From: "Good, Donald" 
To: 'Benjamin Robbins' , atm{at}shore.net
Reply-To: "Good, Donald" 


120 degrees is only one arrangement that works, forming an equilateral
triangle.  It is the ideal arrangement for a scope tube that points in any
direction like the Mag 1 PortaBall scopes (see
http://www.mag1instruments.com example only - not advertizing them).  An
Alt-Az scope (e.g. dobs) tilts only in one direction and the base swivels
to take in the whole sky.  Since it tilts only in one direction, you can
use an isoceles triangle (2 sides equal, one shorter).  This will allow you
to put the base "top" chain mounts in the corner.

You might also consider using just 3 eyebolts with larger eyes on the base
such that 2 turnbuckle hooks could go in each eyebolt.

I have not used a string scope, but here are some thoughts on adjusting the
tension, others might comment:
1. In the vertical, adjust the tension just enough to eliminate slack and
the cage is square and centered.  Have an eyepiece in plus any other cage
mounted equipment that you normally use. 2. Swing the scope horizontal and
the cage will droop a little. 3. Adjust the load bearing turnbuckels to
bring the cage center again and just a little higher.
4. Swing it vertical again and using just the other turnbuckles (not the
load bearing ones), center the cage again. 5. Repeat 2, 3, and 4 until the
droop is reduced to an acceptable range.  If the PVC begins to bend
significantly, then you probably will have to go to aluminum.  If the PVC
does the job, then you are set.  To make this go faster the next time,
swing the scope horizontal and loosen the other turnbuckles (not the load
bearing ones), until the tension is close to when you started, close to
slack.  Note the position above center that the cage is now.  Use this
position as the first one for step 3.

This should set the tension at an acceptable level while bending the PVC the least.

Clear skies,
Don



-----Original Message-----
From: Benjamin Robbins [mailto:benjaminr{at}highwire.com]
Sent: Monday, January 06, 2003 4:08 PM To: Good, Donald; atm{at}shore.net
Subject: RE: ATM - Travel Scope


Donald,

How is it that we can arbitrarily move our chains from their current
location? Here is a quote from Jane's 16 web page
(http://www.tms-usa.com/grayarea/janes16/jane16.htm):

"Probably the most unique part of the design of Janes telescope is
that instead of trusses between the mirror box and the secondary cage,
there are 3 triangles of Spectra fiberglass string, and two fiberglass
spring poles. The concept behind this is the fact that the fiberglass
string has near zero stretch characteristics. The two fiberglass poles are
spring loaded, and keep the fiberglass strings taught. 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 taught, 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."

Our design is based on the chains being rotated 120 degrees from each
other...if we move them to the corner than that is all out of whack isn't
it?

Thanks for all the feedback!
Benjamin

--- BBBS/NT v4.00 MP