The Double pass idea...

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TheSkyBurner
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The Double pass idea...

Post by TheSkyBurner » Thu Aug 16, 2018 5:59 pm

So nobody out there has a working double pass system that I am aware of, so I thought I would share this.

The double pass system is actually very simple to make if you do not mind being resourceful.

You can essentially modify a "flip mirror" to house your cube prism. The mirror is attached via meade eyepiece projection camera adapter and an m42 metal cap.
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The mirror simply goes inside the metal cap and threads onto the back of eyepiece projection barrel.
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Whatever filter you use, will be inside the camera projection adapter just before the threaded mirror cap..
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I used a 2" polarizing prism that I got from ebay. It was used surplus and Im not sure of the specs. This was just an experiment to see how everything is intended to operate in the real world versus a diagram on a piece of paper. (lucky enough this did what I wanted it to)
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Now I have yet to try imaging with this, but I have no reason to believe it will not serve its intended purpose. Performance is a mystery right now. The device clearly passes light through the downward angle facet of the prism where it then clearly hits the mirror in the m42 cap. From here the light bounces off the mirror at 180degrees and returns the light to the topface prism angle where it then goes vertical out of :geek: the eyepiece holder.
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But now that this ball is rolling somewhere , I pass it onto you!

If you want to make any suggestions, additions or know of better parts to mount a cube; please post!

A ~40mm circular 1/20 zero expansion mirror would be a killer find to cement directly onto the m42 cap.



I do not know if there is supposed to be a collimated f30 beam injected into the prism, or if there is supposed to be an f30 telecentric.

"Polarising cube beamsplitters are designed to split incident light into its polarised components rather than by simple transmission and reflection. The cube allows P-polarised light to pass undeviated through the cube while S-polarised light is reflected at 90°. The quality of the cube is typically defined by the extent and accurate beam deviation of the transmitted and reflected polarised light and by the extinction ratio of the cube."
Last edited by TheSkyBurner on Sun Aug 19, 2018 7:25 pm, edited 2 times in total.

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Re: The Double pass idea...

Post by TheSkyBurner » Thu Aug 16, 2018 6:02 pm

premounted cube example, but this is not the one i used.

https://www.ebay.com/itm/36-6mm-Cube-Mo ... 2094296329

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Re: The Double pass idea...

Post by p_zetner » Thu Aug 16, 2018 10:03 pm

Hi.

Do you actually see light coming from the top (exit) part of the flip-mirror housing? I'm surprised if you do since the design, as you've presented, doesn't work in principle. Here's a diagram of what you're proposing:
pol cube beamsplitter - double pass - pt1_siz.png
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In the diagram, BS is a polarizing cube beamsplitter and M is the mirror. The vertical blue arrows represent vertically polarized light and the red circle with concentric dot represents light polarized out of the figure plane (horizontal). In (1), unpolarized light (as a combination of incoherent vertically and horizontally polarized components) from the telescope enters BS from the left. In (2), BS does its job and separates these two components into the reflected and transmitted paths. Reflected light (red - horizontally polarized) is directed vertically downward and gets absorbed while transmitted light (blue - vertically polarized) propagates to M. In (3), M reflects the vertical component back to BS and, according to the operation of BS, this component is transmitted through (into the telescope direction) without any reflection.

If you do see light the conclusion would be that you don't have a polarizing cube beamsplitter.

Cheers.

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Re: The Double pass idea...

Post by p_zetner » Thu Aug 16, 2018 10:23 pm

If you're interested in exploring this idea further, I'd suggest the following approach. This has been on my bench for a while but I have not field tested it yet.
pol cube beamsplitter - double pass - pt2_siz.png
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As in the previous diagram, BS is a polarizing cube beamsplitter and M is the mirror. Here, a quarter wave phase retardation plate (QWP) has been added and a filter (F) location is shown. As before, the vertical blue arrows represent vertically polarized light and the red circle with concentric dot represents light polarized out of the figure plane (horizontal). Once again, in (1), unpolarized light (as a combination of incoherent vertically and horizontally polarized components) from the telescope enters BS from the left, then, in (2), BS does its job and separates these two components into the reflected and transmitted paths. The difference here is, that in passing through QWP, the polarization is changed from linear to circular. In (3), M retro-reflects the incident circularly polarized light thereby changing its circular sense (from left to right or vice-versa). Propagation back through the QWP then results in horizontally polarized light propagating back to BS and, according to the operation of BS, this component is now reflected through to the observation direction.

