Click On the Pictures for a Higher Resolution View
Above photo taken by Peter Abrahams at RTMC 2004. Janes
original
string scope (now Peter Abrahams) is the red one in the background.
As far as I know, the first string telescope using this principle was built by me for a friend of mine named Jane (it's the red one in above photo). This was in 1998. The link to this telescope web page has propagated through the ATM community, I still receive emails about it. Since then, I've seen several other successful string telescopes. In the end, Jane sold the 16 inch scope to Peter Abrahams, current president of Rose City Astronomers.
Soon after I published the web page about Janes 16 inch String Scope, Matt Vartanian improved the design by adding a 3rd pole, and placing them at the apex of the strings. As you can see, I adopted this idea on the 28 inch.
How did I come up with the idea of a string telescope design? One day in about 1996, I was brainstorming about a new telescope design at our lunch hour at Technical Marine Service. We have several amateur astronomers here. One person who is not an astronomer, jokingly commented: "Dan, the next thing you know, you're going to build an inflatable telescope!!". This got me thinking, because I figured you actually could make an inflatable telescope, if the rubber inflatable rings were held in place by kevlar strings. This serendipity event led me to the string telescope design.
MORE ABOUT THE PRINCIPLE OF THE STRING TELESCOPE DESIGN:
The following paragraph was taken from the original "Janes 16 inch string scope" web page.
To further explain the concept, imagine two strings anchored in your garage floor about 2 feet apart, and tied together about 5 feet in the air. Grab the knot with your finger, pull the strings taut, and now you can only move the string in an arc, forward and back, not left to right. Now make another pair of strings, and anchor them to the floor, 120 deg's apart from the first pair. Grab the knot with your other hand, and it also is only able to move in it's own arc, 120 deg's from the first arc. Add a third pair another 120 deg's apart, and have your imaginary friend hold it at its knot, and it too has its own arc. Now tie the three knots together with a secondary cage, and now the only way to move the secondary cage is to slack a string. Keep enough tension on it, and it will stay in the same place relative to the garage floor. Now it should be easy to see how it would work on a telescope.
This has five advantages over a truss tube telescope:
1. It will be lighter, you would need at least three pairs of trusses to do the same thing. These trusses can be pretty light if you use expensive carbon fiber, but will certainly be much heavier than string and two spring poles.
2. It is much quicker to set up. The strings are always connected when the telescope is not in use. To set up the telescope, simply grab the secondary cage and extend it, and then insert the two (or three) spring loaded side poles.
3. No collimation is necessary after setup, as the fiberglass strings do not stretch, and will always position the secondary cage to exactly the same place.
4. Less wind resistance.
5. Simplifies the secondary holder design, as the secondary adjustment (in relation to the primary mirror) is done with the strings.
Keep 4 things in mind if you intend to make a
successful
string telescope:
1. Use low stretch string, and double it over many
times.
I'm using 450
PLUS
Archery string, and it's doubled 32 times!
2. Use 3 pairs of strings, and 3 poles. Using 3 poles means your secondary cage doesn't have to be as stiff, and you need 1/3 less pressure to keep the strings taut.
3. Be sure your mirror box is stiff enough not to warp with the pressure from the strings and poles.
4. Make your secondary cage as light as possible!
See links at end of this page for other string telescopes that I know about.
Here's a view of the strings, with the sheathing lifted
to
expose the string.
NOW BACK TO THE 28 INCH TELESCOPE:
At the beginning of 2003, I decided to make a larger string scope for myself. I talked to Steve Swayze about making a 24 inch mirror, and he said he had a 28 that needed a home once he finished it. I told him I would provide that home. He wasn't completed at Oregon Star Party in the summer of 2003, but I took it from him anyway, put a temporary silver coating on it, and I had a nice push around telescope at this star party that was undercorrected and had astigmatism. It sure whetted my appetite for aperature!! After the star party, Steve took the mirror back, and has since completed it.
