Sky & Telescope - January 2024 - 21

mount with worm-and-wheel gears or belt drives, and
without the need for a bulky counterweight. It's possible
to carry the AM5 with a small telescope out to the yard all
assembled, or to pack the components into a small car with
lots of room to spare.
While other brands (such as iOptron, PegasusAstro, and
Sharpstar) also offer strain-wave-driven mounts in the
$1,800 to $3,000 price range, I chose the AM5 for its price, its
compatibility with ZWO's other hardware and mobile apps,
and its simple polar-alignment routine. The AM5 can easily
handle telescopes up to a 130-mm refractor or a 200-mm
Schmidt-Cassegrain, so there's plenty of room to grow.
Of course, no sooner had I purchased my AM5 when ZWO
announced an even smaller and less expensive mount. Its
AM3 sells for about $1,800, bundled with the same tripod.
For our entry-level system, the AM3 is probably the better
choice, bringing the total system cost down to about $3,000.
But for the extra $500 you do get a greater payload capacity:
12.7 kg (28 lb) for the AM5 versus 8.2 kg for the AM3, making
the AM5 more future-proof.
The Optics - $750
For my suggested starter system, I've elected to keep the telescope
as small, light, and affordable as possible without compromising
image quality. I've chosen the Sharpstar 61EDPH
III ($549), a 61-mm triplet-lens apochromatic refractor.
With its matching 0.75× f/4.4 Full-Frame Focal Reducer
lens ($199) - an essential accessory for deep-sky imaging -
the total cost comes to about $750. That's a good price for
such a quality combination when you consider that other
60-mm triplets with flattener lenses sell for up to twice as
much. While some good doublet 60-mm apos are available at
lower cost, not all have a reducer lens, an option I prefer for
the faster photographic speed. As a bonus, the Sharpstar 61
can also be used visually with any 2-inch star diagonal and
eyepiece, unlike some dedicated astrographic refractors.
Synta-standard dovetail shoes
Focuser
with
camera
rotator
0.75×
reducer /
flattener
Reducer's
adapter
ring with
48-mm
filter
Paired with its reducer, the Sharpstar's focal length is 270
mm, which is long enough to record large targets well. Its
f-ratio of f/4.4 is fast enough to keep exposures short at low
ISO settings, which helps to keep camera noise at bay. The
field of view with a full-frame camera is a generous 7.6° by
5°, providing impressive star-filled images. Adding the $199
1× Full-Frame Flattener lens turns the little Sharpstar into a
360-mm f/5.9 instrument with a 5.8° by 3.6° field - a good
option for many smaller targets. Even with a houseful of
larger refractors, I often turn to this scope for a night's shoot.
Guiding and Control - $450
While it's possible to shoot and stack dozens of short (less
than 1 minute long) unguided sub-frames, I find the results
are often underexposed, noisier, and less detailed when compared
to compiling fewer multi-minute images. That's especially
true of older DSLRs, whose sensors are much noisier in
low light and so cannot tolerate underexposure.
However, keeping stars pinpoint in exposures longer than
a minute requires autoguiding to compensate for a mount's
tracking errors. One of the most affordable solutions is
ZWO's ASI120MM Mini, its lowest-cost guide camera, mated
to its f/4 Mini Guidescope, for a package price of $248.
Although its aperture is only 30-mm, I've never found this
setup unable to find and lock onto a guide star.
To control the autoguider, I recommend the newest version
of ZWO's popular imaging computer, the ASIair Mini, for
$199. The ASIair Mini sets up a local Wi-Fi network you connect
to with your smartphone or tablet using the ASIair app
(free for iOS and Android). A mobile device is much easier
to operate and power in the field than the laptop computer
most other autoguiders require. So far in several months of
use, the system has yielded a 100% success rate. Except when
clouds interfered, not one frame has suffered from misguided
or trailed stars, though admittedly, a short-focal-length apo is
quite forgiving of such errors.
48-mm
T-ring
Vixen-standard dovetail bar
DSLR
camera
body
ATTACHING A DSLR The Sharpstar's 0.75× Reducer screws into the focuser's rotator and has an adapter ring that accepts 48-mm filters. Mounting
a DSLR camera requires a user-supplied 48-mm T-ring for the camera mount (Canon EF in this case).
sk yand tele scope .o r g * JANUARY 2024 21

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Sky & Telescope - January 2024

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