Sky & Telescope - February 2021 - 34

S&T Test Report

The 2-inch focuser is also very good
for imaging the Moon and planets with
the small planetary cameras designed to
fit into 1¼-inch eyepiece holders. And
while I wasn't planning to do photography with large cameras, I was pleased
to find that the 2-inch focuser did not
slip when my heavy DSLR camera was
attached, even when the scope was
pointed at the zenith. In my opinion this focuser serves well for visual
observing and astrophotography with
cameras up to and including DSLRs.
The only hardware issue I had with
the Evostar 150 APO involves the tube
rings. With their clamping knobs fully
tightened, the tube sometimes slipped
when the scope was pointed at high
elevations. My solution was to add a
couple of small cork strips around a
portion of the inside of each felt-lined
tube ring to add a little friction.
As mentioned above, the DX model's
hardware upgrades include the heavyduty 3.4-inch focuser. There's no question that this focuser is very nice, and
T As described in the text, the author mounted
the Evostar scope together with his homemade
6-inch f/10 refractor assembled with a conventional achromatic objective made by A. Jaegers
in the early 1970s. While both scopes were
evenly matched in critical resolution tests, the
Evostar consistently delivered images that were
brighter and more contrasty than the 6-inch.

it really is a must-have upgrade if you
intend is to use the Evostar 150 with a
heavy imaging setup, but I didn't find it
a significant advantage for strictly visual
observing. The rack-and-pinion-driven
drawtube has 3½ inches of travel, with
its midpoint being where many eyepieces
come to focus when using a 2-inch star
diagonal. For straight-through viewing
some eyepieces may need a short extension tube. The fine-focus knob has a
firmer feel than the 2-inch focuser, and
there's a small lever that adds friction on
the rack-and-pinion drive, which virtually guarantees it will not slip regardless
of the load on the focuser. This focuser
has the same style dovetail mounting
base for a finder that's included on the
2-inch focuser.
The biggest optical advantage of the
DX focuser is that it accepts the new
Evostar f/6.2 focal reducer and makes
the scope more attractive to people
interested in deep-sky imaging. While
it's called a focal reducer, this accessory also flattens the field quite a bit. At
f/6.2, the Evostar 150 has an effective
focal length of 930 mm and a field of
view covering about 2¼° × 1½° on a
full-frame (36 × 24-mm) sensor. With a
48-mm T-ring on my full-frame DSLR,
there was only minimal vignetting in
the corners of the frame. And though
it's a somewhat subjective call, I found
star images to be very good over more
than 80% of the sensor's long dimension. The bottom line with the focal
reducer is an imaging circle with good
star images approaching 30 mm in
diameter (covering a 1.8° field) and
almost no vignetting. It may not be in
the same class as today's elite astrographs, but it's still an impressive setup
for deep-sky astrophotography.

Optical Performance
There's a bit of a backstory as to why
I passed judgement so quickly on the
Evostar 150 APO's optical quality, and it
involves another telescope. Earlier this
year I began an observing project with
a 6-inch f/10 refractor that I cobbled
together using an A. Jaegers' objective
that had sat on my workshop shelf for
more than 40 years. The project was a
34

FEBRUARY 2 021 * SK Y & TELESCOPE

S While the Evostar 150 models are sold without a finder, there's a common-style dovetail
shoe on the focusers for attaching one.

departure from my typical just-for-fun
observing, which usually involves deepsky objects and reflectors with apertures
of 12 inches and up.
I gained a lot more appreciation for
observers who rave about refractors
and how they excel for viewing the
Moon, planets, and double stars (the
latter being a big part of my observing
project). It also helped that our New
England astronomical seeing accommodates a 6-inch scope far more often than
it does a large reflector. The A. Jaegers
lens proved to be very sharp and clearly
capable of resolving double stars around
the aperture's 0.75-arcsecond Dawes
limit when the seeing permitted. Its
main weakness is color fringing, which
is to be expected for an f/10 crown-andflint achromat.
Given this 6-inch scope's aperture
and performance, it was an excellent
benchmark for comparing to the Evostar 150 APO, and as such I mounted
both scopes side by side on the same
Sky-Watcher EQ8-R Pro German equatorial mount that was reviewed in last
October's issue, page 66. And to help
level the playing field, I selected sets of
eyepieces that gave similar magnifications when paired with the respective
scopes, often using the same brand and
design, including eyepieces from the
Burgess/TMB Planetary Series and Tele
Vue's Radian family.
My first look through the Evostar
150 APO was an eye-opener. Despite
its slightly smaller aperture (5.9 vs. 6
inches), it gave a view of the brilliant star



Sky & Telescope - February 2021

Table of Contents for the Digital Edition of Sky & Telescope - February 2021

Contents
Sky & Telescope - February 2021 - Cover1
Sky & Telescope - February 2021 - Cover2
Sky & Telescope - February 2021 - 1
Sky & Telescope - February 2021 - Contents
Sky & Telescope - February 2021 - 3
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