Sky & Telescope - April 2025 - 64

Gear Improvement
a small circle revealing imperfect sensor
alignment. You would then incrementally
adjust the camera's tilt plate until
the reflected pattern converges on the
smallest area. This test is quite sensitive
and can detect tilts of less than a fraction
of a degree with ease.
Constructing the test rig is straightforward.
I built mine from a few 1-by-6
pine boards along with wood screws,
three 2½-inch machine screws with wing
nuts, three large nylon washers, Elmer's
carpenter's glue, and a small piece of
Masonite to be used as a smooth rotation
surface. I made our rig 24 inches (61 cm) tall, but you
can increase the accuracy of the rig by making it even taller
(though this was more than adequate for my testing). You'll
also need a pen-style laser pointer clamped to a small aiming
yoke. Finally, some sticky backed note paper for tracking the
dot's rotation is really handy.
While many of the small parts could be made of metal or
wood, I found that highly accurate 3D-printed parts worked
exceptionally well. The three parts I printed using standard
PLA filament were the camera holder, a fork mount for the
laser, and the simulated camera back-focus tube for initial
laser alignment. A low-tech clothespin is handy for clamping
the lasers button down while testing.
After assembly, the laser must be aligned so that it strikes
the sensor approximately at its center. To do this, make a target
that simulates the position of your camera's sensor. You can
do this by taking a tube of roughly the same diameter as your
LASER ROCKER The laser mount should allow
you to precisely aim its beam at the test target or
camera sensor. The author 3D-printed this part,
but it can also be made from thin aluminum flashing
or wood.
camera's with a depth equal to the back
focus of the camera under test. Next, glue
a piece of tracing paper or frosted plastic to
the top of the tube with an X marking its
center. This is where you'll aim the laser.
Insert this fixture into the camera mounting
plate, then adjust the laser until it's
aimed at the center of your target's frosted
screen. This ensures that when the camera
is installed in place of the target, the laser dot will strike the
sensor in its center. We're now ready to start testing!
In Action
Once the camera is installed in place of the target and the
laser switched on, examine the reflected laser dot on the tester's
base. The reflected laser beam has multiple components
- understanding what they are and how they move as the
camera is rotated is critical to achieving optimal tilt alignment.
The brightest spot is the reflection from the front and
back of the sensor cover glass window mounted on the sensor
chip. They are merged into one slightly elongated dot because
of the size of the laser beam and the thinness of the cover
glass. This spot is the one to monitor during testing. Ignore
all the other reflections.
Once you understand the projected patterns, testing is
very straightforward. In most cases, the camera's front plate
CAMERA PLATE The camera is dropped into the recess of the camera
mount sensor-side down, and the nylon washer clamps are pivoted
and tightened to keep the camera centered while it's being rotated.
DUMMY SENSOR Make an alignment target the same depth as
the distance between the camera's detector and the front plate of the
camera body. Every manufacturer provides this information to permit the
proper spacing of coma correctors and field flatteners.
64 APRIL 2025 * SKY & TELESCOPE
Sky & Telescope - April 2025

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