Cooling Fan?

[Chrissyo (Chris)]

Hey guys,

As you may know, I have recently been exploring telescope sites (eg/ Andrews and Bintel) and I have found that some 10" and 12" dobs have a cooling fan.

Basicly, I just wanted to know what it does. I can't think of a reason why if you were outside during the cold night that the mirror would overheat?

Just wanted to know what it does.

[Striker (Tony)]

Chrissyo,


Its more to do with the other end...you get peak performance from your scope once your mirror has cooled down to the ambient temperture outside.....if you have stored your scope in the car for EG.... it will take hours before the mirror has cooled.......so this is when the cooling fan helps...the bigger the mirror the longer it takes to cool down..........

[ballaratdragons (Ken)]

Chrissyo,

I can take my 12" scope from a warm house to freezing temperature outside and instead of waiting an hour or so for the mirror to cool the fan does it in 5 minutes.

The reason we worry about the mirror temp is because the image gets distorted as the glass slowly changes shape as it cools down. It is a very, very small amount of movement but it is very noticable. The quicker we can cool it to the outside temperature the quicker the image stabilises.

(and it stops the mirror over-heating from Amatuer Astronomers looking around the sky too fast) lol

[Chrissyo (Chris)]

Okay, that makes sence.

Thanks guys

[astro_south (Andrew)]

In a bit more detail.....

When the mirror is significantly hotter than the ambient air temperature there is a steep thermal gradient formed and a heat transfer takes place from the mirror to the surrounding air. A large gradient can be disasterous causing all sorts of convective turbulences to the boundary layer of air that sits above the mirror surface. The more turbulent this is, the less performance you get out of your scope. Even at mid powers stars are boiling blobs of light instead of the normal pinpoints due to the light scatter (interference) caused by the boundary layer. What Ken said is also true - the figure of the mirror can be distorted due to temperature, but it's contribution to poor performance is not as great as the boundary layer above the mirror surface.

Time will heal this eventually (get your scope out as early as possible), unless the gradient is so steep that it just can't keep up with the ever changing ambient temp. Keeping the mirror above ambient will also keep the dew away to some extent, and ideally you would want your mirror to be no more than a degree or two warmer than the surrounding air.

My dob has fans that blow across the mirror, although I find most of the time I don't need them. Their operation is currently pretty temporary, but once I make the connections more permenant I will probably use it more. I plan on testing this out more (observing with fans on, etc) in the coming weeks and if I find anything of value I will start a thread on it.

There are heaps of discussions going on around the web in regards to where to place the fans and whether to have them sucking or blowing and I think the jury are still out on those debates (do what ever works for you!) as there is not enough evidence either way. There have been some articles in S&T in recent years regarding mirror cooling (I think by Bryan Greer? sp?) and thermodymanics in a close tube situation that are good reading.

[ballaratdragons (Ken)]

Andrew,

I have found that leaving the fan on while viewing gives a 'boiling' image from air turbulants in the tube.

I found this out by accident. I had a boiling image for about 15 minutes, and thinking the mirror was still giving off heat I went to turn the fan on, only to find it was already on.

I turned the fan off and the image cleared in under 1 minute.

We live and learn.

[MiG]

Ken, which way does your fan blow? It is under the mirror isn't it?

[astro_south (Andrew)]

Ken

The 'boiling' wasn't from vibrations from the fan?

The other thing is if the fan is behind the mirror and blows on the back of the mirror (ie up the tube) it is probably causing additional heat to travel up the scope and is also causing more turbulence on the mirror surface as the air comes around from all sides, not to mention the additional heated air in the light path. A fan blowing across the face would produce a laminar (ie not turbulent) boundary layer that is much more consistent to pass light through. In the case of your tube you would need to put some "exhaust" holes on the opposite side of the tube to remove the heated air (at least that is what was done in the S&T article from memory) - not that I am suggesting you do that now (if it ain't broke......). My truss dob is open to the air and the fans blow upwards so it is easy for the heated air to stay out of the light path and escape the mirror box. I haven't tested the fans on while observing though - I might be gazumped by the vibrations.

Here we are talking about heat with our "foot" scopes - what about those "poor" buggers with the 18+ chunks of glass