22" Obsession UC

They would be my 2 biggest worries
and knowing me I'd drop an eyepiece on it
before first light.

Seriously though, you can negate some of the
worries of dew with a bespoke heater
and the optional shroud would also help.

Weight of mirror would be a signifciant issue - weight is approx 30kg. The fact that you would be lifting it from such a low level means you could easily hurt your back. With the 16" scope I built, the mirror weighed less than half that - and it was plenty I can assure you.

Barberskum wrote:

you can negate some of the
worries of dew with a bespoke heater
and the optional shroud would also help.

I find that I cant really use the scope when i'm heating the mirror as it causes terrible distortion, when I do use it it is usually for about 10-15 mins at 3am, for me it isnt somehting I can use all through the night.

If your primary is dewing up on occasion Dave, probably a better solution which kills two birds with one stone, is to implement a Boundary Layer Fan solution.

A mirror doesn't deliver its best views as you know till its cooled down to ambient. This is because when its above ambient and still radiating heat there is in effect a puddle of warm air sitting on the surface (the boundary layer). This has a different refractive index than the rest of the air in the lightpath and thus it is in effect 'bad seeing' inside the scope.

Without active cooling a big 1.5-2" thick piece of glass is a massive heatsink and will take hours to cool down. It is only when it cools down to ambient that the boundary layer air dissapates by itself. With active cooling in the form of a large fan blowing air on the back of the primary, one can shorten the time that the mirror takes to reach ambient when the boundary layer dissapates. With a mirror that large and even with a cooling fan, you are still talking over an hour at least before the mirror reaches ambient and the boundary layer dissapates.

The purpose of Boundary layer fans is to actively scrub off the boundary layer air from the get go. No need to wait 3 hours for the fanless mirror to reach ambient and the Boundary layer to dissapate on its own. No need to wait at least an hour for the rear fan equipped mirror to cool to ambient when the boundary layer dissapates on its own. Blow that boundary layer off as soon as you set up the scope whereupon the mirror can start delivering its best views from the off despite the fact that it is only starting to cool down. So you are getting an extra 2-3 hours of great views from the very beginning of your observing session. If you are still there after 3 hours, well then the mirror actually will have reached ambient so that a boundary layer now wont form when you switch off the boundary fans to conserve battery power.

Of course, one is still subject to atmospheric seeing and there is nothing we can do about that, but I would prefer to have planetary viewing ruined by bad atmospheric seeing but knowing my scope was not in any way responsible, than to have great atmospheric seeing that may be fleeting that I can't take advantage off for 3 hours because my 'scope seeing' is lousy because of boundary layer issues.

Remember, all this scope cooling to ambient malarkey has very very little to do with temperature differentials of a cooling mirror glass substrate affecting the wavefront. The effect on views is barely measurable and certainly not noticable in the eyepiece. The reason cooling to ambient is so important is waiting for the boundary layer to clear. Why not speed up the process by adding a cooling fan or totally eliminate the problem straight away with a boundary layer fan solution.

Oh yeah, back on topic The air blowing across the mirror surface from the boundary layer fans also prevents dew from forming on the primary. Two birds with one stone!

Hi,
There is already a fan behind the mirror and I find that the scope is completely unusable when it's on, in this design air can flow between the back and front of the mirror and it wrecks the star images.
I keep the mirror in an environment that's not heated, so it's not "hot" after setup, it isn't like Arizona where there might be 20 degrees of a swing between daytime and nighttime.
I estimate that the mirror reaches nighttime ambient within an hour or so, the mirror box acts like a dew shield and like any dew shield will work only for so long.
"Heating" it at 3 in the morning is a "last chance salon" solution, once its turned off the views are not affected as I'm not really heating the mirror, just raising it above the dew point.

Maybe if I win the lotto I'd consider it.
Dave has talked sense into me.
I'm now keeping an eye out for a decent sized regular dob.
More bang for the buck.

Although the smaller UC might be a nice
travelling companion.....

:silly: