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Dew, Swamps, and Size Matters

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#1 Linwood

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Posted 27 January 2021 - 06:38 PM

Someone asked for this offline, and since I had a photo I thought I would post it. 

 

I image in Florida.  By the wee hours dew is almost indistinguishable from rain, except whether you can still see the stars.

 

I found the "Big" Dew Heater from R-Sky, and it appears (only one night so far) to have solved the problem.   R-Sky quotes 3.2, AStrozap in comparison is 2.3.  Not a big difference but it helps.  

 

SizeMatters.jpg

 

Roman was also kind enough to replace the long dangling RCA cord with a regular 55/21 plug more compatible with the CEM70 I had, since the on-saddle power passthrus were all that size.  Now I just use a short extension the right size for the OTA, and nothing dangles down.

 

Takes a while to get to the US from Russia, but ... no dew!  So far. 

 

FYI in case anyone else is in a dew challenged swamp.


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#2 alphatripleplus

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Posted 27 January 2021 - 08:10 PM

Someone asked for this offline, and since I had a photo I thought I would post it. 

 

I image in Florida.  By the wee hours dew is almost indistinguishable from rain, except whether you can still see the stars.

 

I found the "Big" Dew Heater from R-Sky, and it appears (only one night so far) to have solved the problem.   R-Sky quotes 3.2, AStrozap in comparison is 2.3.  Not a big difference but it helps.  

 

 SizeMatters.jpg

 

I assume the quotes are  for the current (in Amps) that these use when run at 12V?  



#3 Linwood

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Posted 27 January 2021 - 08:12 PM

I assume the quotes are  for the current (in Amps) that these use when run at 12V?  

Ah, yes, what's obvious when writing does not somehow make it out the finger tips.   Both per manufacturer site, I have not measured.


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#4 alphatripleplus

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Posted 30 January 2021 - 09:38 AM

Thanks. Hopefully, your initial finding that the R-Sky heater has solved the problem is borne out by subsequent use. 



#5 Andrew Brown

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Posted 31 January 2021 - 09:36 PM

Good Find. Now...must....resist....for now..



#6 luxo II

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Posted 31 January 2021 - 10:08 PM

So.. you’re putting in 40 watts of heat. Can’t help wondering if this is creating thermals from your scope and what this does to the seeing... though obviously anything ie better than dew.

Edited by luxo II, 31 January 2021 - 10:11 PM.


#7 Linwood

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Posted 31 January 2021 - 10:25 PM

So.. you’re putting in 40 watts of heat. Can’t help wondering if this is creating thermals from your scope and what this does to the seeing... though obviously anything ie better than dew.

Indeed, I did not go this route because I thought my rig looked cold, but because of seeing dew despite dew despite a dew strip.

 

I think there's a fundamental flaw in dew strips.  At least on the surface they seem to scale in current draw linearly with length, roughly.  But aperture's surface area subject to dew scales with the square.  So my 11" dew strip at about 34" is about 3 times longer than a 4" refractor at 12" and puts out about 3 times the heat.  But the surface area is about 7 times as great.

 

So going up in wattage more than linearly seems a reasonable step. 



#8 luxo II

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Posted 01 February 2021 - 12:03 AM

That’s not what I meant... I use a 10” mak and I have a 12W heater strap. On the occasions as I’ve turned it on I have noticed the result is terrible seeing soon after due to the thermals rising from the scope.

I’m visual though, with a scope that is generally at 250X or more, 300-350 is pretty normal and sometimes at 500X.

Leaves me feeling it’s a bit of a lose-lose situation... on a calm night when the seeing is very good, either risk the dew, or turn on the heater and only use low power (100X) because the heat causes thermals...

But you’re imaging so I guess you don’t notice.

Edited by luxo II, 01 February 2021 - 12:05 AM.


#9 Napp

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Posted 01 February 2021 - 12:28 AM

luxo II, I am mainly a visual observer.  I don't use my 8' SCT as much as I used to.  But like Linwood stated the dew here in Florida, especially later in the night, is bad.  With my SCT if I don't have the dew heater controller turned up to maximum by about midnight the night is usually over.  There have been a few nights even that was not enough.  I've tried lower settings but then I have to break out the hairdryer to remove the dew.  Dew stops visual as quickly as astrophotography. 



#10 luxo II

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Posted 01 February 2021 - 01:35 AM

Ah that bad.

Is the OTA naked, or do you wrap it with insulation over the heater strap ? This makes a significant difference ...

Edited by luxo II, 01 February 2021 - 01:36 AM.


#11 Linwood

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Posted 01 February 2021 - 09:44 AM

That’s not what I meant... I use a 10” mak and I have a 12W heater strap. On the occasions as I’ve turned it on I have noticed the result is terrible seeing soon after due to the thermals rising from the scope.

I’m visual though, with a scope that is generally at 250X or more, 300-350 is pretty normal and sometimes at 500X.

Leaves me feeling it’s a bit of a lose-lose situation... on a calm night when the seeing is very good, either risk the dew, or turn on the heater and only use low power (100X) because the heat causes thermals...

But you’re imaging so I guess you don’t notice.

Well, bloated stars are not desirable which is the same result you see as waves I think.  So I have a similar concern if not the same way to tell. 

 

 

Ah that bad.

Is the OTA naked, or do you wrap it with insulation over the heater strap ? This makes a significant difference ...

On the C11 I use a Celestron flexible dew shield, which has a padded (and thus I think somewhat insulated) ring that ends up over the dew heater, and extending down over the corrector.  How good it is I'm not sure but there's a lot of bulk in that area (and worked around the dovetail).  Not sure how practical yet more insulation is there.

 

But... I'm also not sure how helpful.  The dew that forms for me with dew heat/shield on starts around the secondary mirror area and grows outward.  I.e. the problem is that radiative cooling is occurring in the center of the objective and the dew heat that conducts in from the outside edge (or convective heating inside the OTA) is not keeping up.  More heat from the outside is the only alternative I see.  I'd welcome a better alternative (that doesn't involve moving 500 miles north). 

 

Heat currents are bad -- I get that.  Dew is worse.

 

The Pegasus temp/humidity meter hits 99 (its max) by around midnight and stays there until after sunrise.  It's not a high humidity area, it's a swamp here. 

 

By the way, one thing I like about this is the extra heat is also spread out a bit more. Intuition (though not real knowledge) tells me that keeping a wider area equally warm rather than a more narrow area hotter is better for currents. But I don't know.  I welcome other advice.



#12 luxo II

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Posted 01 February 2021 - 04:04 PM

Sounds like you’re wrapping the heater around the corrector cell - this would only be effective if the corrector was thermally conductive - which glass isn’t. Secondly, by leaving the OTA bare you’re allowing it to lose heat whatever heat it has very fast - both by convection and radiation. Thus achieving very little IMHO.

I’ll suggest trying this - position the heater further down - about 4”-6” down from the cell. Once hooked up, wrap the OTA in something that will keep the heat in - a piece of reflector and Velcro tabs or a strap will do nicely. Auto sun shade, even a picnic rug would be better than nothing.

The aim is to warm the bulk of the OTA - being metallic the heat will spread and warm the air inside the top of the OTA and thus warm the whole corrector by convection, fairly evenly. If the scope is alt-az it even makes sense to heat the underside of the OTA - not the top - since the warm air inside will tend to rise.

After an hour or so unwrap a little to check the effect - the metal tube should be warm to the touch, even with just 12W the difference is noticeable.

Edited by luxo II, 01 February 2021 - 04:11 PM.


#13 choward94002

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Posted 01 February 2021 - 04:23 PM

Sounds like you’re wrapping the heater around the corrector cell - this would only be effective if the corrector was thermally conductive - which glass isn’t. Secondly, by leaving the OTA bare you’re allowing it to lose heat whatever heat it has very fast - both by convection and radiation. Thus achieving very little IMHO.

I’ll suggest trying this - position the heater further down - about 4”-6” down from the cell. Once hooked up, wrap the OTA in something that will keep the heat in - a piece of reflector and Velcro tabs or a strap will do nicely. Auto sun shade, even a picnic rug would be better than nothing.

The aim is to warm the bulk of the OTA - being metallic the heat will spread and warm the air inside the top of the OTA and thus warm the whole corrector by convection, fairly evenly. If the scope is alt-az it even makes sense to heat the underside of the OTA - not the top - since the warm air inside will tend to rise.

After an hour or so unwrap a little to check the effect - the metal tube should be warm to the touch, even with just 12W the difference is noticeable.

This is effectively a "hot tube" which is how I run my scopes ... you put heating loops (big ones, like yours) around both the front and the rear cells of the scope, then cover everything with a reflectix shield, like this ...

 

Telly.jpg
 
If you look carefully you can see two wires coming out under the Reflectix at the wind cutout ... those are my heating wires for the front cell.  What you are shooting for is the entire system to be nice and warm, at least 10deg warmer than the outside air.  There are two benefits to this:
 
- No dew, ever.  Dew forms when a surface temperature drops to the dewpoint temperature, which changes by temperature and humidty (there are online calculators for that).  With the OTA surfaces like the corrector nice and warm even if there is a fogbank that rolls in it won't condense on the corrector (and at my wet site that's exactly what I get at 4am every evening)
 
- No thermals, ever.  Thermals happen when the primary cools at a slower rate than the air space inside the OTA ... the primary warms the air around it, that air travels up the focus baffle tube and you have a thermal.  With a hot tube, the primary never cools down ... it stays the same temperature as the OTA air, nice and warm.  Same temperature, no thermals ... you're at thermal equilibrium.  There *are* thermals on the outside of the corrector inside the dew shield, but the wind slits ensure that the air mixes very nicely at the surface of the corrector and again, no thermals
 
I have a computer that manages the heaters and a thermal sensor inside the OTA so everything stays about 10deg hotter than the outside air, and that any temperature change down is less than the glass cooling rate (so if the mirror cools at 10deg per hour then even if the outside air cools at 15deg per hour my OTA will be allowed to cool at 8deg per hour) 


#14 Linwood

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Posted 01 February 2021 - 04:54 PM

Sounds like you’re wrapping the heater around the corrector cell - this would only be effective if the corrector was thermally conductive - which glass isn’t. Secondly, by leaving the OTA bare you’re allowing it to lose heat whatever heat it has very fast - both by convection and radiation. Thus achieving very little IMHO.

I’ll suggest trying this - position the heater further down - about 4”-6” down from the cell. Once hooked up, wrap the OTA in something that will keep the heat in - a piece of reflector and Velcro tabs or a strap will do nicely. Auto sun shade, even a picnic rug would be better than nothing.

I try to put the dew strip right at the corrector with its bulk behind (toward the back).  But this is different, I had never heard anyone say go WAY back.  I'll give that a try.

 

@choward94002, won't quote the whole thing, but that's... really different.  Nothing I had read indicated a goal to heat the OTA that substantially, I always had the impression the goal was basically to restore it to ambient or a degree or so above, rather than let radiative cooling bring it below ambient.  "Hot Tube" is a new concept to me. 

 

How much power does it take to keep it that warm?



#15 choward94002

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Posted 01 February 2021 - 05:28 PM

I try to put the dew strip right at the corrector with its bulk behind (toward the back).  But this is different, I had never heard anyone say go WAY back.  I'll give that a try.

 

@choward94002, won't quote the whole thing, but that's... really different.  Nothing I had read indicated a goal to heat the OTA that substantially, I always had the impression the goal was basically to restore it to ambient or a degree or so above, rather than let radiative cooling bring it below ambient.  "Hot Tube" is a new concept to me. 

 

How much power does it take to keep it that warm?

