Nemo's Astronomy World

8 Inch Portable String Telescope

(Completed September 2005)

The Story - This is my first telescope making project. I had originally planned to build a scope when I first got interested in astronomy in 2003, but came across a very nice used 10" Dob for sale and decided to learn how to use a telescope before building one (a smart move I now realize).

In the Spring of 2004 I acquired the "50 cent" 20 inch mirror which re-kindled my desire to build a telescope. But, in another smart move I now realize, I decided to build a "practice" scope before tackling something as large as a 20-incher.

The size and design started only logically (in my way of thinking) with a carrying case. I came across a nifty storage trunk (hard plastic, with wheels for $9.00) at Fred Meyer's and decided that it was perfect for a portable travel scope. Since the largest dimension of the case was about 11 inches, I figured an 8 inch mirror would work just fine, so I started buying the parts and working out the design.

Belonging to a great Astronomy Club (Rose City Astronomers of Portland, Oregon) with some innovative telescope makers - Mel Bartels, Dan Gray, Matt Vartanian - I had already pretty much decided my 20" was going to be a string telescope, having admired Dan and Matt's handiwork and as I studied some of the features and benefits of a string scope - in this case, the ability to disassemble and stow easily. Plus, string scopes are sort of a new and emerging design and I like to try new things. So, an 8 inch string telescope it was to be.

  

Construction Details

The Case

The case is molded plastic manufactured by Contico. Definitly not indestructible strength plastic, but not lightweight either. I would later line the case with dense foam pieces.

It feels strong enough to easily survive taking in and out of a trunk, and hopefully strong enough to check as baggage on a plane.

It has two small wheels on one end so it can be rolled over smooth surfaces, and a sturdy handle.

Inside dimension are 31" long x 14" wide and 11" deep. These dimension would dictate the eventual size of the components.

I didn't buy from Walmart, but it's listed in their catalog.

Mirror Cell

How do you fit a round mirror cell into a square hole? Like this I guess.

My primary mirror is an F6 Orion purchased used from a private party off of Astromart, but never used.  I wasn't into fabricating my own mirror cell, so I purchased one from University Optics - a round one perfect for a tube reflector, but not exactly the ticket for the square mirror box I would eventually end up with. I made a round adapter, countersunk for the bolts to the cell and the box and everything fits real snug.

[NOTE: 1/2" baltic birch (9-ply) used throughout, except for pole holders which are poplar.]

Mirror Box

After building the mirror cell holder I made the mirror box knowing the exact dimensions needed so the mirror cell holder would fit snug. [Box is 10.5" square and 8.5" deep]

I spent a little extra time making the mitered corners so it will look a little classier when finished. The 1.5" x 1.5" corner braces (glued and stapled) helped square up the box, give extra strength to the corner joints, and would serve as pole holders.

Most string telescopes have only 3 poles (or trusses), but given I wanted this all to fit in the storage trunk, I needed to keep the blocks on the inside of the box. If I had used 3, they would have been spaced at points other than the box corners and probably obstruct the mirror, and also not help reinforce the corners - so I decided to go with 4. There is some thinking that the geometry of the strings with 3 poles is better (being farther apart). So we'll consider this an experiment to see if it makes any apparent difference. My hunch is that on a scope this small, it isn't a big deal.

 

Secondary Cage

Two more round adapter rings held together with wooden dowels and two mitered small panels to hold the focuser and finder. Holes countersunk to hold mirror frame bolts.

Measuring the Focal Length

I could have done the math, but I saw this trick in the Kriege and Berry book: The Dobsonian Telescope Book and I figured it would be more reliable than my math.

My target turned out to be a storefront approximately 5 miles away. With the secondary clamped I moved the mirror box back and forth until I was able to get focus over the full range of an 8-24 zoom eyepiece.

This gave me the ultimate distance - 27.5" - between the top of the mirror box and the bottom of the secondary. This measurement established how long the string pieces needed to be (after considering the length of the eye bolts used to hold the string).

String Material

According to other string scope builders - and the pro at the archery supply store - BCY 450 Plus Bowstring is the "lowest stretch" string available, a primary criteria. I suppose one could use any other material that didn't stretch - like wire rope - but a primary advantage of string I believe is that when you disassemble the scope, the strings easily fold away. It's also obviously very light.

 

Stringing Jig

I asked the pro at the archery store how archers tied the end loops on their strings, and he gave me a quick lesson.

You build a rectangular jig with four posts - with the circumference/2 = the desired finished length of the string.

The two posts on either end should be 2-4 inches apart to define the end loop that will get reinforced.

I used dowels for the posts and put a short piece of plastic pipe over each dowel so the string would remain at a workable height for the later steps.

I drove in a finishing nail (i.e., no head so string can be removed easily) near the edge of the board to use as a starting anchor and for finishing the loop.

 

String Making

This turned out to be a dirty job, as whatever black dye or wax the string is coated with ends up all over everything.

Start by tying a slip knot in the end of the string and tighten over the nail. Keep tension on the string as you wind around the posts. This is very important as you don't want any slack in any of the strands when you're done.

How many strands should you use? I'm not sure that has been scientifically determined for a telescope, but the string manufacturer recommended 12 strands for a bowstring, so that's what I did - figuring the finished length of my string would be somewhat shorter than a bowstring. Six on each side to equal 12 when combined. I'd probably add more strands proportionately for a larger length.

Cut the string about 12" longer than the last corner post and tie a half-hitch knot around the combined strands at the corner post and cinch tight. Then start winding the leftover string around the multi-strands between the end posts in a tight pattern. If you have enough string, do the same in reverse. Tie a knot when you're done and cut the excess. Use the string attached to the nail to do the same thing on the other end.

