[PDL]

This issue is well known among users of Schmit-Cassegrain telescope users. When you are setting up your scope it is part of the initialization process to take the lens cap off of the front element (in my case it is an 8" telescope) and point it skyward for 15-20 minutes at least. The issue is with the large mirror at the base and the air inside the telescope tube and the air need to equalize with the external air temperature.

The hot/warm air inside the tube will cause the image to blur. If you are going to jump outside and start looking through the telescope, expect the focus to change. It takes about 30 minutes to set my telescope up (set up tripod, get scope out of trunk - yeah trunk, orient the scope and telescope to the proper alignment (True North and Latitude) and set up the power to run the clock in the base of the telescope). Once you get the latitude set in, you pop the front lens cap off and run around doing other things for 20 minutes. Not a big deal, process is everything.

[rogerstg]

I've run into this shooting eagles in ~20f temps, or stepping out from the house in winter to take a shot of an interesting bird. I thought my 50-500 sigma was fickle and/or my technique flawed, because the lens often performed very well at other times.

No hood, 15 degrees F and breezy. Stepped out onto the deck in shirtsleeves.
K3iii and Sigma 50-500mm APO DG OS HSM, cropped to 33%
1/2000, ƒ/6.3, ISO 2500, @ 500 mm, handheld
Decent resolution.