Re: [ATM] Gluing this mirror

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

Mark,

His 305mm/12.008" diameter mirror at 38mm/1.89" thick is almost a 1:6 
thickness ratio (12.008 at 1:6 is 2.0013" thick).  Since Zerodur is stiffer 
than Pyrex and he is essentially at a "full thickness" piece of glass 
already, the mirror itself should be very stiff on its own.  That's not 
saying anything about astigmatism or other factors that can affect the 
figure of an optic but a cell can't correct for inherent glass-related 
issues anyway.  Unless he is using active supports on this to control the 
figure, which is not the stated case.

What type of environment is he/she using this optic in: a lab, outdoors 
(many of us on this list) or extreme temperature conditions.  You state ATM 
so I'll assume they are using this outside.  By "extreme" temperatures I 
mean locations where temperature drops are fairly large, greater than say 
20F in four hours, and/or where summer versus winter temperature 
differences are fairly large.  There is one location in Canada where it 
swings roughly 160F (-65F in November to +95F in August).  The reason for 
this question is that if the CTE of the mirror support/cell material used 
does not match Zerodur's essentially zero CTE, then the cell will more than 
likely distort the mirror's figure, irregardless of the orientation of the 
mirror.  The greater the temp swing and the greater the difference between 
the CTE of the cell's material and Zerodur's, the greater the (potential) 
problem.

If this person is currently in winter, then they will know immediately 
whether the factors listed in the previous paragraph cause issues.  They 
would more than likely bond this mirror to its support indoors at roughly 
68F.  More than likely both the support and the Zerodur would be at room 
temperature.  If it is 30-40F outside, then they will already be exposing 
it to the kind of temperature extremes I mentioned above.

If the support/cell is already completed, it would not be too much effort 
and time to go ahead with mounting and testing.  They can always remove 
them from each other.  As Dan F has pointed out numerous times on this 
list, surface prep is very important.

Because of the thickness of their mirror and material (Zerodur), I would 
suggest CTE differences between the support and the glass will play a 
(potentially) larger role in whether the figure is distorted than 
gravity/torque while pointing horizontally.

As with any optical system, it is nearly impossible to make blanket 
statements about what would be the largest factor.  There are way too many 
variables to consider to make such blanket statements with limited 
information like this.

I hope this helps.  Thank you.

Sincerely,
Shane Santi - Owner
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833




At 07:54 AM 12/10/2005, you wrote:
>Off-list, I have an inquiry from an atm who is considering a 6 point
>Plop optimized cell for this mirror:
>
>primary mirror diameter 305mm, thickness 48mm (Zerodur), focal length
>1136mm (f/3.72)
>
>He wants to silicone the mirror to the cell and is a little worried over
>the possibility of deformation due to torques when the mirror is off
>vertical.  He anticipates doing a fair amount of viewing near the horizon.
>
>Would those of you that have done similar cells care to hazard a guess
>whether this one will have excessive deformation.  I do not know what
>structure the plans.  Obviously, to minimize torques, it needs to be
>kept as short as possible.
>
>--
>Mark Holm
>mdholm@telerama.com

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