Re: APO cooling down

I'm not sure there is a "formula" to get a number. Typically, cool-down depends on things like size of the objective, thickness of objective, how it is mounted (insulation from atmosphere)... etc etc.

I'd assume that a 5" triplet apo, air spaced in a massive objective cell would cool slower than a 5" doublet in a cheap/small cell with little or no insulation from the atmosphere.

Maybe one of the refractor gurus will step in. FWIW, a SCT with enclosed back and enclosed front will cool MUCH slower than a comparable sized newt.

I have a Takahashi TOA-130, which is a five-inch air-spaced triplet APO refractor. That scope has a lot of glass in the front end of the tube, and definitely requires some cool-down time before critical use. I've found 30 to 45 minutes to be adequate. For instance, at the monthly Denver Astronomical Society open houses, I start setting up the mount & tripod around sunset, and put the Tak on the mount. By the time I install the finder, diagonal and adapters, get out my eyepieces and accessories, plug in the controller & battery, do a rough polar alignment, and visit with other DAS members who are also setting up, 30 minutes or so have passed and the Tak is ready to go.

I also have a Tele Vue TV85 doublet APO refractor. Cool-down time on this is a matter of a few minutes in most cases.

Ron