Don's 8" F6 Stacked
String Telescope

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PURPOSE

This document describes my 8" F6 stacked (Dobsonian) string telescope.  Note that this telescope has a counterbalance spring (Tom Krajci's design).  Here is the presentation I made during the 2010 OSP telescope walkabout.

What makes this telescope different from other string telescopes?  Different or unique features. 

DETAILS

The stacked string design came to me as I sat in the OMSI auditorium waiting for the monthly Rose City Astronomer's meeting to start.  While working on the "String Telescope Concepts" webpage I realized that rigidity of a string telescope depends on the angle of the strings with respect to the light path.  The stacked string design has twice the string angle (the same rigidity for half the strut compression) as an equivalent non-stacked string telescope.

Larger F-number telescopes are a good candidate for the stacked string design.  I decided to do a full scale mockup the stacked string for my 8" F6 telescope.

My initial plan was to use the same string length for both the upper eight strings and the lower eight strings.  Whichever strings have the shallower angle with respect to vertical determine the rigidity of the stacked string telescope so it would not make sense to choose significantly different string lengths for the upper and lower sets of strings.

At the time I made the first eight bow strings I had not determined the final string anchor designs.  It turned out that the upper set of strings was a different length than the lower set of strings, but this worked out for the best.  I decided to put the longer strings above the middle ring and the shorter strings below the middle ring.  This is beneficial because the middle ring has spring clamps that capture the lower halves of the struts.

If you look closely at the following photo you'll notice that each strut has an upper half and a lower half.  By putting the shorter strings on the bottom, the spring clamps on the middle ring capture the lower halves of the struts.  The spring clamps are 3/4" plastic plumbing pipes. 

I used Google SketchUp 3D CAD to draw the stacked string design.  It is EXCELLENT, easy to learn, and FREE.  I drew the middle and upper rings and then printed them out full-scale.  The left printout is the top upper ring.  The middle printout is the bottom upper ring (The "flex ring" design has a top and bottom ring.).  The right printout is the middle ring. 

The following photo shows the middle ring.  Notice that the spring-loaded clamps in the corners are facing to the left and the right.  I did this because the mirror box is 11"x10 3/4", but the middle ring is 10 5/8" x 10 7/8".  This allows the middle ring and the upper ring to nest in the mirror box for transport. 

I don't have a table saw so I used a jigsaw to cut the upper and middle rings oversize, and then I used a router to cut all the surfaces of the rings.  It is a little time-consuming, but you use the tools you have. 

TELESCOPE SETUP

• This telescope can be assembled by one person without assistance.  I have assembled and disassembled the telescope in 10-15 minutes including mounting the Telrad and dew / light shield and cover.

• Here is the telescope on a fold-up hand truck.  The mirror box nests inside the base box.  The pedestal fits over the mirror box/base box.  The altitude bearings are on top of the pedestal.  The struts and attaching parts are in the black bag on top of the pedestal.  The Telrad and eyepieces are in a separate hard case.  The dew shield and cover are not shown.

• This photo shows the outrigger legs attached to the pedestal.  The counterbalance spring and pulleys are already mounted.  The strings permanently attach the upper ring and middle ring to the mirror box.  The middle ring nests inside the mirror box right side up.  The upper ring is turned upside down in the mirror box so that the secondary mirror does not contact the cover over the primary mirror.

Notice that the mirror box is resting in the base oriented 90 degrees from the final position so that the altitude bearings can be attached.

Notice the (two-part) struts and the altitude bearings.

• • • This is a view of the nested telescope as seen from above.

• The following three photos show the telescope:

• About 10 degrees past zenith

• At zenith

• At about 30 degrees

• Here are some photos of the plastic dew shield/cloth cover.  The dew shield/cover acts as both a dew shield for the secondary and primary mirrors and a light shield to block nearby lights in our neighborhood. 

• The photo at the left shows the cloth cover on the ground (left of the telescope) and the plastic dew shield on the ground (right of telescope).

• Next, the plastic dew shield is mounted (it just slides over the upper ring).

• Then, the cloth is attached to the plastic dew shield with Velcro.

• All dressed up and ready to go.  The cloth cover is fully installed.

• Here are views of the upper ring with the Telrad mounted.  The Telrad must be removed to mount the dew shield.

• Here are views of the plastic dew shield.

• The first photo shows the dew shield stretched out flat.  It also folds up to 11"x6" for transport.

• The second photo shows the dew shield ready to install.  Each section of the dew shield is attached to the adjacent section by two small black tiestraps at the upper and lower corner of each section.  There is a large white tie-strap halfway up each corner.  The large tie-straps slide over the thumbscrews at the four corners of the upper ring.  The large tie-straps support the dew shield vertically on the lower part of the upper ring.

• The third photo shows the dew shield resting on TOP of the upper ring, not yet installed.

• The forth photo shows the dew shield installed.  The dew shield slips down over the upper ring. 

• • • The plastic dew shield and cloth cover fold up and nest inside the mirror box for transportation.

