Disassembling a Coronado 90

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Disassembling a Coronado 90

Post by piekielrl »

Has anyone here ever disassembled a Coronado 90? My Coronado 60 comes apart easily. The front lens cell unscrews from the brass tube, and the brass tube unscrews easily from the rear etalon housing. None of the parts on the 90 will budge, as if it's put together with some kind of thread-locking adhesive.

I've disassembled stubborn thread-locked parts before by heating the pieces with a heat gun, but don't want to start trying things with the '90 until perhaps someone could explain how they're put together and if there's a better way to take them apart.

(The reason I'm doing this is to take measurements and such about the internal parts. For example, the tuning collar on the SM 60 appears to have 50 threads-per-inch... One of my observations).

Any suggestions you could give would be most appreciated!

Thanks, Bob Piekiel


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Re: Disassembling a Coronado 90

Post by Merlin66 »

Bob,
Welcome on board!
I hope someone who has an SM90 will answer with more accurate details...
On the smaller etalons, there's a sneaky very small grub screw (on the "face plate" to the T-Max adjuster) which locks the brass sleeve from turning and unscrewing...
Just check that this is not the problem.....


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Re: Disassembling a Coronado 90

Post by JohnM »

And the access hole to it plugged with black silicone.
It can make it very hard to see.


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Re: Disassembling a Coronado 90

Post by solarchat »

Bob is talking about the Coronado SolarMax 90 scope, not the etalon. he wants to unscrew the focusing assembly and the internal etalon holder.


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Re: Disassembling a Coronado 90

Post by Merlin66 »

Ohhh
Thanks for that...it wasn't obvious from his description.


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Re: Disassembling a Coronado 90

Post by marktownley »

No direct experience of this myself, but with the PST with the threads with thread lock on i've used boa straps to release them.


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Re: Disassembling a Coronado 90

Post by piekielrl »

Since my initial post on Nov 13th, I have successfully disassembled my Coronado SMII 90 down to the last individual pieces except for the etalon (I'm obviously not going to touch it or try to remove it from the cell).
The entire scope is put together with red threadlocker adhesive. I carefully heated the individual joints with a propane torch to about 180-200 degrees F (measured with an infra-red thermometer) and was able to unscrew everything with either a gloved hand or a strap wrench. It is actually quite simple in construction, and of course very similar to the SM 60, which (in my case) was NOT assembled with threadlocker.

Eventually I plan on putting together a video of this, or at least a write up of sorts. If anyone's interested in the details in the meantime, just write and I'll be happy to explain further.
Thanks, Bob Piekiel


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Re: Disassembling a Coronado 90

Post by Merlin66 »

Bob,
Some photos of the various bits would be appreciated.


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Re: Disassembling a Coronado 90

Post by swisswalter »

Hi Bob

great. Is putting together as easy ? Yes pics would be fine


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Re: Disassembling a Coronado 90

Post by marktownley »

Thanks Bob, that would be great! :)


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Re: Disassembling a Coronado 90

Post by piekielrl »

Putting it together is a cinch. You just screw the pieces back together by hand - it's mostly just threading the rings and fittings on to each other. I will have some pics up by later tonight. I did not use any Loctite when re-assembling it. It works just fine by hand-tightening the pieces. The only "critical" parts are the small blocking filter elements in the diagonal. They are mounted in the barrels at a slight angle, so you need to mark the rotational positions of the barrels before you heat and unscrew them, so you can install them back in exactly the same orientation. The tilt is to prevent reflections from going back and forth along the optical axis.

On another note: The rear half of the etalon is held into the collar by blobs of black RTV. This RTV is more for keeping the glass centered rather than gluing it in place. The front half of the etalon is only attached to the rear by the molecular bond of the spacers. I would HIGHLY RECOMMEND that you always keep the tuning collar in the TIGHT position (where the central post is physically touching the etalon) when transporting the scope, rather than leaving it in the loose position. This way, even if the scope gets jarred, the etalon is "secured" by the tuning post pressing the halves together. With a big etalon like the 90 has, the larger inertia of the glass makes it more susceptible to de-contacting from a jolt, in my opinion.
Thanks for all your interest!
Bob


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Re: Disassembling a Coronado 90

Post by piekielrl »

