When i connected the telescope and camera today with my computer, the TCF Focuser temperature readout showed the temperature of the telescope to be 18 C. I thought let's experiment on the temperature and defocusing simple relation.

I focused the telescope on a readout at 4,000, which is the position on the TCF auto focuser. Kept on imaging for a while and when the temperature went 2 degrees below at 17 C. I refocused again and got a number of 4,499.. so the focus has been shifted around 450 values.. this is a huge difference!

The following picture will illustrate it better.. Notice the pointed star in both images. The left shows the FWHM value of 3.45 at the temperature of 17 C, this is rather a blurry star now but it was at tight focus when i focused it at 19 C.

On the right is the same star at FWHM of 1.88 at 17 C but after refocusing.. this is much much better.

So with 2 degrees of temperature drop, the FWHM of the star increased from 1.88 to 3.45! This translates as a very accurately focused start to a poorly focused star. What i want to show is we need to constantly keep an eye on focusing. That is why the high end focusers come with the temperature compensation option.. I always keep my system at the temp compensation mode. This keeps on changing the focuser position by noticing the change in the temperature. TCF can detect a a change of a tenth of degree!

Oh and btw, for those who don't know know about it. FWHM is a widely used number to see how small a star is in any image. The lower number is always better. Web has a lot of sources about it.