Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Questions About Insulating SCT's with Reflectix

catadioptric DIY equipment SCT
  • Please log in to reply
333 replies to this topic

#76 Sarkikos

Sarkikos

    ISS

  • *****
  • topic starter
  • Posts: 30442
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 27 November 2018 - 02:19 PM

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

But then you have the other half to deal with also.  Will having the reflective surface of Reflectix directly on the OTA significantly conduct heat away from the OTA?  Would it be better to use the Single Reflective Insulation, with the single reflective surface outermost, rather than Reflectix, which has a reflective surface on both sides? 

 

Also, even with the Single Reflective Insulation, would there be a problem with heat conductance due to the plastic bubble material directly on the surface of the OTA?

 

Mike


Edited by Sarkikos, 27 November 2018 - 02:21 PM.


#77 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 7090
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 27 November 2018 - 02:40 PM

First of all, conduction is the process of energy transfer through a solid.  Convection is the transfer of heat energy between a solid and a gas.  When the system comes to equilibrium under the sky, you determine the surface temperature by adding the net heat lost to the sky via radiation with the heat added by convection with the air.  Once everything stabilizes, you want the telescope to sit at the temperature of the air so preventing convective heat transfer with the air is not a desirable side effect that comes with Reflectix--and that's why adding fans to circulate air inside the tube is a good idea (essential IMO.)   Frankly, if you can't paint it, simply wrapping the tube in aluminum foil is a better strategy than using Reflectix; but, Aluminum foil isn't very durable.  On the positive side: The insulating properties of Reflextix do help to slow cooling of the OTA when you take it from a warm environment suddenly into a cold environment--and that initially helps to reduce air currents; however, without fans, the system will be unstable for a long time.  The fact that Reflectix (or aluminum foil) is reflective on both sides doesn't matter.

 

John


Edited by jhayes_tucson, 28 November 2018 - 12:17 AM.

  • Sarkikos likes this

#78 Rock22

Rock22

    Ranger 4

  • *****
  • Posts: 338
  • Joined: 28 Aug 2017
  • Loc: Diamond Bar, California

Posted 27 November 2018 - 02:53 PM

Mike, just do what many of the rest of us have already done... make it, test it, modify it, test it, and then decide to keep it or trash it.  Here is the rationale behind Reflectix from the Reflectix site: An Old Concept

 

If you don't like the look of the shiny surface of Reflectix (the double layer type works because of the air spacing already provided; I wouldn't use the single layer), wrap it with some thin, colored fabric to avoid messing with whatever functionality the outer reflective surface has. If you don't care about the functionality of the outer reflective surface, paint it, and see if it affects the views.

 

As for the shiny side of the Reflectix toward the OTA or in a dew shield, do not paint it.  This was mentioned in the insulation thread that was locked a while back.  The link above to the Reflectix site illustrates this.

 

There are varying outcomes, so find out how it works best for you.

 

And for added reading pleasure, here is another document from the Reflectix site that defines their terminology (Enjoy!): Simplified Physics


  • Sarkikos, terraclarke, jhayes_tucson and 1 other like this

#79 555aaa

555aaa

    Vendor (Xerxes Scientific)

  • *****
  • Vendors
  • Posts: 1392
  • Joined: 09 Aug 2016
  • Loc: Lynnwood, WA, USA

Posted 27 November 2018 - 03:31 PM

A couple thoughts - the radiative heat transfer goes as the cube of the temperature difference (derivative of an equation that has T^4) and the second temperature is space (an optically complex atmosphere and then space) so there is a huge temperature difference, which is why it's so effective, so I suspect John is right there with saying it's half radiative. In my 16" SCT, when it is first cooling off, I can place my palm in front of the corrector plate and I swear I can feel the radiative heat from it(maybe it's psychological). So during cool-down, I point the OTA at zenith which is the worst for air cooling but it is the best radiatively. Why is your head so dang cold at night? Because it is pointed at space. On a cloudy night, you have a radiative boundary there which is at a modest temp (like freezing) so the delta T is much less so you don't get the radiative transfer and hence frost that you have on clear nights. I still do cooldown even with the insulation because I'm trying to get to a stable equilibrium without forming frost. I can't get there in winter without insulation.

