[Gargoyle_Steve (Steve)]

The "heat shimmer" that you're referring to (often really obvious when looking at the moon) is possibly due to atmospheric conditions - especially if you are looking at an object lower in the sky (ie looking through more atmosphere to see it) or looking across a neighbours roof, etc, where heat from the sun that was soaked up all day is still bleeding up into the air.

A warm mirror does give distorted views, but seeing as you're in Brisbane and we haven't had any nights that you'd call cool at all in recent times I suspect it's more the atmospheric shimmering that you noted.

Mirror temp is all about how warm the mirror is relative to outside air temp - the guys down south have more trouble because sometimes in winter the ambient temp drops faster than the mirror can cool - ie the difference between mirror temp and air temp actually increases, even though the mirror is cooling.

The reason the battery holder has 8 batteries is that the fan is a 12v unit - 8 batteries at 1.5 volts each gives the requird 12 volts. As was mentioned above many of us have bought either 12v rechargable batteries, or one of those 12v jump start units from a car accessory place. (The single battery is cheaper and more compact by the way.)

One reason that you may not be seeing much difference between viewing at the beginning and end of the night is possibly the collimation (read: optical fine tuning) of the scope. A reflector (be it dob mounted or other) needs to be well collimated to get really good results, it could be that the difference visually between a warm mirror and one at or near ambient temp is being lost due to the collimation not being as good as such a great scope deserves!

For what it's worth I took the standard fan off my 12" dob and replaced it with a up-specced computer fan - larger diameter, more aggressive blade design, blows more air to cool quicker - and I wouldn't be without it.

Cheers!