Obsession's new Ultra-Light Dob

Excuse me if I sound like an idiot but one of our members has a 12" Lightbucket..sorry....bridge and he has real bad problems with dewing and it's only a 12".

I mean something that size in this climate? How on earth could you keep the dew off it? I've enough diffs with the 105!

Thin mirrors (1" thick plate) cool down quite fast and tend to dew up. Thick mirrors (like my 20") take hours to fully cool down and consequently don't dew up until much later in an observing session. It also means that you are potentially not getting the best images until the wee hours. Low-expansion glass (like my 20") minimises this problem, but you still can get an unstable boundary layer of warmer air close to the mirror that needs to be blown off with a fan. Fans behind the mirror also speed up the cool-down time.

Phil.

The thickness of my mirror is 4 inches. I use 3 fans extracting the air from the rear of the cell. There are 2 fans opposite sides of the cell some inches above the primary mirror. These side fans blow the air against each other. This I find overcomes the dewing problem.

I also use heat resisters around edge of the diagonal flat which is 2.5 inches thick. The generous use of black flock felt on metal supports appears to reduce dew and increase contrast.

Eamonn

The thickness of my mirror is 4 inches. I use 3 fans extracting the air from the rear of the cell. There are 2 fans opposite sides of the cell some inches above the primary mirror. These side fans blow the air against each other. This I find overcomes the dewing problem.

I also use heat resisters around edge of the diagonal flat which is 2.5 inches thick. The generous use of black flock felt on metal supports appears to reduce dew and increase contrast.

Eamonn

Oooooo-K... that's quite a serious spec - I guess you won't be humping that lot into the back of the Micra!

P.

The fans I use are extremely light. They are about 4 inches in diameter. weigh about 100 grams each. Balancing the truss should not be a problem. Depending on design box and rear cell layout, installing should be easy.
Vibration should be minimal depending on the solidity of the box chamber.

The Dob is still extremely light when integrating these fans and fixtures.

Eamonn A

Excuse me if I sound like an idiot but one of our members has a 12" Lightbucket..sorry....bridge and he has real bad problems with dewing and it's only a 12".

I mean something that size in this climate? How on earth could you keep the dew off it? I've enough diffs with the 105!

With the 20" I have here, it has never dewed up during use, there is a builtin 12v fan behind the mirror for cooling purposes. Mirrors this size have a massive thermal lag so can take many hours to cool down, all it need be is a few degrees above ambient and it wont dew over, you only get dew when the mirror becomes cold enough for condensation.

I haven't seen any image problems due to temperature imbalance, the scope here was designed to work in Arizona where there can be a huge difference between day and night time, so the fan is fairly effective for removing heat from the mirror, nut I never let it remove too much.

The real problem with a mirror this size is in the morning, you have a cooled mirror and warm air from the sun, this can cause condensation on the mirror.

The secondary mirror comes with a dew heater pack, but I've never had to need to use it.