 

Secondly, your surroundings cool down a lot during the night and the insulation helps "insulate" the OTA from the change in temp over time, not just the one-time change in temp going from inside to outside.

 

Third, in winter time especially there seems to be a pretty substantial dynamic in the atmosphere for several hours after sunset where you get really lousy seeing from sunset for about the next four to six hours and then after that you get nice stratification and seeing can become excellent. For me it seems to be more pronounced than in summer. You don't seem to have the furious twinkling of stars as much in summer either.

 

One more thing - a lot of fabrics are pretty transparent in the IR hence the rationale for adding the metalized layer rather than pure fabric. 

 

-Bruce.


Edited by 555aaa, 27 November 2018 - 03:35 PM.


#80 precaud

precaud

    Aurora

  • *****
  • Posts: 4932
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 27 November 2018 - 04:31 PM

A couple thoughts - the radiative heat transfer goes as the cube of the temperature difference (derivative of an equation that has T^4) and the second temperature is space (an optically complex atmosphere and then space) so there is a huge temperature difference, which is why it's so effective, so I suspect John is right there with saying it's half radiative

 

Cube of the Tdelta but inverse square to the spacing between the objects, yes? Since you'd have to give "space" an infinite value, that pretty much wipes out any influence it might have... making it infinitely large gets some of that back...
 

Secondly, your surroundings cool down a lot during the night and the insulation helps "insulate" the OTA from the change in temp over time, not just the one-time change in temp going from inside to outside.

 

That's been part of the discussion. Same rules, just a lower Tdelta at that point.



#81 luxo II

luxo II

    Apollo

  • -----
  • Posts: 1014
  • Joined: 13 Jan 2017
  • Loc: Sydney, Australia

Posted 27 November 2018 - 06:09 PM

Cube of the Tdelta but inverse square to the spacing between the objects, yes? Since you'd have to give "space" an infinite value, that pretty much wipes out any influence it might have... making it infinitely large gets some of that back...
 

 

That's been part of the discussion. Same rules, just a lower Tdelta at that point.

 

You guys have really diverged from the primary objective of this thread - which is the use of insulation to eliminate the thermal plume from the internal baffles in SCT's and maks, and reduce/eliminate the dewing of correctors.

 

Evidence is everything - ie firsthand testing. It works. This is no longer hypothetical and there is no need for extended academic agonising about what might, or what might not be the case.


Edited by luxo II, 27 November 2018 - 06:40 PM.


#82 precaud

precaud

    Aurora

  • *****
  • Posts: 4932
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 27 November 2018 - 06:28 PM

You guys have diverged from the primary objective of this thread - which is the use of insulation to eliminate the thermal plume from the internal baffles in SCT's and maks, and reduce/eliminate the dewing of correctors.

 

Huh? Not at all.



#83 Peter Besenbruch

Peter Besenbruch

    Fly Me to the Moon

  • *****
  • Posts: 7011
  • Joined: 21 Aug 2014
  • Loc: Oahu

Posted 27 November 2018 - 10:12 PM

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.  

If you use a thermal blanket the scope will never reach equilibrium. You are slowing the scope's cool down to such an extent that thermals don't form. A side benefit is greatly increased dew resistance.

 

Basically, there are two ways to go, the insulation way, or the cool to equilibrium way. One or the other.

 

As for how much Reflectix, people have had good luck with two layers, particularly in more severe situations. I use two layers with a single layer over the rear casting. I also use an Astrozap dew shield with a single layer of Reflectix over that.


  • Jon Isaacs, Sarkikos, R Botero and 1 other like this

#84 Peter Besenbruch

Peter Besenbruch

    Fly Me to the Moon

  • *****
  • Posts: 7011
  • Joined: 21 Aug 2014
  • Loc: Oahu

Posted 27 November 2018 - 11:16 PM

I have had a very different experience from John Hayes. I get stable viewing from the beginning, and it doesn't get worse as the night progresses. John says the best situation is a combination of fans, coupled with insulation. He typically deals with larger SCTs. I have tried the insulation on a 180mm Maksutov.