A big difficulty with this scheme is finding affordable optical components (BS, QWP) which are large enough (40mm would be great) and of sufficient optical quality with regard to transmitted wavefront distortion.

Cheers.

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Re: The Double pass idea...

Post by TheSkyBurner » Fri Aug 17, 2018 3:24 am

yes I do see an image coming out of the top eyepiece holder and it is greatly attenuated (my cube and mirror are positioned exactly the same as your diagram; it is just sideways.)

When pointed at a light bulb; I see two green reflections from whatever the coating is on this particular prism, one is on top of the light bulb image and the other is on the bottom of the centered lightbulb image.

The center image is the actual image, and this is not obscured in any way by the two green reflected images.

Is the actual purpose of the QWP intended to extinct the back reflection from going through the prism a second time through the ota and toward the objective?


I can get some 25mm 1/2 and 1/4 and 1/8 waveplates on ebay but they are all spec for 532nm, they are unmounted and made of mica so are extremely fragile.


Im not taking this project too seriously, since i do not have an h-alpha filters. This housing is definitely on par with what the system may look like however and thats what my goal was. The physical object model using real world parts.


I am still working out a couple bugs on my xtruder, so that currently has 75% more of my attention :)

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Re: The Double pass idea...

Post by mdwmark » Fri Aug 17, 2018 4:29 am

double pass has been used with Lasers for a long time, if your working with a point source you fine. A star is a good point source. The sun is not, it has field angle. Lunt tried it with his 8" reflector and it didn't work. If it would have worked it would have been in uses years ago. That .25 deg will get you every time with an etalon.
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Re: The Double pass idea...

Post by Valery » Fri Aug 17, 2018 7:28 am

Apollo, this is a very old idea. In addition to Mark's W. note, I would say that this system is not much cheaper than a two 50mm etalons DS system and it has 2x lower light.
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Re: The Double pass idea...

Post by TheSkyBurner » Fri Aug 17, 2018 8:21 pm

Valery : An Old idea revitalized with modern technology may leap the hurdles that plagued it from production.
Mdwmark : from what i understand after some research, lunt has a 60mm refractor prototype that is fully functional,

Double pass maybe the only way to get a sub angstrom 854.2nm system for less than $10,000. So it is for sure worth exploring in time.

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Re: The Double pass idea...

Post by TheSkyBurner » Sun Aug 19, 2018 4:36 am

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Re: The Double pass idea...

Post by marktownley » Sun Aug 19, 2018 7:20 am

Keep us updated!
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Re: The Double pass idea...

Post by antonello » Mon Aug 20, 2018 7:15 am

Yes, this system is old, but I have not seen many experimental applications... most it is remained in the design phase ...
I myself am working for a prototype, just for the curiosity to see if work well...

As Zetner said, if the beam splitter prism is polarized, the system will NOT work. To work it is necessary to use a non-polarizing prism (it exists, but it costs a lot), or use a polarizing prism with a lambda / 4 delay plate ... as Zetner said.

There is a third solution, which fortunately is the most common, that is to use a semi-polarized Beam splitter, as is the majority of beam splitter prism that can be found in the surplus market, but the problems with all these solutions is brightness and reflections.

The final luminosity of a non-polarizing beam splitter eventually becomes about 10% (more or less, it is a value that I assume, based on my experience, it is not a measured value) of that of the image in "mono tasking" (ie H alpha with only etalon) ... If the beam splitter prism is of a semi-polarized type the brightness is even more lower... I doubt that with this low light the system can be used, but I want to see it with my own eyes .

Then there is the problem of reflections ... A expert German amateur astronomer as confessed to me that he had experimented with the system and found it unusable because of the reflections ...

Another thing: for the system to work, the flat mirror for reflection must be finely "tiltable".

I am experimenting with this solution with a professional approach (hight mechanical quality, where all is adjustable). In particular, I am using a 40x40 mm semi-polarized beam splitter with a lambda / 4 edmund achromatic retarder plate (it costs very little). Unfortunately I am also busy building my house, then my telescope experiments goes slowly.

However, as soon as I have some images and also some results, I will share it here, in any case, without embarrassment.
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Re: The Double pass idea...

Post by TheSkyBurner » Tue Aug 21, 2018 12:55 am

The polarization is what blocks the reflections via extinction ratio.

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Re: The Double pass idea...

