David Lewis is mentioning that a 4 point support should be on .509 radius and that it is about two times better than a 3 point support !!!

From time to time some messages do appear on the list ! An interesting one :

Just a remark about the 3-point *axial* support at 40% the outer radius:
be carefull with the *lateral* supports. If these supports generate
friction (due to a small displacement, for example differential thermal 
expansion of the mirror cell with respect to the rest of the telescope tube) on 
the edge of the glass in a direction parallel to the optical axis, then a 
bending moment would appear at mirror edge and a possible strong astigmatism 
would destroy the mirror figure.

To avoid this problem, the lateral support should be made with approx 5cm
long piano wires (3 at 120deg), 1 or 2 mm in diametre. These piano wires
are clamped in the telescope tube, point towards the mirror center, and have a 
small part of nylon (or any other soft material) at the free extremity to be in 
contact with the glass. These piano wires are very stiff in compression (they 
are good for lateral supporting), but deformable in the transverse direction to 
avoid any strong bending moment to be applied to the mirror. Note that they must 
be clamped carefully to avoid buckling (`flambage' in french) when loaded 
axially.

Luc
 
                 _____________________________________
|                |                              //
| piano wire     |
|---------------||      //      mirror
|                |                      //
|                |__________________________________
telescope
tube

Two new members on the list : Alain Spang of France working at the 
 'observatoire de la Cote D'Azur" in France 
...

"I'm 33 years old and I am currently working at the Calern location of
the observatory as an observer and maintenance technician of the
satellite laser ranging station.

I'm an amateur astronomer since 20 years and my principal interest goes
to atm (I'm on the atm list). I have made several mirrors and my actual
telescope is a 300 mm dobsonian. I hope to have the opportunity in the
future to make larger ones.
I have the wish to get to know the fundamentals of FEM (and related
calculation) and the topic of this mailing-list is clearly in my field
of interest! :) "

and Stephane Guisard working in Chile on La Silla and Paranal :

I am Stephane Guisard, Optical Engineer from the Institut d'Optique in Paris. I
have been working at ESO (European Southern Observatory) for 4 years now. I
live in Santiago (Chile) and work both at La Silla observatory and Paranal on
the VLT. I am involved at work, with Image quality, mirror support and active
optics (VLT) mainly.

I am also amateur astronomer for 12 years now and have built several telescope.
I am now designing the cell for my 530 mm Ritchey Chretien telescope. I am
writing a software to calculate mirror flexures. This software takes account of
: transverse shear effect, central hole, mirror curvature.... The back of the
mirror is flat. It will have the possibility to optimise the radius of the
supports and load fraction on each ring....Later I will add the possibility to
calculate the deflection of the mirror between each support point.

On june 10th the e-mail list mirrorcelldesign@digilife.be was set up by me !

The first message was :

Hi,

sooner than expected I've been able to set up our e-mail list mirrorcelldesign@digilife.be !!!
Thanks to Philippe Vercoutter chairman of AP&P and the Astronomical Contact Group vzw for hosting this list.

If you do receive this message answer to me personnally at jvb@digilife.be so I can find out if everything for everyone is alright.
Everyone that wants to subscribe should send his question to me personnally. No one who is not subscribed can post a message on the list.
Everyone can unsubscribe himself.
This way we can keep it ..., let me quote Mel Bartels :

Hello Johan
I am always for more participation amongst amateurs.  There is already the big_dob and big_glass_pushers lists.  If you keep it tight and very focused and private, it should not impact the atm list.  Of course, discussions can also be carbon copied to the atm list when appropriate too.  By the way, please sign me up!

Clear skies, Mel Bartels

or Luc Arnold :

Johan,
A list mirrorcelldesign@digilife.be is a good idea, and I would subscribe !
A friend of me is a list manager in the amateur CCD field. I know the list gives him a lot of work (350 participants), especially because e-mail adresses of some people change without notice, or commercial messages  use the list as a source or their annoucement. You'll have to put some efficient filters to avoid this kind of problems.

Luc

I hope this way everyone will be pleased.

So, welcome to the mirrorcelldesign e-mail list !
This is a fairly private list of people seriously interested in the calculation and design of better mirror cells and related things. Purpose is to have more and easier contact between all people on the net interested in this stuff. That way we can cooperate, have fun  and perhaps make some advancements.
We can only hope everyone will check his e-mail from time to time and answer in a decent time as most of us are busy doing other things.
Messages that are interesting for every ATM will be CC'd to the ATM-list itself, where an archive is kept.
I hope you will like this list and perhaps you can put from time to time a brick on our "wall" too. You can find the" wall"at http://www.digilife.be/club/johan.Vanbeselaere/atm/optics/mirrorcell/Mirrorc ell.htm .
I'll try to put all interesting results on this page in the time I can spent to it.
Have fun !

So far the following people are subscribed : I proposed them to put up this list and they expressed their interest. I guess everyone is knowing everyone a bit, if not so you can find perhaps some information and/or link on the mirrorcell-homepage !
Mel Bartels, owner of the ATM-list. (USA)
Luc Arnold, professional at the Observatoire de Haute Provence (France)
Richard Schwartz, ATM busy with alternative mirror cell designs. (USA)
Toshimi Taki, ATM, author of FEM-program plate, appeared twice with it in S&T (Japan)
David Lewis, ATM, adapted plate to C and optimilisation is possible (Canada)
Johan Vanbeselaere, ATM, owner of the Javascriptcalculator based on plate (Belgium)

Vanbeselaere Johan
ACG vzw Belgium
list-owner
any remarks and/or problems send please to jvb@digilife.be

On june 1st 1998 an announcement was made on the ATM e-mail list by David Lewis about his website at click ! This goes a lot further than the article of Toshimi Taki where my website is a summary of ! So have a look at it. I hope to expand my mirror cell design calculator using it in the coming weeks !

This section is an attempt to describe what is going on in the world of amateur mirror support as far as I know !

In one of the ATM-books of Ingalls there's the design of Hindle using the equal weight principle for flat disks.

In one of the ATM-magazine numbers David Chandler published a BASIC-program to calculate flotation systems using the equal weigth principle for disk with a paraboidal concave front-side. This is the same as the program cell.exe .

People a bit familiar with mechanical engineering know it's a bit more complicated than just equal weight.
Andre Couder was the first who uses bending of thin plates to propose better supports, but uses lever-systems. His calculations were for continuous ring supports. He deduced the mirror deformation scaling law.

Selke further refines taking shearing effects into account. Schwesinger is even taking into account discrete points instead of a continuous ring.  Malvick and Pearson did already take into account a central hole  and concave front surface. Shear begins to play a role if the thickness of the mirror if more than 1/10th of the distance between points giving raise to thick plate models besides thin plate models.

Nelson et al. do provide a basic study of thin plate models at discrete points , extended by Luc Arnold for central holes and thick mirrors.

Nowadays these theories are used for input-values for FEM-analysis. It's exactly this method that's used by the amateurs Taki and Schwartz. Luc Arnold being a professional does use both systems and is working on active mirror support optimization but was not forgetting his amateurroots !

Let me remark that this is perhaps a very bad review and far from complete. Any corrections and suggestions are welcome.

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