 

I can't add Tempest fans to mine, and shoving a fan up the baffle tube simply takes away from my viewing time. I am wondering what difference the scope type makes, and also the size when it comes to cooling. Insulation has transformed my rather temperature finicky scope to a set up and use item.

 

Another question is what difference the dew shield makes in cool down of the corrector or meniscus. I'm assuming that it slows convection some, but really reduces the area the glass can radiate to.


  • Sarkikos and Axunator like this

#85 555aaa

555aaa

    Vendor (Xerxes Scientific)

  • *****
  • Vendors
  • Posts: 1392
  • Joined: 09 Aug 2016
  • Loc: Lynnwood, WA, USA

Posted 27 November 2018 - 11:51 PM

Cube of the Tdelta but inverse square to the spacing between the objects, yes? Since you'd have to give "space" an infinite value, that pretty much wipes out any influence it might have... making it infinitely large gets some of that back...

No, the distance doesn't matter, it's the solid angle subtended by the object which is the sky which is a half-sphere. It makes no difference if the old surface is a meter away or a light year if it covers the same angular extent. When the second body is treated as the full sphere then the result only depends on the are of the hot body:

 

https://www.engineer...sfer-d_431.html

 

If you plug the numbers into the calculator and just take the heat transfer from a 1 sq meter area into deep space it's 157 watts for 0.5 emissivity. If you change the cold to only say -60C (stratosphere at night), telescope at 0C, 0.5 emission is 99W for 1 square meter, but if you use the shiny stuff, the heat loss goes way down.


  • precaud likes this

#86 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 7090
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 27 November 2018 - 11:51 PM

A couple thoughts - the radiative heat transfer goes as the cube of the temperature difference (derivative of an equation that has T^4) and the second temperature is space (an optically complex atmosphere and then space) so there is a huge temperature difference, which is why it's so effective, so I suspect John is right there with saying it's half radiative. ....

 

-Bruce.

Bruce,

The radiative heat transfer in Joules/sec (= Watts) is proportional to the fourth power of the temperature of the surface--not the cube.  Under most conditions, the temperature difference between the ambient air and the sky turns out not to be as big as you (and I) might guess.  In my previous posts, I said that the sky temperature is typically about -30C, which I got from the sky monitor out at DSW.  After doing a bit of research I found that that's actually not a correct value.  The sky monitor uses an IR sensor to look for clouds and doesn't correctly compute the radiometric temperature value for the sky.  The sky is indeed typically colder than the air temperature (and under very dry conditions it can be quite cold) but it turns out that the difference in emissivity between the sky and glass is what really accounts for the heat loss to the sky.

John

 

 

Cube of the Tdelta but inverse square to the spacing between the objects, yes? Since you'd have to give "space" an infinite value, that pretty much wipes out any influence it might have... making it infinitely large gets some of that back...
 

 

That's been part of the discussion. Same rules, just a lower Tdelta at that point.

 

In order to compute the power (in Watts) radiated from a surface into a hemisphere you have to integrate the radiance of the surface, N (in W/m^2-ster,) over the whole hemisphere.  When you do this calculation, the radius of the hemisphere drops out and you find that the power radiated is given by N*pi (W/m^2.)  So, it doesn't matter how far away "space" is to compute the power flux radiated into (or from) the sky.

 

John


Edited by jhayes_tucson, 28 November 2018 - 12:13 AM.

  • terraclarke likes this

#87 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 7090
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 28 November 2018 - 12:12 AM

I have had a very different experience from John Hayes. I get stable viewing from the beginning, and it doesn't get worse as the night progresses. John says the best situation is a combination of fans, coupled with insulation. He typically deals with larger SCTs. I have tried the insulation on a 180mm Maksutov.

 

I can't add Tempest fans to mine, and shoving a fan up the baffle tube simply takes away from my viewing time. I am wondering what difference the scope type makes, and also the size when it comes to cooling. Insulation has transformed my rather temperature finicky scope to a set up and use item.

 

Another question is what difference the dew shield makes in cool down of the corrector or meniscus. I'm assuming that it slows convection some, but really reduces the area the glass can radiate to.

