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Re: [APML] FL change with temperature
Don,
The tube tends to shrink with decreasing temperature. The optics may also change.
You want to have a focus position readout. This can be a dial micrometer attached to the focuser using custom parts. Or, you could install a robofocus. You also need a method to identify best focus, for example, a knife edge focuser. Then you measure and record (change of) best focus position for a variety of temperatures. You can graph this and fit a curve to it.
Mike Cook did this for his A-P 130 f6:
http://af9y.com/focus1.htm
Matt
At 11:29 18-03-03 -0800, westergren wrote:
>Hi Optics designers,
>
>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.
>
>Thanks in advance,
>Don
--
Matt BenDaniel
matt@starmatt.com
http://starmatt.com
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