Post by antonello » Tue Aug 21, 2018 6:13 am

The issue is much more complicated than you see it. It is not always possible to put a polarizer. In the system in question, removing the reflection is not unlikely that the image will be removed ... and even if the image will not be removed, its brightness becomes even lower ... and it is already low due to the beam splitter. ..
Unfortunately, the issue is complicated by the availability of three types of beam splitter prisms and the semi-polarized beam splitter prism is the most complicated: it is difficult to say that problems will arise without direct experimental proof.

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Re: The Double pass idea...

Post by TheSkyBurner » Sat Aug 25, 2018 10:58 pm

When you have three polarizers in a train, the glass goes clear and transmission is high. The same is true for 6 polarizers, or 12 polarizers. The rotation has to be clocked for each turn! Just like a lyot filter.

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Re: The Double pass idea...

Post by antonello » Mon Aug 27, 2018 5:38 am

At what price?
The fact remains that if your beam splitter is semi-polarized, the image and all the reflections are semi-polarized and it is enough that a reflection has the same angle of polarization of the image to make the project a failure.
However I have not discouraged you in your work (I myself am carrying out the same experiment), but only by pointing out that those who have already tried it have failed and that it does not seem to me that someone has managed to make work this system. Certain, spending a lot of money (non-polarized beam splitter, etc.) is possible to succeed, but the price will be much higher than that of a two-etalon system ... Is it worth it?

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Re: The Double pass idea...

Post by TheSkyBurner » Mon Aug 27, 2018 2:34 pm

yes i agree here, if you are going to spend x 2000 dollars to just experiment with this dont bother.

A lyot filter design can cost $20,000, and although could very well be made to perform better than a a dual etalon system this is very very unreasonable for anybody but a government agency, or a science research facility on a university grant.

Because the brightness factor is a strange issue, you can understand why lunt used an 8" mirror system for its first choice (but then again he has a 60mm working prototype with a patent associated with it) He also has his own coating lab and cnc mill so he can make whatever parts he needed, as needed..

Most solar observatories using beam splitters have 1 meter objectives so brightness is just not an issue to them.

It will likely be just an educational pursuit for amateurs.

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Re: The Double pass idea...

Post by antonello » Tue Aug 28, 2018 5:32 am

Hi TheSkyBurner,
because we are trying to do the same thing, I with pessimism and you with optimism, we stay in touch. In the coming days I can put some pictures of my experiment ...

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Re: The Double pass idea...

Post by TheSkyBurner » Tue Aug 28, 2018 12:27 pm

So i just attached a negative 200mm element on the nose piece of the module and was able to see my neighbors lamp without reflections.

Using a 90mm x 800mm scope with a 25mm eyepiece I can focus without problems. There are zero reflections, however when i attached my ccd camera to get an image the magnification was out of this world and exposure time was so high it was unfeasable. This is without a refocus lens and without a filter.

Visually without a filter it actually does work, and you can see perfectly fine!

The view is definitely 50% less bright immediately however, yet color is unaffected.

I will try adding a baader planetarium g-ccd filter in front of the mirror next and see what kind of reflections are added. This filter is relatively transparency so it is super high intensity and will allow me to still see the light bulb without much affect on the bright..

When I find the position for a refocus lens I will also try getting an image for you.

I am confident that if this does not work for h-alpha it will definitely work for calcium k-line, and white light.

The issue with h-alpha is going to be the blocking filter. The h-alpha etalon itself is super high transmission and transparent so the 50% reduction in brightness is not "useless", it just needs to be incorporated as part of the functionality at the eyepiece.

A new blocking filter design with high transmission for example would be a must.

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Re: The Double pass idea...

Post by TheSkyBurner » Tue Sep 04, 2018 2:01 am

alright I found all the working distances for the double pass system with parts I already have on hand. the tilt module is installed, I am going to test it out with the 1 angstrom calcium filter.

On the nosepiece is an INFRARED CUT coated negative 200mm pcx lens, the skybender tilt module has the chroma ct393.37bp.1 , the retro reflection mirror is a 25.4mm newport UV aluminized 99% reflective for 300-600nm at the back of the skybender. The refocus lens is 27mm x 250mm standard green coated achromat; positioned at the top of the cube.

Now because the metalized coating on the filter is going to cast reflections, I made sure there is plenty of room to where the BOTH reflection will not hit the prism, nor the retro reflector.


So that is that, All I need now is free time and sunshine. (and to clean up my garage!)
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Re: The Double pass idea...

Post by marktownley » Tue Sep 04, 2018 10:41 pm

Keep us posted! :)
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