Ah, thanks...you've raised a good point here. A Maksutov and a refractor are slightly different animals than a SCT.  The SCT corrector plate is quite thin compared to either a Mak or a refractor, which significantly changes the heat transfer characteristics of the front element.  Also, for most SCTs, we are talking about a thin aluminum OTA tube.  If your Mak has a white carbon fiber tube, that will change things as well.  Furthermore, larger scopes will always be much harder to get to equilibrium than smaller scopes.  The underlying basic principles are all the same but the results will vary depending on the size and configuration the of the scope.  That applies to the most effective strategies to combat dew as well.  What works on a SCT might not work well on a high-end air-spaced APO system with a Mak falling somewhere in-between the APO and SCT methods.  Radiative cooling will still affect your Mak (as you've experienced) and I think that fans are a good thing, but if you aren't having big problems without the fans, then you probably don't need them.

 

What works well for preventing dew in all cases is a dew shield.  The dew shield does nothing whatsoever about convection but it does limit the field of view of the sky from the front element and that reduces radiative cooling.  Making the dew shield as long as you can get away with is very effective; but, it makes many systems much more susceptible to wind.  I image at an EFL of nearly 4m so wind is a huge concern.  Heating the dew shield directly is a very effective way to prevent dew on the front surface of a refractive or catadioptric system.

 

John


Edited by jhayes_tucson, 28 November 2018 - 12:13 AM.

  • Sarkikos and Adun like this

#88 R Botero

R Botero

    Vanguard

  • -----
  • Posts: 2249
  • Joined: 02 Jan 2009
  • Loc: Kent, England

Posted 28 November 2018 - 03:23 AM

I observe doubles with a 10” f/20 Maksutov which has a big, heavy corrector. I have the scope permanently mounted in a RoR observatory which is not too far from ambient temperature at any time (max 5-7 degrees) since our winters are not as harsh as in other countries and summers are warm not hot, there is no need for air conditioning, etc.
I used to need to leave the roof open at least 1 hour before observing if I was to get any stable images during the nights of most significant change in temperature (spring, autumn). Since I am looking at very close doubles (<5 arc-sec separation), I need good observing conditions.  Atmospheric seeing is the main determinant factor, but image stability at the eyepiece is a close second. 

I installed insulation around my OTA this summer (material similar to Reflectix; just standard radiator wall insulation) and the improvement has been noticeable. Star images are stable from the start of my observing sessions even if I have not managed to open the roof of my observatory in advance.
This effect is similar to the very beginning of some observing sessions when, not having cooled the telescope, star images would be very good for a short 5-10 mins before thermals appeared inside the OTA. A pre-thermal shock nirvana kind of scenario.
An added bonus of the insulation, which has been mentioned in other posts above, is that my corrector doesn’t dew. I don’t use, and have never used, a dew shield and typically rely on a good dewheater band just behind the front glass.  Humidity is high in England (average >60% and in some nights >90%).  With the insulation material wrapped in a double layer around the OTA, the dew heater band is not very effective, yet my corrector is free of dew. I again, suspect this being down to the much slower cooling rate of the whole OTA. Note the insulation material doesn’t extend more than 1 inch beyond the corrector; ie no Reflectix dew shield.

 

To all considering this solution, I say, go for it!

Roberto


Edited by R Botero, 28 November 2018 - 03:41 AM.

  • Jon Isaacs, Skywatchr, eros312 and 1 other like this

#89 precaud

precaud

    Aurora

  • *****
  • Posts: 4932
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 28 November 2018 - 08:10 AM

Good link, thanks.

 

No, the distance doesn't matter, it's the solid angle subtended by the object which is the sky which is a half-sphere. It makes no difference if the old surface is a meter away or a light year if it covers the same angular extent. When the second body is treated as the full sphere then the result only depends on the are of the hot body:

 

https://www.engineer...sfer-d_431.html

 

If you plug the numbers into the calculator and just take the heat transfer from a 1 sq meter area into deep space it's 157 watts for 0.5 emissivity. If you change the cold to only say -60C (stratosphere at night), telescope at 0C, 0.5 emission is 99W for 1 square meter, but if you use the shiny stuff, the heat loss goes way down.



#90 precaud

precaud

    Aurora

  • *****
  • Posts: 4932
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 28 November 2018 - 08:21 AM

Would like to see pics of this....

 

As for how much Reflectix, people have had good luck with two layers, particularly in more severe situations. I use two layers with a single layer over the rear casting.



#91 Skywatchr

Skywatchr

    Mercury-Atlas

  • *****
  • Posts: 2843
  • Joined: 03 Jun 2006
  • Loc: North-Central Pa.

Posted 28 November 2018 - 08:42 AM

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 outer/inner surface of the reflectix is irrelevant in this application.  The closed tube telescope needs a blanket no matter what it's color. Reflectix is convenient because it's surfaces make it easy to attach it.  You can tape it, put self adhesive velcro on it, or rubber bands if you want.  The only properties you need to look at are it's R value, and how easy it is to use in this application. All this talk of "radiational cooling" is just babble. grin.gif  Once you cover the telescope tube with any type of insulating material, the tiny amount of heat actually radiating from the tube is effectively stopped. However, what you do slow down to a greater extent, is convection heat transfer.  The tube of a telescope is effectively a heat sink.  And a heat sink relies on convection to carry away the heat.  Remember, we are on Earth surrounded by air. tongue2.gif  Stop the air flow across the surface of the heat sink (telescope tube), and you will effectively stop the loss of heat from the tube only, since the corrector and rear cell will still be open.  Trap the air at the tube's surface with a thin foam, then wrap that with another layer of Reflectix or any other thin insulation with a surface that makes that easily attached, and you'll probably find nothing better.  This isn't "rocket science" folks (though some make it sound like it), it is common sense.

 

Someone brought up the point that having the metallized surface against the tube will conduct heat from the tube.  This is correct, the metallized layer will actually conduct heat.  However since the surface is not smooth, it will "reflect" *some* heat back.  The overall effect has efficiency loss. Mainly because air can still flow between the Reflectix and tube, thus carry heat out the open ends.  What you really want is a non-metallized surface against the tube that can trap the air and stop convection.  Using a foam with only one "finished" surface will perform far better.

 

The analogy of the "emergency" or "survival" blankets raises a point.  They are called that for a purpose.  They are used in emergency situations only.  They are convenient, lightweight, small and can be stowed away in much smaller areas.  BUT, a good ol' wool blanket will keep you much warmer.  Why?  Because it provides a much better insulation and conforms to your body.  Which is exactly what you want for your scopes.


  • Sarkikos, precaud and Dynan like this

#92 555aaa

555aaa

    Vendor (Xerxes Scientific)

  • *****
  • Vendors
  • Posts: 1392
  • Joined: 09 Aug 2016
  • Loc: Lynnwood, WA, USA

Posted 28 November 2018 - 10:42 AM

For me the insulating drapery material works a little better than reflectrix, It has more bulk to it but is has an internal metalized layer also.
  • Skywatchr and Sarkikos like this

#93 Sarkikos

Sarkikos

    ISS

  • *****
  • topic starter
  • Posts: 30442
  • Joined: 18 Dec 2007
  • Loc: Per sylvam ad astra

Posted 28 November 2018 - 10:51 AM

What works well for preventing dew in all cases is a dew shield.  The dew shield does nothing whatsoever about convection but it does limit the field of view of the sky from the front element and that reduces radiative cooling.  Making the dew shield as long as you can get away with is very effective; but, it makes many systems much more susceptible to wind.  I image at an EFL of nearly 4m so wind is a huge concern.  Heating the dew shield directly is a very effective way to prevent dew on the front surface of a refractive or catadioptric system.

 

John

I've put dew shields on every type of instrument.  I've made dew shields for binoculars.  I even put a dew shield on my 10" solid tube Newt to prevent dew on the secondary and block ambient glare.  In my experience they work both as dew shields and as light shields. 

 

At my dewy sites, a warming strip around the base of the dew shield is necessary if I want to observe more than an hour or so with an SCT or refractor.  Only the finder scope can go long with just the dew shield.  But dew will even catch up to the finder given enough hours without a warming strip.

