50% reduction in the cost of a double stacked etalon

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50% reduction in the cost of a double stacked etalon

Post by Merlin66 » Sun Jul 08, 2012 6:41 am

While investigating the design and construction of the spectrohelioscope and the associated Coelostat, I stumbled across an other idea....
OK, here we go..
Consider a Marcon type solar mod - a large refractor (R1) forms the first image of the sun, then a second (smaller)refractor (R2) is used as a collimator. The collimated beam is then directed to a bare etalon (E1) (either a Coronado or Lunt external type filter.) and finally a third refractor (R3) is used to re-image the suns disk onto the CCD etc...Big bold and heavy..

What if the last imaging refractor (R3) is replaced with a front surface mirror, and the collimated beam redirected back through the etalon (E1) and refocused Littrow style by the second refactor (R2).
This seems to give the potential to have a double stacked etalon arrangement, with one less refractor and could give a reasonable compact arrangement AND a significant improvement in bandwidth!!!

50% saving in the cost of a double stack!! :hamster:

( The angles through the second refractor (R2) in the Littrow mode need to be very small...the aluminised 90 degree prism idea that Manning used in his spectrohelioscope seems to meet the bill.)
What do you think? B)
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Re: 50% reduction in the cost of a double stacked etalon

Post by Photonist » Sun Jul 08, 2012 11:02 am

Very interesting, Ken.

I built the Marcon style SW150 mod using a Lunt 60 front filter last autumn, but then came the winter and then the Lunt 60 mod and then the TS102 mod, so I haven’t tested it more than once yet. But I have the parts for your idea except the surface mirror.

I have to consult your Spectroscopy book for the Littrow setup. A sketch of your idea would be great!

Viljo

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Re: 50% reduction in the cost of a double stacked etalon

Post by Merlin66 » Sun Jul 08, 2012 1:10 pm

Here's a sketch of the proposed "Marcon-Merlin Mirror Double stack"
Any questions/ comments/ concerns etc just let me know.




The focal ratio of R1 and R2 shoul be similar.
The pick off mirror needs to be at least 12.5mm ( a small secondary mirror would do the job.) Place close to the R1 focus, but far enough away to allow you to get the camera into the final focal plane.
The Littrow angle needs to be as small as possible to maintain a collimated beam through the etalon.
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Re: 50% reduction in the cost of a double stacked etalon

Post by Merlin66 » Sun Jul 08, 2012 1:55 pm

Joe,
Do I take that as a tentive, possible nod, and that this concept could probably work???
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Re: 50% reduction in the cost of a double stacked etalon

Post by rsfoto » Sun Jul 08, 2012 4:00 pm

Here's a sketch of the proposed "Marcon-Merlin Mirror Double stack"
Any questions/ comments/ concerns etc just let me know.



The focal ratio of R1 and R2 shoul be similar.
The pick off mirror needs to be at least 12.5mm ( a small secondary mirror would do the job.) Place close to the R1 focus, but far enough away to allow you to get the camera into the final focal plane.
The Littrow angle needs to be as small as possible to maintain a collimated beam through the etalon.

HI,

After reading this I was all excited as I have a 90mm f/5.6 = 500mm as well as a 78mm f/8.0 = 630mm foal length laying aorund. Also a diagonal and a big OAG from Hutech, all collecting dust in my Observatory.

Now after reading the following

I tried the double pass thing with zero success.( using a beam splitter cube)

If you don't have polarizers, and waveplates, the metallized surface of the etalon reflects back off at 100% before the light even enters the filter.


There is some trick regarding attenuation and proper reflection dumping, it doesn't just work on the principle of a mirror by itself.

All my exictement went down :blink:

If I understood well an Etalon works in the same way as an electrical Diode ? It lets light through only in one way ?

There is a mentioning of polarizers and waveplates. Where would this go and what are waveplates ?

Sorry for my poor knowledge but sounds interesting :hamster:
regards Rainer

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Re: 50% reduction in the cost of a double stacked etalon

Post by colinsk » Sun Jul 08, 2012 4:13 pm

There is an old method of double passing a pulsed laser beam through an amplifier gain media. It is where I got the waveplate and polarizer idea to begin with. Then later I found the the Mica etalons had been using a similar method to eliminate back reflections for a very ling time.

The Litrow idea is a good one. The angle through the etalon would have to be kept very small making the second telescope very long. I would have to draw it to see what the values of R2 would need to be to keep the angles in the etalon small as well as reimage without aberrations.
Thank you, Here are some references I have collected.



