RE: ATM - Travel Scope

["Good, Donald" <dgood@aha.org>]

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@highwire.com]
Sent: Monday, January 06, 2003 4:08 PM
To: Good, Donald; atm@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