ADDITIONAL COMMENTS

I built a full-scale mockup before building the final version of this telescope.  To save effort I started with my spring counterbalance scope and just replaced the 8-strut truss and upper ring with a 4-strut stacked string design.  One of my design requirements was to use the existing optics.

The biggest irritation with the 8-strut truss version of the assembly and disassembly time.  The string design reduces assembly time from about a half hour to about 10 minutes.   

++++++++++++++++++++++++++++++++++++++

The "stacked" string telescope concept is described on the "String Telescope Concepts" webpage.  

What makes this telescope unique - different? 

• Stacked String Telescope, like two string telescopes that are stacked.

• Flexible - Rigid Pairs:

Telescopes typically have rigid members.  However, Dan Gray's 28" telescope has a "flex plate".  The flexible assembly works because it is backed up by a rigid ground board.  The flexible member is paired with the rigid member.  Dan's flexible-rigid pairing led to the "Flex Ring" idea and caused me to think about the advantages of selectively choosing flexible-rigid pairs.

With this telescope, the middle ring is flexible in the vertical direction (rigid in the lateral direction).  This flexibility is paired with the rigidity of the mirror box.  The lower strings attach the middle ring to the mirror box.

The upper ring BOTTOM ring is flexible in the vertical direction (rigid in the lateral direction).  This flexibility is paired with the rigidity of the mirror box, through the middle ring, connected by upper and lower strings.

The struts are two half-struts that stack one above the other.  The struts are flexible in the lateral direction due to the pin between the top and bottom halves.  This flexibility is paired with the rigidity (in the lateral direction) of the middle ring.  The middle ring captures the struts laterally.

• Flex Ring:

The "flex ring" (noted above) has a TOP and BOTTOM ring.  The strings and struts only contact the flexible BOTTOM ring.  The spider, focuser and finder scope only contact the TOP ring.  The TOP and BOTTOM rings are connected by three spacers (three points define a plane).

• Rigid, Light Weight String Anchors:

Each of the 24-string anchors is a #4 chain link and a #10 carriage bolt.  This design is inexpensive, compact, lightweight, and works well.  The upper and lower eight chain links are long #4 links that are bent in a vice.  The middle chain links are short #4 links. 

• Minimized Bending Moments:

The upper and lower chain links are located to minimize/eliminate bending moments in the upper ring and mirror box.  By minimizing the bending moments I prevent the need to have a more rigid (heavier or more expensive) upper ring and mirror box. 

• No String Adjustment:

This telescope has no turnbuckles or eyebolts to compensate for variations in string lengths and string anchors.  I intentionally allow the middle and upper rings to flex in the vertical direction only to compensate for these variations. 

Here are some sketches:

Two-part struts work well.  They take up less storage space.

• I'm using the "flex ring" design that I have previously used on my 12.5" F4.5 telescope.  This design uses two 1/4" "rings".  The string anchors and struts contact the lower of the two rings.  The optics attach to the upper of the two rings.  The two rings are connected by three struts.

• • • Upper ring assembly upper (optics platform) ring.

• • • Upper ring assembly lower (flexible) ring.  Notice the chain anchor links.  The thumbscrews are used to compress the struts.

• • • Here is a sketch of the string attaching detail at the mirror box.

Making String Anchors

• Here is how I made the string anchors from chain links.  Seven of the links are marked (see small "Sharpie" marks) for bending.  The eighth link is already bent with a carriage bolt through it.

• The middle photo shows a link that has been hammered to about 45 degrees.

• The right photo shows the link hammered to 90 degrees.

String Anchor Study

• This is a "string anchor study" that graphically shows the importance of locating the string anchors to minimize adding a bending moment to the upper ring (or mirror box).  Minimizing the moment is significant if the upper ring is flexible.  If the bending moment is NOT minimized, the upper ring should be "enhanced" to make it more rigid, adding to the weight of the upper ring. 

(Note:  For both photos, the strut compression eyebolts are tightened to give the same strut compression.  The upper ring is 5 mm thick plywood.)

• GOOD - In the photo at the left, the centerline of the string crosses the centerline of the strut at the upper ring.  Note that the ring is NOT bowed.

• NOT OPTIMUM - In the photo at the right, the centerline of the string crosses the centerline of the strut a couple of inches ABOVE the upper ring.  This introduces a bending moment when the strings are tensioned.  Notice the bow in the ring.

Making Strings

• The bow strings are made using a jig based on David Nemo's instructions.

• For this design, I have wrapped six loops of bow string (BCY 450 Plus) around four hex-socket capscrews.  One of the capscrews has a knob for releasing the string assembly after the ends are tied.

• Use the following formula to determine screw spacing. 

• Total Finished String Length = "X" length between capscrews + "Y" length between capscrews

• I have made 5 of the lower 8 bow strings.  There will be 8 upper bow strings in addition to the 8 lower bow strings.

• • • The bow strings are inserted into the parachute cord to prevent the strings from vibrating on the assembled telescope.

What would I do differently next time?:

If I was starting from scratch I'd use larger side bearings and eliminate the counterbalance spring.