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Questions About Insulating SCT's with Reflectix

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#51 cytan299

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Posted 27 November 2018 - 10:09 AM

The Good Year blimp is shown during the day.  The Hubble Space Telescope is in sunlight.  Amateur astronomers observe at night ... usually.  Sure, white or shiny surfaces would minimize heat loss during the day, or in any case when the object is in view of the Sun.  But during the night?  I don't see it.  Neither does the cold sky.

 

Mike

Hi Mike,

  The amount of radiation that is lost from any surface is *independent* of whether it is night or day. You take a warm telescope out from the house, it starts radiating straight away and starts cooling. To stop it cooling, you can wrap pink insulation around it. Pink insulation reduces heat loss because it traps air between the OTA and the cold outside air.

 

At this point, there are two modes of heat loss from the outer surface of the pink insulation: radiation and conduction/convection to the outside cold air. You can't do too much about the heat conduction/convection between the pink insulation and the air. But you *can* reduce radiative cooling: by painting the pink insulation silver. 

 

cytan

 

P.S. Your reasons why the blimp and Hubble are silver is incorrect. The silver is to *insulate* the insides, i.e. to keep the insides cool. Black is a good heat radiator and a good absorber. Silver is a bad heat radiator and bad absorber. For our application, we want silver because it is a bad radiator.


Edited by cytan299, 27 November 2018 - 10:18 AM.

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#52 Sarkikos

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Posted 27 November 2018 - 10:24 AM

The fact that a shiny surface will radiate less infrared than a black surface is important to know. 

 

But is this a universal law or a rule of thumb?  I wonder if this is true of all black surfaces?  Visible light is not infrared.  Isn't there some kind of paint or material that is black to the eye but will still reduce radiative cooling?

 

And how important really is the radiative cooling from the surface compared to the heat retention of the insulating material?

 

Mike


Edited by Sarkikos, 27 November 2018 - 10:32 AM.


#53 precaud

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Posted 27 November 2018 - 10:34 AM

But is this a universal law or a rule of thumb?  I wonder if this is true of all black surfaces?  Visible light is not infrared.  Isn't there some kind of paint or material that is black to the eye but will still reduce radiative cooling?

 

It is universally true, diminished a bit if the surface is highly polished (smooth).

 

But I maintain that the "radiated heat loss" portion of this event is teeny compared to the convective heat loss. And a single layer of Reflectix is a poor way of attenuating either.


Edited by precaud, 27 November 2018 - 10:35 AM.

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#54 Reid W

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Posted 27 November 2018 - 10:46 AM

Tried the "old school" method last night.  

 

I set my "non-insulated" C8 out before sunset.  About 17:00.  Temps started at 44.

 

At 20:00 temp was down to 36.  

 

At 21:00 temp was near 34.

 

Views were nice, but out-of-focus views *at the end of the evening* of Almach clearly showed the heat.  

 

I have enough material left to insulate the C8 tube and its dew shield.  Will see how that impacts the views.  It was a big time improvement on the C11.  


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#55 Bean614

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Posted 27 November 2018 - 11:28 AM

Starman876 wrote:  "This is all great news.   However,  if one keeps the scope outside and let it settle to ambient temps would a thermal blanket still be needed?  Or are we insulating the scope from out body heat.   I would think if it was that cold out we would be wrapped pretty good also."

 

You don't quite get it yet.  Just as when YOU go out in the cold, you put your winter coat on BEFORE you go out, so that the COAT keeps the heat in!  Please.... Just FORGET the words 'Cooling' and 'Ambient'.  The idea here is to NOT cool the scope!  And with the Reflectix added, your scope will, just like your own body under a warm coat, RETAIN the heat it had when indoors.  It will radiate the heat VERY slowly, thus producing ZERO air currents, and thus you will have sharp views.

 

 Please refer to Peter's post, #30 above.


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#56 starman876

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Posted 27 November 2018 - 11:34 AM

It is universally true, diminished a bit if the surface is highly polished (smooth).

 

But I maintain that the "radiated heat loss" portion of this event is teeny compared to the convective heat loss. And a single layer of Reflectix is a poor way of attenuating either.

Shiny surfaces radiate less heat so that is why solar collectors are shiny black.   Something to do with emissivity.  That is why the chrome on cars gets so hot.  So I would think a shiny material covering a tube would release less heat and thus keep thermal currents inside the OAT down because less heat is being exchanged with the outside colder temperature.   I guess ideal would be the inside of the OTA being the same temperature as the ambient temperature.   That is why people install fans.   On Newts they install fans to blow cold air across the primary.  However, a Newt is not a closed system.  Would an SCT work better if it was not inside a closed OTA.   Make one using truss tube construction?  That way there would be no tube currents.  Seems we all agree that the tube currents inside a closed OTA really degrades the image.  Of course this is not much of a problem in the summer as it is in the winter.   Maybe and SCT is only designed for warmer climates and in the north we should stick to newtonians and refractors.  



