Sky & Telescope - April 2025 - 63

Diagnosing The Problem
There are two common sources for a tilted focal plane. The
first source is due to poor collimation of an optical system.
This often is caused by the misalignment of the focuser
relative to the optical axis. With a Newtonian reflector, I
can shim the focuser and laser-align the optics to bring this
under control (S&T: March 2024, p. 60). Some premium
refractors include a push-pull adjustment cell for aligning
the objective lens with the central axis of the focuser.
But there's also a less well-known reason why the focal
plane could be tilted: The camera's sensor could be installed
incorrectly. While the plane of the electronic sensor's
surface should be parallel to the front plate of the camera,
often it is not.
With either cause, the only way to narrow down the source
of misalignment in your system is to align the camera's tilt
plate before you can diagnose if your instrument's collimation
needs more critical alignment. Recently, I purchased a
new camera for use in our f/4 Newtonian astrograph and
found that right out of the box the sensor was tilted by a small
amount. This triggered our pursuit of a simple and fail-safe
method to square up any sensor misalignment.
Standard Technique
Ever since large detectors became widely available to amateurs,
manufacturers have struggled to ensure they were
installed in the camera bodies correctly. This proved a challenging
task that likely added significant cost to the final
product. A more effective solution was to add an alignment
plate to the front of the camera body to give users the ability
to correct the tilt themselves.
The recommended fix for sensor tilt is to install your
camera on the telescope and then use tiny wrenches and a
flashlight at night to tweak the tilt plate adjustment screws
on the camera while taking test exposures to see the results
of the adjustments. Having tried this procedure myself, I
found it extremely difficult to correct the tilt with my fast
optical systems under such conditions. If only I could make
the tweaks in the daytime, in my workshops with full access
to all the adjustment screws and without the camera being
installed on the scope!
After a great deal of thought, I came up with a solid,
easy-to-use and accurate testing rig that's simple to build
using readily available materials and hand tools. The resulting
tester allows rapid alignment of many cameras regardless
of body shapes.
Base plate
A Simple Rig
The basic concept is to mount your camera facing down over
a properly sized hole, and shine a low-powered laser up at
an angle to reflect off of the sensor cover glass, which then
reflects back down to produce a pattern on the base of the
fixture for evaluation. By rotating the camera slowly, your
sensor will reflect a stationary dot if it's parallel to the camera
front plate. More often than not, this motion traces out
TESTING WITHOUT OPTICS This side view shows how the test rig
works. The user aims a laser pointer at the center of the camera's sensor.
The beam is then reflected onto the tester base where it remains stationary
as the camera is rotated provided that the sensor is mounted parallel
to the camera's front plate. The reflected dot from the optical window
on the front of the camera is dimmer due to anti-reflection coatings and
should be ignored. Reflections from the sensor are omitted for clarity.
sk yand tele scope .o r g * APRIL 2025 63
TINY TILT This array shows nine areas from a single wide-field image
and reveals uneven focus from upper right to lower left due to a tilted
sensor in a ZWO ASI071MC camera. The camera was paired with the
same 135-mm f/2.8 lens used to image IC 1396 on the facing page.
Camera
Sensor
Sensor window
Camera window
Laser
UNLESS OTHERWISE NOTED ALL IMAGES ARE COURTESY OF THE AUTHOR;
DIAGRAM OF ALIGNMENT RIG: LEAH TISCIONE / S&T
Beam reflected off sensor window
Beam reflected off camera window
Beam to target
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Sky & Telescope - April 2025

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