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Antique
4.5" f16 Refractor |
 I recently
aquired an antique 4.5" f15 refractor from a man who got
it from the widow of the man who made it. The telescope is about
60 years old, and is very well made with lots of brass. The man
that made it, Thomas Wilbur Stone, poured his own iron into sandcast
molds for the equatorial mount, turned and milled his own brass
gears, and according to the person I bought it from, made the
main lens as well. The ad for the telescope said the main lens
was poor. Indeed, as I first looked through it, I could see it
had some seriuos problem optically. The stars all had tails and
were flared grossly to one side. Due to the poor optics and very
heavy mount (the guy didn't want to ship it),he accepted my really
low offer.
Well, after some reading on objective lenses, I took the main
lens apart and closely examined its two elements. It appeared
to be a standard doublet-with the thin crown element up front,
and the thicker flint element behind. It was also air-spaced,
and had numerous confusing markings all along the edges of both
lenses as if they had been dissasembled many times in the past
decades.
I cleaned them, and decided on a certain orientation keeping
the crown element forward (closest to the sky). I also aligned
the two elements with the axis of the telescope tube using a
chesire eyepiece. Looking through the telescope showed the problem
was still there. Almost ready to accept the lens was bad, I kept
thinking why so much prescision would go into making such a beautiful
telescope-but place a terrible lens in it. I decided to reverse
the lens-that is-to place the flint element towards the sky.
The result was spectacular improvement. Nearly perfect color
correction-and pinpoint stars. Star testing at high power gave
perfect Airy patterns-with only a hint of spherical abberation!
The lens is a gem, and consistent with the workmanship on the
rest of the telescope!
I gazed at Saturn for a long time- at 175X its rings were magnificent.
Cassinis Division was ink black, and the globe was belted with
subtle hues of yellow and brown! Five pinpoints of light attended
the planet(its moons)!
I still need to do some restoration work-the mount needs painting,
and I would like to make a nice tripod for it-but I plan on using
this scope for our clubs outreach programs. I will also bring
it to a meeting within the next couple of months. |
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Ultra
Portable 14.5" Telescope |
 As
an avid telescope builder, I read with great interest Richard
Berry and Dave Kriegeâs landmark book on building a large
dobsonian. I was so inspired by their work that I built two classical
truss-tube dobsonians -- a 12.5" f6.8 and then a 16"
f4.5. Although I am able to transport these telescopes in my
Honda station wagon, they are nonetheless large and bulky. I
wondered just how much further I could push the envelope of portability
while still maintaining a fairly conventional design using tried
and true methods from my previous experience. About this time
George Uhl, a friend who has a fine 14.5" f4 sonotube dobsonian
lamented his increasing difficulty in getting away to dark skies.
His scope was purchased from John Hudeck, founder of Galaxy Optics.
The telescope was Johnâs personal scope for a number of
years before he moved up to a larger aperture. While optically
excellent, it was nonetheless impractical to transport due to
the large one-piece tube, groundboard and high-sided one-piece
rockerbox. Certainly it would not fit on an airliner, which was
on George's wishlist for a travelscope. And so during a stargazing
trip to Virginia's Shenandoah Mountains, George and I hatched
a plan to rebuild this scope into a portable truss-tube telescope
that would be easy to transport and be possible to take through
airports. I would design and build it, and George would pay for
the materials (and throw in a 6" f/15 Jaegers achromat to
boot).
The first component I made was the mirror box. I used ¸
inch HVSC Birch Plywood from a local hardware store. The dimensions
are 17X17X6 inches. The idea was to keep the thickness down as
much as possible for transport. A thin mirror box would also
allow for rapid thermal equilibrium. In the classical truss-tube
dobsonian, the mirror box is made tall enough so the balance
of the telescope is at the top of the box. Placement of the side
bearings is possible at the balance point thereby eliminating
the need for counterweights. However, with the low profile mirror
box I built, the balance point of the telescope was about 8-inches
above the box. To achieve balance without using too much counterweight,
I used large 18-inch diameter altitude bearings and mounted them
as far up the mirror box as possible while still maintaining
enough contact with the box for rigidity. This got me to within
4 inches of the balance point. The rest of the imbalance was
taken care of by attaching two extension springs mounted on each
side of the rockerbox. One end is attached to the altitude bearing,
while the other mounts to the bottom edge of the base of the
rockerbox. The are aligned in such as way that when the scope
is pointed straight up, there is no tension and they point to
the center of rotation of the truss-tube. As the scope is lowered,
tension gradually builds up that counterbalances the increasingly
top-heavy upper end. Trial and error resulted in a nearly perfect
match. The slight residual imbalance can be easily taken out
by using a "roostertail", a thin plywood plate inserted
between the mirror box and mirror cell that supports a small
amount of counterweight, such as a copy of Burnham's Celestial
Handbook. To allow easy installation and removal of the side
bearings, I used threaded inserts which accept 1/4-20 knobs.
