Mirror Edge Support Calculator

[Orion]

Found this on the Bigdob Yarhoo forum.
It is a Mirror Edge Support Calculator by Robert Houdart.

It might be useful to some.

http://www.cruxis.com/scope/mirroredgecalculator.htm

[h0ughy]

looks like a nice tool Ed for those who can make such beasts!

[Satchmo]

Thanks for posting that Orion. I tried to look at the case for your 1.1" thick X 24" primary but it won't accept the numbers. The next version will allow evaluating a sling not on the centre of gravity so it may be very uesfull for you to evaluate your sling placement tolerances for your telescope.

I'd add that the choice and end result for `cable sling' is essentially the same as a normal sling edge support. A cable sling is usually chosen for a large thin mirror as it is easier to position exactly at the mirror edges Center of Gravity than a normal wide cloth/ nylon sling.

This calculator is of use not only for telescope makers, but anyone who wants to evaluate the surface flexure due to edge supports ,with optical axis at the horizen, of any commercail mirror and mounting system. Just divide the Nanometre out put figure by 550
to get a figure for surface flexure in fractions of a wave.

I'm greatful for this calculator as it can demonstrate what I have known for a long time qualitatively through practical experience. While 2" thick sling mounted mirrors may before as excellent `ballast' to keep the balance point low in a modern truss Dobsonian, there is a price to pay in thermal behaviour, and thinner 1.6" thick mirrors up to 24" do still perform as superbly at the eyepiece in regard to image quality provided the mirror is excellent in the first place. The views do not suddenly fall apart from astigmatism or `potato chipping' as modern commercially driven folklore might suggest :)

[Merlin66]

I did my Honours degree ( many years ago now) on the 98" Isaac Newton Telescope; this was to investigate the structural elements beng used.
One of the interesting things I found was, as a mirror is just a solid/ liquid piece of glass, it wants to flow and sag. For edge suports, the centreline of the support mechanism should be in line with the neutral axis of the disk.

Also if you can imagine a mirror sitting on edge and cut through the diameter ( ie a half moon resting on a half moon) then the theory says that the bottom half should be supported such that the loading varies from zero at the diameter to a maximum at the bottom ( a sine curve) similarly the top half should be "PULLED" upwards in the same fashion. This loading will ensure that there are NO excessive point loads applied to the mirror to cause distortion.

Astronomers applied this theory by gluing lugs radially to the mirror and pivoting weights ( remember the total weight of the mirror on the radial supports also varies with the vertical tilt of the telescope. When the mirror is pointing upwards, there's effectively no radial loading, whereas when its pointing horizontal there's maximum radial loads) This is the system used on the 200" as well as most others above 100" diameter. Another alternative was to "float" the mirror on a tube of mercury located arround the mirror ( many large secondary mirrors are mounted this way.)

A dobbie belt goes somewhat towards providing reasonable radial support for the BOTTOM half of a mirror,but the top half still sags.

In the late 70's I published a paper in MNASSA, ( and in the BAA journal at the time) where I proposed a system using opposing balanced hydraulic cylinders mounted radially, say 16/24 of them, such that they were interconnected across the mirror ie when one was "pushed" down by the weight of the mirror the opposing one would "pull" by the same amount. An additional bebefit was that they were self-centering. The movement is really "virtual" so the cylinders are just small diaphrams.
This provides the Schwesinger loading pattern with minimal additional weights.
This, combined with "air bag" back supports would be the ideal combination, with minimum moving parts and additional weight.
Just my 2c worth.

[Suzy_A]

Hi People,

I use Graphical PLOP - http://www.davidlewistoronto.com/plop/

See the example plot of the stress points on a mirror..