 

Heating the dew shield directly makes sense to prevent dew on the objective or corrector.  But there are some naysayers who claim that - at least when wrapping the OTA with Reflectix - the warming strip should not be on the dew shield, but on the OTA behind it.  If true, this would be very inconvenient for me, because I have a couple AstroZap dew shields with built-in warming strips.  The internal warming strip is positioned about at the corrector.  So do you think I should rip the strip out and install it behind the dew shield, or leave it where it is, inside the shield? grin.gif

 

Mike


Edited by Sarkikos, 28 November 2018 - 10:59 AM.

  • Jon Isaacs and Dynan like this

#94 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 7090
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 28 November 2018 - 11:44 AM

I've put dew shields on every type of instrument.  I've made dew shields for binoculars.  I even put a dew shield on my 10" solid tube Newt to prevent dew on the secondary and block ambient glare.  In my experience they work both as dew shields and as light shields. 

 

At my dewy sites, a warming strip around the base of the dew shield is necessary if I want to observe more than an hour or so with an SCT or refractor.  Only the finder scope can go long with just the dew shield.  But dew will even catch up to the finder given enough hours without a warming strip.

 

Heating the dew shield directly makes sense to prevent dew on the objective or corrector.  But there are some naysayers who claim that - at least when wrapping the OTA with Reflectix - the warming strip should not be on the dew shield, but on the OTA behind it.  If true, this would be very inconvenient for me, because I have a couple AstroZap dew shields with built-in warming strips.  The internal warming strip is positioned about at the corrector.  So do you think I should rip the strip out and install it behind the dew shield, or leave it where it is, inside the shield? grin.gif

 

Mike

 

Mike,

Warming the tube of a SCT works, but under most conditions and on its own, it's probably not the best way to prevent dew.  The problem is that it relies on convection and radiation to heat the corrector plate from the inside.  That means that the inside surface has to be warmed more than necessary to bring the temperature of the front of the corrector up to the outside air temperature.  Remember, it's the front surface that counts and you want to use as little power as possible to minimize convective turbulence.  In either case, we probably want to raise the temperature of the front surface a little above the air temperature (by maybe 1-3 degree) to provide a small safety margin against dew.  One other thing that I haven't mentioned is that when it's cold, frost forms at a higher temperature than the dew point so that's another reason to provide more margin when it's cold.  

 

The best way to heat a dew shield is to heat it uniformly with more than a single strap and to control its temperature with respect to the air temperature.  I am currently using two straps on my C14 controlled with a Dew Buster that has a heat sensor under the straps.  The Dew Buster allows setting a temperature differential between the heater strap and the ambient air temperature.  I also have a heater positioned about 4-5 inches behind the corrector on the tube and here's why.  In order to minimize the cross section of my scope for wind resistance, I use a relatively short dew shield, which requires more heat than might be good for minimizing air movement.  I'm still trying to evaluate whether or not I'm going to pull this heater from the tube and move it to the shield.  Unfortunately, even though I have temperature sensors all over my scope, I don't have a way to remotely control any of the heaters independently or it would be easy to experiment to see what works best.  I do have the ability to measure the temperature of the front corrector and the air temperature and I can remotely turn the whole system on or off.  I've found that my system typically holds the corrector at about 1.8 degrees C (+/- ~0.2) above the air temperature.  There have been some pretty pronounced dew conditions out at DSW (RH > 90%) this fall and so far I haven't see a trace of dew or frost, which indicates that it's working pretty well.

