I went out last night to shoot the Orion nebula, using a bahtinov mask (sharpstar 2) to get critical focus. When I went out an hour later to check on it, all of the photos had gotten out of focus. It was around freezing, so I'm guessing that this happened because of the temperature change. When I went to refocus, my lens (Sony 200-600 G) at max infinite focus couldn't quite get the cross hairs caused by the bahtinov mask to line up, but it was close enough to get good photos.
I have a few questions. First did the focus shift because of the temperature change (house was ~70 degrees F, outside was ~30 degrees F) or did something else possibly cause that.
Second, if the focus shift was caused by a temperature change, would a dew heater strap help keep focus? I'm in Colorado, and its pretty dry here, so I normally don't have to deal with dew too much.
Lastly, if the lens couldn't quite focus to infinity after the focus shifted, is there a way to fix that? I know macro tubes allow lens to focus closer to the subject than the lens normally can, but there doesn't seem to be a way to make it focus past infinity more without some extra optics.
EDIT - This was on an A7iii, not a dedicated astro cam, so I don't think an electronic auto focuser would be an option. the standard helical focuser did not slip (it was taped in place)
Edited by mtn_nut, 02 March 2021 - 01:22 PM.