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Harry

The relationship is between lamp life and brightness setting will not be
linear. A reasonable reduction in brightness will increase life but the
(halogen) lamps may not like being constantly run too cool.

I am surprised that your projector doesn't adjust low enough in
brightness using the built in brightness controls. Perhaps a combination
of setting low brightness on the projector and some mechanical
brightness reduction will work for you. One option is to use some
neutral density material.  If it is a DLP then a simple piece of
Polaroid will give you a factor of 2. Putting a diaphragm in front of
the lens will give you adjustable control but will create some
vignetting. You may find this acceptable depending on the particular
projector - place the diaphragm as close to the lens as possible - try
it will various sized holes in cardboard diaphragms first.

While you are projecting a video image, is the appearance of the
starfield so important? Attention will be on the image. There is a trade
off when the projected image loses its impact in order to preserve the
starfield. Ideally it would great to have a bright image against a dark
starry background.  However the crosstalk effect, with a image lighting
up the rest of the dome, gets more noticeable as the dome size reduces.

Of course if you can brighten up the starfield this will help. That is
another issue.

Glen Moore
Science Centre and Planetarium
Wollongong


Harry White wrote:

>ISEN-ASTC-L is a service of the Association of Science-Technology Centers
>Incorporated, a worldwide network of science museums and related institutions.
>*****************************************************************************
>
>Does anyone out there use data projectors in their Starlabs ?
>
>If so, have you investigated de-rating the bulb at all ?
>
>The projectors are far too bright to go happily with the starfield projector
>and reducing the current should greatly increase the bulb life.
>
>I haven¹t tried it yet and I suspect some clever electronics will shut the
>projector down as soon as the current falls.
>
>Has anybody out there tried it  ?
>
>Thanks
>
>

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