[ATM] Re: Second Thoughts on Mirror Support

[stuart452 at earthlink.net (Stuart Hutchins)]

----- Original Message -----
From: "Steve Houlihan" <sho@surfnetUSA.com>
To: <atm@atmlist.net>
Sent: Saturday, February 07, 2004 11:40 AM
Subject: [ATM] Re: Second Thoughts on Mirror Support


> Another thought (Third?) on mirror support.  Do all of the equitorial
mounted
> telescopes out there need to use some sort of positive attachment system
> such as RTV? I cant see a sling being used in these scopes, but am not to
> familiar with anything other than Dobs.
>
Steve,

No telescope "needs" a positive attachment system.  The principle behind all
successful mirror cells is "flotation". This means supporting the weight and
controlling the position precisely
while letting the mirror assume its own stress-free shape. The principles
are fundamental, and the successful designs date back well over 100 years.
Whiffle trees support the back in a uniform manner, mimicking flotation in a
liquid.  Another system at the edges keeps the mirror from sliding in any
direction, but with no pinching and with all force vectors passing through
the center of gravity.

Any sort of  "positive attachment" violates the fundamental requirement of
"flotation".  That 's why you hear all the howling when you say RTV.  No
professional grade instrument with a solid mirror uses glue to hold it down.

Unlike glues of the past, RTV adheres well to glass and it is more compliant
than epoxy.  Being easy to use, it is tempting to see how far its use can be
"stretched" by ATMs.  It is a compromise that is not too harmful up to a
point.

If your goal is to have the highest quality telescope, you will be OK using
the cell shown on your site if you add
 rigid supports around the edge of the mirror and do not use RTV.  For an
alt-az mount, like a Dob, two contact points near the center of the mirror
thickness are sufficient.  They should be about 45 degrees either side of
the low point of the mirror. This spacing minimizes the potato chip effect,
The sling also works.

For an equatorial mount, 4 edge restraints is a minimum, and the mirror
should be free to slide about 5 mils (the thickness of an index card)
between them.  The system may be elaborated by putting a whiffle at each of
these positions.

You do very nice looking work.

Stuart Hutchins