Re: The Double pass idea...

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
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p_zetner
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Re: The Double pass idea...

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

NOTE TO READERS: This is a response to an original post by another contributor. This original post has been deleted.
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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
pol cube beamsplitter - double pass - pt1_siz.png (50.05 KiB) Viewed 1565 times
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.
Last edited by p_zetner on Wed Nov 28, 2018 3:02 pm, edited 1 time in total.

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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
pol cube beamsplitter - double pass - pt2_siz.png (53.93 KiB) Viewed 1560 times
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 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 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.
Last edited by antonello on Tue Aug 28, 2018 5:28 am, edited 1 time in total.

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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 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 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 marktownley » Tue Sep 04, 2018 10:41 pm

Keep us posted! :)
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Re: The Double pass idea...

Post by highfnum » Mon Nov 05, 2018 11:57 pm

I played around with this idea a while back

didn't do much for me
but I most likely missed something and did something wrong
dpass.jpg
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Re: The Double pass idea...

Post by highfnum » Tue Nov 06, 2018 8:40 pm

hey thanks for posting this thread

I took mine apart and cube was in opposite of yours :oops:

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Re: The Double pass idea...

Post by antonello » Fri Nov 23, 2018 10:40 am

Hi
As I have already written, despite negative experiences of other amateur astronomers more competent than me (even a German guy who deals with professional Optics), also I, few months ago, I have decided to experiment the possibility of making a double stack with a beam splitter cube.

There are many problems, some have already been illustrated:

1) Brightness. It is important mainly for photographic use. If one only wants to visual observation, in my opinion, there is still enough light.

2) Reflections. They are a big problem. They can be minimized with a good construction and with a lengthening of the optical path of the light that passes through the beam splitter (that is the distance between the first negative lens and the second positive lens of the collimator). If you use the same lenses as the standard collimator (for example, the same as the PST), unfortunately this lengthening it must be as short as possible because plus greater is the separation between the two lenses, the lower the image quality. In practice, a compromise will be necessary.

3) The collimator must be calculated with an optical software. You can not put any achromatic lens and another any simple lens. A good collimator for etalon (in a commercial telescope) consists of a simple negative lens and a simple positive lens placed as in the image.

4) A precise system for tilting the flat mirror is needed after the beam splitter.

5) For a semipolarized or polarized beam splitter, a retarder plate 1/4 phase is necessary.

All these problems can lead to failure.

Despite all this, I have decided to experiment this solution, aware that to obtain something, it is necessary to work in a professional and non-artisanal way. At the moment I have made almost the whole system. I just have to finish gluing the Lamda/4 achromatic retarder plate between an optical window and the flat mirror, as shown. I'm not in a hurry to finish the experiment because now I have as a priority the end of the construction of my house (and my telescopes are packed). Now I will finish the whole block, and then, in the next few months, I will experiment with my system.

Of course, it will be a pleasure to tell the result.-)

My realization is shown in the images (with 40x40 mm beam splitter).
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Re: The Double pass idea...

Post by antonello » Tue Nov 27, 2018 7:50 am

Yes, sure.
A 25x25 mm beam-splitter Edmund, NOT polarized, costs only $ 220 and therefore has an affordable price. Who has a PST can certainly experience it.

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