Mechanical Gun Aim
If your projector has lens tilt rings, set them to factory spec for your
projection distance before doing any aiming.
You must first mechanically aim the guns properly. Most CRT projectors have
red and blue lens/CRT assemblies (aka gun assembly) capable of swinging
left/right. Up/down mechanical aim is accomplished by altering projector tilt.
These movements mechanically aim the center of all three guns so they converge
at screen center. Typically four bolts accessible at the bottom edge of the gun
assembly secure the gun. These are loosened to free the lens and gun assembly
and allow left/right swing. Do NOT REMOVE all four screws. Merely loosen them.
Check in your installation manual to verify which screws need to be loosened.
Some projectors only have a few fixed swing angles which are secured by placing
a locking screw through specific drill holes. Others allow infinitely variable
convergence angle and are secured by placing the locking screw through an arc
shaped slot instead of a hole. The green gun usually isn't adjustable left/right
so getting the projector centered and square to the screen during mounting is
vital.
The guns sometimes stick in their prior position even when the screws are
loosened. Controlled pressure can free a gun, but never force the guns because a
sudden give way motion could snap a tube neck. One method of producing
controlled pressure is to place some fingers between the lenses and gradually
form a fist. This allows considerable pressure to be exerted without risk of
sudden excessive motion. Once freed, just point each gun exactly and lock them
in position. How that is accomplished comes next.
Marking Screen Center
Mark the your projection screen's exact midpoint of each edge (top, bottom,
left, right) with a triangle of 3M blue easy release tape. Then mark the exact
center with a LIGHTLY applied triangle of the same tape. This tape, if left for
less a few hours, should not mar a screen. At least it never has in my hands. I
cannot say the same for other brands so I suggest no substitutes. Finding screen
center can be accurately done using a laser pointer placed at one screen edge
midpoint marker and bouncing the beam off the screen surface to make a spot at
the opposing midpoint marker. If a laser is not available, a tape measure can
suffice so long as care is take to avoid marking the screen with tape measure.
Aiming the Guns
The traditional method of setting mechanical aim is to display a center
cross, center the cross in each phosphor and then swing the guns and tilt the
projector to place the projected red, green, and blue crosses all at screen
center. The problem with the traditional technique is the difficulty in judging
when the center cross pattern is accurately centered on the phosphor. I prefer
an alternative technique which is easier to visually judge centering using
edges. Basically, this takes advantage of how easy it is to accurately judge
evenness of the gap between test pattern edges and the phosphor edges.
Turn down contrast and display a white field pattern and shift the raster to
make the edges of the pattern equidistant relative to the phosphor edges. Look
into the lenses while doing this, not at the screen. You will need to do this
for each gun. The pattern will be widest at the CRT bottom assuming ceiling
mount. Adjust size and position left/right to assure the pattern is well clear
of the phosphor edges and equally spaced from the edges at its greatest left and
right extents. Adjust the pattern up/down to achieve even spacing for the top
and bottom edges as well. Once that is done, the pattern is precisely centered
left/right and top/bottom on the phosphor surface. All you need to then is
mechanically aim the guns to project that centered pattern so its edges are
centered relative to the projection screen.
Look at the top of the projection screen and examine the left/right
relationship of the projected pattern edges and the screen edges. Swing the gun
left/right to equalize the edge relationships. Don't worry if the red and blue
guns are not yet the same width. Just make sure the edges are balanced
left/right for each gun. Lock the CRT's into position. They are now precisely
mechanically aimed left/right.
Next, pay attention to how the projected white field pattern is positioned
vertically using just the green gun. You should have previously set the pattern
so is centered up/down on the phosphor. Adjust projector tilt up/down to make
the projected green white field edges equally balanced relative to top and
bottom screen edges. Now for more explanation.
If you happen to know your center cross pattern is already precisely centered
you can just use the center cross to aim the tubes. Unfortunately, the most
accurate way I know of doing that is to pull the lenses off, center the cross on
the phosphor while measuring with a ruler, then remounting the lenses. The white
field pattern edge comparison method described above allows easy, accurate
physical aim and centering on the phosphors without pulling the lenses. This
also gives a subtle plus for the red and blue guns as I'll explain later.
I know this method seems backwards, but balancing the edges of a white field
pattern against the edges of the phosphor and then the projected edges relative
to the screen edges achieves precise mechanical aim in an easy manner. The
advantage to this method is basically the difference between having someone mark
the middle of a piece of paper without aid of a ruler vs aligning a slightly
smaller piece of paper so it is uniformly spaced inside the larger piece of
paper. The latter is easier to do accurately.
Consider the off center red and blue guns. If you aim the actual center of
the phosphor of those tubes to project at the center of the screen, you'll note
that the phosphor usage distribution is unequal left/right due to the throw
angle. Graph it out and you will see that the farther half of the screen gets
illuminated with a smaller area of phosphor. Ever notice how the side of the
screen opposite the side of the gun is less well focused? This is part of the
reason.
Centering a field pattern relative to the phosphor and then using those lit
up edges to guide lens aim will actually place the red and blue guns so they are
mechanically slightly off true center. The left lens ends up pointed slightly
left of center and the right lens ends up slightly right of center. At first
blush, this seems wrong, but this can actually be advantageous because it makes
the raster usage, resolution, and illumination more uniform across the screen.
Less horizontal linearity compensation and lens flapping are needed.
Now if you are a traditionalist and want the center of the phosphor actually
aimed at the center of the screen, you can pull the lenses off, set the center
cross with great precision and then use the projected center cross position to
guide mechanical aim. This is the usual way things are done, but I present an
alternative approach with some advantages.
Note: Mechanical Aim is Not Scheimpflug (Lens Flapping)
Novices confuse the two, but mechanical aim is not the same as lens flapping
(aka Scheimpflug). Mechanical aim of the CRT/lens assembly is akin to taking a
telescope and physically pointing it at something. Lens flapping adjusts the
mounting angle of the end lens without changing the direction the telescope is
pointed. Lens flapping doesn't change the direction of aim (well just a little),
but compensates for the planes of the screen and phosphor being non-parallel. If
the lens were kept parallel to the phosphor surface, it would be impossible to
focus throughout the screen at the same time. The flapping places the lens into
a plane intermediate between those of the screen and phosphor and that makes
global focus possible..
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