Sky & Telescope - September 2020 - 69

t The Sharpstar's 420-mm focal length and
f/2.8 speed are ideal for capturing wide-field
targets, offering a field of view of 4.8° by 3.2°
with a full-frame sensor. This deep image of
M45, the Pleiades, is a stack of eight 8-minute
unfiltered exposures at ISO 400 with a Canon
EOS Ra mirrorless camera. Inset: The author
tested the Sharpstar Hyperboloid Newtonian
astrograph using a Canon EOS Ra camera, on
an Astro-Physics Mach1GTO mount guided
with the now-discontinued SG4 standalone
autoguider from Santa Barbara Instruments.

16 mm or so will produce a view with
a dark shadow from the large secondary mirror ﬂoating in the middle of
the ﬁeld. This is not an instrument
intended for wide-ﬁeld views of the
night sky. However, it does work
superbly for its intended purpose: wideﬁeld astrophotography.

Optical Performance
Jiaxing Ruixing Optical, the company
manufacturing the Sharpstar line, has
created an instrument with an optical
design similar to the legendary Takahashi Epsilon 130 and 180 Newtonian
astrographs. Like the two Taks, the
Sharpstar 150 HNT uses a hyperbolic
primary mirror matched to a corrector
lens in the focuser to yield a wide ﬁeld
largely free of off-axis coma and on-axis
spherical aberration, all while offering
tremendous speed at f/2.8 for a focal
length of 420 mm.
The HNT projects an image circle
I measured to be 55 mm, though the
more conservative speciﬁcations state
a fully illuminated ﬁeld of 44 mm. The
latter is sufﬁcient to ﬁll a full-frame
(24 × 36 mm) sensor. I performed most
of my testing using the full-frame Canon
EOS Ra mirrorless camera I recently
reviewed (S&T: Apr. 2020, p. 64).
However, while the corners of the
frame were certainly illuminated, some
minor light fall-off occurs toward the
frame edges, as should be expected
with such a fast optical system. I measured just over one stop of darkening
at the extreme corners of a full-frame
image, a surprisingly low level of
vignetting for an f/2.8 reﬂector. The
generously sized 70-mm (minor axis)
secondary mirror is one key factor to
keeping the vignetting low. Users can

compensate for the vignetting with the
usual practice of taking and applying
ﬂat-ﬁeld calibration frames.
Stars were also tight to the corners,
though not perfect. With the corrector
at its factory-installed position down
the focuser, stars did exhibit a slight
amount of what looks like coma at the
extreme corners. Even so, the amount
of aberration was no more than what I
see with some of the refractor and ﬁeldﬂattener combinations I use.
Achieving that level of performance
did require careful collimation of the
primary mirror - not surprising after
the instrument had been shipped across
the continent. Out of the box, images
exhibited asymmetrical aberrations,
with stars on one side of the frame
more distorted than on the other.
To collimate the HNT, the instructions recommend removing its corrector
lens. As this is threaded directly into
the focuser, I was reluctant to remove it
early on in the testing, not knowing if
I could replace it properly. As it turned
out, my fears were groundless, though
removing and replacing the corrector is
not an operation I'd want to perform in
the ﬁeld.
As a result, I left the corrector in
place and used the eyepiece adapter
to inspect a star at high power, while
adjusting the primary mirror's collimation screws to make the star's image
symmetrical both in and out of focus.
That did the trick.
In subsequent testing the instrument
never traveled farther than from my
house to the backyard each night, so I
did not subject it to ﬁeld trips on rough
roads. But over two months of testing
on cold winter nights I found no need
to re-collimate the HNT. Performance
remained consistent and reliable.
I shot several ﬁelds, such as the Belt
of Orion, that included bright stars. I
did see dim blue ghost reﬂections from
bright stars in some cases. However, the
multi-coated three-element corrector
did a good job suppressing ﬂares. With
bright stars in the ﬁeld, what I did see
were diffraction spikes. These spikes
were not just around the stars themselves, a characteristic of any reﬂector

A

B

C

D

p (A) The astrograph's corrector lens
comes threaded into the barrel of the
focuser. (B) A recessed well on the corrector
lens accepts 48-mm filters that sit on the
corrector. (C) The included thin M63-to-M48
adapter ring then holds the filter in place.
(D) A user-supplied T-ring with 48-mm
threads screws onto the adapter for attaching DSLRs. Mirrorless cameras require a
custom adapter or additional extension.
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