My wife Roberta mentioned to me that she didn't want
to
climb a tall ladder to look through the telescope, thus the folded
newtonian
design. I knew I might have trouble designing a good light
baffle, but
that didn't stop me from building it anyway. My first baffle was
a
complete failure, and I thought I was in trouble. Then, one day,
just
two days before oregon star party 2003, I was looking through the
focuser,
and noticed that my spider vanes blocked a lot of the light, so I
thought
about continuing the spider vanes above the secondary mirror, and also
below
the existing spider vane, and it turned out to be a very successful
baffle.
I had to add a smaller baffle behind the secondary mirror, but the
infocus
star image doesn't suffer too much. I'll be comparing this to
Howard
Banich's 28 inch at OSP this summer. He's using a conventional
newtonian
design, so it will be an interesting comparison.
While Steve had the mirror again, I worked on
finishing
the telescope and designing hardware for a new control system for
it.
Mel Bartels Scope II
software
controls the hardware that I designed.
Setup Time:
One of the main advantages of a string telescope is
setup
time. Since the strings are always connected, you simply have to
put
the poles in and "unscrew" them to make the strings tight. This
telescope
lives in a dedicated utility trailer when not in use. When I
drive
up to a star party, I can be observing with a telescope that is
tracking
the stars in 15 minutes or less!! I think that's pretty good for
a
28 inch telescope.
SECONDARY MIRROR AND SPIDER:
The spider is made out of a sandwich of two thin (.030")pieces of carbon fiber
epoxied
to a 1/8th inch foam plastic. The carbon fiber is the size and
shape
it needs to be to support the secondary. The 1/8th" foam plastic
extends
beyond the carbon fiber, both below the carbon fiber and above it, and
becomes
the light baffle.
The secondary mirror is not adjustable. Instead of adjusting the secondary mirror to align it with the focuser, you do the opposite, adjust the focuser until it's in the proper place relative to the secondary. Instead of tipping the secondary to align to the primary, you adjust the length of the 6 strings. This is another advantage of the string telescope concept, you don't need to make an adjustable secondary mirror.
My secondary cage weighs in at about 10 lbs, including all hardware (without eyepieces or telrad). The secondary ring is made from two rings of 2 inch foam. I cut the rings out of the foam using a compass arrangment and a hot wire. The two rings were glued together, then wrapped with 1 layer of carbon fiber. The ring itself weighs in at 4 lbs. and is really stiff.
CLUTCHES:
I knew I would need clutches, so I designed a clutch
system
that seems to work pretty nicely. In order for the clutches to
not
slip during tracking or slewing, the telescope needs to be on ball
bearings,
the friction is provided by the clutch. The clutches need to be
adjusted
so it's still easy to push the telescope, but the motors will move the
telescope
without slipping the clutches. To accomplish this, my scope is a
bit
stiffer than I would like, but the nice thing, is, it really isn't all
that
stiff, I've seen a lot worse, and when you point the scope near your
object,
you can center it with the radio handpad. It's really great to
use.
The clutch uses teflon and ebony star formica. I've always wondered about ebony star, and the fact the so many ATMers use it, but it really does work a lot better than regular formica.
The drive gears are timing belts turned inside out, and stretched around a disk. This makes a really economical gear. My Azimuth gear is really a piece of particle board I routed in a circle until I could stretch the 758 tooth timing belt around it. The particle board cost about $15, and the belt about $45. It's 49 inches in diameter.
Why did I use gears instead of rollers? Rollers have the advantage of low periodic error. Rollers have a disadvantage of slipping, which isn't too much of a problem with a scope that has telescope position encoders on it, but the other main problem was how to add the friction necessary to drive the scope without slipping in azimuth, and adding instability by using the weight of the scope on the drive roller. This is why I chose a gear for the azimuth. I'm not worried about periodic error, it's not even noticable for visual use, and I can use Mel Bartels software to program it out anyway if I decide to try unguided astrophotography. I did use friction for the altitude. I'm driving one of the shafts that supports the telescope, so it drives the scope with two rollers.
The other shaft is an "idler" shaft, and is
connected to
the altitude shaft encoder. Since there is little friction on
this shaft,
there is no detectable slippage between the shaft and the telescope
altitude
bearings.
Weight:
The telescope assembly (side bearings, mirror frame,
cell,
and secondary cage) weighs in a bit less than 90 lbs. The mirror
weighs
93 pounds, total of 183 pounds to lift. This is easy for two
people,
but I've created a wheelborrow arrangement so I can now set it up by
myself.
