi-res imaging system, it's a great low-power grab and go scope! My next task will be to cut a nicer rubber gasket (:-), and replace the B600 rotating helical focuser with a non-rotating model so that some imaging can be more easily performed. Etalon tuning is a snap - not tilting, and just a ~ 1/4 turn of the etalon cover gets you nicely on-band.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by MalVeauX »

Fantastic! Love the simple setup.

I really wish there was a fairly straight forward way to re-do Solarmax etalons to improve their pressure tuning; the center pin thing kills them. They're so bad.

Very best,


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by christian viladrich »

A DIY pressure tuner. Excellent Bob !


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by thesmiths »

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.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Carbon60 »

This is great information, Bob. As a user of Lunt 35s this is particularly relevant.

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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by AndiesHandyHandies »

thesmiths wrote: Mon Sep 06, 2021 4:00 pm 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.
In a recent thread it seems clear to me that the PST is tuned by pressure on the foam ring compressing the three spacers between the thick etalon glass plates.

CV calculated that the force will give a 0.5A worth of compression in the spacers.

Any tilting is likely to break the molecular bond holding the plates on to the spacers. No reports of that spontaneously happening.

Non-uniformity in the foam ring or un-even re-bound could result in the sweet spot moving about a bit.

Cheers. Andrew.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by thesmiths »

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).


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by AndiesHandyHandies »

thesmiths wrote: Thu Sep 09, 2021 8:39 pm 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).
I suppose the small amount of tilt would be accomodated by the molecular attraction force.

Cheers. Andrew.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Carbon60 »

Parts purchased and installed. Just waiting to give it a trial run when the Sun returns :)

Bob, I bonded each large washer to black felt and then cut away the excess around the outer and inner perimeters when the glue set to provide soft surfaces to mate with the etalon glass cover plate and around the red filter.

Fingers crossed and hope it works.

I’ll post some images.

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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

Hi Stu,

Very interested to see your implementation and results - thanks for trying this out ;)


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by rsfoto »

Hi Bob,

Interesting but I really did not understand what you are doing with the etalon ?

Is it correct for me to say that you press togeter the two glasses and reduce the gap between them ?

I think the is a way to create a good pressure spring with a 3D printer. The question is to design one for a space of ~ 5mm ...

BTW I just did get a 3D printer :lol:

Rainer


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

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.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

Bought the parts I need to do this with my Lunt50 etalon, only £14 inc delivery. I will report back how I get on...

https://simplybearings.co.uk/shop/p9636 ... _info.html


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Carbon60 »

Bob Yoesle wrote: Sat Sep 11, 2021 2:42 am Hi Stu,

Very interested to see your implementation and results - thanks for trying this out ;)
Hi Bob,

Here's my first result. The conditions were not ideal with scudding clouds and quite a bit of haze, but after a bit of tweaking/tuning by compression I got both etalons on band with no tilt at all. Usually I have to tilt the front etalon to get on band.

Image20210914_Ha_Compression Tuning_Test by Stuart Green, on Flickr

This was imaged using a 2.5x PM. When I removed this and tried to image a wider field of view I did actually struggle and ended up with some out of tune regions, but I think a lot of that could be down to my scope design. Usually I always remain with a sweet-spot using the 2.5x or 5x PMs and for this the solution works perfectly well. The limb in this DS configuration is nice and solid.

I'm looking forward to further optimising when the conditions allow, but this is in my view an excellent start.

Thanks for sharing your design concept, Bob.

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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

That's a pretty nice hi-res image Stu! A lot depends on the native contrast uniformity of the etalon itself, and if the methodology really can produce near-equal compression forces on the etalon. So it's an experiment... but your results also look promising. I'm contemplating of how to do something similar along these lines with larger etalons, perhaps using Teflon ring bearing surfaces instead of roller needle bearings, etc.

BTW - I understand for another potential modder with an LS50 that the tilted ERF retaining ring stuck up too far to use the first baring race. I suggested removing the ERF retaining ring and using a soft silicone to secure the ERF in its cell so as not to present an uneven surface for the bearing race to seat against, or making a thinner retaining ring to accomplish the same thing.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

Very promising Stu!

