Re: ATM thermal equation

["Richard" <richas@earthlink.net>]

The number you need is called the thermal coefficient of expansion, k.   Its
value depends on the material.  According to
http://megspace.com/science/sfe/opt_fem_data.html, plate glass had a k of
8.3E-6/degC, pyrex has a k of 3.25E-6/degC. For other materials, you will
have to search in common engineering handbooks.

The focal length change df is given by

df = k * f * dt

f   is the focal length
dt   is the temperature change

When you work this out, you must subtract the change in length of the
telescope tube.

Note that the glass thickness, the focal length, the mirror diameter, the
radius, and all other dimensions of the mirror change in exactly the same
proportion.

----- Original Message -----
From: "Eldereis de Paula" <eldereis@dfm.ffclrp.usp.br>
To: <atm@shore.net>
Sent: Friday, July 13, 2001 4:22 AM
Subject: ATM thermal equation


>
> Is there an equation that gives me the focal
> lenght change as a function of  temperature?
> Say, how much the focal lenght will increase (or decrease)
> if the mirror temperature rises 1 degree?
>
> Thanks you all,
> Eldereis
>
>
>