74 replies to this topic

[speedster]

The internet is a wonderful place to share ideas at the speed of light but sometimes AN idea becomes THE idea when repeated enough and it may help to examine its premise.  Maroubra_boy made an excellent post about preventing SCT dew formation without heat. 

 

https://www.cloudyni...refractors-too/

 

Sundown at Kitt Peak means the mountaintop peace and solitude is broken by the roar of fans blowing air across glass not just to prevent dew but more importantly to improve seeing.

 

“When the optical face of the mirror is even 1-2 ºC (2-3 ºF) or more above the ambient air temperature, observable image degradation will exist.  The wavefront will be deformed as it passes through temperature gradients near the face of the primary mirror.” Bryan Greer, September 2000 Sky & Telescope  describes how this was quantified using the rainbow schlieren test method.  The same applies to corrector plates. 

 

To see the effect of the boundary layer, http://autocostrutto...ng-mirrors.html

 

Greer continues, “Fans have three benefits. Most importantly, they increase the rate of heat loss from the primary mirror. This results in the mirror being able to track the falling nighttime temperature more closely, as well as reducing the initial cool-down time when the scope is first taken outdoors.  Another benefit is possible when fans are directed at the front face of the mirror where they can scramble the optical structure of the boundary layer. The boundary layer is a thin layer of warm air hugging the mirror's entire outer surface. All of the convective heat transfer from the mirror to its surroundings takes place within this layer. The portion of the boundary layer on the front face of the mirror is where the image degradation occurs. Chopping up the structure of the boundary layer has the effect of restoring high resolution to the image.  While fans do reduce the duration of the initial cool-down period, this is not their main purpose. The night air temperature is constantly falling, and the mirror cannot track along with the ambient closely enough without forcing airflow over the mirror. Thus, the fans should be left on throughout the observing session.”  Again, the same thing applies to corrector plates.

 

Why apply heat to a corrector when it works against reaching and maintaining ambient temp, is completely unnecessary for dew prevention, and does nothing to reduce boundary layer optical issues?

 

Dew and frost happen when a surface is below the dew point temp.  The dew point temp is always below ambient and you’ll know when they get close to each other as you’ll be standing in a fog.  Dew is simply excess water vapor condensing on a surface that is above freezing while frost is the same excess water vapor condensing on a surface that is below freezing. But, in all cases, the dew point temp is below ambient.

 

With a fan, you are blowing ambient air at the precise temp you want the corrector to be.  The forced air flow disrupts the boundary layer, increasing seeing while preventing dew.  And, it’s simpler, cheaper, and consumes less power. 

 

Maybe dew strips prevent dew at the expense of all other factors while fans tick all the boxes.

 

Lack of fan vibration, even with dirt cheap fans is discussed in other threads here on CN. I work at long focal lengths and still worry about vibration so I like magnetic levitation fans.  Here’s one solution: standard Celestron C11 dew shield, 40mm 12V mag lev fan $25, plug adapter glued to the side of it $2, and it runs on 5V USB power.  If you want the full airflow at 12V, use a USB step-up cable to convert 5V to 12V, $8.  Of course an AC converter also works.  Since the fan is disrupting the boundary layer, keeping the corrector at ambient, and improving seeing, it runs all the time, even when dew is not a threat.

 

Edited by speedster, 19 February 2021 - 01:03 AM.

[TOMDEY]

I've wondered about that forever --- like we knock ourselves out to (attempt to) thermalize our entire OTA to ambient --- but then HEAT the optics to prevent dew... never made any sense to me. My biggest scope is a 36-incher in a 24-foot dome. What I found works the best is to blow the snot out of the entire interior (shutters open, of course) and also at the telescope... to bring everything to ambient, over a couple of hours (starting before sunset). That gets the wavefront behaving wonderfully... the doors and shutters remain open for observing (of course) and the fans are fine off then. My fans are two giant BOX Fans that blow in through the personnel door... That, plus the chimney effect causes the warmer air to rise up and out through the open shutters. It makes a GIANT difference in the seeing. Goes from rather pathetic to quite spectacular!    Tom

 

~click on~ >>>

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[ayadai]

Thanks for posting this, speedster. This is the route I decided to go a couple of weeks ago, given our climate. I'll post the results of my fan design experiments when they are completed and tested.

[petercoxphoto]

I know this is the wrong scope for this forum, but after reading the linked post I decided to install a large fan on my 8" (200mm) Newtonian. It's a premium 140mm low-speed fan - found here: https://noctua.at/en/nf-a14-pwm

 

 

It's configured to suck air down the tube and expel it from the rear. I bought a sheet of acrylic and cut it to size with three flanges fitting between the collimation screws on the rear cell. I then cut a 140mm circle in the center of the plate which matches the fan's diameter. It's fixed to the rear cell with high-strength 3m double-sided foam tape, and the fan likewise is attached to the acrylic by the same method. 

 

This seals the back of the scope well so that the only source of air for the fan to move is through the tube itself. The fan has a native speed of 800rpm, and comes with an adapter cable that reduces the speed to 650rpm using a simple resistor on the positive wire. As far as I can tell, the fan is totally vibration free and nearly silent. I have it connected to the Pocket Powerbox for power.

 

Sadly the weather has been truly terrible here for the last few weeks, but I look forward to doing some comparison tests at the first opportunity.

 

On a related note, I have a GSO 12" RC on the way, which should arrive next week. Does anyone know if the fans in the back plate blow inwards to the mirror, or out the back of the OTA? Hopefully the former, as I really don't relish the idea of opening everything up to flip them around.

 

Cheers,
Peter

[Old Speckled Hen]

I've wondered about that forever --- like we knock ourselves out to (attempt to) thermalize our entire OTA to ambient --- but then HEAT the optics to prevent dew... never made any sense to me. My biggest scope is a 36-incher in a 24-foot dome. What I found works the best is to blow the snot out of the entire interior (shutters open, of course) and also at the telescope... to bring everything to ambient, over a couple of hours (starting before sunset). That gets the wavefront behaving wonderfully... the doors and shutters remain open for observing (of course) and the fans are fine off then. My fans are two giant BOX Fans that blow in through the personnel door... That, plus the chimney effect causes the warmer air to rise up and out through the open shutters. It makes a GIANT difference in the seeing. Goes from rather pathetic to quite spectacular!    Tom

 

~click on~ >>>

 

 

Now that is what you call STYLE!

 

saluté

[Don W]

Interesting but I think you should be mostly concerned with dew prevention rather than elimination. Once your optics reach the dew point it's very difficult to get rid of it.

[speedster]

This is prevention.  I should have been more clear.  This eliminates dew forming in the first place.  Forced air will also get rid of it if you allow it to form although far better to prevent it in the first place.

[maroubra_boy]

Great to see that people are taking up my idea on how to implement fans on their Maks and SCT's

 

If I may add a few things here:

 

The use of fans is ONLY intended once dew starts forming, not before, for three reasons:

1, You don't want the fans to force cool the corrector as this will induce a thermal strain in the glass thereby distorting its figure.  And if you are insulating your Mak or SCT meaning you can rip high magnification straight away on setting up without a cool down period, you don't want there to be any heat strain on the optics.

2,  Once you notice dew starting to form, the fan is switched on and this will dry off what little dew has formed in a jiffy.  There is no heat in this process so no heat strain is induced on the glass either.  It is not the same situation as when using dew straps to try to dry off a dewed up corrector - that is hard, slow and uneven.

3,  If the night is not dewy, then you don't need to turn the fans on at all.  Not using power unnecessarily is to your advantage as well.

 

Prevention and elimination are shades of grey of the same equation.  It means being in tune with your environment and not doing things "just because", and understanding the why so when you implement these systems you are not introducing other problems.

 

The use of fans on Maks and SCT's is most efficient when combined with insulating of your OTA.  Not necessary to do it, but insulating your OTA means that there is no waiting for a cooling period, so your productivity all of sudden gained anything between 1 to 2 hours.  For me, those two hours IS ALL THE TIME I HAVE at home with my scopes.  Stupid I would be not to insulate my scope knowing there is a way I can actually eliminate this cooling period...

 

This use of a fan is not limited to SCT's & Maks.  Remember it is just as effective with refractors.

 

Petercoxphoto, your photo is not out of place here.  It is the use of fans in my Newts that inspired me to work out a way to do so with my closed tube OTAs.  Like you the fan on my closed tube Newts, closing off the rear of the OTA, drawing air down through the tube and out the back end, NOT blowing onto the primary mirror.  And the use of fans is how things are also done in professional observatories - NO HEAT in these places - and this is ground zero for me in all of this, as it was looking at how the Big Boys do things and how WE as amateurs can apply the same principles.

 

Alex.

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Edited by maroubra_boy, 19 February 2021 - 05:29 PM.

[maroubra_boy]

What IS CRITICAL with using fans in Maks, SCT's and refractors is the fan must be placed in such a way in the dewshield that it generates a vortex of air inside the dewshield.  This ensures even movement of air all around inside the dewshield and does not generate potential cold spots on the corrector.  A cold spot is a thermal strain, and this distorts glass, just like the application of heat does.  So it is important to place the fan 50mm to 75mm in front of the corrector as well, not immediately beside it.

 

Alex.

[speedster]

To break up the boundary layer and any funky wavefront, run the fan all the time.  Otherwise no improvement in seeing.  No glass strain as the air being blown is the same as the air already in the dew shield:  no temp difference.  Vortices will happen whether the fan is aimed straight in or at an angle.  Sweeping air on the dew shield doesn't do much as there is little, if any, temp difference across it.  You could make a case that a low emissivity surface on the outside of the dew shield would be beneficial.  Dew, and seeing issues, are happening at the boundary layer which is only about 1-2 cm thick and not a consistent thickness.  There will always be a boundary layer as there will always be air stuck to the corrector but a fan can reduce the thickness to microns.