Yes Bob, tilted retaining ring will be removed and silicone (or probably blu tak with me! :lol: ) used in place. It might leave the 5mm thickness of the bearing and 2 washers a little shy but that's not a problem and easy enough to make a packing gasket to bring up to desired thickness.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bastelhannes »

Next was deciding how to apply uniform pressure, and I struck upon the idea to use a very thin needle trust bearing with even thinner thrust races that would total only 4 mm thick when combined, which also had the appropriate inside and outside diameters, and leaving plenty of room for a thin (~ 2 mm) rubber gasket to apply uniform pressure to the etalon periphery.
Bob,

I am struggling for a few days with my mind. Why are you using the needle bearing? Is it really necessary or would the washers/ trust races be sufficient? The pressure should be uniform over the area of the ring...

Sorry for asking this dumb question...

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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by rsfoto »

MalVeauX wrote: Fri Sep 03, 2021 6:45 pm
I wonder if a pretty specific 3D printed housing could make this work. I also wonder if a large PVC cap with threads could be used for the threaded part, and that a 3D printed unit could be placed inside that PVC cap to hold the etalon exactly. This would be possible I think, and very inexpensive. And then much easier to put onto any donor OTA with a nicer focuser and stuff.

Hi Marty,

While it is still cloudy at my place i am having fun with 3D printing. Yes that could be possible, BUT, yes a big BUT, the temperature inside the housing ¿ how high is it ?

Printing in ABS or Nylon could be possible but not with the normal PLA, starts to get thermoplastic at 50°C, or maybe PETG ... Make a search in Internet to get an idea of the temperature stability of this materials ...

Perhaps a resin printer would be better ¿? What I have seen Resin printing is a bit messy :mrgreen:

Have seen a lot of videos in the last weeks and since Saturday I am printing pieces in order to learn how this new ¿? technology behaves. Got a printer for a volume of W235 x L235 x H250 mm ~ 9.25" x 9.25" x 9.84". Lots of fun :o


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

Why are you using the needle bearing? Is it really necessary or would the washers/ trust races be sufficient? The pressure should be uniform over the area of the ring...
Hi Bastelhannes,

My experiences and thought processes led me to believe that without a lubricant (and the resultant possible contamination of the optical parts) thrust races alone would have considerably more friction when used alone to apply pressure than with the needle roller bearings. As I noted, rotational torque applied to the rubber gasket and underlying etalon face was something I wanted to minimize as much as possible, hence the roller bearing cage is perfect for the application - and was quite inexpensive. Additionally, the additional 2 mm of thickness they offer are in this case beneficial, and the whole assemblage was cheap and required no real tooling, etc. other than to cut a piece of rubber sheet into a compression ring with scissors.

Of course you could possibly use multiple metal washers with some kind of lubrication, or other materials (ultra high molecular weight plastics such as Teflon, polyimide, PEEK, PPS, Nylon, Acetal and Polyester are also known for their good sliding friction properties) for thrust bearings, if constrained to do so. Indeed, this may be a solution I try for larger etalons. But these will require custom fabrication, and before going down that path I wanted a proof-of-concept demonstration with optimal mechanical performance and minimal time and material costs.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bastelhannes »

Hi Bob,

Thank your for the explanation. I totally forgot tightening a screw means a rotation vector on the layer below.

Did anyone play with piezo actors so far? Yes, they are a lot more expensive...


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bastelhannes »

Hi Marty,
Printing in ABS or Nylon could be possible but not with the normal PLA, starts to get thermoplastic at 50°C, or maybe PETG ... Make a search in Internet to get an idea of the temperature stability of this materials ...