#57 cytan299

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Posted 27 November 2018 - 12:20 PM

It is universally true, diminished a bit if the surface is highly polished (smooth).

 

But I maintain that the "radiated heat loss" portion of this event is teeny compared to the convective heat loss. And a single layer of Reflectix is a poor way of attenuating either.

I think your speculation is easily tested with an IR temperature gun and three tubes: bare, wrapped with reflectix and wrapped with relectix but painted black. Well, if only someone is willing to do it ...

 

cytan


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#58 precaud

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Posted 27 November 2018 - 12:22 PM

Shiny surfaces radiate less heat so that is why solar collectors are shiny black.   Something to do with emissivity.  That is why the chrome on cars gets so hot.

 

Yes, emissivity is the term for radiation characteristic. It's basically an inverse curve to absorption.

 

So I would think a shiny material covering a tube would release less heat and thus keep thermal currents inside the OAT down because less heat is being exchanged with the outside colder temperature.

 

While "true", the problem with that description is that it implicitly assumes that "radiation loss" is the only heat loss mechanism at work. Radiated heat loss is transfer between two bodies, a radiator and an absorber. And it is relatively inefficient when the  temp differential between the bodies is small. By far the largest heat loss component in this event is convective, from the OTA coupled to the air mass.

 

You can see these realities in the wood heating world. Wood stoves are black and often with a textured surface to maximize radiation efficiency. Very few white stoves, except for the "warm furniture" class. If you need to minimize clearance of pipe to combustibles, you use either chromed or white-painted pipe. In both of these situations, high delta-T's exist, enabling plenty of radiated heat transfer.

 

 

But from, say, an SCT OTA that is gradually losing heat to an air mass that is gradually declining in temperature, the delta-T's are low and heat loss by radiation is teeny. The convective, i.e. air-coupled, loss is by far the dominant component.

 

And we know the way to deal with that dynamic by the way we dress for cold. Fibrous insulating material covered by an enclosed air stop is most effective.


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#59 precaud

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Posted 27 November 2018 - 12:27 PM

I think your speculation is easily tested with an IR temperature gun and three tubes: bare, wrapped with reflectix and wrapped with relectix but painted black. Well, if only someone is willing to do it ...

 

Not entirely. That only measures surface temperature. Radiated heat loss also requires the presence of a nearby absorber, and its characteristics factor into the equation.


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#60 Sarkikos

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Posted 27 November 2018 - 12:36 PM

Shiny surfaces radiate less heat so that is why solar collectors are shiny black.   Something to do with emissivity.  That is why the chrome on cars gets so hot.  So I would think a shiny material covering a tube would release less heat and thus keep thermal currents inside the OAT down because less heat is being exchanged with the outside colder temperature. 

So maybe split the difference and paint the outer surface of the Reflectix insulator glossy black?  

 

If shiny chrome colored surfaces do radiate the least heat, why are solar collectors shiny black rather than shiny chrome colored?  Maybe because black also absorbs the most heat.

 

Mike


Edited by Sarkikos, 27 November 2018 - 12:37 PM.

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#61 Sarkikos

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Posted 27 November 2018 - 12:46 PM

I think it is better to just fuggedabout the "reflective" aspect of the material. which, in my experience, is mostly hyped in significance. The basic event is a relatively high-mass metallic item (the OTA) immersed in a cold air mass that is declining in temperature. This is convective heat loss. Radiated heat does not play much of a role in this event.  ANY insulation material, reflective or not, slows down the "heat loss" of the OTA to the air. The higher the R value, the slower the transfer.

 

If you place the reflective side against the OTA, to "stop" the radiated heat loss, you now have conductive heat transfer into the insulation material, swamping any meager benefit of trying to block the radiated loss. The honeycomb structure of Reflectix is only isulative where the enclosed air bubbles exist. The solid part of the material is a "thermal bridge" for conductive heat transfer.

 

A few years ago I tried Reflectix and some other materials to insulate the air feeds from a solar heater to/from a house. Used by itself, Reflectix most underperformed its R value rating. But it worked well as the outer layer of a two-part insulator, because it stops the insulated air mass from mixing with the insulated one. This assumes that the object being insulated is covered completely. Any gaps in the insulation (in our case, at either end of the OTA) lower the R value dramatically.