 

Insulation is not necessarily our friend.  It's the low emissivity aluminum foil that does the trick.  Consider a few cases:

 

First case:  SCT with no insulation.  Move it from a 72 degree room to a 45 degree backyard.  The temp difference is largest, and seeing is the worst, when it first goes outside.  Let's say the seeing is "level 1 seeing" and it takes sixty minutes to cool within 2 degrees of ambient for the best seeing  and call that "level 10 seeing".  30 minutes in, call that "level 5 seeing".

 

Second case:  exactly the same as the first but the SCT is insulated with bubble wrap (just like Reflectix but without the foil bonded to it).  The tube is about R-2 and the bubble warp, twinwall, signboard, etc., adds about R-1.  The insulation slows the cooling so this tube starts out just like case one but doesn't get to level 5 seeing temp for 45 minutes and doesn't reach level 10 for 90 minutes.  Insulating to slow the conduction just stretches out the time till best seeing.  That's thermodynamics we just can't get around.

 

Third case:  same setup as the second but this time with Reflectix instead of plain bubble wrap.  Same longer cooling time but the low emissivity foil greatly reduces the radiant transfer and evens out the radiant transfer so the tube is more uniformly changing temp.  Still changing slower than the first case but it's doing it more evenly.  That unevenness is a big player and foil is helping reduce it so seeing is better faster even though the time to ambient temp is longer.

 

Fourth case:  best of both worlds but not nearly as convenient, wrap the tube in aluminum foil.  Speediest ambient equilibrium, as in case one, but also much less radiant transfer as in case three.  With so much anecdotal thinking about Reflectix, I can imagine the discussion "just wrap it with a space blanket" would generate.

[luxo II]

FYI the stuff used in space applications is similar to 6 space blankets scrunched up the get them crinkled, then laid out , stacked, and stitched together.

Edited by luxo II, 21 February 2021 - 12:03 AM.

[Rustler46]

What IS CRITICAL with using fans in Maks, SCT's and refractors is the fan must be placed in such a way in the dewshield that it generates a vortex of air inside the dewshield.  This ensures even movement of air all around inside the dewshield and does not generate potential cold spots on the corrector.  A cold spot is a thermal strain, and this distorts glass, just like the application of heat does.  So it is important to place the fan 50mm to 75mm in front of the corrector as well, not immediately beside it.

Thanks Alex, for this most useful information. So the fan should be 50-75mm above the corrector, further up the dew shield? Also to tip the fan to create the vortex, what is shown in the photo in the original post of this thread is not as effective. How much does to need to be tipped? If just 10° then just having some extra spacers on one side of the attachment bolts would do. If something like 45-60° is needed, then what is shown in the photo in #8 reply would do.

 

I've got a 40mm Maglev fan coming on a slow boat from China (just $4). I'm looking forward to trying this on my SCTs. This technique looks to be "game-changing". No dew + No heat + better local seeing - well all that sounds like a winner to me!

 

Best Regards,

Russ

[Rustler46]

Great to see that people are taking up my idea on how to implement fans on their Maks and SCT's

 

If I may add a few things here:

 

The use of fans is ONLY intended once dew starts forming, not before, for three reasons:

1, You don't want the fans to force cool the corrector as this will induce a thermal strain in the glass thereby distorting its figure.  And if you are insulating your Mak or SCT meaning you can rip high magnification straight away on setting up without a cool down period, you don't want there to be any heat strain on the optics.

2,  Once you notice dew starting to form, the fan is switched on and this will dry off what little dew has formed in a jiffy.  There is no heat in this process so no heat strain is induced on the glass either.  It is not the same situation as when using dew straps to try to dry off a dewed up corrector - that is hard, slow and uneven.

3,  If the night is not dewy, then you don't need to turn the fans on at all.  Not using power unnecessarily is to your advantage as well.

 

Prevention and elimination are shades of grey of the same equation.  It means being in tune with your environment and not doing things "just because", and understanding the why so when you implement these systems you are not introducing other problems.

 

The use of fans on Maks and SCT's is most efficient when combined with insulating of your OTA.  Not necessary to do it, but insulating your OTA means that there is no waiting for a cooling period, so your productivity all of sudden gained anything between 1 to 2 hours.  For me, those two hours IS ALL THE TIME I HAVE at home with my scopes.  Stupid I would be not to insulate my scope knowing there is a way I can actually eliminate this cooling period...

 

Alex.

Alex, thanks so much for bringing this dew-prevention technique to our attention. I can't wait for my 40mm Maglev fan to arrive from China. In the meantime I've got plenty of other projects to keep me occupied on a cloudy night. But the prospect of little or no added heat to keep dew away from my SCTs is exciting.

 

With a DewBuster system already installed, it will easy to empirically gauge the effectiveness of this method by applying less and less heat to see if dew sets in. Now it takes keeping the OTA 5-7° above ambient air temperature to prevent formation of dew. Once I set that temperature differential to zero, there is no heat being added. So if I'm not standing in fog, air temperature is above the dew point. So the trick is to prevent the corrector from cooling below the dew point via radiation to the night sky. Blowing air on the corrector will work toward keeping it at ambient air temperature via convection and conduction. 