Pictures:
Here is the altitude servo motor engaged on the
altitude combination
clutch/gear with the "gear" being made from a timing belt that has been
turned
inside out and stretched around an aluminum disk.
In this photo, notice that the shaft the altitude
bearing
is resting on, is connected solidly straight down to the azimuth
bearing.
The middle section is called the "flex plate", as it doesn't need to be
stiff
at all. It just holds the roller shafts in position (Another
design
by Ed Harvey). You can also see the timing belt turned into gear
for
the azimuth gear.
You can see the ebony star on the face of the "gear"
and
you can see one of the teflon pads that's bolted to the smaller plate.
Here's a view of the altitude clutch sandwich.
The mirror cell point position was determined from PLOP. The
mirror
is collimated from the front end. I made a long 9/16 socket (it's
about
6 feet long) by welding a socked to the end of a 1/2 inch
conduit. You
can collimate the scope while looking through the eyepiece. You
can
see the bottom end of one of the collimation bolts on the right hand
side
of the above picture.
The disk at the center of the mirror is a 3 inch
aluminum
disk that is slightly cone shaped, it's really thin at the edge and
it's 3/8"
thick in the middle. It is Siliconed to the mirror, and has a
hollow
lamp socket thread which you can see sticking out the back of the
mirror
cell. There is a aluminum large keeper washer on the end of the
shaft.
This is setup was made really lightweight to keep the mirror from
stressing
with the weight of this keeper mechanism. This is my insurance
for
my mirror, in case someone moves the telescope altitude down too low.
Here is the circuit board for the two servo motors, and
also
for the other two telescope shaft encoders.
Connectors:
Right Side Top: 12-24 volt D.C. power
Right Side Middle: RS232 Serial Port
Right Side Bottom: Hand Pad Controller
Left Side Top: Altitude/Dec Servo Motor and Encoder
Left Side Middle: Azimuth/RA Servo Motor and Encoder
Left Side Bottom: Telescope encoders (both Altitude and
Azimuth)
Other Pictures:
The base is made out of two 2 inch rings of styrafoam, which were fiberglassed.
You can see the protoype HandPad Reciever
hanging from the side bearing. It works great!
Another view of the baffle, and unmovable
secondary.
It's adjusted by string lenth, and the focuser position instead of
adjusting
the secondary.
Looking down the tube end. The strings look
like
they're loose. Really they're tight, it's just the outside
sheathing
that's loose.
Here's two photos taken through the focuser. The one
on
the left is centered in the focuser. Notice no light from the
park setting
is seen. The one on the right is offset, so you can see part of
the
light baffle, but again, no park!
Pictures taken at Hammerle Park in West Linn,
Oregon, on
5-23-04
Stolen Ideas:
I think Greg Babcock
made
the first telescope with the side bearings similar to mine.
A co-worker at Technical Marine, Ed Harvey invented the flex plate design that uses a solid connection between 4 contact points on the altitude bearings straight down through to the ground board. This creates a remarkably stable telescope. I've seen this idea used a lot since then.
Ed also helped me with the mirror cell design, and many other ideas were brainstormed with him. He's currently making a 14" telescope.
OTHER STRING TELESCOPES:
Matt
Vartanian
Craig Combes
Wow, a 16" airline transportable and only weighs 27 lbs!!!
Craig
Colvins 8 inch string travel scope.
The following were emailed to me by Peter
Abrahams:
--6
inch
by Charlie Wickes, at RTMC 2000
--13 inch f6
by
Kim Hyatt & William Kelley; RTMC 98 & 99,
flex
mirror
--13 inch by
Michael
Lindner rebuilt 13" coulter
and these three that are in progress, or were a couple years ago:
--Chuck Faytak 2, 16 inch in progress
--John Swenson in progress
--Robert S. Williams completed
Tom Simmons made a nice 12.5 inch F6. He found
my
website, and emailed me. He doesn't have a web page, but here's a
couple
of pictures:
Picture 1
Picture 2
If you know of more, let me know and I'll add them
to
the list
