Here is something weird I discovered a few days ago. I set up the 8SE with the dew shield on because the dew point was high enough to worry about. During alignment I noticed the collimation was off with the donuts bent a bit to one side. This seemed odd because I had just set this new scope up a few weeks ago and meticulously adjusted it until I thought it was perfect.
Thinking I might have to tweak it again I removed the dew shield and magically it was back to fine. This is the stock Celestron velcro lens shade and I installed it lined up as evenly as I could with the front ring on the scope. I also discovered that tipping the front of the shade up a bit got rid of most of the distortion.
Maybe the shade is just a bit out of round and intruding on the light path? The only thing I could think to try, other than adjusting it until it looks better, is to re-fasten the velcro so that the "front" end is bit wider to cut down on the chances of it interfering.
Is this a common trick to do so that the shade doesn't wreak havoc on the image? I don't think I will need it today since the T/D is more than 15°F, but I might experiment if I have time.