The Filters We Need With Digital...

Started Oct 5, 2008 | Discussions thread
OP Joseph S Wisniewski Forum Pro • Posts: 34,130
Don't point it at the sun!

Henrik Andersson wrote:
Thanks Joseph, Boris and Gary for the feedback.

Joseph S Wisniewski wrote:

Henrik Andersson wrote:

... or will there be a lot of reflections from the polarizers,
destroying the contrast if the image at the focal plane?

Yes, this technique increases the problems with reflection. It also
causes three other things to go wrong...

What happens if I put these after a reflector telescope instead?
(Meade LX400 or similar).

In the eyepiece reflector slot, or between your eye and the eyepiece?

I guess that the light isn't much polarized after the reflections due
to the angles ... so no quartz plate is needed to start with?

It's still polarized. The only way to depolarize the light is with either a 1/4 wave plate, or something that scatters and randomizes it. It's not the polarized nature that causes reflections, it's the concept of two parallel plains of glass.

But you will at least eliminate reflections between the polarizer and front element of the lens.

Would it it help if I tilt the filters slightly to get rid of some

Yes. Tilt the polarizers out of square with the optical path, and tilt one out of square with the other.

I do that in my epi-illumination system on my microscope.

Just remember the golden rule: crossed polarizers do not block infrared or ultraviolet (and can be used as an improvised filter for IR or UV photography). Having IR to UV substantially above visible light levels can do considerable damage to your eye. So you don't use them to cut the brightness on a scene that's too bright to view. That's why we have ND filters specifically for moon viewing, and solar filters over the front of the scope for solar viewing.

Also, if you let very intense light into a scope, you can overheat the optical components that experience high light concentrations (secondary mirror, eyepiece, and eyepiece filters). There's also a danger of exposure of your eye to concentrated light if a light limiting component such as a filter fails.

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Rahon Klavanian 1912-2008.

Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.

Ciao! Joseph

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