 

In regard to your statement above, blowing air on the corrector will not necessarily cool it, but will just serve to keep it at the same temperature as the air blowing on it. Now if the corrector is warmer than air temperature, then blowing air on it will cool it down toward ambient air temperature. This heat transfer comes about by temperature differential. When heat is being radiated away to the night sky, the airstream will supply the needed heat to balance that lost, keeping the corrector at air temperature.

 

Now if there is dew on the corrector being dried off by the air stream, there will be cooling by evaporation of the water. But once the dew is gone, no further cooling will take place by that means. At that point the air stream will serve to keep the corrector at the same temperature as the air stream. Of course the above scenario is simplified, since nighttime air temperature is usually falling, and there will be residual heat in the OTA itself. The corrector could well be warmer than the air temperature. In that case cooler air would cool the corrector, but to a temperature no lower than air temperature.

 

In my case living in a very moist marine environment, dew begins to form right from the start on most nights. Often I only notice dew has formed at the end of the observing session when breaking equipment down. Sometimes I do climb a ladder to look down toward the corrector to look for dew. But usually I'm too focused on the observing to take time for that. Typical observing conditions begin relatively clear and fog-free. But as the night progresses the air starts to become "thick" with moisture. Before long I'm in a fog. Such is the life of an astronomer observing in the cool, moist Pacific Northwest.

 

So I plan to employ dew-prevention techniques before dew forms. This would include the fan above corrector and heat applied to the OTA. Once dew forms outside, there is often condensation inside the OTA, which is difficult to remove except by heating via a hair dryer. This of course plays havoc with image quality. As I experiment and find that no heat input is necessary, I'll be quite pleased. My DewBuster system supplies a convenient 12-volt power source right on the OTA. This will be used to power the fan. 

 

In any case Alex, I would welcome your comments on my thinking above. Also I expect Speedster and others will add a lot of useful insight as well.

 

Best Regards,

Russ

[maroubra_boy]

If there is no dew from the start, there would not be any heat transfer.

 

If there is dew, as the fan evaporates the dew there will be some heat transfer.  However, once dry the vortex of air is then keeping the corrector from developing dew.  Would this constitute a warmer mirror?  No.  If air is moving this will prevent dew from settling, but this is not because of any heating occurring to the corrector.  Dew point is an equation involving humidity and temperature.  Dew formation however also involves air speed.  If air speed is above a certain level, then dew will not form, and may evaporate what dew has formed.

 

On one occasion at my dark sky site, the night was bone dry with a steady breeze blowing and a crystal clear sky.  It was in the middle of winter and the temperature was around 2°C.  At one point the breeze stopped and it became overcast.  I had a look at a friend's vinyl seat that was black to see if dew was forming on it.  Instead of dew frost was forming on it and all other black coloured surfaces, but no dew on other colours or materials.  Half an hour later the breeze thankfully picked up again, the clouds cleared off and the frost on the seat sublimed away!  What is important to note here is not just that the breeze picking up was enough to dry things off, but that the black colour of the vinyl seat drops the temperature of the material to below freezing, and that a breeze was enough to dry off frost.  There is no heating of the seat occurring here.  All air speed and how it impacts on the evaporative capacity of air.

 

The mechanism around the evaporative capacity of moving air is not totally understood by me, and something I am further investigating.  Engineers that have designed professional observatories do understand this as they are the ones who have developed the use of fans for these structures.  I'll see what I can dig up around this.

 

Lastly, Russ, by all means try the combination of heat and fan.  I'm sure it will work.  There is also a point when dew formation is so intense that not only will it be difficult to curb, but if you pay attention to the sky, such heave dew will be severely compromising transparency, ie foggy conditions, and really this means time to call it a night.  Keeping our optics dew-free can be achievable using all manner of techniques and combinations there of, but we should also not forget to look up to gauge conditions too!  The purpose of fans though is to get away from the use of heat.  Heat is an inefficient method of dew control.  Sure it works, but at great cost of power requirements and the effects of uneven heat distribution on optics.  So keep this in mind.  But by all means post your findings.

 

Alex.

[speedster]

Air holds less and less water vapor as it cools.  At some point, the dew point temp, it can no longer hold all its water vapor and the excess vapor begins to condense on cooler surfaces.  Frost and dew are both the same expelled water.  Dew if it forms on a surface above freezing and frost if it forms on a surface below freezing.  Dew point temp is always below ambient temp (unless you are standing in a fog).  Even when the ambient temp is below freezing, the dew point is lower. 

 

If all the surfaces are at ambient, no dew/frost will form.  But, surfaces are below ambient because they are loosing heat by radiation to the sky (and to a lesser extent to line of sight terrestrial objects).  When that radiant loss causes the surfaces to drop below the dew point temp, dew/frost forms.  There is always a boundary layer of air attached to the surfaces.  Air movement can reduce that boundary layer to a very thin film but it's still there and the air in that boundary layer can expel water when it hits the dew point.