OK guys, here's the pics. I have it pretty much stripped to the core, but in the photos I didn't remove the lenses from the cells (the front objective, the front and rear collimating lenses), but I do have all the retaining rings loosed and removed. If I wanted to take out the lenses, I would just need to pull them out with a vacuum-parts handler, or simply turn the cells over and let them fall out, I plan on measuring the radii of the lens surfaces and the focal lengths, then trying to calculate the type of glass and/or the refractive index of each piece. The lenses are very heavily multi-coated.
Of particular interest is the diagonal - the front filter element (in the chrome barrel) is transparent gold colored that makes everything look deep red when viewed through it directly. This filter is mounted in the barrel at an angle, but the light coming through it does not change color when you tilt it. The tilt seems to be just for preventing unwanted, on-axis reflections. The rear filter, the red-colored one, makes everything look green when viewed through it directly. The diagonal mirror itself is some sort of "cold" mirror - it has a deep purple coating, and makes everything look purple / pink when used alone. If the chrome barrel is attached to the diagonal mirror, light coming through it is a deep red. Once the rear filter is screwed in place, the resultant light throughput is extremely dim, deep red. Obviously the "blocking filter assembly" is designed to cut UV, IR, and further trim and drop the intensity of the light coming through the etalon.
The four tiny screws that hold the focuser into the 2" barrel thread into tiny brass or steel inserts laid into channels cut into the drawtube. The screws are so short that they only "bite" in barely ONE THREAD when screwing them in. They are hence, VERY easy to strip.
The hardest piece to initially separate was the large, black adapter that holds the etalon assembly into the gold tube. It has the most mass, so it took the most amount of heat to soften the threadlocker. The front objective cell was the second hardest part to remove, again due to high metal mass. Once I got the scope apart, it takes about 2 minutes to completely disassemble it and lay the parts out as you see in the photos. It takes less than three minutes to re-assemble it, and I see no reason to ever thread-lock the parts again!

Before you jump at me with your criticism, I did this because "Inquiring minds want to know!" My SM60 comes apart easily, with no threadlocker except for the focuser and diagonal, so I wondered "What was so special about the 90 that they make it difficult to get inside?" Nothing - as far as I can see - they're virtually identical, except for the size of the parts.

Enjoy, Bob Piekiel









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Re: Disassembling a Coronado 90

Post by solarchat »

AS THE PERSON BEARING ALL LIABILITY FOR ANYTHING ON THIS FORUM, I STRONGLY ADVISE THAT YOU DO NOT DO THIS TO YOUR SOLAR TELESCOPE UNDER ANY CIRCUMSTANCES.


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Re: Disassembling a Coronado 90

Post by astroshot »

Holy smokes! I hope you aren't left with any spare parts when you reassembled it!


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Re: Disassembling a Coronado 90

Post by piekielrl »

Hi Steve, I didn't know this topic would be against the rules here because of the many other similar topics I've browsed. After I posted my initial question asking if anyone else had taken one of these apart, several people expressed interest in my results, so I followed through with the pics. I've seen Mark's video on youtube showing how to completely disassemble a PST, and all of the various mods that come up with here from time to time.
My main reason for doing this was to find out why Meade / Coronado would freeze the threads on the SM90, but not on the SM60 or others, which can be taken apart by just using your hands to unscrew them.
My pics (as well as my original inquiry) were for educational purposes, not to try to get others to change or alter their scop
So I will concur with your statement: DO NOT TRY TO MODIFY A COMMERCIAL SOLAR SCOPE, OR SERIOUS AND PERMANENT EYE DAMAGE MAY RESULT.

Lunt's / Coronado's patents (at least four of them) are available online, describing in more detail the construction of the etalon assembly and tuners, as well as a statement from Coronado describing eye safety guidelines established by professional opthamologists, and how Coronado's factory-made scopes exceed these guidelines by many factors. Those wishing further info should study these papers.
Bob Piekiel


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Re: Disassembling a Coronado 90

Post by solarchat »

I never said it was against the rules. Just want to get my 2 cents in for the inevitable mishap occurs and someone holds me responsible. No biggie...


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Re: Disassembling a Coronado 90

Post by Merlin66 »

Yeah, Lunt and Coronado patents are available (copies in the library) but they don't reflect the actual construction.
I can see the etalon in the cell, and what looks like the front lens with a central screw (?). I assume this is the rich field "connector" ?
Can you set up this assembly left to right to show the relative positions. Where's the tilt arm adjuster?
Just trying to visualise how the etalon cell is adjusted......


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Re: Disassembling a Coronado 90

Post by piekielrl »

let me try to describe the cell for you, left to right (that is, eyepiece end to objective end:

At the left end is a convex lens that re-focuses the parallel light coming out of the etalon. It's held in by a metal retaining ring and rests on a thick foam gasket ring. You can remove the lens by unscrewing the ring and lifting it out.
Next in line is the rear-facing half of the etalon. It is held in the cell by a series of small setscrews that lightly touch and center it, and blobs of black silicone RTV.
The front half of the etalon is attached to the rear half only by 6 spacers around the edge, and presumably one in the center, under the "obstruction." Like said elsewhere, it is only half to the rear half by molecular bonding of the spacers.
In the front of the etalon comes the combination ERF and concave (negative) collimating lens. The lens takes the focused light from the objective and makes it parallel to go through the etalon. This assembly is fitted into a separate aluminum cell that screws on to the rear half. That's what turns when you adjust the tuning lever. It rotates this front cell by a small portion of a full circle.
There is a nylon-tipped setscrew mounted through a hole bored in the center of the glass of this front lens assembly. This tiny screw presses on the front half of the etalon when the tuner is tightened and squeezes the spacers so the air gap changes, thus tuning the cavity. The center screw is the ONLY thing that touches and compresses the etalon. There is NO contact made at the outer edge of the tuning collar to the outer edge of the etalon. (I'm assuming such contact could force the etalon to rotate and de-contact, so they only use a center screw).