You've got two essentially unrelated things going on that folks conflate ... dew prevention and thermal management

 

Dew prevention means keeping the corrector plate warmer than the dewpoint temperature .. and note that dewpoint temperature is a function of both temperature and humidity.  That's why you can be doing great on temperature and when a fogbank rolls in you get dew ... you've got to keep the corrector warmer than that dewpoint even when the humidity shoots up. 

 

People handle that in two ways ... first they try to slow down the passive cooling of OTA compared to the cooling outside air temperature with dew shields, a reflectix coat, etc. and second they try to warm up the corrector plate with dew heater (actually warming up the air around the corrector plate, since the cork gaskets holding in the corrector plate make terrible thermal conductors).  Keep the corrector plate warmer than the dewpoint temperature and you'll never have dew

 

 

Thermal management means keeping the OTA temperature higher than the primary mirror temperature ... if it's lower then you will get heat plumes from the primary up the focus baffle tube.  The bigger the primary mirror the slower it cools, but the interior OTA temperature will cool at the same rate as the outside air.  That's why it takes so long to acclimate a C14 compared to a C6, that's a big chunk of glass that needs to cool.  That's also why you will sometimes get thermal plumes if there is a cold front that moves in ... the colder outside air will cool down the OTA to below the primary temperature and once again, thermal plumes.

 

People try to handle this in two ways; first, they try to force the primary to cool faster by setting up some serious convection currents in the OTA with outside air ... like blowing air on your soup to cool it down.  That's the whole idea with fans, really blow that cold air onto the primary and cool it down faster than it would otherwise.  You'll still get thermals, but they will be much briefer until the mirror cools down ... that's the tempest fans.  Another approach is "crash cooling" which is what the planetary folks use (where thermals REALLY make a difference), essentially cooling down everything to below what it would ever get at night.  With this approach the cooling night air doesn't cool down the system, rather it's actually heating it up.  I've written stuff about that, and Kokatha has posted about it as well.

 

Unfortunately if you try each separately you run into issues ... using Reflectix and dew shields and heater is great for dew management, but doesn't help with the primary mirror cooling down so you're stuck with thermals.  Putting on tempest fans to cool down the system, or crash cooling, is great until the humidity rises and you get dew.

 

The "hot tube" approach came out of my experiments with thermal management a year or so ago, and so far I'm quite happy with the results.  I haven't had dew problems, even at my "wet site", for over a year now and that's in the face of a 4am fog bank at the "wet site" and cold fronts moving through during monsoon season at the dark site.  I also haven't had thermal problems (after John Hayes showed me what to look for) using the hot tube approach

 

For your other question, each heating strap draws 2A [https://www.amazon.c...e?ie=UTF8&psc=1], four straps total so it's an 8A draw per OTA ... those aren't powered from a USB hub obviously, I have some power relays on the thermal management computer that handle this ... and those straps are under Reflectix, so they definitely keep the OTA toasty!


Edited by choward94002, 01 February 2021 - 05:50 PM.


#16 alphatripleplus

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Posted 01 February 2021 - 05:45 PM

I also have my C8 wrapped in a double layer of Reflectix and use one dew heater behind the corrector. Under my conditions, the Reflectix slows cooling, and the dew heater does conduct heat to the OTA via conduction.



#17 luxo II

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Posted 01 February 2021 - 11:51 PM

OP there’s another solution as well - see https://www.cloudyni.../#entry10851513

This might be worth trying.

8A... 72W woah ... are you toasting marshmallows with that ?

#18 choward94002

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Posted 02 February 2021 - 02:02 AM

OP there’s another solution as well - see https://www.cloudyni.../#entry10851513

This might be worth trying.

8A... 72W woah ... are you toasting marshmallows with that ?

8A at 5v is 40w, not 72w at 9v but it's still a good draw ... remember it's a C14 with quite a bit of thermal inertia to overcome.  I used Comsol Multiphysics to model the thermodynamics of the system and it worked out that 20w on the rear cell and 15w on the front cell would take care of things, but I wanted a bit of a fudge factor so I went 20w on both ends ...