 

Heating is the ultimate mechanism at work eliminating dew/frost and the the heat comes from the ambient air being blown over the surfaces.  Something has to partially replace the heat lost by radiation.  That can be dew strips, hair dryer, etc., or a fan blowing ambient air which is above the dew point.  We don't have to be warm, we just have to be above the dew point and, since ambient is always above dew point, all we have to do is be ambient.  Ambient also happens to provide the least wavefront distortion.  Even when air movement is not needed for control of condensation, it minimizes the thickness of the boundary layer which minimizes wavefront distortion.  I'll never get to the point when I can say, "Gee, that's a perfect image except for the corrector's boundary layer distortion" but, when minimizing that distortion is a free effect of controlling dew with a fan, I'll gladly take it.

 

Dew heaters certainly work and if you throw enough money at controls, they can predict when to come on and when to cycle power to try to not overshoot ambient but, ordinary free air and a $10 fan does more and does it more accurately.  As Alex points out, major observatories, with all the money they want for the stuff of dew control, opt for simple fans.  We are programmed to think what more money = better performance and that is generally true but sometimes there is a cheap, simple, elegant solution that beats big bucks.

 

Turning on the fan when dew forms works.  But, if you start a 5 hour imaging run and don't want to babysit it waiting for dew to form, you can start the fans, get the seeing advantage up front, and go to bed.

[Rustler46]

To break up the boundary layer and any funky wavefront, run the fan all the time.  Otherwise no improvement in seeing.  No glass strain as the air being blown is the same as the air already in the dew shield:  no temp difference.  Vortices will happen whether the fan is aimed straight in or at an angle.  Sweeping air on the dew shield doesn't do much as there is little, if any, temp difference across it.  You could make a case that a low emissivity surface on the outside of the dew shield would be beneficial.  Dew, and seeing issues, are happening at the boundary layer which is only about 1-2 cm thick and not a consistent thickness.  There will always be a boundary layer as there will always be air stuck to the corrector but a fan can reduce the thickness to microns.

 

Insulation is not necessarily our friend.  It's the low emissivity aluminum foil that does the trick.  Consider a few cases:

 

First case:  SCT with no insulation.  Move it from a 72 degree room to a 45 degree backyard.  The temp difference is largest, and seeing is the worst, when it first goes outside.  Let's say the seeing is "level 1 seeing" and it takes sixty minutes to cool within 2 degrees of ambient for the best seeing  and call that "level 10 seeing".  30 minutes in, call that "level 5 seeing".

 

Second case:  exactly the same as the first but the SCT is insulated with bubble wrap (just like Reflectix but without the foil bonded to it).  The tube is about R-2 and the bubble warp, twinwall, signboard, etc., adds about R-1.  The insulation slows the cooling so this tube starts out just like case one but doesn't get to level 5 seeing temp for 45 minutes and doesn't reach level 10 for 90 minutes.  Insulating to slow the conduction just stretches out the time till best seeing.  That's thermodynamics we just can't get around.

 

Third case:  same setup as the second but this time with Reflectix instead of plain bubble wrap.  Same longer cooling time but the low emissivity foil greatly reduces the radiant transfer and evens out the radiant transfer so the tube is more uniformly changing temp.  Still changing slower than the first case but it's doing it more evenly.  That unevenness is a big player and foil is helping reduce it so seeing is better faster even though the time to ambient temp is longer.

 

Fourth case:  best of both worlds but not nearly as convenient, wrap the tube in aluminum foil.  Speediest ambient equilibrium, as in case one, but also much less radiant transfer as in case three.  With so much anecdotal thinking about Reflectix, I can imagine the discussion "just wrap it with a space blanket" would generate.

Thanks for the insight, Speedster. Your practical application of the physics involved is appreciated. In my case my SCTs (C-8 or C-11) are kept outside in garage at a cool temperature. An night the temperature is not much cooler than daytime temps. So there is less adjustment needed. 

 

I'd like your assessment of my thinking on how the fan works:

• Corrector lens radiates heat to space, cooling it to below ambient air temperature down to the dew point.
• Air in contact with cold lens is cooled to dew point, and dew begins to form.
• With fan operating the air provides a bit of heat (forced convection & conduction) to counteract radiant heat loss and keep corrector lens above dew point.
• No dew forms as long as ambient air temperature is above dew point.
• Moving air has some impact on whether dew begins to form at dew point. Alex has commented on this.

This last point may be in play in commercial orchards when there is danger of frost. The large fans employed keep the air moving, delaying onset of dew and frost.

 

Is my thinking correct on this. In any case, what really matters is the dew doesn't form even when not heat is added via a dew-heater.

 

Russ

[speedster]

Yep, that's pretty much right on the money.  Check out post #15.

