master the adjustment of the optical
systems, it is recommended to have the following pattern perfectly
to get the best focus on all parts of the dome and also correct the
of the image.
of the projector
of the DLP technology and
the need of most classical projection rooms, the image is not centered
in the lense axis causing the image to be projected with an angle. To
this in our case, the rear of most projectors should be raised to
horizontally. With some projectors, tilt corrections can be diminished
with mechanical levers or software adjustments.
on a small screen as a reference.
Put the altazimuthal grid and compass cardinal points on and suppress
atmosphere in Stellarium360. By moving the screen, the image should
in or out but the center and the E/O points must remain on the same
this isn't the case, adjust by
tilting the VP, for example in using the adjustable feet.
the height of the block so
that the circular image is formed at the center of the lens. Warning,
four legs must be at the same height to avoid creating extra angle.
a bubble level for horizontality can be useful.
can use a paper, masking with
it half of the lens to see if the circular image is perfectly centered
with the optics.
on the type of Heligon
or the camera lense, you'll have to put the block close to the
For others, you'll have to adjust the distance (between 10 and 15 cm
some Heligons). Try as much as possible to remain in the optical axis.
of the block
you place the optical block,
most of the time it isn't aligned. The result is a staggered pattern
the dome. You can help the adjustment with a translucent bulb for the
fine adjustment, the easiest
way is to gently move the projector to perfect alignment: Left / Right
Rotation for the horizon I / O, up and down to the horizon N / S
must get something like this
in the end...
the projector in the dome
uncritical to be slightly shifted
from the center of the dome. If the offset is too important, some parts
of the picture will appear brighter than other parts (the side the
is close to).
to the optics, you will
discover that you have sometimes to cheat and reach a compromise,
the edges will be sharp but the zenith will be blurred or sometimes
one side of the horizon. It's due to spherical aberration of the lenses.
options: use a diaphragm or more
radically change optical elements for something better.
basic rule: always check the horizon
in front of you, not just on the horizon to your left or right.
the image appears blurred on one side, but when you shift to the
way, the sharpness becomes acceptable.
aberrations with a baffle
with a Heligon condenser, the sharpness control on the entire image can
be difficult. Most of the time, the horizon is sharp for 3 points of
compass (N, E, O), but not the last (southern). By changing the
it merely shifts the problem.
a Baffle at the entrance of the condenser, it's possible to improve the
quality of the projection. Of course, you lose (a little bit) in
but most projectors are almost "too bright", so a small loss is
(you can compensate by setting up the level of intensity of the
of the central opening: 30
of the center: +4 mm high
How to build a
Open Office Draw, it's very easy to generate a page with circles of
sizes. Just change the properties of the inner circle: diameter and
and print it all. For those not fluent software, here are some PDF
Baffle : Download
cut each drawing with scissors, following the best circular
The optical beam must be nearly
centered with the Heligon. The development must be done on the dome at
the peak, using the grid altazimuth.
Using a centered Baffle, we get
a clear picture on the horizon initially unclear, but the opposite
is then partly hidden. Make several trials with different sizes to
the minimum diameter (5 mm). With Stellarium360, the grid will be
on altazimuth 360 at height 0 (do not forget to remove the landscape
atmosphere). On the system used by the Bressan's planetarium, the ideal
diameter is 30 mm.
Without the baffle, Try by successive
tests the gray area to the southern horizon. To facilitate future
also display the menu M and alignment of constellations.
Gradually decentralize the Baffle
up to try to determine the offset necessary. on Bressan's Planetarium,
this offset is 4 mm.
With appropriate software (eg
open office), make some copies with different offsets (center opening
If one of the Baffles is satisfactory,
the double-felt-to stiffen, and avoid stray reflections.
of projection horizon south-south
southern horizon is slightly
blurry, the menu text "drools", the stars are imbedded.
the baffle, the text is clearer,
the stars are less drooling.