Sky & Telescope - March 2024 - 64

Gear Improvement
With the laser now aimed precisely at the primary mirror's
center, adjust the primary's tilt to return the laser
beam to the exact center of the white target on the laser.
When the mirror is properly adjusted, the return beam will
merge with its source.
Focus on the Focuser
The scope is nearing excellent collimation. It's now time to
check and adjust the tilt of the focuser - an extremely critical
task for imaging. With the super collimator in the focuser
and turned on, rack the focuser all the way inward until it
stops. Readjust the tilt of the primary if needed to return the
beam exactly to its pinhole source. Now rack the focuser all
the way out. You can see the returning beam on the painted
foil target by looking at its reflection in the primary mirror or
using a small inspection mirror held inside the tube. The laser
dot should still be exactly in the center. But more often than
not, it's offset in one direction, and this indicates a tilted
focuser or one with unacceptable mechanical slop. If this
alignment is not spot on, you must adjust the four tilt-adjust
screws found along the base plate of the focuser in order to
recenter the beam onto the target. If your scope's focuser
doesn't include angle-adjustment screws, you'll need to make
the adjustments using shims made from plastic or metal.
To find out which ones need adjustment, simply push
the focuser drawtube on its side with your thumb and
observe the motion of the laser dot. When you find the side
that pushes it back to center, that's the side which must be
raised up. Turn the tilt-adjustment screws on the focuser no
more than half a turn at a time. Lock the focuser tilt-adjust
screws back down and then repeat all the collimation steps
- changing the focuser's tilt negates all the earlier secondary
and primary alignments.
You may have to repeat this process a few times before the
laser dot stays put at both the inner and outermost positions.
You can increase the accuracy of this alignment by adding
a 2-inch spacer tube inserted into the focuser drawtube
to increase the distance between the inner- and outermost
points of measurement. If you do this, be sure to alternate
the clamping screws on the extension tube with the focuser
clamp screws to even out any potential offset errors.
Testing the Results
To use your camera and evaluate your astrograph's collimation,
select a rich star field or open cluster and point the
telescope in the general vicinity of your target. Take a few
short exposures, being careful to accurately focus on a star
as close to the center of the frame as possible. It's best to
also guide your exposure on the star near the center of the
field to avoid field rotation, which can ruin the accuracy
of this test. When the exposure is complete, examine the
image closely at 100% magnification or greater. Look for
round stars at the center to ensure that the tracking was
accurate, then inspect each of the four corners of the field
one at a time. Any residual distortion should be similar in
64 MARCH 2024 * SKY & TELESCOPE
FOCUSER ADJUSTMENT Using the methods described in the article,
the author determined he needed to raise one side of the focuser mounting
plate to correct the tilt of the focuser in his 8-inch f/3.9 astrograph.
This particular focuser didn't include adjustment screws, so the focuser
was raised using thin, metal shim material.
all four corners, with stars in the center of the frame sharp
dots. Even the best coma correctors will still have less sharp
or slightly distorted stars in the corners, but our goal here is
to even out the residual distortions so everything is symmetrical
about the center of the field.
For PixInsight users, there are two tools available in the
current version to aid analysis. One is the AberrationInspector
script (Script > Image Analysis > AberrationInspector), and
the other is the FWHMEccentricity inspector (Script > Image
Analysis > FWHMEccentricity). Run the latter and hit the
support button to generate a map of your distortions. A tilted
focuser reveals itself in this script as a distorted area on
one side or corner of the image. The program CCDInspector
(ccdware.com) is also a great tool for analyzing your test
results. In fact, that's the only thing the program does, and it
does it very well.
In Closing
Proper collimation of your fast Newtonian astrograph will
vastly improve the quality of your deep-sky astrophotos and
should be repeated whenever stars begin to appear asymmetrical
in your images. This technique is just as useful for visual
observers, as well-corrected stars across the eyepiece are a
pleasure to behold. The rewards of this effort are razor-sharp
star images, not to mention the intricate details of nebulae
and galaxies over the maximum amount of the field your
scope can produce.
¢ Retired engineer CHRIS SCHUR continually brainstorms
ideas to perfect his astrophotography. See his work at
schursastrophotography.com.
http://www.ccdware.com http://www.schursastrophotography.com
Sky & Telescope - March 2024

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