72 September 2014 sky & telescopeGary Seronik
Telescope Workshoptelescope makers are also very
active observers. For them, designing and building a new
scope is a means to an end. They push the boundaries of
what is thought feasible, and innovate when current solu-
tions no longer suffi ce. Oregon ATM Mel Bartels clearly
falls into this camp, and his startlingly short 6-inch f/2.8
Newtonian is proof.
“My initial interest was to build the largest-aperture,
no-ladder telescope possible — a goal that I am still
working toward,” said Mel. “I decided to experiment with
smaller apertures to fi nd out how much I’d like the views
produced by an f/3 or faster mirror, and to try my hand at
parabolizing such a fast mirror.”Many of the best
A Short & Sweet 6-Inch RFT
This ultra-fast Dobsonian is purpose-built for stunning wide-fi eld views.
More than simply a test bed, Mel’s 6-inch f/2.8 is a
nifty rich-fi eld telescope (RFT) — an instrument that
delivers the maximum amount of starlight for a given
aperture. For optimum results, the telescope and eyepiece
must be considered together as part of a single optical
train. That means selecting an eyepiece that simultane-
ously gives the widest fi eld of view at the highest magni-
fi cation while matching the maximum size of your eye’s
fully dark-adapted pupil.
Mel started with the eyepiece (a 21-mm Tele Vue
Ethos) and determined that an f/2.8 mirror used with a
Tele Vue Paracorr Type-2 coma corrector (which increases
the primary’s focal length and f/ratio by a factor of 1.15×)
would be ideal for his eye’s 6.5-mm pupil. “Having made
RFTs in the past, I was quick to realize the advantage of a
modern eyepiece with a 100°° apparent fi eld coupled with
a super-fast primary,” says Mel. A subtle, but important
consequence of these new eyepieces is that they permit
a wider true fi eld of view with a given aperture than ever
before. Equipped with the 21-mm Ethos, Mel’s 6-inch
provides a true fi eld that’s 4.9° wide at a magnifi cation of
23 ×. By comparison, typical astronomy binoculars deliver
that same celestial real estate, but at less than half the
magnifi cation and with anemic 2-inch objective lenses.
It wasn’t long ago that a mirror faster than f/4 would
cause even experienced ATMs to tremble with apprehen-
sion. Now, f/3 mirrors are the new f/4. Such fast mir-
rors are challenging, and ones faster than f/3 are largely
uncharted territory for amateurs. So what was it like
to make an f/2.8 mirror? “I was very worried about my
ability to generate a smooth curve at the steep deviations
from spherical that such a paraboloid calls for,” he recalls.
The key to success was an oversized pitch lap — one with
a diameter slightly greater than that of the mirror. This
enabled Mel to fi gure the mirror without introducing the
kind of surface roughness often produced with the sub-
diameter tools typically used to fi gure fast mirrors.
But for a truly fi ne optic, the mirror must be smooth
and accurately fi gured. “The fi guring was certainly touchy
— polishing for only seconds with diluted, high-quality
cerium oxide was enough to cause subtle changes,” Mel
reports. But in most respects, the f /2.8 was similar to
making an f/3, which Mel had done previously (S&T: Jan.Oregon ATM Mel Bartels displays his ultra-fast 6-inch f/2.8 rich-
fi eld telescope. Used in conjunction with a 21-mm Tele Vue Ethos
eyepiece, the scope delivers a binocular-sized fi eld of view for
breathtaking deep-sky vistas.MEL BARTELSTW_layout.indd 72 6/23/14 12:31 PM