My first Magazine Sky & Telescope - 02.2019 | Page 72

ASTRONOMER’S WORKBENCH by Jerry Oltion
A 25-inch Breakthrough
p Mel Bartels with his
25-inch f/2.6 scope.
MEL BARTELS HAS BEEN ON some-
thing of a crusade lately. Long a propo-
nent of large, thin mirrors, he has also
become a proponent of fast mirrors,
resulting in what he calls “richest-fi eld
telescopes.” You saw his 13-inch f/3
scope in our January 2012 issue, and
his 6-inch f/2.8 scope in our Septem-
ber 2014 issue. He’s since taken it up
a notch both in size and speed: He has
just fi nished a 25-inch f/2.6.
That in itself would be exciting
enough, but Mel has come up with an
innovative new twist on the standard
alt-azimuth mount that completely
eliminates “Dob’s hole” (see sidebar).
But wait, there’s more! Pierre Lemay
(originator of the tracking ball scope
70
FE B RUA RY 2 019 • SK Y & TELESCOPE
years before I came up with the same
concept, most recently featured in our
March 2016 issue) has come up with a
new two-speed helical Crayford focuser
for the same scope. Together, these
The solution was simple,
elegant, and apparently
brand-new: add a second
altitude axis!
design elements have created a com-
pletely new observing experience.
Mel caught the thin-mirror bug from
Oregon ATM David Davis, who did
much of the pioneering work in slump-
ing thin glass in a kiln and finishing
out mirrors as thin as a quarter-inch
over 16 inches of diameter. With that
success, Mel reasoned that he could get
away with ¾″ plate glass for a mirror
up to at least 42 inches. Mel ground a
13-inch f/3 first, then pushed faster to a
10-inch f/2.7 and a 6-inch f/2.8 that he
made in tandem.
All three scopes were wildly success-
ful, so he decided to push onward with
larger, faster mirrors. Next up was a
25-inch blank that was slumped to f/2
by ATM Greg Wilhite, who presented it
to Mel as a gift. F/2 was way faster than
Mel wanted to go, but as he says, “I do
not look gift horses in the mouth and
took the glass with many thanks.”
This innovative scope incorporates new ATM advances on three separate fronts.