[Rustler46]

Russ, by all means try the combination of heat and fan.  I'm sure it will work.  There is also a point when dew formation is so intense that not only will it be difficult to curb, but if you pay attention to the sky, such heave dew will be severely compromising transparency, ie foggy conditions, and really this means time to call it a night.  Keeping our optics dew-free can be achievable using all manner of techniques and combinations there of, but we should also not forget to look up to gauge conditions too!  The purpose of fans though is to get away from the use of heat.  Heat is an inefficient method of dew control.  Sure it works, but at great cost of power requirements and the effects of uneven heat distribution on optics.  So keep this in mind.  But by all means post your findings.

 

In my climate I often have to endure what I call "thick air" - that is when fog is beginning to form in the air. Some of it is actual micro droplets of water, some is salt crystals from surf spray evaporating. Often when driving on a sunny day on the Oregon coast highway, there is a residue of salt on the windshield. The water evaporates leaving salt crystals on exposed surfaces or as nuclei upon which fog can form.

 

At night if I quit observing at the outset of thick air, I would miss what few semi-clear hours I have available. Yes, sometimes it quickly fogs up and observing comes to an end. Other times I just cover the optics and wait a while. Occasionally a thick fog later settles down below my elevation (185 ft., 56 meters), leaving a clear sky. We all have different observing conditions. Such things as day/night temperature differential, relative humidity, how quickly temperature drops at night, et cetera, all have an effect on the situation. I believe this is why there are so many differences of opinion as to what is necessary to prevent dewing.

 

So I'm hopeful I can dispense with heating the optics. My fan has been shipped and is on the slow boat from China.

 

Best Regards,

 

Russ

[InductorMan]

Hi Alex, 

 

This is brilliant!

 

However I want to quibble with the mechanism at work here.

You say: 

 

If there is no dew from the start, there would not be any heat transfer.

 

 

That's not the whole story. I guess I'm belaboring the point since others have said this... but I guess I think it's useful to say the same thing a couple of different ways when it's this weird/hard to wrap the mind around; maybe you've totally grasped it already but some other reader might click with yet another explanation. 

 

The glass of a corrector has an emissivity of about 0.8 or higher: it's black, just like the car seat you mentioned in #14, at least in the thermal infrared spectrum. So it's radiating heat out to space. That's what provokes dew formation in the first place. First the radiation drives the temperature below ambient, then the dew begins condensing as the water vapor gives up heat to the glass (which has now dropped below the dew point). 

 

If you blow air from a fan on the glass, you're increasing convective heat transfer. The glass is still radiating the same amount of heat out to space, but now it's got an enhanced effective thermal "connection" to the ambient air, so it drops less in temperature before an equilibrium is reached. But there is still heat transfer. That radiation is still happening, so the glass must be at least a little below ambient, in order to extract an equal and "opposite" amount of heat from the forced convection air stream.

 

So basically I agree with Jim: what you're doing is in fact heating the corrector. The corrector wants to be at the radiation temperature of whatever it's pointed at, but you're forcing it to be much closer to the ambient temperature of air by dumping tons of ambient air on it. The air is hotter than the corrector wants to be, and so there is a constant heat flow from the air to the corrector. I mean, it's sort of weird to call it "heating". Maybe we could call it "heating back up", since it obviously never gets hotter than ambient!

 

Regardless, it's brilliant, and I'm extremely glad you discovered and shared this!

 

 

Russ, you asked: 

Moving air has some impact on whether dew begins to form at dew point. Alex has commented on this.
This last point may be in play in commercial orchards when there is danger of frost. The large fans employed keep the air moving, delaying onset of dew and frost.

 

Is my thinking correct on this. In any case, what really matters is the dew doesn't form even when not heat is added via a dew-heater.

 

 

There really isn't any effect from moving air per se.

 

Yes, it absolutely makes a difference in practice, but just by keeping the radiating surface better coupled to the bulk of ambient air; air that's warmer than the dew (or frost) point.

 

(edit: to counterpoint is that if a surface were artificially held at the same below-dew-point temperature regardless of the air blowing over it, movement of the air would actually increase the condensation rate by bringing more moist air to the surface; not reduce it. Like an air conditioner evaporator coil. So it's just the heat transfer effect; moving air itself doesn't change whether or not dew or frost forms)

 

With something like a vineyard (or orchard, but I'm from the SF Bay where the fans are more frequently used on vineyards!) we have this large, flat field which can gather a blanket of cold air over itself as it radiates to space. That's a stable configuration: there's nothing driving convection, the cold air will just sit there, and there's not very much mass of air to be cooled down, so it doesn't take that long for the plants to start freezing.

 

Those fans are typically mounted at a larger height than this layer (although I've seen low ones too, but the taller ~25 foot ones seem preferred), and what they're doing is actually mixing the air vertically as well as moving it horizontally.

 

They increase the air speed at the surfaces of the plants, which keeps those radiating surfaces better coupled to the ambient air. And then they stir the ambient air to mix it vertically, so it doesn't have a chance to get stratified like I described, and that cold blanket is disrupted an mixed with a larger bulk of air. That larger mass of air is harder to cool.