Perhaps a resin printer would be better ¿? What I have seen Resin printing is a bit messy :mrgreen:

Have seen a lot of videos in the last weeks and since Saturday I am printing pieces in order to learn how this new ¿? technology behaves. Got a printer for a volume of W235 x L235 x H250 mm ~ 9.25" x 9.25" x 9.84". Lots of fun :o
I am working with high temp PLA. This is printed like normal PLA, after the print placed in your oven (including the support structures) and heat it at 80°C for 15 minutes. After cooling sown the crystal structure in the PLA has changed and the PLA now handels 140°C.

Unfortunately my oven is too random keeping the correct temperature. I am looking for a special oven...

Also I have got good results using PETG with infused carbon fibres...


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

Bastelhannes wrote: Thu Sep 16, 2021 6:49 am Did anyone play with piezo actors so far? Yes, they are a lot more expensive...
Nope, what you read is what has happened so far with this, it's a new thread.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

Hmmm.

This is not as straight forward with a Lunt50C. As noted previously the ERF is tilted, as is the ERF retaining ring, which then sits proud of the inner face of the ERF housing (red screw on section). This is remedied by putting a riser plate around the ERF retaining ring to create a flat surface for the bearing to sit on.

There is no more than 5mm clearance between the inner face of the etalon housing and the face of the etalon. Tight, but doable.

Now, here comes my 'doh' moment. The edge of the etalon housing (the black bit with the tilter on visible after the red housing is screwed off) sits maybe 1/2mm proud of the face of the etalon. Soooooo, my 78mm bearing and washers rides the edge of the etalon housing, not the face of the etalon.

Back to the drawing board.

I need a bearing that is smaller than the 78mm I ordered - something more like 74mm (simplybearings.co.uk are going to love me :lol: ).

Doable, just different component parts...


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

Ok, sometimes you just need to sleep on something... Think I have a solution for this, just hope i've got enough laser cut rubber left to make a different sized gasket. Should be able to use existing bearing without buying new one...

Guess this is my project for the weekend now...


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Rusted »

Is there anything to gain from using PTFE on PTFE to lower the torque on the etalon and rubber ring?
It might make for a much simpler bearing with thickness becoming a free choice compared with needle roller thrust bearings.

BTW: Are the needle rollers tapered to match the difference in turning radius along their length?


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

Rusted wrote: Fri Sep 17, 2021 6:10 am Is there anything to gain from using PTFE on PTFE to lower the torque on the etalon and rubber ring?
It might make for a much simpler bearing with thickness becoming a free choice compared with needle roller thrust bearings.

BTW: Are the needle rollers tapered to match the difference in turning radius along their length?
Regards the PTFE tape - honestly no idea?

Regards the bearings being tapered, visually, and bear in mind each roller on the ones I purchased is about 5mm long and 1mm wide (estimate), they don't look to be tapered, but without doing the maths I don't know how much taper you would expect to see in the geometry of it all.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

An update on this with my Lunt50C:

So I laser cut a new rubber washer that sits between the face of the etalon and the back side of the bearing, this works and the bearing sits above the etalon housing which is slightly proud of the face of the etalon. That problem solved...

Next work around... There really isn't many mm of space between the etalon face and the back of the ERF housing, maybe 4mm, and this wasn't helpful in fitting everything in. However, the ERF cell screws into the red anodised barrel of the etalon housing. In true fashion with solar kit the thread is 'locked' on this with a couple of liberal smears of silicone seal. Picking this out with a pin to free the threads, and then using a lens spanner you can back off the ERF cell housing towards the front of the scope to give more space internally.

Soooo, now, from the face of the etalon I have rubber washer, bearing, metal washer, rubber riser plate around the ERF cell to give a flat surface. I woke this morning to the sun shining around the edge of the bedroom curtains, it's not going to be a wall to wall sunshine day as there is cloud floating around but I am optimistic I should get to test it... Watch this space!

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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

And the proof of the pudding... My Lunt LS50C etalon needs no tilt for it to be on band, see pic below:

ImageHa-FD-SS-BW by Mark Townley, on Flickr

Using the compression method described in this thread indeed shifts the centre line of the tuning, but in my case as the etalon needs no tilt the method is not needed. Pic below shows with compression applied.

