Re: [APML] FL change with temperature

[Aplanatic@aol.com]

Hi Don,

> Can you tell me how the focal length of a refractor objective changes with, say 10°F temperature change?  I can account for the expansion of the aluminum tube of the OTA, but I wonder if the FL changes at all, or slightly, or a lot?  In particular, I have an AstroPhysics 180/f9 fl=1620mm.  I would like to calculate the focus adjustment needed to compensate for 
> temp change through the night.

Without knowing the details of the design and the glass used in the objective, it's not possible to give you a very precise answer.  My guess would be about 6-12 ppm/C increase in focal length with increasing temperature.  This offsets the change in length of the aluminum tube, focuser, etc., which has ~24 ppm/C TCE.

Let's say that the *differential* change in focus is 15 ppm/C.  A 130 mm f/6 scope will change focus by ~12 microns per degree C.  Very roughly, you can stand 5-8 degrees C change in temperature before re-focusing.  Refractors are very good in this aspect compared to uncompensated Cassegrains.

-Dave Rowe-
_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo