Sky & Telescope - September 2020 - 70

S&T Test Report

with spider vanes holding the secondary
mirror in the optical path. Diffraction
spikes were evident across the ﬁeld far
removed from the source star. While
"content aware" cloning during image
processing could take care of these, they
are an annoying artifact to deal with.

Filter Use
The fast photographic speed of the
HNT makes using light-pollution and
narrowband ﬁlters more necessary but
also practical. Even using dense ﬁlters,
exposures can still be reasonably short,
allowing the capture of lots of images in
a single night. But how do you physically place a ﬁlter in the path? There is
no ﬁlter drawer or camera adapter onto
which you can thread a ﬁlter.
Instead, the top of the corrector
lens in front of the camera has a slight
recess into which you can drop a standard 2-inch (48-mm) ﬁlter, which is
then held in place by the supplied thin
M63-to-M48 adapter ring, which in
turn accepts a 48-mm threaded T-ring
or 48-mm nosepiece.
The method works but requires
removing and replacing the camera for
each ﬁ lter change. For DSLRs and mirrorless cameras, using a clip-in ﬁ lter
in the camera body itself is another
choice, but one that still requires
removing the camera to swap ﬁ lters
(and adds more vignetting into the
optical path). Cooled astronomical

u The primary mirror cell contains three collimation screws and matching lock knobs. All
are recessed so the tube can be placed on its
end or inserted into its case without accidentally affecting the collimation.

cameras with thin ﬁ lter wheels
between the camera and focuser should
come to focus with the 150 HNT.
The issue is that, unlike most other
telescopes with add-on ﬁeld-ﬂattener
lenses, the HNT's corrector lens doesn't
just slip into or screw onto the focuser
- it is screwed directly within the
threaded interior of the focuser.
While the corrector lens can be
removed outright (as suggested for
collimating the mirrors) or adjusted in
its position, doing so requires using a
supplied lens spanner wrench. I tried
raising the corrector up the focuser,
reducing the distance from the lens to
the camera sensor by a few millimeters,
to see if that improved the ﬁeld ﬂatness. It did. As such, users may wish to
experiment with the spacing.
In my case, I would have left it
there except that decreasing the lensto-camera spacing meant that when a
2-inch ﬁlter was placed on top of the
corrector lens, the camera's T-ring would
not screw onto the focuser as far as it
would without the ﬁlter in place, placing
the camera at a very different rotation
angle. This made shooting aligned sets
of ﬁltered and unﬁltered images virtu-

ally impossible. The image examples here
were all taken with the corrector at its
"factory-set" position down the focuser.

Mechanical Features
The telescope's focuser is a 2½-inch,
dual-speed rack-and-pinion design
with 30 mm of travel. A DSLR with
its back-focus requirement of approximately 55 mm reaches focus with the
focuser extended only 10 mm. So while
the HNT is optimized for DSLR use,
there's enough travel and back focus to
accommodate most other cameras. The
focuser proved solid and precise, and it
locked down well to prevent slippage.
The one feature the focuser lacks is a
convenient camera-angle adjuster. Turning the camera to frame ﬁelds requires
loosening three slot-headed screws at
the base of the focuser, most not very
accessible, and with slots so wide I
found my attempts to turn them strip-

q Left: The corrector lens can be removed but only by unthreading it with the supplied spanner wrench. Collimation is still possible using a star test
with the corrector in place. Using a laser collimator will require removing the lens. Right: The gold slot-headed screw is one of three that allow the
focuser to rotate for framing the field. However, access to them is awkward, making camera rotation inconvenient.

SEPTEMBER 2 02 0 * SK Y & TELESCOPE

Sky & Telescope - September 2020

Table of Contents for the Digital Edition of Sky & Telescope - September 2020