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High-transmission polarizing filter?

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#1 Leaf

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Posted 10 November 2021 - 03:18 PM

Hi guys,

 

I'm looking for a solution to variably attenuate brightness in real time, without having to swap anything out. The main one I've seen is just using a pair of polarizing filters, where you can twist the eyepiece to adjust brightness, but most seem to cap out at 40% light transmission (also, I'll most likely be using a turret, so I'd need to buy several). I'd reeeeaaaally like something that can push 90%+ light transmission, or close to it, so I can switch between viewing bright and dim objects without removing anything (a standalone cell could also be nice). Does anyone know if it exists?

 

EDIT: Bad wording on my part, I'm open to anything that gives me the above capability, not just variable filters.


Edited by Leaf, 10 November 2021 - 04:05 PM.


#2 Tangerman

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Posted 10 November 2021 - 03:50 PM

A polarizing filter selects one polarization of light to allow through. With a single polarization filter and unpolarized light, the best you can do is 50% transmission (because, in a simplified explanation, half of the light is polarized one way and half the other way, and you're letting through half of it). Add a second filter in front and now that filter takes out 50%, the one after takes out a certain percentage based on how it's rotated compared to the first one. So no, you can't push 90% transmission with a polarization filter (assuming your polarization filter works like it ought to).


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#3 Justin Fuller

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Posted 10 November 2021 - 03:50 PM

Physics is against you on this one. If randomly polarized/ unpolarized light strikes a polarizer, the probability is high that most of the light will be attenuated to some degree, from 0% to 100%. The only way you'd get 90% transmission from a polarizer is if you were looking at already polarized light and your polarizer is aligned to the correct axis for that polarized light to transmit though the polarizer. There is a physical reason those variable polarizers only transit ~40% at best.


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#4 Lazaroff

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Posted 10 November 2021 - 03:50 PM

Seems impossible with polarizers, so how about a filter wheel with a set of neutral density filters at several densities?


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#5 Leaf

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Posted 10 November 2021 - 04:10 PM

A polarizing filter selects one polarization of light to allow through. With a single polarization filter and unpolarized light, the best you can do is 50% transmission (because, in a simplified explanation, half of the light is polarized one way and half the other way, and you're letting through half of it). Add a second filter in front and now that filter takes out 50%, the one after takes out a certain percentage based on how it's rotated compared to the first one. So no, you can't push 90% transmission with a polarization filter (assuming your polarization filter works like it ought to).

Physics is against you on this one. If randomly polarized/ unpolarized light strikes a polarizer, the probability is high that most of the light will be attenuated to some degree, from 0% to 100%. The only way you'd get 90% transmission from a polarizer is if you were looking at already polarized light and your polarizer is aligned to the correct axis for that polarized light to transmit though the polarizer. There is a physical reason those variable polarizers only transit ~40% at best.

Unfortunate, but good to know. Thanks for the explanation, guys.

 

Seems impossible with polarizers, so how about a filter wheel with a set of neutral density filters at several densities?

Could do. I'll have to see if I have enough in-travel to accommodate a wheel. Always the catch with Dobs grin.gif



#6 MitchAlsup

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Posted 10 November 2021 - 05:22 PM

You can't get more than 50% (polarization physics) out and you can't get less than 2% (coherence of the source) out--that is simply the nature of the polarization beast.

 

If you need to use a polarizer and it is currently too dark in your scope, you need a bigger scope.............



#7 Leaf

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Posted 10 November 2021 - 05:50 PM

You can't get more than 50% (polarization physics) out and you can't get less than 2% (coherence of the source) out--that is simply the nature of the polarization beast.

 

If you need to use a polarizer and it is currently too dark in your scope, you need a bigger scope.............

I dunno, do people use ND filters for looking at DSOs? The 50% ND I have feels like it'd be too dark for already-faint-from-light-pollution targets, especially if I use a nebula filter on top of it. 8" is a pretty solid aperture I think.



#8 Justin Fuller

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Posted 10 November 2021 - 06:25 PM

No, there is no reason to use ND filters on DSOs. I personally don't see the utility in ND filters at all, your eye can accommodate differences in brightness pretty well with time. The moon dazzles when you first look at it, but after about ten seconds your eye will adapt. Going from looking at a full moon to a DSO will take your eye around 30-45 minutes to properly adapt.
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#9 havasman

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Posted 10 November 2021 - 06:40 PM

No, there is no reason to use ND filters on DSOs. I personally don't see the utility in ND filters at all, your eye can accommodate differences in brightness pretty well with time. The moon dazzles when you first look at it, but after about ten seconds your eye will adapt. Going from looking at a full moon to a DSO will take your eye around 30-45 minutes to properly adapt.

waytogo.gif waytogo.gif

 

IME the Lumicon variable polarizing filter is the best of that lot. If they make an ND it's likely the best of that sorry lot too. After years of failing to improve anything by using them I have found that, as recommended by Lunt, a Lumicon variable polarizing filter is useful with the Lunt Hershel wedge/NP101is white light solar setup.