 

So a less-conductive material (more fibrous) under a layer of Reflectix will give better results.

Forget about Reflectix.  What would be the ideal insulating system for Cat OTA's?  

 

Maybe the layering should be something like this (from OTA out to environment):

 

- high R fabric layer

- insulating air layer (bubble wrap?)

- shiny Mylar or similar layer

 

Reflectix is just cheap and everywhere.  It doesn't have to be Reflectix.

 

How about this?  Single Reflective Insulation.  https://www.reflecti...ive-insulation/

 

The product consists of one 96% reflective layer of film bonded to two internal layers of heavy gauge polyethylene bubbles (total thickness 5/16”).

It appears to be like Reflectix but without the inner reflective layer.  So use a high R fabric layer directly on the OTA, covered by the Single Reflective Insulation.  Or maybe the double layers of bubbles would be sufficient without the high R fabric?

 

Mike


Edited by Sarkikos, 27 November 2018 - 12:54 PM.


#62 cytan299

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Posted 27 November 2018 - 12:56 PM

Not entirely. That only measures surface temperature. Radiated heat loss also requires the presence of a nearby absorber, and its characteristics factor into the equation.

To be clear, the experiment will measure temperature in let's say 1 min intervals using the same IR sensor pointed at all three tubes. The temperature fall rate should depend on how the surface is treated. By carting out the three tubes at the same time (so that all three tubes have the same initial temperature) and location, the nearby absorber (whether it is an radiative absorber, wind currents etc) is irrelevant because all three tubes will see the same external environment.

 

In the best case scenario:

  • No reflectix: temperature drops fastest.
  • Black reflectix: temperature drops second fastest.
  • Silver reflectix: temperature drops the slowest.

If both silver and black reflectix temperature drops at the same rate or let's say within 10% of each other then I'd say that whether the reflectix is painted black or not it's not too important and your speculation is correct: that the dominant heat loss mechanism is conductive/convective.

 

cytan


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#63 starman876

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Posted 27 November 2018 - 12:57 PM

This is a great thread.  Makes us think.  


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#64 cytan299

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Posted 27 November 2018 - 01:02 PM

So maybe split the difference and paint the outer surface of the Reflectix insulator glossy black?  

 

If shiny chrome colored surfaces do radiate the least heat, why are solar collectors shiny black rather than shiny chrome colored?  Maybe because black also absorbs the most heat.

 

Mike

The solar collectors, are painted black to absorb as much solar radiation as possible to heat the water. In this scenario, black is used as an absorber, not for insulation.

 

 

cytan


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#65 precaud

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Posted 27 November 2018 - 01:04 PM

Forget about Reflectix.  What would be the ideal insulating system for Cat OTA's? 

 

I didn't say "forget about Reflectix", I said, forget about dealing with radiated heat loss by using "reflective" materials. Not because they are no good at it, but because the radiated heat loss in this situation is teeny.

 

Maybe the layering should be something like this (from OTA out to environment):

    - high R fabric layer

    - insulating air layer (bubble wrap?)

    - shiny Mylar or similar layer

 

Or more simple, some sort of insulating layer/blanket (without plastics, which are good conductors), covered by an airblocking layer (plastic ok here).

 

I think the confusion here is a conceptual one. It's easy to think that, just because the OTA is cooling inthe cold air, that it is "radiating" heat. While that may be true in simplistic terms, it doesn't necessarily help with finding the best way to deal with it. There are very different ways to deal with radiating vs convective vs conductive heat transfer.



#66 Sarkikos

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Posted 27 November 2018 - 01:05 PM

The solar collectors, are painted black to absorb as much solar radiation as possible to heat the water. In this scenario, black is used as an absorber, not for insulation.

 

 

cytan

So are the solar collectors shiny black to help prevent radiation of heat out from the collector?  Black to absorb heat, shiny to prevent radiation.

 

Mike



#67 Sarkikos

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Posted 27 November 2018 - 01:14 PM

I didn't say "forget about Reflectix", I said, forget about dealing with radiated heat loss by using "reflective" materials. Not because they are no good at it, but because the radiated heat loss in this situation is teeny.

 

Or more simple, some sort of insulating layer/blanket (without plastics, which are good conductors), covered by an airblocking layer (plastic ok here).

 

I think the confusion here is a conceptual one. It's easy to think that, just because the OTA is cooling inthe cold air, that it is "radiating" heat. While that may be true in simplistic terms, it doesn't necessarily help with finding the best way to deal with it. There are very different ways to deal with radiating vs convective vs conductive heat transfer.

