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Old 01-04-2015, 08:16 AM
jjz (Joe)
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Cooling a Dobsonian Q's

Sorry if this has been mentioned before, did a search couldn't find anything.

Under what conditions do I need to cool the mirror on my 10 inch Dob?

ie If theres say > 5 degree difference between the location the scope was stored and the viewing location?

How long do I need the run the fans for?

How can you Identify that the scope needs to be cooled? Is there anything you can see through the eye piece.


TIA for your help

Jjz
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  #2  
Old 01-04-2015, 09:21 AM
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mental4astro (Alexander)
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Hi Joe,

The mirror doesn't need fan cooling. What needs disruption is the air inside a solid tube instrument. It traps warm air that is inside the tube and the residual heat in the mirror, which can create convection currents (thermal currents) that can disrupt the image quality.

But I just said the mirror doesn't need cooling, and in the next sentence I say the mirror is warm!?!?

No, I'm not crazy!

You cannot use a fan on glass and expect it to cool uniformly. It will only cause a cold spot right where the fan is blowing, but the rest of the mirror is still warm. This creates strain within the mirror as it shrinks where it is cold, distorting the mirror surface.

Remember, glass is a poor conductor of heat, and borosilicate glass types (like Pyrex) even more so. You cannot force a mirror to cool faster than it will allow heat to dissipate from itself!

So why do scope manufacturers put fans on scopes? Two reasons 1: Misunderstanding of the thermal dynamics, 2: Marketing - people see a fan and they get all excited and think that 'this must be a good scope - it's got a fan!'.

These rear mounted fans will also not cool the inside of a tube. They can't - air will not go around a mirror if the cell holding the mirror is open in the first place!!! This is another misconception. You cannot force air to go around something if there is another place of least resistance for it to go - straight back out through the mirror cell!

If you look at professional observatories, they do use fans. But look closely at how they are used. Not one single fan is blowing onto the primary mirrors directly!!!! The fans are used to blow across the face of the mirror to blow away any dew that can form on it. But wouldn't the mirror get warm during the day when not in use? No, because these observatories are giant cool rooms, air conditioned to the temperature the evening is expected to drop to, so these huge mirrors do not get warm, ever.

An even worse mistake is if your scope is an open tube/truss type. There is no closed tube system then to trap warm air in the first place. The heat coming off the primary mirror is disrupted straight away, venting straight through the truss system.

Letting the mirror cool by itself not only does not create internal stresses and so figure distortion, but the mirror cools evenly. Any, any change in the shape and so tiny change in the focal length, this is negated as we constantly change eyepieces and so need to shift the focuser. Plus, the change in the focal length is so minute, our eyes won't see the difference. The only instance when it can make a difference is with imaging as this tiny, tiny shift in focus can be picked up. Also, keeping a fan on a mirror during the night will maintained the stress on the mirror as it will have a constant cold spot where the fan is focused, and the rest of the glass stays warmer. This will deform the image too.

Cooling of mirrors is more critical for imaging. But it needs to be a controlled cool. If you have a closed tube scope, reflector, refractor, SCT, Mak, etc, letting your scope cool to the ambient temperature is advantageous as I mentioned above if you are viewing under high magnification. If your scope is an open tube/truss one, a cooling period is not necessary.

If you would like your scope to cool prior to using it, this will take anything from 1/2 hour to a couple of hours, all depending on the temperature differential between the ambient and that of the mirror, and the size of the mirror - small mirrors will lose heat faster than large ones. A cooling period is also good for closed tube scopes. Just remember, you can still use your scope, open tube or closed, from the moment you've finished setting up!

It took me a while to rationalize the misuse of fans on scopes. Particularly now that I also build scopes, how glass behaves thermally has become more significant in my mind in order to best design the instruments I build.

Cheers,

Mental.
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Old 02-04-2015, 12:07 PM
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ZeroID (Brent)
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MMmm, interesting. Looks like I will be taking the wee muffin fan off the back of the Serrurier and saving a bit of weight.
What about dew control ? I have an even smaller fan directed at the secondary from one side. Not sure how effective it is but as it's spread would be far greater than the secondary area I would expect very little possibility of uneven thermal movement.
Yeah, I know we're splitting hairs here but it's an interesting discussion. I think I'll be removing both fans now anyway.
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Old 02-04-2015, 10:10 PM
sharpiel
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Quote:
Originally Posted by ZeroID View Post
MMmm, interesting. Looks like I will be taking the wee muffin fan off the back of the Serrurier and saving a bit of weight.
What about dew control ? I have an even smaller fan directed at the secondary from one side. Not sure how effective it is but as it's spread would be far greater than the secondary area I would expect very little possibility of uneven thermal movement.
Yeah, I know we're splitting hairs here but it's an interesting discussion. I think I'll be removing both fans now anyway.

