My 8” F6 Rotating Tube
All telescopes have a story behind them; all ATM built
scopes seem to have more than one.
This is mine.
I started in this hobby in
2002 when my wife took an astronomy course at the local community college and
then decided to join our local club.
For Mother’s day that year I gave her a
pair of Oberwerk 15x70 binoculars.
One night later, I was hooked.
I started
my hardware collection with a little 4.5” F8 reflector but as everyone knows,
aperture fever soon took over.
An upgrade to an 8” scope seemed like the right
next step.
I began the project by doing the usual research, pricing
components and weighing my options.
One night as I’m trying to decide between
the E-bay 8” F4 and the Orion 8” F6 my wife, looks at me and says “Spend the
money to get something you will be happy with.
If you put all this effort into
building a telescope and don’t get something you can be proud of you will just
be disappointed.”
She’s the best.
The next day I called Mark Harry and ordered an 8” F6.
I’ve
never looked back.
So now that I have a premium mirror on order I knew I would
need a premium mount to do the mirror justice.
I also knew that with the
Missouri weather I would end up spending more time looking at the telescope
than looking through it.
After many hours of web searches and some hints from
the folks over at Astromart I found Black Knight Telescopes, run by Paul Stock.
If you hang around on enough mailing lists for long enough, just about every
telescope maker out there will surface after a while and one day I saw a post
from Paul describing a couple of his main secrets.
He described how he makes
his tubes look so good and how he makes them rotate in the cradle (more on that
later).
I took that chance to contact him and ask a few questions.
It was at
about that time I realized what a unique world I was about to enter.
TMs,
professional or amateur, are always more than ready to share their ideas with
anyone who asks.
Paul was very helpful and offered me many of his trade secrets
while asking nothing in return except that I give credit where credit was due.
Thanks Paul.
As a builder I don’t generate many original ideas myself,
but I can copy other ideas very well.
What follows is a description of what I
consider to be a copy of a Paul Stock Black Knight Telescope.
The key features
of this telescope mount are its height and the tube finish and ability to
rotate.
The wood finish makes everything look nice and really tops things off
but it isn’t what makes this scope a dream to use.
The Mark Harry primary goes
without mention as a fabulous mirror.
I get comments on its performance almost
every time I take it out.
I am still amazed at what is visible in an 8”
mirror.
The Tube
So what makes the tube look the way it does?
Simple, post
form Willsonart.
Willsonart makes the ATM staple Ebony Star.
They also
manufacture most of their other patterns and colors in what’s called post form
thickness.
Post form is flexible enough to wrap around a post (or tube)
without cracking or breaking.
The trick is to cap both ends with a stiff ring
in order to prevent splitting.
In my case I used a plywood circle notched with
a router to fit over the tube ends.
The laminate is held to the tube with a
row of pop rivets along the seam.
Because my scope is designed to be used
either right-handed or left-handed I put the seam opposite the focuser.
Next
time I would put the rivets under the tube based on the users preferred
eyepiece location.
The following picture shows both the rivets and the top end
ring.
The inside of the tube is flocked with black velvet.
The
fabric is held in place with Elmer’s Spray Glue.
This glue works better than
the 3-M products for this application as it doesn’t dry rigid, soak through and
it can be worked for several minutes before it sets.
Tube Rotation
Possibly the best feature of the telescope mount is the
rotating tube.
Being able to adjust the eyepiece position by simply turning
the tube to the appropriate location makes observing a dream.
There is no need
for an adjustable chair (I use a shortened bar stool) and I can sit for the
whole observing session.
The original idea came from Phyllis Lange in an
Astronomy Magazine article a few years ago and was perfected by Paul Stock on
his fabulous Black Knight telescopes.
The mechanics are simple, if not a bit
heavy.
The tube rests inside a box that has holes cut in it larger than the
tube diameter.
Furniture pads placed on the inside of these cutouts allow the
Willsonart to slide will very little effort.
The altitude bearings are
attached to the outside of the box.
It makes for some nice square surfaces to
attach everything.
The handle attached to the top makes it easy to carry.
The tube is held in place with a plywood clamp ring, backed
with Ebony Star that rests against three furniture sliders.
Inside the ring is
a strip of leather that allows me to put quite a lot of even pressure on the
tube with the ring.
I recently added an adjustable latch that makes setting
the proper tension easy.
This approach also allows for simple, in-the-field
balance adjustments.
Simply loosen the latch, slide the tube in or out and
reset the latch.
Spider and Secondary Holder
Once again I need to thank Paul for providing the spider
design.
The plans had been on his website before he shut it down.
The basic
design is simple.
Connect two fender washers with a central bolt about an inch
apart.
Use a dowel rod with a hole drilled in it for a spacer.
Hold them
together with three screws and have three more pass through them to adjust the
secondary holder.
The secondary holder is a piece of PVC cut at a 45-degree
angle with another fender washer glued to the square end.
That washer allows
the central bold to connect the whole thing together.
The key to the adjustment
system is a rubber washer that goes inside the secondary holder and is kept in
compression by the central stalk.
It provides enough give to allow for easy
adjustment but keeps everything stiff enough that you don’t have to collimate
the scope every time you use it.
The spider veins are made from metal banding strap from your
favorite home store that are bent at one end for the connecting bolts and
attached to thumb screws at the other end to provide a mounting mechanism.
My
spider is attached with brass furniture mounts, but a simple º-20 nut would
work just fine.
The two pictures below show the spider and secondary holder
just before installation and then after installation in the well-flocked tube.
Focuser
Normally the focuser doesn’t get much mention but due to the
special mounting for mine I thought I spend some time on it.
The Kineoptics
HC-2 is a wonderful focuser.
Machined to extremely tight tolerances it works
flawlessly.
However, the HC-2 is best mounted on a flat focuser board.
There are
several methods for mounting the HC-2 to a round tube but the standard ones all
involve drilling three precise mounting holes through the edge of the easily
frayed Sonotube.
During my tests every hole frayed to be unusable.
I had been
drilling the focuser hole first, followed by the mounting holes.
I might have
had more success drilling the holes in the opposite order, but I am happy with
my solution.
The solution I used was to make a mounting block from plywood that would mount
to the inside of the tube through a 3” hole. The block would then have a flat
face toward the focuser that could have all of the mounting holes drilled on
the drill press. The most difficult part was cutting the correct curve in the
mounting block. The process involved setting the depth on my router to the correct
curve radius and running the block through several times for each depth. Once
everything was drilled and tested I glued the block to the inside of the tube
and mounted the focuser to it. I’ve had no difficulties at all with the focuser
mount in almost three years of use. Both pictures show the outside face of the
mounting block. The inside of the block is covered with the tube flocking so
it doesn’t show in any of the pictures.
Base
The base seen in these pictures is actually the second base built for this telescope.
The first was built from home-store quality birch plywood. After all of the
“beauty” cutouts were made it was quite flexible and had a noticeable backlash
while moving the scope. After several attempts to stiffen the base with aluminum
channels and angles I decided that a rebuild was in order. This time a sheet
of æ” Baltic birch was used. At one time I thought that the wood choice was
overrated, but after using the higher-grade plywood I am convinced that you
get what you pay for. Baltic birch is a joy to work with and it’s quality helped
speed the rebuild time for the base. This base would end up being slightly wider
than the old one because I laminated two pieces of plywood together to get 1
º” thick rocker box sides. The reduction in the flexure and the elimination
of the backlash, even at high power, is remarkable.
The finish consists of a coat of white stain and then six
(yes six) coats of gloss water-based polyurethane.
I liked the ease of use of
the water-based poly but since I used a painting pad for the application the
finish has a few bumpy spots.
I will probably go with standard polyurethane
for the next scope.
The mirror cell is the standard University Optics 8” mirror
cell.
It works with no problems whatsoever.
Optics and Performance
A telescope is never really finished and this one could use
a touchup or two to the finish, some of the baffling could be glued down
better, the azimuth movement could be better and I want an equatorial platform
under it so I can take full advantage of the Mark Harry mirror on Mars this
fall.
However, this scope brings a smile to may face every time I use it.
A
good telescope is a joy to use, and a good one you build yourself is even
better.
I can easily split the double-double at about 100x and my recent tour
of the Hershel objects in Virgo confirmed that this scope is one worth keeping
for years to come.
I am extremely happy with the results of this project.
I’m
a much better wood worker now than I was when I started and I understand a lot
more about telescope making.
The scope looks great and its performance is
outstanding.
I would like to dedicate this article to my two youngest
children, Elizabeth and John, who remind me every day that there is always
something new to see and learn.
Thanks kids.
