Disassembling a Coronado 90

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Re: Disassembling a Coronado 90

Post by Merlin66 »

Thanks Bob....that answers that question ;-)


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Re: Disassembling a Coronado 90

Post by swisswalter »

Hi Bob

thank you very much for the wealth of information you gave.

However, beeing a strong believer of the tilt tuning I still can not see that you squeece the distance of the etalon plates by just applying some force to the center of the outer etalon plate.

On top of thet, the Etalon sits on a foam, so all pressure applied to it will be passed right away into the foam, hence no change to the space between the etalon plates


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Re: Disassembling a Coronado 90

Post by marktownley »

Well hasn't this turned into an eyeopening thread ;)

Bob, thank you for all this information and all these pics. Karma to you 8)

Before I go any further here, I just want to reiterate to anybody who reads this thread in the future the point Stephen made about the potential risks with a disassembly...

Now, i'm really glad you posted this when i'm on holiday as I feel I can give a decent reply to this. So, before I go any further please be aware this is me on my soap box, it is a Townley philosophical ramble, and that the views expressed are my own and as such not everyone will agree with them, but, hey ho!

If we look back in time to the pre-meade days at say their 90mm external etalon, I think(?) these were a really high price compared to now - $14k? - somebody correct me if i'm wrong. The reason I think for this high price is that each etalon was precisely tuned to the Ha line by the use of exactly the right thickness spacers in between the etalon plates - this precise spacer thickness is the difficult thing to replicate, and was the thing that made it expensive as pretty much each etalon is a handbuilt item rather than the batch production of the more modern etalons of today. This as a result, as many pre-meade etalon owners will testify to, gave fantastic views. This is great bar 3 factors; the etalons were manufactured to operate at a certain altitude above sea level, the etalons were manufactured to operate at certain atmospheric pressure and thirdly they were damn expensive. The first 2 points are addressed to some degree with the tilt tune thumbwheel on the etalon. This method of etalon production is exactly the same method that Ken Huggett of Solarscope uses on the Isle of Man, and guess what, the prices for them are high but then again so is the quality - go figure...

Now, when Meade takes over Coronado there is a commercial decision is made that the big Ha scopes have to become a lot cheaper - at prices in excess of $10k they aren't going to sell as many scopes if the selling price can be brought down to $3-4K. Throw in competition now from Lunt too and you can see why the guys at Meade were on a cost cutting mission.

So, how do they cut costs? Well simple really, it's all down to these etalon spacers... As long as all the spacers on an individual etalon are the same thickness it is ok if they aren't precisely tuned to 656nm, infact, ever so slightly above is what they're aiming for. If the next etalon that is made is tuned ever so slightly different than the previous one that is fine too... What has happened here is the quality control has now got a much wider tolerance range in terms of what is acceptable, if this was a pre-meade etalon it would have been rejected as it was out of spec and would have to be 're-spaced', which, in turn makes it more expensive...

So how do Meade use these etalons that aren't all quite tuned to exactly the Ha line? Well this is where their Rich View tuning method comes into play. What is it and what does it do? Well it makes use of a property of Fabry Perot etalons in that they are ever so slightly wedge shaped. The 2 inner surfaces are (obviously) perfectly parallel to each other, however the outer face of each etalon is at a slight angle to the inner face, it has to be, to stop endless reflections. Meade sits the back edge of the etalon on a soft foam ring - they do this on the PST, you can see it in Bob's pics above, i've also seen the same thing on walters Richview 90mm external etalon too. The principle is straight forward really, if you push down on a wedge on a soft foam ring then the whole etalon 'tilts' with respect to it's at rest position. The effect of this is that when you tilt an etalon you are blue shifting the tuned frequency, so if we have an etalon that is tuned slightly high we can tune it back to 656.28nm to get on band.

This is not at odds with the patents either, in effect this is compression tuning, or depending on how you view it tilt tuning...

Seems perfect doesn't it! However, why don't we use the tuner wheel to apply this tilt (in the case of external etalons)? well you can, but the Richview method is much more fine, almost like a vernier in its operation. In the case of the internal etalons like Bobs in the pictures above, the pressure to the etalon is applied in the centre as the centre of the etalon isn't used, it is uncoated and houses the central spacer - it's the obvious place to apply the pressure as it applies it evenly across the etalon. It's pushing this wedge shaped etalon into the soft foam...

It's all good and well me saying this, but what evidence do I have to back it up? Well, with a PST or solarmax richview scope, as you twist the tuning ring the view goes from red wing to on band to blue wing. How do I know this, well you can see and people have photographed ellerman bombs doing this. There is another clue too that can be seen in peoples images and also reports about the SMii scopes, and that is they are not as evenly on band across the disk as the Lunt scopes are; this is because the etalon is slightly tilted (not perpendicular as the pressure tuned Lunts) and the result of this tilting can manifest itself in a sweet bar or slight change of band across the fov. This variance of band across the fov between different scopes is a result of the etalons all being tilted by slightly different amounts to get them on band - a result of the wider tolerances in the accepted thickness of spacers, it in turn a direct result of Meade cost cutting. So with the SMii scopes you may get a good one, you may get one not quite as good...

Regards your discussion about the ERF on DS etalons, well again this is back to cost, it is just cheaper to manufacture an etalon with an ERF that can be either a standalone etalon to go on another refractor or screw directly onto the SMii scope. If there was 2 versions then it would cost more, and as the ERF is what applies the pressure for the richview tuning system a complete redesign would be needed = more cost. Interestingly the solarscope double stack etalons are ERF-less.