 

In any case, what really matters is the dew doesn't form even when not heat is added via a dew-heater.

 

 

And you're right: really, who cares! It'll work. I haven't tried it, but I know it'll work, both because of everyone's experience and because it's a physically elegant and near perfect solution, and I'm very excited to try it. 

Edited by InductorMan, 07 March 2021 - 11:45 AM.

[Rustler46]

Hi Alex, 

 

This is brilliant!

 

However I want to quibble with the mechanism at work here.

You say: 

 

 

That's not the whole story. I guess I'm belaboring the point since others have said this... but I guess I think it's useful to say the same thing a couple of different ways when it's this weird/hard to wrap the mind around; maybe you've totally grasped it already but some other reader might click with yet another explanation. 

 

The glass of a corrector has an emissivity of about 0.8 or higher: it's black, just like the car seat you mentioned in #14, at least in the thermal infrared spectrum. So it's radiating heat out to space. That's what provokes dew formation in the first place. First the radiation drives the temperature below ambient, then the dew begins condensing as the water vapor gives up heat to the glass (which has now dropped below the dew point). 

 

If you blow air from a fan on the glass, you're increasing convective heat transfer. The glass is still radiating the same amount of heat out to space, but now it's got an enhanced effective thermal "connection" to the ambient air, so it drops less in temperature before an equilibrium is reached. But there is still heat transfer. That radiation is still happening, so the glass must be at least a little below ambient, in order to extract an equal and "opposite" amount of heat from the forced convection air stream.

 

So basically I agree with Jim: what you're doing is in fact heating the corrector. The corrector wants to be at the radiation temperature of whatever it's pointed at, but you're forcing it to be much closer to the ambient temperature of air by dumping tons of ambient air on it. The air is hotter than the corrector wants to be, and so there is a constant heat flow from the air to the corrector. I mean, it's sort of weird to call it "heating". Maybe we could call it "heating back up", since it obviously never gets hotter than ambient!

 

Regardless, it's brilliant, and I'm extremely glad you discovered and shared this!

 

 

Russ, you asked: 

 

There really isn't any effect from moving air per se.

 

Yes, it absolutely makes a difference in practice, but just by keeping the radiating surface better coupled to the bulk of ambient air; air that's warmer than the dew (or frost) point.

 

(edit: to counterpoint is that if a surface were artificially held at the same below-dew-point temperature regardless of the air blowing over it, movement of the air would actually increase the condensation rate by bringing more moist air to the surface; not reduce it. Like an air conditioner evaporator coil. So it's just the heat transfer effect; moving air itself doesn't change whether or not dew or frost forms)

 

With something like a vineyard (or orchard, but I'm from the SF Bay where the fans are more frequently used on vineyards!) we have this large, flat field which can gather a blanket of cold air over itself as it radiates to space. That's a stable configuration: there's nothing driving convection, the cold air will just sit there, and there's not very much mass of air to be cooled down, so it doesn't take that long for the plants to start freezing.

 

Those fans are typically mounted at a larger height than this layer (although I've seen low ones too, but the taller ~25 foot ones seem preferred), and what they're doing is actually mixing the air vertically as well as moving it horizontally.

 

They increase the air speed at the surfaces of the plants, which keeps those radiating surfaces better coupled to the ambient air. And then they stir the ambient air to mix it vertically, so it doesn't have a chance to get stratified like I described, and that cold blanket is disrupted an mixed with a larger bulk of air. That larger mass of air is harder to cool.

 

 

And you're right: really, who cares! It'll work. I haven't tried it, but I know it'll work, both because of everyone's experience and because it's a physically elegant and near perfect solution, and I'm very excited to try it. 

Thanks Inductorman for the clear explanation of the processes in play. At first I thought the fan would cause dew to form. But on understanding the heat flow processes it does make brilliant sense. I'm excited too. I hadn't realized the point you made about the corrector being slightly below ambient for heat to flow from air to glass. This differential would be small, but important for dew prevention.

 

I'm thinking that changes in the air's transparency to IR radiation might occur from hour to hour, affecting the rate heat is lost to space.

 

Russ

[InductorMan]

I'm thinking that changes in the air's transparency to IR radiation might occur from hour to hour, affecting the rate heat is lost to space.

 

 

I'm sure! I believe that optically transparent water vapor is supposed to be able to absorb significant amounts of IR heat radiation. 

 

I believe that's also part of the mechanism of another older vineyard protection mechanism we see around here, the smudge pots. They're some sort of kerosene or fuel oil burning stove. They position them throughout the vineyard, especially at the edges, and try to: a) blanket the vineyard with smoke and vapor, which makes the air more opaque and helps retard radiation, and b) create vertical convection currents which help mix the air vertically, disrupting that blanket of cold air, just like the fans do.

