Mirror Support Points

[Aster]

Late last year I came across an article and a nifty program which calculated the ideal support points for parabolic mirrors and explained all the relevant theory. Like, 9 point better than 6 or three for X diameter and focal length.

Anybody know what I am talking about ???

Because I lost the www address and articles/program I downloaded.

Thanks

[RB]

I seem to remember something like that in an old thread here on IIS.
Try doing a search on here.

[xstream]

Plop is what you want Alexander.

http://www.davidlewistoronto.com/plop/

[Satchmo]

Plop is a great simulation tool.

If allowing for refocus, 6 points are generally better than 9. Just arrange them equidistant at the 58% radius zone.

I can recommend 6 points. Provided the back is flat you can also use a continuous ring support instead of 6 points ( ie Silastic or Velcro ) . My 14" F5 X 1.5"thick is attached by a continuos ring of Velcro and holds figure and collimation excellently, and no clips to cause diffraction effects.

[Aster]

Thank you fellows, plop sounds just about right.

[Aster]

Hi Mark,

I was thinking of supporting it on a continuous ring just inside of the outside diameter, around the 90 - 95% zone.
Leaving as much of the back exposed to the air as possible.
Mirror is only 31mm thick, moulded pyrex. Will it deform,sag to much from a parabola ?

Your silicon/velcro idea sounds great. Would save me a lot of work making side supports and clips.

Have it mounted at present with a standard 3 point system made out of particle board, which naturally doesn't keep it's collimation.

[Satchmo]

Hi Alex, without knowing your specs exactly I'd say you would end up with a slightly overcorrected figure just supporting at edge.More important it will warp in odd ways when not pointing vertically .

Its best to keep the ring surface area minimum so it doesn't warp the mirror. My 14" uses about 18 X 1 inch square pads in a 60% zonal ring. Good industrial Velcro too you get cut off a roll at the hardware , not the litle packets.

If Silicon don't let it compress under1/4".
Remember it means the cell will have to go under the tap too when you wash the mirror, which is why I like Velcro. It has a little give in it and yet holds collimation really well. I think another IIS guy used it on his 12" GSO with great results. getting the mirror off takes good nerves . You need to apply a wooden wedge under one side and keep up a sustained force for a while to make the velcro come apart.

[Aster]

Thanks Mark, thought as much. 250mm 1530mm FL. about
1/20 wave front, strehl .987 I think one of the best mirrors I ever made (by hand too no machine) ,and I made a few :)

[GrahamL]

hi alex:)

With some helpfull advice from mark I used it on my 12" gso without a problem.

My cell is a bit of a mess as I changed my mind a few times while makeing it but the velcro works well
You can see some edge supports there , they don't actually do much , barely touch the mirror
I put them there so i could remove and replace the mirror in the same position ..The velcro seems to have a fairly passive
hold on the mirror in that i can easily lift it a little in the cell ..but at the same time its fairly difficult to seperate it
So much so I clean my mirror in the box these days.

In shear or upside down I couldn't see the mirror ever falling out .. .. would a full thickness 6 :1 mirror work in this arrangement mark?.. mine is quite heavy ..and I could imagine a 2" being heavier still.

[PhilW]

Here is my latest approach. It is more suited to a truss than a tube, for reasons that will become apparent.

The back of the mirror simply sits on six teflon sliders spaced at 58% (those grey buttons). The side supports are bearings spaced at 90 degrees, and placed on the lateral centre of gravity of the mirror (those white things to the right of the picture). You can calculate where the lateral centre of gravity is using Houdart's calculator:

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

I also have a restraining post at the upper edge of the mirror with felt inside, which just clears the side of the mirror. This prevents the mirror sliding around the mirror box while in transport.

The obvious limitation of this approach is that you can't invert the scope, else the mirror will fall out, & that would not be good. Since a tube is usually transported horizontally, that rules it out for a sonotube dob. Also, I wouldn't be quite so confident using it for a small mirror (this is a 14"). But it has been working well for me & holds collimation. You can see there's plenty of ventilation around the back. I also have a fan on the rear of the scope (not attached in this photo).

Phil