If the inner metallic layer of Reflectix actively conducts heat out of the OTA, then maybe we should forget about Reflectix.  Sounds like we shouldn't have a heat-conducting layer in direct contact with the OTA.  We want to keep heat in the OTA, not conduct it out. 

 

That's why I thought the Single Reflective Insulation made more sense.  Keep the reflective layer for the outer layer, but get rid of it for the inner layer.  (On the other hand, if the radiative property of the Reflectix shiny surface is no more than 10%, then maybe chuck both Reflectix and the Single Reflective Insulation.)

 

But since the plastic bubble wrap material is also a good conductor of heat, as you say, have the innermost layer be a high R fabric.

 

Maybe I should use the word "emit" rather than "radiate."  The average layman - myself for example - does not always use technical words according to the narrow technical meaning of the words.  

 

Mike


Edited by Sarkikos, 27 November 2018 - 01:22 PM.


#68 precaud

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Posted 27 November 2018 - 01:16 PM

So are the solar collectors shiny black to help prevent radiation of heat out from the collector?  Black to absorb heat, shiny to prevent radiation.

 

Mike

 

Bingo. Shiny also to collect less atmospheric contaminants and stay cleaner over time.

The tradeoff to this approach is, the surface becomes increasingly reflective at low angles of incidence to the sun. So it is best used with tracking collectors.


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#69 precaud

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Posted 27 November 2018 - 01:20 PM

If the inner metallic layer of Reflectix actively conducts heat out of the OTA, then maybe we should forget about Reflectix.  Sounds like we shouldn't have a heat-conducting layer in direct contact with the OTA.  We want to keep heat in the OTA, not conduct it out. 

 

That's why I thought the Single Reflective Insulation made more sense.  Keep the reflective layer for the outer layer, but get rid of it for the inner layer.

 

But since the plastic bubble wrap material is also a good conductor of heat, as you say, have the innermost layer be a high R fabric.

 

Maybe I should use the word "emit" rather than "radiate."  The average layman - myself for example - does not always use technical words according to the narrow technical meaning of the words.  

 

Mike

 

I think you got the concept.

 

This sounds like a job for "Shrouds by Heather"! I wonder if she knows a market exists for it?


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#70 Sarkikos

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Posted 27 November 2018 - 01:29 PM

It seems like we've deconstructed the entire concept of Reflectix as an insulator for Cat OTA's.

 

- The inner metallic shiny surface directly on the OTA will conduct heat out of the OTA.

- The bubble wrap, since it is plastic, will continue the conduction of heat away from the OTA.

- The outer metallic shiny surface will reduce radiative loss of the heat that has been conducted away from the OTA.  But the reduction in radiative loss might be small.

 

On the other hand, many observers have shown good results wrapping their OTA's with Reflectix.

 

Maybe it's just that some other system will work even better.

 

Mike


Edited by Sarkikos, 27 November 2018 - 01:29 PM.


#71 jhayes_tucson

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Posted 27 November 2018 - 01:30 PM

Yes, of course.  The question I pose is:  Does the outer color of the Reflectix really matter at night?  I don't think it does.  But some people act as if the outer color is the most important factor!

 

Mike

Mike,

You do not understand radiative heat transfer.  First of all, we aren't talking about visible light.  Almost everything around the telescope radiates at a temperature of about 300K which peaks at a wavelength of 9-10 microns.  To understand this stuff better, first go stand in front of a fire in a fireplace as you read this.  The amount of power transmitted from a surface is determined by something called the emissivity, which describes how well the surface compares to a perfect black body.  The color of the surface relates directly to the emissivity of the surface.  Under stable conditions, the emissivity of a surface equals its absorptivity, so it emits as much as it absorbs.  A shiny, aluminum coating (like on the surface of a mirror) or a very clean, white surface has a very low emissivity.  As you stand in front of the fire, you'll notice that on the side of your body facing the fire, your skin is getting quite warm.  That's due to the fact that your skin is absorbing heat in the form of radiation from the fire.  There might be a bit of conduction as well but that's minuscule compared to what you are getting from the radiation generated by the hot embers.  But don't go away thinking that this effect is only significant while standing in front of a hot fire.  Radiative heat loss between your body and the walls of your house play a significant role in your comfort level at a given room temperature between winter and summer.  This is why a thermostat setting of 70F in the summer might seem a bit warm; whereas, you might be too cold at the same setting during the winter.