ImageHa-FD-SS-Bob-Mod-BW by Mark Townley, on Flickr

So, morale of the story, this method works really well, but only if you are having to tilt your etalon to bring it on band.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bastelhannes »

Because I am not aware, if the Lunt etalon is a different type compared to the PST etalon: would it be worth to take some time to try this with PST etalon? If Mark punches my idea balloon again with his reality check needle.. I could save some time, if this idea is too ridiculous... (No offend Mark, I really appreciate your knowledge and am happy, that people like you share your knowledge. In my case it's like having a fantastic idea while being drunk and the next morning I discover being sober: what a sh't...)


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

Bastelhannes wrote: Sun Sep 19, 2021 6:05 am Because I am not aware, if the Lunt etalon is a different type compared to the PST etalon: would it be worth to take some time to try this with PST etalon? If Mark punches my idea balloon again with his reality check needle.. I could save some time, if this idea is too ridiculous... (No offend Mark, I really appreciate your knowledge and am happy, that people like you share your knowledge. In my case it's like having a fantastic idea while being drunk and the next morning I discover being sober: what a sh't...)
Tee hee hee! :lol:

The PST etalon is tuned by compression, so the same principle is already happening here.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

Those are very good demonstration images Mark!

The methodology works, but as you note the etalon is already on-band without tilt, and compression only further blue-shifts the CWL.

Applying heat via some sort of electrical resistance similar to a dew heater, or the Sun itself (ERF removal), might serve to warm the etalon feet/spacers sufficiently to raise the CWL and allow the etalon to have "room" for compression tuning. A longpass filter could be employed for environmental protection of the front etalon surface, and perhaps give a bit of “greenhouse effect” for etalon warming.

As with any heat applied to an etalon, you’d have to give it a bit of time for the etalon to stabilize thermally for optimum results. And as I discovered with the DSF100 filters, the larger the etalon, temperature stabilizing becomes more difficult and takes longer, and etalon gap uniformity may not be as easily obtainable. Then again there's a big difference between 50 mm and 100 mm...


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by AndiesHandyHandies »

Hi

I think in the thread close by thread we have established that the PST is tilt tuned.

Cheers. Andrew.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by AndiesHandyHandies »

Hi

Are these the only tuning methods available:

Tilt tuning, to make the space between the plates on axis longer - PST

Heating the mica spacer - Quark

Pressurising the air in the etalon air gap to change the refactive index - Lunt

Patented RichView tuning using novel spacers and actuators to mae the space smaller - Coronado DS90.

And so the whole no of wavelengths of light which will fit in and result in constructive interference

For all of these a comb filter is needed to pick out the required h-alpha wavelength, and reject off band wavelengths where different nos of waves fit in the gap.

Professionals seem to tilt the Fabray-Perot etalons to tune them.

The patent seems to preclude any other commercial H-Alpha filters using compression and I cannot find any other reference to compression tuning.

Cheers. Andrew.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

I think in the thread close by thread we have established that the PST is tilt tuned... Patented RichView tuning using novel spacers and actuators to mae the space smaller - Coronado DS90... The patent seems to preclude any other commercial H-Alpha filters using compression and I cant find any results for compression tuning.
Hi Andrew,

The "rich-view" spacer feet are not "novel." They are fused silica as is used by Coronado, Lunt, and Solarscope.

Those denying the compressibility of fused quartz are mistaken. I know it sounds counter intuitive that compressing the thin glass spacer feet could be accomplished in order to tune an air-spaced etalon - but that is the reality. The gap spacing is so sensitive that only small gap changes results in a CWL shift. And these spacers, even though made of low-expansion materials, are sensitive to both a thermal expansion and mechanical compression because no material is impervious to compression or thermal expansion. The quality and uniformity of the spacers are just as important to etalon performance as the etalon plates themselves.

1. I have proved mechanical compression tuning works on a Lunt etalon quartz spacers conclusively with the etalon fringes shown in the first post in this thread. Mark's post above confirms this. Before I implemented vacuum tuning, my DS140/90 telescope's internal quartz-spacer etalon was compression tuned around the periphery.

