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Re: Window Protection Or Not

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Posted by P.Halsted/">P.Halsted on August 22, 1999 18:53:52 UTC

: Hello,

: Could I put a window on the front of a telescope to protect it against dust? : Is it possible for Newton and Cassegrain?

: Thanks, : Noper.

O.K. I'm late to the discussion on this one, but I asked myself the same question just a month or so ago... I got a whole lot of nothing in response, so I tried an experiment. I went out with my telescope and did star tests first without any glass in front, and then with a cheap piece of window glass over the front of the scope.

I found that there was no measurable light loss on even VERY dim stars, that there was no visible added color, and that there was a tiny loss of contrast due to rough optical surfaces. I could not discern enough drop in contrast to force me out into far sub-zero temperatures, but it was there. I think it would be minimal with flat surfaces on the glass.

I got a hold of some ray trace software and modeled the system with and without the window in place. I found that there is no significant color or coma added to the non-window scope, so the concerns are strictly light loss (very little) and optical smoothness.

I'm looking into the tube current/cool down thing now, and I am of the opinion so far that a sealed tube with internal baffles would have less tube current trouble than an open ended tube.

The tube should not be a great heat conductor such as sheet metal, but should be paper or other composite or wood construction. The tube would come to an internal equilibrium that should be sufficent to stop tube currents although the entire tube assembly might be slightly above ambient outdoor temp. This should actually help with keeping the dew off. About the only better setup would be a forced fan like the Intes scopes. Note that an open truss design allows a lot of stray currents in the optical path even though cooling is quick, and it is pretty well understood that good baffles give a big boost to contrast. (baffles are not possible in an open truss, because stray light can come from anywhere.)

The improvement in the image contrast by eliminating the spider is so slight that taken alone it is not a compelling reason to use a window to seal the tube, but is rather a bonus side effect.

Hey, I'm convinced that windows are good for small to medium Newts,(up to 10 or 12") but may be too hard to execute for bigger arpertures.

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