Edited by choward94002, 02 February 2021 - 02:03 AM.


#19 smasraum

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Posted 03 February 2021 - 03:39 PM

8A at 5v is 40w, not 72w at 9v but it's still a good draw ... remember it's a C14 with quite a bit of thermal inertia to overcome.  I used Comsol Multiphysics to model the thermodynamics of the system and it worked out that 20w on the rear cell and 15w on the front cell would take care of things, but I wanted a bit of a fudge factor so I went 20w on both ends ...

So what you're saying is that it was just a WAG.  wink.giflol.gifgrin.gif
 



#20 choward94002

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Posted 03 February 2021 - 06:41 PM

So what you're saying is that it was just a WAG.  wink.giflol.gifgrin.gif
 

Sure ... Comsol is commonly used where WAG's make all the difference ... [https://www.comsol.c...ench-simulation] or [https://www.comsol.com/video/inductive-heating-billet-tutorial] and who could miss [https://www.comsol.com/video/thermal-stress-analysis-turbine-stator-blade] where WAG's are made all the time ...



#21 smasraum

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Posted 03 February 2021 - 06:58 PM

Sure ... Comsol is commonly used where WAG's make all the difference ... [https://www.comsol.c...ench-simulation] or [https://www.comsol.com/video/inductive-heating-billet-tutorial] and who could miss [https://www.comsol.com/video/thermal-stress-analysis-turbine-stator-blade] where WAG's are made all the time ...

Basically the magic 8 ball of thermodynamics is what you're saying. 

 

Now I need a CFD analysis of the air flow inside the OTA.



#22 choward94002

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Posted 03 February 2021 - 07:15 PM

Basically the magic 8 ball of thermodynamics is what you're saying. 

 

Now I need a CFD analysis of the air flow inside the OTA.

Comsol does that as well [https://www.comsol.com/cfd-module], when I was designing the windbreaks for my OTA's as well as my dewshield slits (what, you thought the location and size of those dew shield gaps as well as the length of it was just happenstance ...) and when I was modeling the thermal plumes for my thermal analysis discussion a while back I did just that, modeling the airflow inside and outside of the OTA ... :)

 

Great program, that ... combine it with AutoCad and Solidworks for the physical dimensions and it almost becomes trivial to do that kind of modeling ..



#23 smasraum

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Posted 03 February 2021 - 07:21 PM

Comsol does that as well [https://www.comsol.com/cfd-module], when I was designing the windbreaks for my OTA's as well as my dewshield slits (what, you thought the location and size of those dew shield gaps as well as the length of it was just happenstance ...) and when I was modeling the thermal plumes for my thermal analysis discussion a while back I did just that, modeling the airflow inside and outside of the OTA ... smile.gif

 

Great program, that ... combine it with AutoCad and Solidworks for the physical dimensions and it almost becomes trivial to do that kind of modeling ..

ROFLMAO!  Outstanding!



#24 Andrew Brown

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Posted 03 February 2021 - 08:10 PM

You know what would be good? 

 

If people put the altitude of their observing site[s] in then we could, using judgement, have half an idea if their solutions would be transferable to their own locations.



#25 Napp

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Posted 03 February 2021 - 09:10 PM



You know what would be good? 

 

If people put the altitude of their observing site[s] in then we could, using judgement, have half an idea if their solutions would be transferable to their own locations.

Agree but more info than that is needed from anyone posting a "solution".  I'm in Florida.  Where I observe is a high area - over 150 feet in elevation in a national forest.  Except in the middle of winter the nights are warm to hot with high humidity.  Air temperature from around midnight on tends to stay very close to the dew point which means incredible dew.  I just shake my head when I see someone post that a dew shield is all you should ever need to prevent dew formation.  Usually, they are out west in a very dry area and their "solution" works quite well there.  But no way will it handle conditions here.




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