The Filters We Need With Digital...

Started Oct 5, 2008 | Discussions thread
Henrik Andersson Senior Member • Posts: 2,454
Re: Don't point it at the sun!

Joseph S Wisniewski wrote:

Henrik Andersson wrote:

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 will only use the telescope with a camera, without an eyepiece. Called prime focus? A RedOne for instance. Maybe I start with a 5DmkII.

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.

I meant from the reflections in the telescope. On the primary and secondary mirrors. I guess that those reflections doesn't polarize the light much thanks to the angle of the reflections.

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

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.

Even though I will use cameras I guess that I might try to fit a UV and IR block filter too into the light path ...

Now the light path look like:
Maybe a weather proofed housing
Telescope
IR/UV-blocking. Maybe this should be in front of the telescope ...
"variable-ND"-servo solution
Servo board with three settings 0.63x focal reducer, none and 2x tele extender
Camera on a focus table

I have to check on how much back focus that is available ...

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.

Many thanks for all the info.

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Henrik

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