I had an old no-name variable polarizing filter I dug out and tried against the Lumicon. After I did I threw the no-name in the trash.


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#10 Leaf

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Posted 10 November 2021 - 07:04 PM

No, there is no reason to use ND filters on DSOs. I personally don't see the utility in ND filters at all, your eye can accommodate differences in brightness pretty well with time. The moon dazzles when you first look at it, but after about ten seconds your eye will adapt. Going from looking at a full moon to a DSO will take your eye around 30-45 minutes to properly adapt.

waytogo.gif waytogo.gif

 

IME the Lumicon variable polarizing filter is the best of that lot. If they make an ND it's likely the best of that sorry lot too. After years of failing to improve anything by using them I have found that, as recommended by Lunt, a Lumicon variable polarizing filter is useful with the Lunt Hershel wedge/NP101is white light solar setup.

I had an old no-name variable polarizing filter I dug out and tried against the Lumicon. After I did I threw the no-name in the trash.

And that's why I want a brightness filter I can turn off grin.gif I personally find the Moon eye-searing to gaze at no matter what, and Jupiter and Saturn look overly bright. In your experience, do you think 40% would be "enough" light for looking at (at least some) DSOs, or even dim planets like Neptune and Uranus?



#11 havasman

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Posted 10 November 2021 - 07:16 PM

And that's why I want a brightness filter I can turn off grin.gif I personally find the Moon eye-searing to gaze at no matter what, and Jupiter and Saturn look overly bright. In your experience, do you think 40% would be "enough" light for looking at (at least some) DSOs, or even dim planets like Neptune and Uranus?

Absolutely not. Never.

Perhaps you should consult an ophthalmologist.



#12 MitchAlsup

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Posted 10 November 2021 - 07:49 PM

No, there is no reason to use ND filters on DSOs. I personally don't see the utility in ND filters at all, your eye can accommodate differences in brightness pretty well with time. The moon dazzles when you first look at it, but after about ten seconds your eye will adapt. Going from looking at a full moon to a DSO will take your eye around 30-45 minutes to properly adapt.

Your eye will never adapt to a full moon in a 20" or larger telescope !

 

You need an ND4-ND6 filter at this level of aperture. Some people use 2 polarizers and adjust the angles to permit the proper amount of light through.

Jupiter needs a ND2-ND3

Saturn can use an ND1-ND2.

 

Using such filters reduces or eliminates the time it takes you eye to re-adapt.



#13 Justin Fuller

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Posted 10 November 2021 - 08:26 PM

Your eye will never adapt to a full moon in a 20" or larger telescope !

 

You need an ND4-ND6 filter at this level of aperture. Some people use 2 polarizers and adjust the angles to permit the proper amount of light through.

Jupiter needs a ND2-ND3

Saturn can use an ND1-ND2.

 

Using such filters reduces or eliminates the time it takes you eye to re-adapt.

I disagree, I've looked at the moon with a 60" f/16 with no ND and survived, I think use of ND filters is a personal preference. Certainly you never need an ND filter with DSOs, no matter the aperture.



#14 Leaf

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Posted 10 November 2021 - 08:37 PM

Absolutely not. Never.

What I expected. And so the search continues...

 

Your eye will never adapt to a full moon in a 20" or larger telescope !

 

You need an ND4-ND6 filter at this level of aperture. Some people use 2 polarizers and adjust the angles to permit the proper amount of light through.

Jupiter needs a ND2-ND3

Saturn can use an ND1-ND2.

 

Using such filters reduces or eliminates the time it takes you eye to re-adapt.

I disagree, I've looked at the moon with a 60" f/16 with no ND and survived, I think use of ND filters is a personal preference. Certainly you never need an ND filter with DSOs, no matter the aperture.

It will vary from person to person no doubt, our eyes are all made different after all. For me, it's uncomfortable to look at the Moon without one, and I can see more detail in Jupiter and Saturn with one. So I like to have one... though I wish I didn't, that'd make things a lot simpler lol.gif


Edited by Leaf, 10 November 2021 - 08:39 PM.



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