I hope this helps. Taking pics of it from the side won't show much, because all you'll see is the outer edges of both barrels, and not the inside. If you look at the Coronado patents for the tuning mechanism, it gives a pretty clear line drawing of what the thing would look like in cross section.

If you need more clarification, just write back and I'll try to explain further.

Thanks, Bob


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Re: Disassembling a Coronado 90

Post by Merlin66 »

Bob,
Thanks for that.
Hmmmm
Sounds like we're back to tuning by compression of "flexible" spacers....again!!!
Mark - what do we do now??


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Re: Disassembling a Coronado 90

Post by piekielrl »

Yes, Coronado has always used the compression spacers for the tuning effect.
I wanted to add something to my previous post where I described the etalon housing, and pose a question here to those who are reading:

I talked about the ERF in combination with the front collimating lens - Look closely at my pics, and you will see that this ERF element is really a heavy multi-layer coating, and it is a gold / transparent color. It is NOT the deep red color of the ERFs on the external double-stack etalons. The internal construction of the double-stack units is otherwise EXACTLY like the internal etalon units here in these scopes.

That brings me to my question: A Coronado scope is self-contained, and safe & ready to use visually, just as it its made. When you buy an external etalon, for double-stacking, you have to buy a blocking diagonal for it if you want to use it on an aftermarket scope, the deep red ERF up front is not enough by itself. HOWEVER, if you use one of these external units as a double-stack system, why should there be an ERF up there at all? When you double-stack, you seriously drop the brightness, because less light is coming through the two etalons. Since your parent Coronado scope is designed to function as a stand-alone instrument, when you double-stack it, it doesn't need any "extra" light reduction. You're adding the external etalon to narrow the bandwidth.

It would seem to me that a double-stacked Coronado would work better (brighter) if they made an external etalon SPECIFICALLY for double-stacking WITHOUT an additional ERF. You're not putting any extra light into the parent scope - in fact, the second etalon is REDUCING the light coming in. The added ERF reduces the light further...

Anyone have an idea about this?

Thanks, Bob


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Re: Disassembling a Coronado 90

Post by Merlin66 »

Bob,
There has been ongoing debate about the tilt/ squeeze tuning in the PST.....
Up until the introduction of the "Richview" tuning I believe all the etalons were tilt tuned.
The red ERF's need the added ITF to reduce the bandwidth and suppress IR.
A Baader D-ERF doesn't really need a supplementary ITF...but these are generally "built-in" to the blocking filter assembly and tolerated.

The light loss in the DS is due to the etalons and the final bandwidth. A second ERF just reduces a small amount -less than 10%. (Having said that, many members have removed the ERF for DS - let's see what they say...)


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Re: Disassembling a Coronado 90

Post by piekielrl »

THANKS! That answers my question and makes perfect sense. I was also thinking about removing the front ERF from a double-stacking etalon, but I didn't want to say that here as it might sound like I'm advocating altering stock solar equipment...
Having said THAT, you would need to replace the ERF in an external Coronado Rich-View tunable etalon with a plane glass optical window, and affix a tuning screw at its center so as to function the same way as the ERF mechanism (to put pressure on the etalon to tune it when the collar is tuned).

Take care, Bob


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Re: Disassembling a Coronado 90

Post by solarchat »

That's exactly how they were made before Meade took over. None of my pRe Meade DS90s are safe to use without the external etalons attached. The internal etalons are fixed and non tunable. The SolarMax II you have here has the same internal etalon tuning mechanism as the PST. Compression against foam spacers by turning an external ring, or stick. A double stacked SolarMax II has redundant ERFs.


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Re: Disassembling a Coronado 90

Post by Merlin66 »

Bob,
I'm not a 100% sure, but the added front plate may not be necessary....
I think the Richview "connection" is on the rear (inside surface) of the external type.....


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Re: Disassembling a Coronado 90

Post by piekielrl »

I have two SM II external etalons - a 60 and a 90. Both have the tuning screw on the front, directly attached to the red ERF. It appears to be glued to the filter glass, so of you removed the filter, you'd lose the tuning screw.

Best, Bob


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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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