2. Coronado has implemented this compression methodology over the years with quartz spacer etalons - first with their internal double stacking modules when based in Tucson, and then with the SMII and SMIII "rich view" tuning implemented by MEADE Coronado. It is widely known the PST etalon is usually tuned via edge compression (some early PST's may have been tilt-tuned), and edge compression is shown in the original 2006 patent (link below). Christian Viladrich has confirmed central compression pressure tuning for the "rich view" etalons. Disassembly of the RV etalon can plainly see how it is accomplished (imperfectly) with mechanical pressure applied to the central spacer area.

SM60II richview etalon.jpg
SM60II richview etalon.jpg (115.2 KiB) Viewed 5633 times
Note the pressure adjustment bolt in the center of the internal SM60II collimator lens, which if over tightened can actually fracture the collimator lens.


3. "Professionals" generally seem to use piezo-electric tuning for the most precise applications of fine-tuning etalons, and this is indeed making very small changes to the etalon gap spacing as is done with mechanical compression.

4. Mechanical compression tuning was patented by Lunt et.al. at Coronado in 2006. US patents are generally in effect for 20 years (this one expires in 2024). Lunt may then opt to implement this as well for their front tilt-tuned etalons. Nothing precludes DIY implementations for non-commercial applications.
Last edited by Bob Yoesle on Tue Sep 21, 2021 5:10 pm, edited 1 time in total.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by AndiesHandyHandies »

Bob Yoesle wrote: Tue Sep 21, 2021 2:10 pm
I think in the thread close by thread we have established that the PST is tilt tuned... Patented RichView tuning using novel spacers and actuators to mae the space smaller - Coronado DS90... The patent seems to preclude any other commercial H-Alpha filters using compression and I cant find any results for compression tuning.
Hi Andrew,

The "rich-view" spacer feet are not "novel." They are fused silica as is used by Coronado, Lunt, and Solarscope.

Those denying the compressibility of fused quartz are mistaken. I know it sounds counter intuitive that compressing the thin glass spacer feet could be accomplished in order to tune an air-spaced etalon - but that is the reality. The gap spacing is so sensitive that only small gap changes results in a CWL shift. And these spacers, even though made of low-expansion materials, are sensitive to both a thermal expansion and mechanical compression because no material is impervious to compression or thermal expansion. The quality and uniformity of the spacers are just as important to etalon performance as the etalon plates themselves.

1. I have proved mechanical compression tuning works on a Lunt etalon quartz spacers conclusively with the etalon fringes shown in the first post in this thread. Mark's post above confirms this. Before I implemented vacuum tuning, my DS140/90 telescope's internal quartz-spacer etalon was compression tuned around the periphery.

2. Coronado has implemented this compression methodology over the years with quartz spacer etalons - first with their internal double stacking modules when based in Tucson, and then with the SMII and SMIII "rich view" tuning implemented by MEADE Coronado. It is widely known the PST etalon is usually tuned via edge compression (some early PST's may have been tilt-tuned), and edge compression is shown in the original 2006 patent (link below). Christian Viladrich has confirmed central compression pressure tuning for the "rich view" etalons. Disassembly of the RV etalon can plainly see how it is accomplished (imperfectly) with mechanical pressure applied to the central spacer area.


SM60II richview etalon.jpg

Note the pressure adjustment bolt in the center of the internal SM60II collimator lens, which if over tightened can actually fracture the collimator lens.


3. "Professionals" generally seem to use piezo-electric tuning for the most precise applications of fine-tuning etalons, and this is indeed making very small changes to the etalon gap spacing as is done with mechanical compression.

4. Mechanical compression tuning was patented by Lunt et.al. at Coronado in 2006. US patents are generally in effect for 20 years. Lunt may then opt to implement this as well for their front tilt-tuned etalons. Nothing precludes DIY implementations for non-commercial applications.
Hi

Thanks for the clarification.

1. Can I tell from my serial no if my PST if it is an early one, it has the rusted yellowish coating. Do the compression tuned PSTs still have the orange foam?

