Polarizing filters : safe or not for solar observation

Use this section to discuss "standard" Baader/Coronado/ Lunt SolarView/ Daystar, etc… filters, cameras and scopes. No mods, just questions/ answers and reviews.
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christian viladrich
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Polarizing filters : safe or not for solar observation

Post by christian viladrich »

Hi,
Polarizing filters are sometimes used to modulate the brigthness of images in complement to neutral filters (then two polarizing filters are used) or on a "Brewster" solar wedge (in this case only one polarizing filter is used).
It is taken for granted that they are safe for solar observation.
However, when we look at this transmission curve measured by Peter Hobel, we can see that while transmission in white light can be ajusted by rotation of the two polarizing filters, the combination is virtually transparent to IR :

Image

This is not an exceptional case. Here is the transmission profile of a polarizing set I have. The transmission is modulated in white light by rotation of the pair of filters (up to about 800 nm), then transmission rises again :

Image

All of this is a bit annoying since the modulation of brigthness in visible light gives the user a feeling of safety while transmission in IR is still > 80%.

Now, if we look at Edmund Optics catalog, we can see that most polarizers are meant to operate between 400 - 700 nm, others between 1000- 2000, only one covers 400-2200 nm:
https://www.edmundoptics.fr/f/near-infr ... ilm/39534/

It would be interesting to measure the transmission profile of polarizing filters sold for solar observations. In any case, it is probably preferable to use them with a KG3 filter (to block IR).

BTW, I started a web page on filter transmission :
http://astrosurf.com/viladrich/astro/in ... lters.html


Christian Viladrich
Co-author of "Planetary Astronomy"
http://planetary-astronomy.com/
Editor of "Solar Astronomy"
http://www.astronomiesolaire.com/
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eroel
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Re: Polarizing filters : safe or not for solar observation

Post by eroel »

Christian, if I am not wrong, remember many years ago when groups existed, that we had many talks concerning that problem.
Best regards and thanks for the info.
Eric.


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Re: Polarizing filters : safe or not for solar observation

Post by DeepSolar64 »

Wouldn't the neutral density filter block the IR? My wedge has this in addition to a polarizing filter.


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Re: Polarizing filters : safe or not for solar observation

Post by george9 »

I think it is good that you have reminded people that polarizers are for viewing comfort and optimization, NOT eye safety.

The rule of thumb is that the system should already be blocking sufficient UV and IR before you approach it with a polarizer. Once you know 200-400 and 700-3000 are safe, what is left is visible and you should be able to judge it. But I don't even trust myself on that. My 400-410 is not very sensitive, and I could be staring at dangerous levels without it seeming too bright.

Therefore, go for the ISO eye safety standard for the three ranges (UV, visible, and IR), and then optimize it.

To draw an analogy, for naked eye solar, I bring a #12 welders filter (made by Schott, so quite a good figure, not the sleeked $2 versions). Visible is bright, but UV and IR are about the same blockage as #14. Then you can use it through haze and sunset, and just tilt it to dim enough for bright Sun. Formally, #12 makes the ISO safety standard. So you are assured that you are not hurting your eyes.

I think a Herschel wedge with ND3 makes similar transmission, as DeepSolar64 points out. A little bright in visible but safe in UV and IR. So the polarizer should be fine there.

But I would NOT use a polarizer with Baader photographic film for visual use (I guess it is possible that the Baader film blocks enough UV and IR, but I don't know).

George


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Re: Polarizing filters : safe or not for solar observation

Post by george9 »

And thanks for the filter page. The Astrodon UV filter looks great. And not that much more expensive than filters with only 65% transmission.

George


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Re: Polarizing filters : safe or not for solar observation

Post by krakatoa1883 »

Great work, Christian, thanks for sharing.
george9 wrote: Sat Jul 11, 2020 11:20 pmBut I would NOT use a polarizer with Baader photographic film for visual use (I guess it is possible that the Baader film blocks enough UV and IR, but I don't know).
found a transmission curve at this link, can't tell if reliable or not,


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My solar images and reports with articles on solar equipment
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