Re: Cooldown time?

"Roger Hamlett" <[EMAIL PROTECTED]> kirjoitti viestissä
news:[EMAIL PROTECTED]
>
> "PJ Savolainen" <[EMAIL PROTECTED]> wrote in message
> news:[EMAIL PROTECTED]
> > Hi all
> >
> > Any knowledge of which type/brand of telescopes have shortest cooldown
> time?
> >
> > Regards
> > Pena
> Smaller is allways faster.
> The order of times, for common types, runs something like:
>
> Refractor (least for a given size, _but_ depends on the actual design -
not
> 'zero' though)
>
> Newtonian (depends on the tube design)
>
> SCT
>
> MCT
>
> The last two are 'worst', because of their relatively sealed tubes,
combined
> with the number of times the light passes up/down the tube, and thermal
> distortions of a mirror have a larger effect than the same distortions on
a
> lens. The Mak, generally has more weight of glass, and this takes longer
to
> adjust. However well designed fan cooling, can massively reduce the times
> involved.
> Any form of 'short tube' long focal length design (which SCT's, and MCT's
> are), will involve some form of focal length multiplication - in these
> scopes, the secondary, acts like a Barlow to increase the effective focal
> length. On scopes like this, any 'faults' before the magnification stage,
> will be increased in severity. This includes thermal distortions, and tube
> currents in the scope...
> The 'fastest' cooldown, will be a short focal length refractor.
> Small scopes (perhaps 4" and below), will normally nearly acclimatise, in
> the time needed to set up.
>
> Best Wishes
>
>


Thanks for all for Your answers...
I've learned that some SCT's have up to 2 hours cooldown - way too long for
my use.

I'm now in the market for  8" or 10" Newtonian and would love it to have
goto-system.
How does the GoTo work in cold temps, like -25c which is quite common where
i live?

Pena