2. It seems the Patent I looked at is different to the actual RichView implementation?

3. At a quick look.

4. I cannot get the link to work. Does it refer to novel materials? This is the one I looked at. https://patents.google.com/patent/US20050078906

So if you have an old tilted PST can it be converted to pressure tuned, and is one way of tuning superior to the other?

Thanks. Andrew.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

Hi Andrew,

Item 1. I am not familiar enough with the PSTs to give you guidance here (compression tuning of the PST goes back at least to 2017 if not further), and I similarly can't tell you how easy it would be to convert a possible tilt tuned PST etalon to a compression tuned etalon. Theoretically, a compression tuned etalon would be better if you can uniformly apply pressure that maintains a good etalon gap uniformity verses a tilt etalon. But as with most things there are a lot of variables in the execution that will affect the outcome. Personally I would leave the OEM tuning in place for the PST - which likely is already compression tuned.

Item 2. The patent is generally referring to compression of the etalon spacers, while the "rich-view" implementation applies compression only to the central spacer in the SMII and SMIII. However, in 2017 Meade Coronado also refers to the PST as being "rich-view" tuned, which only involves peripheral spacers. So "rich-view" is really a generic term applied to any mechanical compression methodology used by Meade Coronado.

Item 4. The link is to a pdf file that opens fine in Chrome, here's the source link, and you can see the pdf by clicking on that download option. This is the final approved patent by the patent examiner, your link is to the patent application.

I quoted you with regard to the term "novel." The patent refers to both conventional etalons (as I have used in this thread):
In the preferred embodiment of the invention, the device consists of a housing that includes a hollow cylindrical component and a cap that define an interior space adapted to receive a conventional air-spaced etalon [emphasis added]. The cylindrical component and the cap are coupled by means of a very finely treaded connection which, through rotation of the cap, allows the axial adjustment of the length of the interior space and the corresponding uniform compression of the etalon to fine tune the optical length of its cavity.
as well as "individual pressure actuators" (e.g. the SMII/III "rich-view" method), and use of other compression methods and materials.
Last edited by Bob Yoesle on Tue Sep 21, 2021 6:27 pm, edited 1 time in total.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bastelhannes »

Bob,

Thank you very much for all the details and the links to the documents.

Looks like Mark got a new "reality needle" competitor...


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by marktownley »

Bastelhannes wrote: Tue Sep 21, 2021 5:51 pm Looks like Mark got a new "reality needle" competitor...
:lol:


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bastelhannes »

Bob,

did you, by accident, measure the movement you induced to the etalon by pressure? This would be very interesting to evaluate a fitting piezo actor...


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by AndiesHandyHandies »

Hi

Merlin in the other thread active now say all the PSTs he has taken apart had the orange ring in.

If they all have the two litle feet on one side as in Marks PST diss-assembly video shows then it seems clear to me they are tilt tuned.

The double stack Coronado filter with richview are compression tuned as well as tilted. Merlin checked all this with his spectro-graph.

Cheers. Andrew.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Carbon60 »

Just to add to the previously described success with my Lunt 35 on my 150mm/Lunt 35 DS mod (result shown earlier in this thread) and Mark's Lunt 50, here's my Lunt 60 front Etalon mod using Bob's compression method.

I used the same components, just larger (60mm inner diameter/85mm outer).
Lunt 60 Etalon_Bearing and Washers.JPG
Lunt 60 Etalon_Bearing and Washers.JPG (116.86 KiB) Viewed 5564 times
The washers were bonded onto fabric (piece of blackout curtain) to provide a soft surface to mate against the glass and inner surface at the front of the removable red etalon housing.
Lunt 60 Etalon_Washer Bonding.JPG
Lunt 60 Etalon_Washer Bonding.JPG (92.46 KiB) Viewed 5564 times
I cut around the inner and outer perimeters once cured to remove the excess fabric, leaving a bit of a boarder around the top washer to help hold it in place during assembly into the red etalon housing.
Lunt 60 Etalon_Components.JPG
Lunt 60 Etalon_Components.JPG (104.67 KiB) Viewed 5564 times
Lunt 60 Etalon_Bearing Assembly.JPG
Lunt 60 Etalon_Bearing Assembly.JPG (105.79 KiB) Viewed 5564 times
The housing was screwed into place leaving a few threads to fine tune when on the scope.
Lunt 60 Etalon.JPG
Lunt 60 Etalon.JPG (93.66 KiB) Viewed 5564 times
As with my Lunt 35 and as Mark has said with his 50, on-band can easily be achieved without any tilt by simply screwing the housing down on the threads. Too far and it goes off-band, as Mark has shown.

