Focuser moves in as temperature drops?


I am imaging using SGP with an Edge 9.25 and a Feather Touch/Micro Touch autofocuser. I see that the focus moves in as the temperature drops. This seems backwards to me. If the telescope is shrinking as it gets colder, wouldn’t the camera need to be moved out by an equal distance to compensate, not in?



Sometimes when the temperature compensation wizard is used, you end up with a negative TC. If that happens, it needs to be manually corrected to be a positive TC so that the focuser moves out with dropping temperature and a shrinking OTA.



Hi Charlie,
It does this without using the wizard. I refocus after about 1C temperature change, and the focuser has to move in, not out, with dropping temperatures. This appears to defy physics, and I am curious why it does this. :smile:


Are you using the Celestron focal reducer with the EdgeHD? If so, there may be some strange issue with the reducer optics that change the normal colder -> out relationship between the OTA and the cassegrainian optics.



You may have done this already, but one thing to verify is that IN is actually moving the mirror up / closer to secondary. When I got my micro touch, the behavior was opposite and easily corrected via a setting in the MT.


My refractors do this too. We all think of the aluminium tube shrinking when in fact, the focal length of the glass lens changes more!


Hi Charlie,

No, no focal reducer.



Hi buzz,

I never thought of that possibility. That could explain it.



I’ve always been baffled by this too and eventually accepted it as “the way it is”. I’ve heard Buzz’s explanation before…seems that could be verified as the overall focal length would be changing too. So capture the sampling when warm and when cold and see if it’s different? Might not be large enough to measure…



When Roland Christen (founder of Astro-Physics, Inc) was asked why AP did not offer carbon fiber tubes on their APOs, Roland said that a three element APO lens experiences a focal length decrease as temperature drops. The rate of focal length shrinkage of the lens is very close to the shrinkage experienced by the aluminum tubes they use on their scopes. As a result, there is only a small focal plane shift with dropping temps with their scopes. I do not remember which one shrinks more. When carbon fiber is used as the OTA material, it does not shrink and, as a result, there is more focal plane shift with carbon fiber tubes. So, no carbon fiber at AP.



Very interesting. That is likely the best explanation then. Thank you!



That is indeed the case. I have an FSQ and a TOA 130 as well as a Stellarvue Raptor. The Raptor has the most focus shift of the three and it is the carbon tube. Carbon is great for mirror scopes, not so much for refractors. When I talked to Vic at Stellarvue he also told me that the same sized carbon fiber scope takes much longer for the stellar profile to stabilize on a given night because they cool so slowly.

Bottom line, great for refractor weight, look pretty cool, not all that great otherwise.