 

Hope this isn't all too off topic. Basically the upshot is that I believe any mechanism of using forced convection to hold the corrector closer to ambient temperature and not let it drop too far below ought to help dewing. Heck it even could work below freezing, as long as the "dew point depression" is sufficiently large. Same mechanics work for frost/sublimation as for dew/condensation.

 

Basically, if relative humidity is less than 100%, to allow for that slight inevitable temperature drop without hitting dew/frost point, there's room for this mechanism to do its job.

[speedster]

There really isn't any effect from moving air per se.

 

The moving air is what makes this work.  It gets the corrector to ambient faster and keeps it there and also improves seeing by breaking up the boundary layer.  In the orchard/vineyard case, those fans may be blowing air below freezing but the motion itself prevents ice formation on the plants.  If you are observing at -20F, you are blowing -20F air on the corrector but that is still above the dew point.  Another illustration of the effect of moving fluid is ice breakers using air bubbles to prevent ice from sticking to the hull or offshore platforms using propellers to keep fluid (water) circulating around submerged structure to prevent it from becoming encased in ice and reacting to the movement/expansion of the ice sheet.  Swimming pool pumps turn on at low temps to prevent freezing even though the water and equipment temps are well below freezing.  We had the pool issue during the recent Texas stormageddon.  Pool temp way below freezing but no ice because the circ pump was running.  Then, power loss, no pump, no circulation, and 3" of ice the next day.

 

I'm thinking that changes in the air's transparency to IR radiation might occur from hour to hour, affecting the rate heat is lost to space.

 

Yep, there are awhole lot of things in play in the atmosphere and some are constantly changing.  Think about clouds:  a cloud comes by and you go from pretty transparent to virtually opaque in the IR just like in the visual.  Dust is another big deal. 

[InductorMan]

The moving air is what makes this work.

 

I'm sure we're saying the same thing here: I'll just say I agree with your description, and I am trying to describe the same thing. I might have muddled it up the way I worded that.

 

I was just trying to point out that the basic mechanism is improved heat transfer alone, and that motion per se has no disrupting effect on dew/ice formation, at least in the somewhat artificial scenario where we manage to keep the actual surface temperature exactly the same. I could elaborate on that but it seems off topic so I'll stop, and if we're not in agreement on that point, not a big deal: the point remains that this is a super great sounding way to prevent dew!

[Rustler46]

There really isn't any effect from moving air per se.

 

The moving air is what makes this work.  It gets the corrector to ambient faster and keeps it there and also improves seeing by breaking up the boundary layer.  In the orchard/vineyard case, those fans may be blowing air below freezing but the motion itself prevents ice formation on the plants.  If you are observing at -20F, you are blowing -20F air on the corrector but that is still above the dew point.  Another illustration of the effect of moving fluid is ice breakers using air bubbles to prevent ice from sticking to the hull or offshore platforms using propellers to keep fluid (water) circulating around submerged structure to prevent it from becoming encased in ice and reacting to the movement/expansion of the ice sheet.  Swimming pool pumps turn on at low temps to prevent freezing even though the water and equipment temps are well below freezing.  We had the pool issue during the recent Texas stormageddon.  Pool temp way below freezing but no ice because the circ pump was running.  Then, power loss, no pump, no circulation, and 3" of ice the next day.

 

I'm thinking that changes in the air's transparency to IR radiation might occur from hour to hour, affecting the rate heat is lost to space.

 

Yep, there are awhole lot of things in play in the atmosphere and some are constantly changing.  Think about clouds:  a cloud comes by and you go from pretty transparent to virtually opaque in the IR just like in the visual.  Dust is another big deal. 

The nice thing about the fan causing ambient temperature air to move over the lens, is that no matter what changes occur, the effect is the same - dew doesn't form as long as that air isn't fog. The rate of falling temperature, changes in transparency, et cetera - no matter what, the lens will be slightly lower than ambient air temperature and above the dew-point. It's nice to have the laws of physics work in behalf of the astronomer, not against him/her. 

 

I appreciate how different forum members have chimed in to help us get a fuller picture of what is going on.

 

All the Best,

Russ

Edited by Rustler46, 07 March 2021 - 10:27 PM.

[Rustler46]

I'm sure we're saying the same thing here: I'll just say I agree with your description, and I am trying to describe the same thing. I might have muddled it up the way I worded that.

 

I was just trying to point out that the basic mechanism is improved heat transfer alone, and that motion per se has no disrupting effect on dew/ice formation, at least in the somewhat artificial scenario where we manage to keep the actual surface temperature exactly the same. I could elaborate on that but it seems off topic so I'll stop, and if we're not in agreement on that point, not a big deal: the point remains that this is a super great sounding way to prevent dew!

Kudos to Alex for bringing this technique back to the fore. Before this recent discussion, I didn't really believe what he had said previously. Thanks also to Speedster for commenting on this in my thread - Downsizing OTA from C-11 to C-8. I might still be loading the innards of my SCT with lots of heat in an ineffective way to combat dew. Of course until my new fan arrives from China, that's exactly what I'll be doing, to not be dewing. 

 

Russ

 

Russ