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SIMPLIFIED CONDENSOR UNIT
This versatile mount permits
to get an hemispheric projector at low cost. The quality of the projection
depends primarily on the choice of lenses and care in optical alignments.
Even if the result isn't worth a condensor type Heligon Rodenstock, we
tried to get a result as close as possible. We don't need to use the fisheye
diaphragm with this model, which was essential with the basic installation
based on camera lens (but with loss of light). Compared with the Heligon,
we get small chromatic abberations but it's only an expert eye critics.
The optical
assembly includes achromatic doublets, mounted like a Plössl eyepiece,
i.e. 2 achromats opposed.
To reduce optical aberrations, it's
better to build a baffle at the entrance of the condenser.
Characteristics
Values can differ slightly from 10%
for the focal length, and even more in diameter, without fundamentally
changing the process.
Doublet 1: F1 = 200mm approx. Diameter
from 40 to 50 mm
Doublet 2: F2 = 60 mm diameter from
25 to 40 mm
The distance between D1 and D2 must
be adjusted using the projector and the angle of view desired. Do not attempt
systematically to obtain a field of 180 ° because the optical adjustments
are more critical with very large fields.
Distance (d) D1-D2: 80 to 120 mm
typically, adjust for the "zoom effect" wanted
Focal length = (f1 * f2) / (f1 +
f2-d), typically around 80mm
Baffling: to be tested empirically
based on the quality of the lenses used, between 20 and 35 mm in diameter
for what we tested. It must be placed before the doublet D1; it can be
sticked to the lens.
Assembly
One of the difficulties is
to keep all optical components with a perfect alignment. A proposal:
-The condenser should be assembled
as a 2 inches eyepiece, just pushed into a diagonal.
-The centerpiece is a 2-inches diagonal.
-The fisheye is screwed within a
specially designed adapter ring to replace the original 42mm filletage
in the inside, and 50.8 mm (2 inches) at the outside.
-This ring shall be 3-4 mm thick.
Settings
the optical unit is placed in front
of the projector, recoilless.
The focus is done either by moving
the focus of the projector, or by sliding the condensor unit.
The zoom is set to get the better
field and the better adjustment. It can be hard to master a good adjustment
as it'll change each time the position of the bloc compared to the exact
position of the beamer's carriage of lenses which is moving diagonally
(if your beamer don't produce a straight image).
Yves LHOUMEAU
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