Remove the strings from the posts and put one of the reinforced ends over the nail. Pull the other end tight and make sure that reinforced ends have approximately the same length of reinforcement on either side of the center point (should be at least 1" on both sides). Cut a 12-18" length of string and using slip knot, tighten the knot about .5" from the end to form the final end loop. Wind the string tightly away from the end for a couple of inches and reverse and repeat until you almost run out of string. Tie off the end and trim the excess.

The archery "pro" advised me that master string makers use a "fisherman's knot" for this reinforcement wrapping; and they make special "server" string for this purpose - presumably with a tougher finish, though he offered the same string should work fine for my application.

String Holders

I used eyebolts that are about 1.75" long on the secondary which would later give me the ability to adjust the focal point by + .5 inches. No adjustment is possible on the eyebolts attached to the mirror box.

With hacksaw I cut off about 1/8" from the end of each eye so that I could easily slip the end of the reinforced string end into the eyebolt. This meant no turnbuckle or other hardware would be necessary to attach the strings.

 

The Poles

I looked all over for poles that would fit inside the case, and extend to the necessary length when in use. Paint poles are too long and very expensive. I eventually found these brushes that were a perfect length (28") and in my price range ($6.88 each). They are lightweight metal and sturdy enough - with an internal twist lock to hold in place at the extended length.

Pole Holders

Keeping tension on the strings when the telescope is assembled is I've deduced the most important design consideration of a string telescope.  Without 'tight' strings the secondary will tend to twist as the scope is moved and collimation will be lost fairly easily and quickly.  So, the answer is to keep the strings tight - and you do that by extending the poles until the strings are as tight as seemingly possible - and holding the poles in that extended position.

Version #1 - Springy Things

My first invention was a Springy Thing - basically a compression spring inside the pole hole. I struck paydirt on a trip with my pole to Wacky Willies (a recycler/liquidator in Portland, OR. with a treasure trove of miscellaneous electronic and hardware stuff) when I found the perfect spring and some plastic bottle caps that I actually could screw the spring into so the pole would having something flat to push against. Total cost: $2.

This method worked by pushing the pole into the hole with the spring compressing about a half-inch, which was enough to allow the top end of the pole to clear the secondary ring and fit in a small hole. Then the spring would extend and 'push' the pole towards the secondary and tightening the string.

When fully assembled, this method worked reasonably well and held the strings tight. However, because springs move under stress, I could literally 'twist' the secondary. It would 'snap' back into its original position and hold collimation so it is an entirely OK solution for a scope of this size, but certainly not for a larger scope.

 

Version #2 - Nuts and Bolts

After an inspection of my scope by the master, Dan Gray, he quickly identified the Springy Thing as something that definitely could and should be redesigned to give the scope more rigidity by keeping the strings fully taut at all angles of scope movement and stress.  So, it was off to Ace Hardware with an idea for using nuts and bolts.

After walking back and forth through every aisle of the hardware store with my pole, I came across this design.  The pole fits very snugly inside a plastic pipe coupler.  The coupler has a 'lip' in the middle so the pole can only fit in half way.  I found a hex head bolt that also fit very snugly in the coupler, and with a hammer drove the bolt in one end.  In another coupler I drove the nut down to the lip. Then I redrilled the holes in the mirror box corner braces so the nut-coupling fit snug in the hole.

To set up the scope I insert the pole into the bolt-coupling piece and screw all the way into the nut piece - or at least as far as necessary for the other end of the pole to clear the secondary and be placed into the receiving hole.  Then I just 'unscrew' the pipe-bolt-coupling piece until the strings are tight. Time will tell if the nuts and bolts will hold firm in the coupling after repeated use, or start slipping. Worst case is replacing an end for about $1.

When the scope is fully assembled now, I can no longer twist the secondary as all the strings are super-tight and hold the secondary rock solid.  

  

Rocker Box - Ground Board

The rocker box and ground board were pretty straight forward, starting with the maximum dimension that would fit inside the storage case.

I decided to make the box as tall as could fit in the storage case so I wouldn't be laying on my stomach when looking at an object near the horizon. This raised the center of gravity of the assembled scope high enough for the scope to want to tip over unless one of the ground board feet was directly under the lowered string-tube. My solution was to use 5 feet on the ground board and I don't have that problem any more.

 

  

Miscellaneous Things

  • String-tube light baffle is lycra-type black fabric. I roll onto the secondary when stored, and pull down the string-tube when assembled.
  • Secondary light baffle is thin dense foam cut to fit.
  • The scope is way top-heavy to balance normally, so I use springy-cords for tension.  This has been a tricky process to find the right place to locate the spring and amount of tension to use.  Lesson taught by a fellow club member was that the line of the spring should be at a right angle to the ground board when the when the scope is pointing straight up.
  • Focuser is nothing fancy - an Orion 1.25"
  • Finder is a Regal red dot located just off line from the focuser (after some experimentation).
  • I put a 'sleeve' over the strings - some tube-fabric found at GI Joes in the fishing department.  There is some thinking that this reduces vibration from wind.  I think it is probably more important for protecting the strings as the scope is taken apart and assembled over and over.
  • Collimation is easy and relatively close each time re-assembled.
  • The scope performs great and seems to travel well.

All the pieces fit nice and snug into the storage case so my primary goal has been met - as has my secondary objective of gaining experience in building a scope.

Now how much harder could a 20" be? <grin>

Questions?  Contact me:  david (at) nemoworld.com

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