I see you have found the same as I did......and I suggest stripping away the black anodizing on any sunshade you have on the scope for a start. I did that and got a much more subtle shift in temp when exposed to the sun. I left the main body of the scope black to "normalise" when sitting in ambient temperatures.
The sunshade tube seems to act as a big heat generator holding and transferring heat back into the scope body, where it absorbs into the largest mass of the whole assembly......the objective lens. Glass (irrespective of quality) expands under heating, and it is common sense that its dimensions will alter minutely, which may explain small shifts in ranging performance in larger diamater objective lenses. "may explain" is the operative term as I just cannot quantify it. I have found the same results as you, and have calmed and soothed my scope into a modicom of predictable performance using cultural methods rather than radical surgery.
As for a cheap heat shield, try cutting up an ironing board cloth cover,......the silver ones that reflect heat back up off the board into the fabric. Really cheap, and has a foam barrier between the cloth and the scope. Can be kept on in direct sun, or removed in different conditions. Buy a new one tho, don't pinch the missus' one.
I think the main way to deal with this problem is to test in conditions that you expect will prevail at a given competition shoot, and set up a variable range pointer on your sidewheel accordingly ahead of time, prioritising the longer ranges of course.
BTW I am not posting on the SA forum any longer, too many T**ts being negative fir the hell of it......sad as it used to be a great forum. Just waiting for the same said T**t to do the same here.
cheers, glad to see our little side topic bearing fruit. I still reckon there is some light/dark effect on ranging now I have sorted the heat issue.....we get much more UV radiation down here due to the hole in the ozone layer,....so am experimenting with some old camera UV filters now. Getting a marked increase in contrast detail.
Light n dark sorted
just gotta sort the damn loose nut behind the trigger!
Last edited by greyskullnz; 17th June 2010 at 07:45 AM.