Sky & Telescope - June 2022 - 72

S&T Test Report
The small locking ring included with the
Evoguide 50DX fits around 1¼-inch barrels to
preset the focus point of equipment inserted
into the focuser. It's shown here on a small
guide camera that's not fully inserted into the
field flattener for the sake of illustration.
I particularly liked the view with a Tele
Vue 24-mm Panoptic eyepiece, which
provided 10.5× and a 6.1° field - about
the maximum possible with a 1¼-inch
eyepiece fitted to the Evoguide.
Most of us won't look at the Evoguide
and think " telescope, " but looks
are deceiving. When I first got into
astronomy, there were several manufacturers
selling high-quality 50-mm
refractors, including such venerable
companies as Unitron and Swift Instruments
(with the latter's 50-mm model
reportedly made in Japan by Takahashi).
These scopes had the classic look of a
long-focus refractor. But the reason was
practical more than for aesthetics since
their doublet crown-and-flint objectives
required a relatively long focal ratio
to achieve sufficient color correction.
Furthermore, typical eyepieces back
then did not perform well with fast
focal ratios. Neither of these issues is a
problem today. With eyepieces that gave
matching magnifications, the Evoguide
and my Swift 50-mm scope (yup, I own
one) made for an interesting matchup.
And if I had to pick a winner, the prize
would go to the Evoguide, since its fully
multi-coated objective provided slightly
brighter and definitely more contrasty
views. Bottom line: The Evoguide serves
well as a very nice and reasonably
capable 50-mm telescope.
Astrograph Mode
As mentioned earlier, the Evoguide by
itself produces decent star images across
an image circle only about 10 mm in
diameter, corresponding to a 2¼° field
of view. But adding the optional Evoguide
50ED Field Flattener increases the
usable image circle to about 28 mm and
a 6+° field. As the images on the facing
page show, the flattener performs well
with cameras with APS-format sensors.
I did my testing with a ZWO ASI071MC
Pro camera, which has a one-shot-color
sensor measuring 23.6 by 15.6 mm.
While the Evoguide's performance
with the field flattener falls a little
short of systems built from the ground
up as dedicated astrographs (see, for
example, Alan Dyer's review of the
William Optics RedCat 51LX APO
astrograph in our March 2021 issue,
page 66), it's considerably less expensive.
And the Evoguide and flattener
greatly out-performed images I shot
with the same ZWO camera attached
to a Zeiss-made Hasselblad 250-mm
f/4 camera lens stopped to f/5.6. And
for the price I paid for that lens in
today's dollars, I could own several of
the Evoguide astrograph setups.
The main caveat that goes with using
the Evoguide's flattener is its rather short
17.5-mm back-focus requirement for
optimum performance. Many of today's
cameras, like the ZWO model I used,
easily work with the flattener, but there's
not enough back focus for any camera
I know of that also has a filter wheel in
the light path. But that's not the end of
the story. Starizona (starizona.com)
offers a special flattener called the EvoFF
v2 ($99) that has 55 mm of back focus
and will work with many camera/filter
wheel combinations.
The Evoguide's helical focuser has a
very fine thread pitch - it takes more
than 18 turns of the focus ring to span
its 18-mm range. This makes it very easy
to achieve precise focus for imaging,
The Evoguide is a highly versatile scope that, despite not being mentioned in the owner's manual, will work with DSLR cameras (left) as long as
you're willing to accept good star images across only a 10-mm circle covering a 2¼° field at the center of the frame. Fitted with eyepieces (center),
the Evoguide becomes a high-quality finder and small telescope. And, of course, it makes a great guidescope (right). It's shown here with the field flattener,
but the author found no advantage to using it with the small sensors in typical guide cameras.
72 JUNE 2022 * SKY & TELESCOPE

http://www.starizona.com

Sky & Telescope - June 2022

Table of Contents for the Digital Edition of Sky & Telescope - June 2022