Etalon Article Part I

Etalon Article Part II

Solar Patents of interest

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Re: 50% reduction in the cost of a double stacked etalon

Post by Merlin66 » Sun Jul 08, 2012 4:20 pm

Rainer,
Rusty was talking about a completely different layout!
There are no beamsplitters etc in this concept.
The etalon will pass light through from either direction...
I'd love to be able to at least test out the concept but I'm in the process of moving back to Oz.
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Re: 50% reduction in the cost of a double stacked etalon

Post by rsfoto » Sun Jul 08, 2012 4:22 pm

The etalon will pass light through from either direction...


Hi Ken,

Sorry for my ignorance but as Rusty mentioned double pass and in your sketch the light passes twice the etalon I thought it was the same :silly: :blush:

Thanks. So your idea could be tested with the pieces I am having collecting dust ? or are the focal length to short for this ?

Would one use a standard Cornoado etalon included with the ERF ?
regards Rainer

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Re: 50% reduction in the cost of a double stacked etalon

Post by Merlin66 » Sun Jul 08, 2012 4:29 pm

The standard external etalon SM40/SM60 complete with built in ERF or the Lunt equivalent should work.You wouldn't need an ERF on the large aperture "donor" scope.
As mentioned, the f ratios should be similar, and the pick-off Littrow mirror as close the the optical axis as possible (depends on the focal length and the size of the solar disk v's vignetting)
It's a relatively easy set up to trial...if it doesn't work nothing is damaged along the way...(only my ego!!)
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Re: 50% reduction in the cost of a double stacked etalon

Post by rsfoto » Sun Jul 08, 2012 4:35 pm

The standard external etalon SM40/SM60 complete with built in ERF or the Lunt equivalent should work.You wouldn't need an ERF on the large aperture "donor" scope.
As mentioned, the f ratios should be similar, and the pick-off Littrow mirror as close the the optical axis as possible (depends on the focal length and the size of the solar disk v's vignetting)
It's a relatively easy set up to trial...if it doesn't work nothing is damaged along the way...(only my ego!!)

In my case using a 500mm and 630mm focal length the resulting focal length would be 500+630+630 = 1760mm and what f would come out ? Would it be the entrance f or the reduced 60mm of the etalon ?

I assume that the focal point ot the second telescope needs to meet the focal point of the first telescope. Right ?
regards Rainer

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Re: 50% reduction in the cost of a double stacked etalon

Post by Merlin66 » Sun Jul 08, 2012 4:46 pm

To answer the easier second question first...yes.
OK
Assume you use the 90 f5.6, 500mm as R1, then the 78 f6, 630mm as R2.
First thing that happens is the light beam from the R2 is limited by the etalon size...let's assume a SM60, 60mm. So, this part will actually end up as a 60/630 = f10.5 system - that would be what your camera would see and record.
Moving back to the R1....to get maximum throughput this should/ will operate at the same f ratio as R2 ie f10.5, so your 90 f5.6 becomes a 500/10.5 = 47.5mm effective aperture
You end up with a 47.5mm aperture, f10.5 double stacked layout.
Hope this makes sense.
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Re: 50% reduction in the cost of a double stacked etalon

Post by DavidG » Sun Jul 08, 2012 4:51 pm

This method has been around since the 1960's. It works fine for point sources but any extended object has a problem in the field angle. With the Sun being 1/2 degree is diameter the field angle across it's diameter would not be uniform and only the center would show any reduction in band width. Since the light is reflected back thru the etalon any field angle error would be doubled and you have to have some tilt of the mirror to seperate the incoming for the out going images so it can be picked off and viewed. Also you can't use just any tilt of the mirror, it's like when you double stack two etalon the traditional way and you don't get the tilt right, you don't get the reduction in bandwidth.
Lunt had a prototype design for 8" reflector that tried to use this method.

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Re: 50% reduction in the cost of a double stacked etalon

Post by Merlin66 » Sun Jul 08, 2012 4:56 pm

Dave, I hear you loud and clear...
(I thought the original Lunt drawings I saw for the the 8" system involved beamsplitters - and suffered the same issues- or similar to Rusty's venture???)
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Re: 50% reduction in the cost of a double stacked etalon

Post by rsfoto » Sun Jul 08, 2012 5:03 pm

Hi,

I just started to draw my telescopes according to Ken's sketch and I will see what come out :?

Any help for this is appreciated :thanx:
regards Rainer

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Re: 50% reduction in the cost of a double stacked etalon

Post by colinsk » Sun Jul 08, 2012 5:04 pm

The beamsplitter idea was mine. It was a guess of how Lunt might have been thinking. It was drawn long before Lunt had a single telescope. I never heard what the final design was.
Thank you, Here are some references I have collected.



