Sky and Telescope - March 2016 - 64

Gary Seronik
Telescope Workshop

A 20-inch Dream Scope
This instrument is more than a Dobsonian alternative.
A lot of dobsonian-style

PIERRE LEMAY

telescopes have
appeared in this department over the years, and for good
reason. They're popular because they have the potential
to deliver plenty of aperture in an effective, simple-to-use
package. But another design that shares these attributes
is the so-called ball scope. Indeed, but for one significant
detail, they would probably be more popular than Dobs.
And what is that detail? In a word, the ball. More about
that in a moment.
The attributes of the ball-and-socket design are well
demonstrated by this 20-inch, f/3.9 reflector built by
Blainville, Quebec ATM Pierre Lemay. He describes it as
his dream scope, and I suspect his dream is one shared
by quite a few readers. For Pierre, the "dream" includes

Building this 20-inch f/3.9 ball-and-socket telescope took Quebec telescope maker Pierre Lemay took more than 20 years. The fully assembled
instrument tips the scales at 130 pounds, including its innovative, dualaxis-drive platform.

64

March 2016 sky & telescope

large aperture and motorized tracking in an instrument
that is comfortable to use and lightweight enough to be
easily transported and set up. But Pierre's dream took
more than 20 years to come true.
"The project began in 1990 when I saw a classified
ad in Sky & Telescope for a 20-inch, conical Pyrex mirror
blank advertised for $500," he says. "It was a rough cast
piece of glass that needed a lot of work before grinding could even begin." First stop was a local tombstone
maker's shop, where a curve of about the correct depth
was generated by sandblasting. Next, Pierre attacked
the rear surface of the blank with a grinding wheel and
jig to thin it and take the edge thickness down from 1
to ½ inch. This reduced the mirror's weight from 50 to
32 pounds. Additionally, he perforated the center of the
mirror to facilitate its mounting in the scope later. After
that, it was simply a matter of grinding and polishing a
20-inch f/3.9 paraboloid!
Following the primary mirror, the next big hurdle
was fabricating a "ball." This really is the make-or-break
part of the design. Failing to find a suitable ready-made
hemisphere, Pierre resigned himself to making one, or
rather, two. "My first attempt recalls the joke about how
the operation was a success, but the patient died," he
says. "But I learned a lot and developed the methods I
used in my second try."
To begin, Pierre made a crude sphere by stacking
2-inch-thick disks of Styrofoam in incrementally varying
diameters - like a layer cake. He included an aluminum plate on the bottom (to support the mirror cell),
and plywood ring on top for attaching the truss poles.
Next, he used a router mounted in a large, pivoting jig to
generate a smooth shape from the stair-step layers. The
resulting Styrofoam hemisphere would serve merely as
a form, over which Pierre applied auto-body filling putty.
Once the putty hardened, the assembly went back into
the jig for more routing and then sanding. The resulting
hemisphere received eight coats of liquid epoxy resin
(sanded between each coat) for a smooth, strong finish.
All that remained was to separate the sphere from the
form by breaking up the Styrofoam. "I was then left with
a hollow, 3/16 -inch-thick hemisphere weighing about 40
pounds," he says.



Sky and Telescope - March 2016

Table of Contents for the Digital Edition of Sky and Telescope - March 2016

Contents
Sky and Telescope - March 2016 - Cover1
Sky and Telescope - March 2016 - Cover2
Sky and Telescope - March 2016 - 1
Sky and Telescope - March 2016 - Contents
Sky and Telescope - March 2016 - 3
Sky and Telescope - March 2016 - A
Sky and Telescope - March 2016 - B
Sky and Telescope - March 2016 - 4
Sky and Telescope - March 2016 - 5
Sky and Telescope - March 2016 - 6
Sky and Telescope - March 2016 - 7
Sky and Telescope - March 2016 - 8
Sky and Telescope - March 2016 - 9
Sky and Telescope - March 2016 - 10
Sky and Telescope - March 2016 - 11
Sky and Telescope - March 2016 - 12
Sky and Telescope - March 2016 - 13
Sky and Telescope - March 2016 - 14
Sky and Telescope - March 2016 - 15
Sky and Telescope - March 2016 - 16
Sky and Telescope - March 2016 - 17
Sky and Telescope - March 2016 - 18
Sky and Telescope - March 2016 - 19
Sky and Telescope - March 2016 - 20
Sky and Telescope - March 2016 - 21
Sky and Telescope - March 2016 - 22
Sky and Telescope - March 2016 - 23
Sky and Telescope - March 2016 - 24
Sky and Telescope - March 2016 - 25
Sky and Telescope - March 2016 - 26
Sky and Telescope - March 2016 - 27
Sky and Telescope - March 2016 - 28
Sky and Telescope - March 2016 - 29
Sky and Telescope - March 2016 - 30
Sky and Telescope - March 2016 - 31
Sky and Telescope - March 2016 - 32
Sky and Telescope - March 2016 - 33
Sky and Telescope - March 2016 - 34
Sky and Telescope - March 2016 - 35
Sky and Telescope - March 2016 - 36
Sky and Telescope - March 2016 - 37
Sky and Telescope - March 2016 - 38
Sky and Telescope - March 2016 - 39
Sky and Telescope - March 2016 - 40
Sky and Telescope - March 2016 - 41
Sky and Telescope - March 2016 - 42
Sky and Telescope - March 2016 - 43
Sky and Telescope - March 2016 - 44
Sky and Telescope - March 2016 - 45
Sky and Telescope - March 2016 - 46
Sky and Telescope - March 2016 - 47
Sky and Telescope - March 2016 - 48
Sky and Telescope - March 2016 - 49
Sky and Telescope - March 2016 - 50
Sky and Telescope - March 2016 - 51
Sky and Telescope - March 2016 - 52
Sky and Telescope - March 2016 - 53
Sky and Telescope - March 2016 - 54
Sky and Telescope - March 2016 - 55
Sky and Telescope - March 2016 - 56
Sky and Telescope - March 2016 - 57
Sky and Telescope - March 2016 - 58
Sky and Telescope - March 2016 - 59
Sky and Telescope - March 2016 - 60
Sky and Telescope - March 2016 - 61
Sky and Telescope - March 2016 - 62
Sky and Telescope - March 2016 - 63
Sky and Telescope - March 2016 - 64
Sky and Telescope - March 2016 - 65
Sky and Telescope - March 2016 - 66
Sky and Telescope - March 2016 - 67
Sky and Telescope - March 2016 - 68
Sky and Telescope - March 2016 - 69
Sky and Telescope - March 2016 - 70
Sky and Telescope - March 2016 - 71
Sky and Telescope - March 2016 - 72
Sky and Telescope - March 2016 - 73
Sky and Telescope - March 2016 - 74
Sky and Telescope - March 2016 - 75
Sky and Telescope - March 2016 - 76
Sky and Telescope - March 2016 - 77
Sky and Telescope - March 2016 - 78
Sky and Telescope - March 2016 - 79
Sky and Telescope - March 2016 - 80
Sky and Telescope - March 2016 - 81
Sky and Telescope - March 2016 - 82
Sky and Telescope - March 2016 - 83
Sky and Telescope - March 2016 - 84
Sky and Telescope - March 2016 - Cover3
Sky and Telescope - March 2016 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/newtrack/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com