Thank you Andrew,Mark & Brian. I believe I can put this down to frosting on the front window of the camera as per this post on Cloudy Nights;
https://www.cloudynights.com/topic/2...on-my-st8300m/
Regarding the heating option Mark, as far as I know there isn't any but I believe SBIG introduced a "window heating strip " as a customer installed upgrade to combat the problem but I can't find any solid info on it.
As for dew forming on the objective Brian, there was just the beginnings of it but everything else was pretty much soaking wet ( I should have known better
) I think the most likely cause was adding a spacer to the FR which let some moist air into the image train
, I won't be doing that again. I'll change the dessicant & keep the image train sealed whilst outside in future. I appreciate your input guys, you've helped me solve this.
Cheers,
Tony