Three knobs hold each bearing to the side of the mirrorbox. The
front of the box is a thin ¹ inch plywood webbing, which
adds considerable strength and resists flexure. In each corner
behind the webbing are two threaded "T" nuts, which
accept the threaded rod segment which protrudes from the end
of each truss pole. In this way the truss poles can be threaded
into the mirrorbox. Finally, in each of the bottom corners of
the mirrorbox is a 5/16" threaded insert which allows the
mirror cell to be mounted to the bottom of the mirrorbox.
The mirror cell is simple, but functions well. It consists of
a ²ä plywood plate 17X17 inches, and attached is a
conventional 9-point flotation mirror support. Four holes at
the corners allow a 5/16" threaded rod to attach the cell
to the mirror box. Collimation is done using three Allen screws
in a conventional manner. A small muffin fan draws air around
the mirror to aid in cooling.
The secondary cage was designed to be light, and to have a thin
profile so that it would fit in a trunk along with the main mirror.
The cage's most unconventional feature is the curved vane for
the secondary support. I made it according Gary Seronik's article
in S&T, but used heavier 18-gauge steel with a width of two
inches. A dew guard is mounted to the back of the 2.6-inch secondary.
The 2-inch JMI Crayford focuser is baffled at the bottom of the
drawtube, and across from this focuser is a kydex light baffle.
The effect is a well-baffled telescope that can be used near
local light sources without loss of contrast from scattered light.
I mounted a Telrad to the plywood rings, which just happened
to be separated by the same distance as the mounting holes on
the Telrads base.
The truss poles posed some difficulty to transport effectively.
They had to be lightweight, yet still minimize flexure. A full-length
44" pole would not fit well in a suitcase, so I decided
to use a telescoping pole system of some kind. After some experimentation,
I decided to try closet rods. They come in a number of lengths,
are fairly light, and inexpensive. There is enough friction so
they stay together without changing length. To attach them to
the mirror box, I placed a wooden dowel into one end of the rod.
The dowel had a hole drilled through it into which I placed a
round-head screw. Using Locktight glue and a nut to tighten the
screw to the dowel, I screwed this entire assembly into the lower
end of each truss pole. Therefore, each pole can simply be screwed
into the mirror box, and breaks apart into two pieces for easy
transport.
The groundboard is fairly conventional and only ¹ä
thick. The rockerbox is modular, with the front and back boards
removable. This allows the hinged sideboards to fold flat, thereby
lowering the profile of the groundboard/rockerbox assembly to
only 3 inches! The sideboards are ² inch plywood, as is
the bottom of the rockerbox.
The entire telescope fits in one medium-sized Pelican case, and
two suitcases-somewhat more than originally hoped for-but still
practical. The entire telescope fully assembled weighs only 55
lbs. The heaviest component-the mirrorbox-weighs 28 lbs. Before
I gave the finished telescope back to George, I had several opportunities
to try it out . The telescope easily fit into the car, and set
up within a few minutes. Using it was a pleasure; it had that
nice buttery smooth feel of a well-built and balanced dobsonian.
The telescope responds nicely to tracking and slewing movements
applied to the secondary cage. There is very little "elasticity"
despite the lightweight design. The images of stars were sharp
and the lack of noticeable diffraction effects very pleasing.
As of this writing, George is observing the heavens from some
high mountains in Colorado. Along with his family-on the airplane
with him was a 14.5" f4 telescope! |
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