This has really helped get better FDs with my triple stacked Lunt 60 which otherwise suffers from banding. This enabled me to take this multi-pane mosaic with little to no witness lines between panes; something that was really difficult beforehand.

Image20210920_Ha_FD_Mono by Stuart Green, on Flickr

I'll obviously run further trials to give it a thorough test, but this and my Lunt 35 mods are looking good so far.

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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

That's a fantastic mosaic image Stu! And thanks for posting your implementation pics - very helpful to see the fabric centering method you described earlier. Another refinement could be a thin shim of some sort to keep the needle bearing itself centered in the housing and on the bearing races.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Valery »

Bob Yoesle wrote: Tue Sep 21, 2021 5:01 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.


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Carbon60 »

Bob Yoesle wrote: Wed Sep 22, 2021 11:53 am That's a fantastic mosaic image Stu! And thanks for posting your implementation pics - very helpful to see the fabric centering method you described earlier. Another refinement could be a thin shim of some sort to keep the needle bearing itself centered in the housing and on the bearing races.
Hi Bob,
Good thought about a shim, but I think once the compression load is applied through the stack the bearing is effectively held in place by friction. I haven’t seen any evidence of it shifting yet, but will keep an eye on it.

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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Bob Yoesle »

Hi Bob, They referred to the Double Stack 40mm external etalon being RichView tuned. Not to internal tilt tuning etalon. Valery
Hi Valery. The reference to RichView tuning I cited was the PST's internal etalon, then called a "30 mm," which in reality is the same as always 20 mm clear aperture etalon. It is tuned via mechanical compression, which shifts the natively high CWL blueward to be on-band just as occurs with a tilt tuned front-mounted etalon.
The compression system works because it can squeeze the feet due to their Youngs modulus... 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. (Andy Lunt)
Hi Bob, Good thought about a shim, but I think once the compression load is applied through the stack the bearing is effectively held in place by friction. I haven’t seen any evidence of it shifting yet, but will keep an eye on it. Stu.
Hi Stu. Good to hear, but I will note that I completely back off the compression on the LS35 so as to maintain the resilience of the silicone support ring when the scope is not in use, and therefore the smaller diameter of the roller bearing and un-secured bearing races do have some ability to shift slightly within the etalon cover assembly...


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Re: Small Etalon Improvement For Only $15 USD (13 Euros)

Post by Martin_S »

This has been a great thread, many thanks to all the contributors to this thread.
I'm about to try this on my LS50F etalon. I will be trying a slightly different method of assembly.

I have around 100 ,65mm diameter x 1.5mm thick O rings that were sent by mistake on Ebay, so I thought I would try one instead of the rubber gaskets that have been used in previous
posts.

The etalon housing is slightly higher than the top face of the etalon glass by about .5mm, so I'm hoping that this O ring will put even pressure onto the etalon. I'm not sure how much the O ring will compress once everything is assembled. If it works it should be a lot easier than custom cutting rubber gaskets.

Martin
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Screenshot 2021-11-27 121354.jpg
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H alpha : ,Skywatcher 100mm F10 open frame refractor, a tilted 2" 7nm H-alpha filter as a sub energy rejection filter, Baader TZ4 telecentric focal extender, Player One energy rejection filter, Combo Quark Chromosphere, Naked PST etalon for double stacking, ASI74mm camera
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