Re: Rule of Thumb for refocusing an AP160 as a function of temperature change?

Roland Christen

The focal length of the 160 refractor in its aluminum tube changes by approximately 0.5mm for every 10 degrees temperature change. The resultant defocus at F7.9 will be about 63 microns, which is approximately 10 arc seconds. So, if you want to keep the defocus at less than 1 arc sec, you would need to refocus every 1 degree C.

The reason that Apo refractors change focus is due to the difference in thermal expansion of the ED glass versus the mating crown elements. FPL53 has twice the expansion of the mating crown, and unfortunately these ED glasses are needed to produce the color-free images that you need for high resolution imaging.

Using Fluorite in the place of ED glass makes the situation worse, since fluorite has close to 3 times the expansion of the mating crown elements. This is somewhat compensated by the shrinking of the aluminum tube, which cuts the focus shift by about 40% of what it would be if the tube was made of carbon fiber.


-----Original Message-----
From: wbelhaven via <wbelhaven@...>
Sent: Fri, Jun 4, 2021 10:26 pm
Subject: [ap-ug] Rule of Thumb for refocusing an AP160 as a function of temperature change?


Anyone have a reasonable rule of thumb for an AP160, of the form, "If the ambient temperature changes by x degrees, you should refocus", and a suggested value of 'x'?

I'm using it at f/7.9 (measured) with the 160FF in case that matters, and a camera that gives a res of 0.61"/px.

The automation software I'm using can be told to run an autofocus after ever change in x degrees. Wondering what to plug in for x. Does 2 deg C smell right?


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