 

When your scope is under the sky on a still night, roughly half of the heat exchange comes from radiative transfer.  I recently calculated the effect for a piece of glass exposed to the full night sky and found that the front surface will stabilize at between 7 and 11 degrees below the ambient air temperature (depending on the temperature and humidity.)  That's why you get dew on the optics and also it's why the inside of a sealed telescope (like a SCT or refractor) can get wet or frosted (on a cold night.)  It's also a big reason why you get thermals inside the tube.  I've personally seen split imaging from a thermally stabilize C14 XLT with a black tube on cold, humid night due to radiative cooling of the tube, which caused a stable thermal temperature gradient inside the tube.

 

Reflectix does two things:  1) It reduces radiative heat loss to the sky, and 2) It reduces convective heat loss from the tube to the surrounding air.  Both of these effects greatly slow thermal stabilization of the system.  The problem is that the front optical surface will still be exposed to the sky so it's still possible to get natural convective air turbulence within the tube.  It's better (meaning less turbulent) than the currents produced by not having the Reflectix, but in my experience, it simply prolongs the time to get to a well stabilized system--and that's not desirable.  The best solution is to prevent radiative cooling on the tube while using forced convection to more rapidly bring the OTA to thermal stability.  In my experience under a wide range of conditions using a SCT, I've found that Tempest fans are an excellent solution.  They provide only a small amount of air exchange with the outside air and the circulation is very small, but it is perfect for preventing temperature gradients, baffle currents, and for more quickly bringing the OTA into equilibrium with the outside air.  So, in my view, Reflectix on its own is only half of the solution.  To get the best results, you need to do both:  1) Wrap the tube in Reflectix, and 2) Use fans to provide a small amount of outside air circulation within the tube.

 

It's clear that you are not alone in struggling to understand this stuff.  In this thread alone, it's apparent that a lot of folks don't understand the differences between conduction, convection, and radiation and the roles they play with respect to image quality and dew prevention.  The article that I'm working on will explain this stuff in a lot more detail.  Hopefully, going through a bit of the underlying physics will help to dispel myths and raise the collective understanding a bit.

 

John


Edited by jhayes_tucson, 27 November 2018 - 01:33 PM.

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#72 Sarkikos

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Posted 27 November 2018 - 01:32 PM

I think you got the concept.

 

This sounds like a job for "Shrouds by Heather"! I wonder if she knows a market exists for it?

Yep.  If some entrepreneur hit upon a good system to insulate Cat OTA's, that you could just slip over the tube and forget about it, there would be money to be made.  They would get some of mine.  I'm only a DIYer when I have to be!

 

grin.gif

Mike


Edited by Sarkikos, 27 November 2018 - 02:23 PM.

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#73 Sarkikos

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Posted 27 November 2018 - 01:34 PM

Mike,

You do not understand radiative heat transfer.  ... It's clear that you are not alone in struggling to understand this stuff.

A little advice:  it's better to attack the thoughts, not the person.  Or is my saying this an attack on the person? thinking1.gif

 

At least you did make it clear that I'm not the only one who doesn't understand. 

 

I do appreciate the information.  I'll take some time to absorb it. waytogo.gif

 

grin.gif

Mike


Edited by Sarkikos, 27 November 2018 - 01:56 PM.


#74 precaud

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Posted 27 November 2018 - 02:01 PM

When your scope is under the sky on a still night, roughly half of the heat exchange comes from radiative transfer.

 

This sounds too high to me. I look forward to seeing how you determined this.



#75 Sarkikos

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Posted 27 November 2018 - 02:11 PM

In my experience under a wide range of conditions using a SCT, I've found that Tempest fans are an excellent solution.  They provide only a small amount of air exchange with the outside air and the circulation is very small, but it is perfect for preventing temperature gradients, baffle currents, and for more quickly bringing the OTA into equilibrium with the outside air.  So, in my view, Reflectix on its own is only half of the solution.  To get the best results, you need to do both:  1) Wrap the tube in Reflectix, and 2) Use fans to provide a small amount of outside air circulation within the tube.

 

I have an EdgeHD 8" with installed Tempest fans.  So I should wrap the OTA in Reflectix, leaving openings for venting from the Tempest fans?  Run the Tempest fans all night?  (Some say you should not expose the vents while using Reflectix, but cover them up.)

 

I have a AstroZap dew shield for the Edge, with a built-in warm strip.  The warming strip appears to be positioned about at the location of the corrector.  I like the AstroZap shield for its structural stability, excellent flocking and warming strip to prevent dewing on the corrector.  Should I continue to use the shield and the Reflectix wrap?  Extend the Reflectix over all of the shield or only part of it?  Run the warming strip?  (Some say that the warming strip should be positioned on the OTA behind the corrector.  At least one observer I know about has removed the built-in warming strip in order to reposition it.)

 

Thanks,

Mike


Edited by Sarkikos, 27 November 2018 - 02:15 PM.



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