The effect of a circular polarizer in a double stack

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bart1805
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The effect of a circular polarizer in a double stack

Post by bart1805 » Sun Jan 27, 2019 5:19 pm

Hi all,
In the proces of optimizing the adaptor to double stack two PST etalons the question about the circular polarizer popped up. Without a CP you have terrible ghost images and reflections and thanks to this forum I found the solution of the CP. At least Randall, Gus, George and Stu use it.
I experienced myself that mounting a CP between the two etalons worked as it should: the ghosts were gone. But there are a few things I don't understand:
Why is it so that the CP should face backwards? Backwards used a CP can't kill the light of my computerscreen 100%, when I look through it like I should, that is possible. The difference in brightness turning the CP when faced backwards is much less than looking through it the regular way.
Wikipedia is not necesssarily a fantastic source, but here is what you can read there:

Linear polarizing filters can be easily distinguished from circular polarizers. In linear polarizing filters, the polarizing effect works (rotate to see differences) regardless of which side of the filter the scene is viewed from. In "circular" polarizing filters, the polarizing effect works when the scene is viewed from the male threaded (back) side of the filter, but does not work when looking through it backwards.

It states that when looking through it backwards there is no polarising effect. Is that true? And if it is true what is happening then?

There still is a difference in brightness, turning the CP when facing backwards, so has experimented anybody what the effect was of turning the backward faced CP when imaging in H-alpha? Is there an optimised postion to fight the ghosts vs exposure time for example? Or is there no effect at all? If so, it would make the adapter between two etalons a lot easier....

Simply because I can't explain what is happening, would it be possible that the effect is the same (so no ghosts) when using just the quarter wave plate of the CP?
Thanks in advance for your answers!
CS! Bart.

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Re: The effect of a circular polarizer in a double stack

Post by Carbon60 » Sun Jan 27, 2019 7:25 pm

Hi Bart,

You need both the polariser and quarter wave plate- qwp (together the ‘circular polariser’).

The ghost images come from reflections between the two etalons, of course. Light entering through the objective passes through the first etalon and then the second etalon. However, part of this light reflects off the front face of the second etalon towards the rear face of the front etalon where it reflects back down the OTA to the sensor, creating ghost images. Placing the circular polariser between the etalons facing backwards, away from the objective, means that it is actually facing forwards with respect to the front face of the second etalon. In this configuration the polariser and quarter wave plate work together to kill any reflections from the rear face of the first etalon. Basically, the reflected light from the second etalon is circularly polarised as it passes through the circular polarising filter (say, left handed screw) towards the first etalon. It then reflects off the first etalon, becoming rotated in the opposite hand. When it reaches the polarising filter, this time heading towards the second etalon, it is blocked because of its ‘handedness’.

I hope this makes sense.

Stu.
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Re: The effect of a circular polarizer in a double stack

Post by bart1805 » Sun Jan 27, 2019 8:43 pm

Carbon60 wrote:
Sun Jan 27, 2019 7:25 pm

I hope this makes sense.
Stu, thank you very, very much!!
I could not find a logical reason why it worked the way it indeed does, but your explanation is crystal clear.

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Re: The effect of a circular polarizer in a double stack

Post by Valery » Mon Jan 28, 2019 4:53 am

So, which side of a CP filter should look to the second etalon? Male or female?


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Re: The effect of a circular polarizer in a double stack

Post by Merlin66 » Mon Jan 28, 2019 5:22 am

Valery,
It sounds like the female side - the side which would normally face "outwards" in a conventional arrangement (think about screwed into an eyepiece)
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Re: The effect of a circular polarizer in a double stack

Post by bart1805 » Mon Jan 28, 2019 8:11 am

That is correct. Male side is facing first etalon, female side is facing second etalon.

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Re: The effect of a circular polarizer in a double stack

Post by Merlin66 » Tue Feb 05, 2019 5:44 am

Trying to come to grips with this "issue"
I note that some Variable ND filters using in astronomy (ie "Orion Variable Polarizer") are actually two linear polarizers rotating against each other......
OK, I think the "key" is that the reflections from the etalon surfaces change the direction (left and right hand) of the CPL beam.
(See: https://sciencedemonstrations.fas.harva ... larization)
When these reflected CPL beams go through the reversed CPL filter, the outcome is is different from the incoming beam directly through the etalons allowing us to suppress the reflections but not the original beam.
I'll try to put this into a diagram, hopefully for better (??) clarity.
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Re: The effect of a circular polarizer in a double stack

Post by Merlin66 » Sat Feb 09, 2019 3:23 am

Here we go..
The video tutorial from Edmund Optics shows the set-up and answers the question.
The section starts a 2:30
https://www.youtube.com/watch?v=NkfLOUr2Z3g
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Re: The effect of a circular polarizer in a double stack

Post by bart1805 » Sat Feb 09, 2019 10:10 pm

Merlin66 wrote:
Sat Feb 09, 2019 3:23 am
Here we go..
Thanks Ken!

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Re: The effect of a circular polarizer in a double stack

Post by george9 » Mon Feb 11, 2019 1:13 am

Sorry I missed this thread. The circular polarizer is highly efficient at eliminating reflections at a cost of attenuating the main image. My experiments show that it works facing both ways. I place it forward-facing so that I can still use my binoviewer for visual. That way light comes out circularly polarized and equally bright in both eyepieces. If I flip the circular polarizer backwards, then the light comes out linearly polarized and I see through one eyepiece at a time.

