Confused - Focuser Temperature Calibration?

[Jeff]

Hi Folks,

I recently purchased a SharpSky Pro motorised focusing kit, and am quite impressed with its operation. Manual focus operation is still very simple using the setup, and autofocus works a treat from TheSkyX using @Focus2 or @Focus3.

But going through calibration for automated temperature compensation, something has me a bit stumped.

As my scope (Esprit 120 triplet) cools, the focuser need to move INWARDS slightly to maintain focus ... by approx 50 steps per degree of temperature drop.

I expected the focuser would need to move OUTWARDS to maintain focus as the OTA cooled/contracted.

Has anybody else had a similar experience?
Could this be normal for a triplet ... with focal length maybe decreasing slightly as the temp drops due to changed spacing of the elements?

Of course, I may have made a stupid mistake somewhere.

The laws of physics seem to be conspiring against me.
Explanation sought .... so I can sleep.

[Jeff]

I think I may have found the answers below.
https://www.cloudynights.com/topic/1...ted-lens-cell/

[multiweb (Marc)]

For the first imaging sessions disbale the temperature compensation and log the position vs. Temperature. Graph them in excel get your ratio from the area where it's linear then enter it in your config and save the profile. You control the direction with the "reverse" checkbox.

[Jeff]

Thanks Marc.

Done all of that, and it's working well.
I was just surprised at the direction of adjustment.

I was expecting as the OTA cooled and contracted, the refractor focuser would need to auto-adjust outwards slightly to maintain sharp focus at much the same focal length.

As it turns out, my focuser adjusts inwards slightly as the temperature progressively drops by about 15 degrees. The CN thread offers a potential explanation, as the focal length of the optics also tends to shorten a bit as the lens cells shrink slightly when they cool.