66 March 2014 sky & telescopeGary Seronik
Telescope WorkshopTel escope m a k ing is a hobby with
many facets. Some people enjoy assem-
bling scopes with commercial parts,
whereas others want to make everything
from scratch. And there are also the
adventurous ones such as Ed Jones of
Ohio who enjoy creating new designs.
I featured Ed’s Chiefspiegler refl ec-
tor in the November 2008 issue, page 87.
His newest eff ort is a high-performance
design that he calls the Jones Medial
Refractor. It builds on some of his previ-
ous scopes, including the Chiefspiegler,
and it also shares DNA with exotic designs
such as the Schupmann and Honders/
Busack Medial refractors. “When develop-
ing this design,” Ed says, “I started with
the Honders/Busack Medial, but I tilted
the lenses to avoid an obstruction, and IThe Jones Medial Refractor
This optical design builds on the strengths of several previous ones.
used Chiefspiegler-type correctors.”
Ed’s scope has a single-element, 7-inch
objective (made from BK7 or K5 optical
glass). The next element in the optical
path is a 6.66-inch diameter Mangin
corrector — a lens whose back surface is
aluminized (or chemically silvered in Ed’s
case) to refl ect light back through the lens
a second time. As such, it functions as
both a lens and a mirror. The Mangin cor-
rects for the chromatic aberration of the
single-element objective.
In Ed’s design, the Mangin is tilted to
divert the light cone toward the side of
the tube, thus avoiding the need for an
obstructing secondary mirror. But, as is
typical in the optical-design game, the
solution to one problem often creates a
new one. In this case, the new problem
is a tilted fi eld. “To correct for this, I
changed the folding element from a 2.19-
inch fl at mirror to one with a very weak
convex curve,” Ed says. “The result is a
telescope that is highly corrected, like the
Honders, but unobstructed.” Light from
the convex mirror is fed through a 2-inch-
diameter plano-convex fi eld lens that
corrects for lateral color error and astigma-
tism. This is an off -the-shelf component
(part number 011-4660) from OptoSigma
(optosigma.com)
Ed’s scope has fi ve optical surfaces
(excluding the fi eld lens), each of whichEd Jones shows off his 7-inch Medial refractor
that features apochromatic performance.Objective lens Mangin correctorFocal
plane
Field
lens
Relay
mirrorThe Jones Medial Refractormust be generated, polished, and tested.
That sounds like a lot of work, but all the
surfaces are spherical, which makes the
task considerably easier. The extra eff ort
does, however, pay off. The resulting
telescope is compact, features a convenient
eyepiece position, and most importantly
is a superb performer. “The scope works
very well at high magnifi cation with no
hint of color error,” Ed reports. “It puts a
lot of dark sky between all four of the com-
ponents in Lyra’s famed Double Double. I
also enjoy using a low-power eyepiece and
seeing perfect star points all the way out
to the edge of the fi eld.”
Ed’s 7-inch is just one possible variant
of the design; his recipe can be modifi ed
to produce other examples that retain its
excellent performance. For instance, Ed
says that with some changes to the focal
ratio and the spacing of elements, the
design can be scaled up to a 40-inch scope
and still be diff raction limited across the
entire visible spectrum!
Readers interested in exploring the
Jones Medial Refractor further can obtain
detailed information by e-mailing Ed at
[email protected]. ✦Contributing editor Gary Seronik is an expe-
rienced telescope maker and observer. He
can be contacted through his website, http://www.
garyseronik.com.ED JONESS&T: LEAH TISCIONETW layout.indd 66 12/23/13 11:39 AM