Here is what I think happens assuming you put your circular polarizer in backwards (male thread toward the objective), which means the quarter-wave plate first, then the linear polarizer. I welcome corrections. The unpolarized light comes in from the front etalon through the quarter-wave plate but remains unpolarized and all gets through. It then hits the linear polarizer, becomes linearly polarized and dropping half the light. It hits the second etalon and most gets into the etalon. Some bounces back still linearly polarized in the same orientation, gets through the linear polarizer (now heading backwards toward the first etalon), and then hits the quarter wave plate. Out comes circularly polarized light (either left or right handed). That hits the back of the first etalon and some bounces back. When circularly polarized light hits a mirror, it bounces back with the opposite rotation (left to right or right to left). Now it comes to the quarter-wave plate (heading towards the second etalon), which turns it back into linearly polarized light, but it is now 90 degrees off from before. Now when it hits the linear polarizer, it cannot pass through, and you just lost your reflections. The cost was losing about half the good light due to the linear polarizer.

If you put the circular polarizer in forwards, as I do, you should also lose the reflections. Light comes in through the first etalon and then the linear polarizer (losing half the light) and then the quarter-wave plate and comes out circularly polarized. It hits the second etalon. Some goes through and some bounces back with the opposite circular polarization. It hits the circular polarizer and should be heading toward the first etalon linearly polarized 90 degrees off from the linear polarizing filter. So the reflections should stop when they hit the linear polarizing filter and never make it back to the first etalon.

Not sure why backwards is recommended because even without the binoviewer, I see no benefit reversed versus forward. If the light comes through the first etalon linearly polarized, then you would have to rotate the circular polarizer to line up with that light, and that would be the problem. Just looking at some articles, an etalon can produce polarized light due to the non-90-degree incidence angle of short focal-length objectives. That would cause some of the light coming from the objective not to pass, and other parts to pass more. Might produce an odd optical effect. But again, I am not seeing it.

Anyone know the reason for recommending backwards?

George

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Re: The effect of a circular polarizer in a double stack

Post by george9 » Tue Feb 12, 2019 4:13 pm

Just bumping the thread to see if there are any thoughts why the backwards circ polarizer is recommended when forward works great (see last post). George

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Re: The effect of a circular polarizer in a double stack

Post by Carbon60 » Tue Feb 12, 2019 8:54 pm

Hi George,

I guess whatever works, works. A possible advantage of a backwards facing filter Is the fact that the light remains linearly polarised and reflections could be further dimmed with a second linear filter at the camera.

Stu.
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Re: The effect of a circular polarizer in a double stack

Post by Merlin66 » Wed Feb 13, 2019 4:56 am

https://www.edmundoptics.com/resources/ ... aveplates/
George,
Fig 9 shows what I think you are saying....
This infers either way should work.......
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Re: The effect of a circular polarizer in a double stack

Post by george9 » Wed Feb 13, 2019 12:00 pm

Yes exactly. Thanks both. George

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Re: The effect of a circular polarizer in a double stack

Post by marktownley » Wed Feb 13, 2019 8:18 pm

I think this is a great topic and should be one for the library when discussion has finished.
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Re: The effect of a circular polarizer in a double stack

Post by bart1805 » Wed Feb 13, 2019 9:49 pm

I tried both ways. And was convinced that facing backwards worked best. But I did not make any adjustments, so the position of the CP was fixed. Facing backwards, the average lightloss is less than facing forward. Could that be the reason? So not so much the amount of reflections, but the amount of light loss? Less when facing backwards, so that it gives the idea that mounting it that way is best? The only option is trying it out. When the DS adapter is ready I will try both ways with the CP in different positions.

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Re: The effect of a circular polarizer in a double stack

Post by Astrophil » Thu Feb 14, 2019 3:23 am

See if this adds to the confusion. If the circular polarizer (linear polarizer+ 1/4 wave plate) is looked through at a linear or elliptical polarized source and rotated, the brightness will vary if the linear polarizer side of the CP is facing toward the source. Not so much variation if the 1/4 wave side is facing the polarized source. If the first etalon produces a polarized component this could be important. Solid etalons polarize light somewhat and to maximize transmission, direction might be important, but not for suppression of reflections. If the second etalon is also sensitive to the incoming polarization it should benefit from correct orientation of the etalon. The CP will either block (reduce) the reflection off of the second etalon or the backside of the first, depending on whether it's facing forwards or backwards.
In the case of reducing reflections from within the telescope itself I placed the CP to block the reflection from the first surface of the blocking filter in the Quark Combo I use in a Maksutov reflector.

Phil

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Re: The effect of a circular polarizer in a double stack

Post by george9 » Thu Feb 14, 2019 2:31 pm

I agree it is important what the etalons and blocking filters are doing. In my case, the circularly polarized light seems to get fully through the second etalon and blocking filter because the binoviewer, which polarizes when it splits, lets equal light through the two eyepieces.

I checked my notes from my tests, and I did not note a difference in brightness in the desired signal for the two directions. I did, however, record that I thought the residual reflections were slightly brighter (worse) forward than backward, but both were great (nearly black). I am using air-spaced etalons. Someone else noted banding when in the forward direction.

Seems like you just need to experiment a bit and include the option of mounting it either way.

George

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