Etalon Article Part I

Etalon Article Part II

Solar Patents of interest

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Re: 50% reduction in the cost of a double stacked etalon

Post by Merlin66 » Sun Jul 08, 2012 5:35 pm

Thanks guys, being a Scotsman...I never give up!!
David,
The main reflective mirror is not infact tilted relative to the etalon.
The change in the beam direction comes from the Littrow angle.
(I have the same / similar problems when designing spectroscopes!!!)
I do agree, the field angle will be an issue - but is it a show stopper?
Don't know.....yet!
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Re: 50% reduction in the cost of a double stacked etalon

Post by rsfoto » Sun Jul 08, 2012 5:37 pm

HI,

Now I am totally confused as it looks like here is the talk about 2 different designs :unsure:
regards Rainer

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Re: 50% reduction in the cost of a double stacked etalon

Post by Merlin66 » Sun Jul 08, 2012 5:39 pm

Rusty, could we just use this thread for the Marcon-Merlin concept?
If you want to develop your ideas, no problem, but either continue one of your previous threads or start another......
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Re: 50% reduction in the cost of a double stacked etalon

Post by DavidG » Sun Jul 08, 2012 5:42 pm

what if instead of using etalons, you used the baaderplanetarium 2 nm filters, in a lyot style stack? four in a row should plow down the bandwidth one would think,.

the mirror would simulate 8 of them in the retro-reflection.


(its gonna get real expensive, but hey all prototype inventions are)

The problem is that one gets about 0.70 reduction in bandwidth with you stack to two filter, but the third filter needs to have bandwidth that is equal to or smaller then the combination of the first two or you don't get any farther reduction in bandwidth. You also have the problem of the amount of light that is transmitted.

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Re: 50% reduction in the cost of a double stacked etalon

Post by DavidG » Sun Jul 08, 2012 5:57 pm

Thanks guys, being a Scotsman...I never give up!!
David,
The main reflective mirror is not infact tilted relative to the etalon.
The change in the beam direction comes from the Littrow angle.
(I have the same / similar problems when designing spectroscopes!!!)
I do agree, the field angle will be an issue - but is it a show stopper?
Don't know.....yet!

Merlin,
As I said this idea has been around and used since the 1960's. There is a reason why it hasn't been used for solar observing or for extended objects.
If it worked the guys at Daystar back in the 70's and 80's would have used it. In fact I was discussing this concept with Mark Wagner who worked at Daystar about a month ago.
If you want to make it work you would need to do something similar as what is done in a spectrohelioscope were the image of the Sun is scanned over the Etalon and then Etalon is masked down to a slit aperture.

All the Best,
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Re: 50% reduction in the cost of a double stacked etalon

Post by rsfoto » Sun Jul 08, 2012 7:09 pm

To answer the easier second question first...yes.
OK
Assume you use the 90 f5.6, 500mm as R1, then the 78 f6, 630mm as R2.
First thing that happens is the light beam from the R2 is limited by the etalon size...let's assume a SM60, 60mm. So, this part will actually end up as a 60/630 = f10.5 system - that would be what your camera would see and record.
Moving back to the R1....to get maximum throughput this should/ will operate at the same f ratio as R2 ie f10.5, so your 90 f5.6 becomes a 500/10.5 = 47.5mm effective aperture
You end up with a 47.5mm aperture, f10.5 double stacked layout.
Hope this makes sense.

I reread this and I think I understand it :blush:

You mention both telescopes should have the same focal ratio but what happens then if I use my TOA 130 = 1000mm focal length and then make the same calculation I would be getting a 1000/10.5 = effective aperture 95mm :blink:

Would that be possible ?
regards Rainer

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Re: 50% reduction in the cost of a double stacked etalon

Post by Merlin66 » Sun Jul 08, 2012 7:12 pm

yes, you got it!
The longer focal length would ideed give a larger effective aperture...
a TS102/1100 would be even better!
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Re: 50% reduction in the cost of a double stacked etalon

Post by rsfoto » Sun Jul 08, 2012 7:16 pm

Hi Ken,

Well I have no TOA 150 (which I think you meant with 1100mm focal length, but I have a Mewlon 250 with 3210mm focal length :woohoo: ) but a friend does and I guess he will not hand it out :woohoo:

So let me draw the new configuration using a 1000mm, but I guess that will not change anything at R2 in the way my OAG sits there, just make my life more difficult as the diameter of the TOA 130 is bigger :silly:
regards Rainer

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