70 AUSTRALIAN SKY & TELESCOPE February | March 2019
ASTRONOMER’S WORKBENCH by Jerry OltionA giant breakthrough
This innovative scope incorporates new ATM advances on three separate fronts.
MEL BARTELS HAS BEEN ON
something of a crusade lately. Long
a proponent of large, thin mirrors,
he has also become a proponent of
fast mirrors, resulting in what he
calls “richest-field telescopes”. He has
previously completed 32.5-cm f/3 and
his 15-cm f/2.8 telescopes. He’s since
taken it up a notch both in size and
speed: He has just finished a 62.5-cm
(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 box).
But wait, there’s more! Pierre Lemay
(originator of the tracking ball scope MEL BARTELS; FLEX ROCKER: JERRY OLTION; FOCUSERS, BEARINGS: JERRY OLTIONyears before I came up with the same
concept) has devised a new two-speed
helical Crayford focuser for the same
scope. Together, these design elements
have created a completely new observing
experience.
Mel caught the thin-mirror bug
from amateur telescope maker David
Davis, who did much of the pioneering
work in slumping thin glass in a kiln
and finishing out mirrors as thin as a 6
mm over 40 cm of diameter. With that
success, Mel reasoned that he could get
away with ¾′′ plate glass for a mirror
up to at least 105 cm. Mel ground a
32.5-cm f/3 first, then pushed faster to
a 25-cm f/2.7 and a 15-cm f/2.8 that he
made in tandem.All three scopes were wildly
successful, so he decided to push
onward with larger, faster mirrors.
Next up was a 62.5-cm blank that was
slumped to f/2 by amateur telescope
maker 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”.
He decided to grind it back to
f/2.6, which would leave him with a
12-mm-thick edge. He did that in about
a month of hand grinding, then fine-
ground it for another 70 hours. He had
to stop at 20-micron grit, since the thin
mirror flexed too much and scratched
with finer grits.Mel Bartels with his
62.5-cm (25-inch)
f/2.6 scope.