So there you go, my take on it all. Just my thoughts so feel free to throw mud pies or blow me kisses, I don't mind!

Mark :)


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Re: Disassembling a Coronado 90

Post by piekielrl »

Thank you both, Walter and Mark. I now understand what is happening here, with respect to the pressure affecting the tilt of the etalon on the foam, rather than the spacing. Also this explains the presence of the "sweet spot" in the Coronado scopes' field of view.

These construction details (of solar scopes)seem to be one of the best-kept secrets in the astro-community. Even many long-time optics engineers are still puzzled over this.

I'm so glad I joined this forum to be able to talk with all of you and share these things.

OK, here's another question that has been puzzling me for a long time: When you lay two optical flats on each other and observe the fringes under monochromatic light, you get straight, parallel fringes if the flats are good. If etalon surfaces are super-flat, say 1/100th-wave, and precisely parallel, why don't they show perfectly straight fringes? I always see a "perfect" bulls-eye pattern when I test them or see photos of other etalons under similar tests. To us opticians, circular fringes mean a mis-match in radii (convex or concave surfaces). What's going on here?

Thanks again, Bob


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Re: Disassembling a Coronado 90

Post by Suncityan »

Well now, I've just read a modern day "Gettysburg Address"!

Thanks Mark


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Re: Disassembling a Coronado 90

Post by swisswalter »

Hi Mark

:kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss: :kiss:


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Re: Disassembling a Coronado 90

Post by swisswalter »

Hi Bob


a very interesting question. I guess the profs will jump in to answer it


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Re: Disassembling a Coronado 90

Post by marktownley »

OK, here's another question that has been puzzling me for a long time: When you lay two optical flats on each other and observe the fringes under monochromatic light, you get straight, parallel fringes if the flats are good. If etalon surfaces are super-flat, say 1/100th-wave, and precisely parallel, why don't they show perfectly straight fringes? I always see a "perfect" bulls-eye pattern when I test them or see photos of other etalons under similar tests. To us opticians, circular fringes mean a mis-match in radii (convex or concave surfaces). What's going on here

I'll have a go... When you say you're observing using monochromatic light I guess you're talking about using a laser. You get the straight parallel fringes here because the laser is a coherent light source and so is giving out plane waves, when they reflect back off the flats you get this plane wave interference pattern. With an etalon you get the bullseye pattern when it is illuminated with a diffuse light source, as a result this is a spherical wavefront, the waves that are reflected from the front and back surfaces of the etalon plate are two sets of spherical waves that when they interfere with each other form the circular (bullseye) interference pattern.


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Re: Disassembling a Coronado 90

Post by piekielrl »

Hi Mark, I've used both lasers and a diffuse mercury lamp to view the bands on two joining flats, and both of them show straight bands.
I've used both lasers and diffused monochromatic light (the mercury lamp) on the etalons, and they always show circular bulls-eyes.
What am I missing here?

Thanks again, Bob


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Re: Disassembling a Coronado 90

Post by marktownley »

Maybe the circular bullseye interference pattern occurs because the etalon plates are wedge shaped?




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Re: Disassembling a Coronado 90

Post by marktownley »

Here's another graphic I found...




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Re: Disassembling a Coronado 90

Post by Merlin66 »

A circular aperture (like an objective) causes a circular arrangement of fringes.
The plates themselves usually have an outer surface inclined at 3-30 min arc to avoid secondary images.
There's an extensive chapter on etalons in "Practical Spectroscopy" by Harrison, Lord and Loofbourow, Chapter 20, p 558-569.


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Re: Disassembling a Coronado 90

Post by marktownley »

A circular aperture (like an objective) causes a circular arrangement of fringes.
The plates themselves usually have an outer surface inclined at 3-30 min arc to avoid secondary images.
There's an extensive chapter on etalons in "Practical Spectroscopy" by Harrison, Lord and Loofbourow, Chapter 20, p 558-569.

There you go, there's always someone who knows round here ;) Thanks Ken :)


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Re: Disassembling a Coronado 90

Post by Merlin66 »

I found this reference from "Fundamentals of Optics" - Jenkins and White, to show the Fabry-Perot etalon "optics" even clearer....
Hope this helps.



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Re: Disassembling a Coronado 90

Post by piekielrl »

Thanks to everyone for their help and comments. Now I want to "see this for myself."

Does anyone have a decontated or damaged / destroyed etalon that they would be willing to sell me for experiments?

Thanks, Bob Piekiel


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Re: Disassembling a Coronado 90

Post by marktownley »

A circular aperture (like an objective) causes a circular arrangement of fringes.
The plates themselves usually have an outer surface inclined at 3-30 min arc to avoid secondary images.
There's an extensive chapter on etalons in "Practical Spectroscopy" by Harrison, Lord and Loofbourow, Chapter 20, p 558-569.

I've always wondered how much this amount is, thanks Ken :)


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Re: Disassembling a Coronado 90

Post by davelilis »

piekielrl wrote: Mon Dec 23, 2013 5:27 pm OK guys, here's the pics. ........................
Enjoy, Bob Piekiel
Sorry for reviving an old thread, but where are the pics referred to here?, I'd be very interesinted in seeing them !!

Thanks
Dave.


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