Having fans blowing ACROSS the front face of the primary mirror removes the image disruption of a thermal layer/boundary forming in the air column directly above your mirror (forms due to the temperature differential between the warm mirror and cooler ambient air temperature and causes heat shimmers in the optical path). Whilst this fan(s) will have minimal cooling benefit it will definitely improve your seeing and optical performance. You should accordingly move your primary cooling fan just forward and perpendicular to the primary mirror.

There may be a similar optical improvement regarding your secondary mirror fan, depending on the direction of airflow in relation to the secondary light cone. If it's not introducing either vibration or unequal thermal equalising of your secondary or adding undesirable weight to the top end of your OTA it's probably not hurting you.
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Old 03-04-2015, 08:20 AM
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mental4astro (Alexander)
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Yep, I agree with Les.

There are three ways to prevent dew formation, heat, air flow and shelter.

Solid tube Newt's have their primary mirror sheltered down the end of a long tube. Those long 'light shrouds' that are often seen wrapped around open tube Newt's do the same thing, but they introduce other complications as they get wet. I use what I call a 'cuff' that comes up only about half way up the open tube from the mirror box. Since using this on my 17.5" I have never had dew problems plague it. My 12" the same, and save for impossible nights when even fog forms it has also not had dew problems.

Air flow is the way profession observatories stop dew formation on their mirrors. Vibration from the fans is the biggest hurdle here. Primary and secondary mirrors alike. Once the primary is at thermal equilibrium with the surrounds, this 'boundary layer' does not exist that can form from a warm primary. The fans then help prevent dew formation.

Heat can be used on scopes, but it needs to be carefully employed. Secondary mirrors benefit from gentle heating as it eliminates potential problems from fan vibration. But heating a secondary is more complicated than just sticking a heater onto it. The secondary mirror holder plays a vital part in this. The wrong holder and it will render any heating efforts futile. Heating of the secondary is very effective as it is small and the heating system can be designed to quite uniformly and gently heat it.
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Old 03-04-2015, 09:09 AM
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Alex, you should pop that extremely informative post - which is more like an article actually - into our "Projects & Articles" section. It's really good .

If you're interested, email Mike with the pdf.
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Old 03-04-2015, 12:35 PM
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Alex, you should pop that extremely informative post - which is more like an article actually - into our "Projects & Articles" section. It's really good .

If you're interested, email Mike with the pdf.
I agree Suzy, I certainly learnt something from that. It seems the fans on most scopes are not a lot of use
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Old 03-04-2015, 01:20 PM
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mental4astro (Alexander)
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Thanks Suzy for the suggestion and Ian for the encouragement. I've written an article and made the submission to Mike.

Let's see what happens,
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Old 03-04-2015, 06:38 PM
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ZeroID (Brent)
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Thanks for the tips Alex, very useful to know. My Serrurier is all truss, very open with just a black card tube providing a light shield round the primary and open top and bottom. Seems to survive dewing pretty good. EPs and the secondary have been only minor problems so far.
We'll see if I need it when I move onto imaging through the scope.
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Old 04-04-2015, 03:11 AM
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Alex, that's great news and well deserving of it being there in my opinion. I hope it does make it in. There's certainly a lot to learn from it that's for sure.
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Old 04-04-2015, 03:16 AM
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Originally Posted by gaa_ian View Post
I agree Suzy, I certainly learnt something from that. It seems the fans on most scopes are not a lot of use
Hi Ian, yes who knew .

And from my experience so far and from what I've been told, the primary mirror takes a lot more to dew up than the secondary anyway so I've never needed to bother with dew control on the primary either with heaters or a fan.
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Old 08-04-2015, 10:43 AM
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Hi Ian, yes who knew .

And from my experience so far and from what I've been told, the primary mirror takes a lot more to dew up than the secondary anyway so I've never needed to bother with dew control on the primary either with heaters or a fan.
My experience has been to use a heat band on the corrector plate of a SCT (almost essential) and for a Dob to use a 12V hair dryer if dew on the EP or secondary is an issue.
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Old 10-04-2015, 03:14 PM
PSALM19.1 (Shaun)
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No need for cooling fan with a Dob...no way! Thanks Alex!
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