 

John


  • Sarkikos likes this

#95 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 7090
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 28 November 2018 - 12:01 PM

The outer/inner surface of the reflectix is irrelevant in this application.  The closed tube telescope needs a blanket no matter what it's color. Reflectix is convenient because it's surfaces make it easy to attach it.  You can tape it, put self adhesive velcro on it, or rubber bands if you want.  The only properties you need to look at are it's R value, and how easy it is to use in this application. All this talk of "radiational cooling" is just babble. grin.gif  Once you cover the telescope tube with any type of insulating material, the tiny amount of heat actually radiating from the tube is effectively stopped. However, what you do slow down to a greater extent, is convection heat transfer.  The tube of a telescope is effectively a heat sink.  And a heat sink relies on convection to carry away the heat.  Remember, we are on Earth surrounded by air. tongue2.gif  Stop the air flow across the surface of the heat sink (telescope tube), and you will effectively stop the loss of heat from the tube only, since the corrector and rear cell will still be open.  Trap the air at the tube's surface with a thin foam, then wrap that with another layer of Reflectix or any other thin insulation with a surface that makes that easily attached, and you'll probably find nothing better.  This isn't "rocket science" folks (though some make it sound like it), it is common sense.

 

Someone brought up the point that having the metallized surface against the tube will conduct heat from the tube.  This is correct, the metallized layer will actually conduct heat.  However since the surface is not smooth, it will "reflect" *some* heat back.  The overall effect has efficiency loss. Mainly because air can still flow between the Reflectix and tube, thus carry heat out the open ends.  What you really want is a non-metallized surface against the tube that can trap the air and stop convection.  Using a foam with only one "finished" surface will perform far better.

 

The analogy of the "emergency" or "survival" blankets raises a point.  They are called that for a purpose.  They are used in emergency situations only.  They are convenient, lightweight, small and can be stowed away in much smaller areas.  BUT, a good ol' wool blanket will keep you much warmer.  Why?  Because it provides a much better insulation and conforms to your body.  Which is exactly what you want for your scopes.

 

I don't mind opinions but it is very discouraging to see any post like this one that actively discourage the value of basic physics in favor of "common sense."   For anyone with a technical education, "common sense" includes the principles of physics.  I'm not going to comment on all of the incorrect points here but I will point out that the 'R' value of any insulator takes into account both conductive insulation and radiative effects.  They don't make Reflectix shiny just because it look pretty.

 

John


  • Jaimo!, 1074j, munirocks and 2 others like this

#96 TG

TG

    Vanguard

  • *****
  • Posts: 2357
  • Joined: 02 Nov 2006
  • Loc: Latitude 47

Posted 28 November 2018 - 02:55 PM

Hi Mike,

   I have to disagree with what you wrote: the key to reflectix is the the shiny outer layer because it reduces radiative cooling. If you paint it black, radiative cooling *increases* (c.f. black body radiation and the reason why emergency blankets used by EMTs are silver and reflective), which means that the reflectix is less effective as an insulation when black. As for the inner surface of the reflectix that touches the OTA, heat transfer is via conduction -- this means that the shininess of the surface is not important. What reflectix does here is to increase conduction resistance having an air layer. Air is a very bad heat conductor.

 

  The bottom line: if you want an effective insulation of the OTA, keep the reflectix silver on the outside and try to keep the reflectix tightly wrapped onto the OTA.

 

cytan

If you paint the outer part black, the inner part is still shiny and since it's the second layer separated from the first one by air bubbles, I think conduction will be minimal. I have a reflectix shipping "box" for shipping foods that I might experiment with...



#97 Skywatchr

Skywatchr

    Mercury-Atlas

  • *****
  • Posts: 2843
  • Joined: 03 Jun 2006
  • Loc: North-Central Pa.

Posted 28 November 2018 - 06:14 PM

I don't mind opinions but it is very discouraging to see any post like this one that actively discourage the value of basic physics in favor of "common sense."   For anyone with a technical education, "common sense" includes the principles of physics.  I'm not going to comment on all of the incorrect points here but I will point out that the 'R' value of any insulator takes into account both conductive insulation and radiative effects.  They don't make Reflectix shiny just because it look pretty.

 

John

I really don't care what you "don't mind".  You don't need a degree in thermodynamics, statistical mechanics, and kinetic theory to understand how to put a blanket on a telescope, professor John.  Reflectix is a product that was developed for the construction, and packing industry, not for amateur Astronomy.  Though many who possess common sense have learned how to adapt it even without your superior intellect.  I didn't "actively discourage" anything, except perhaps, over-thinking. Interestingly, the cave dwellers knew enough to put a "door" on a cave, and to wrap themselves in an animal's pelt with the fur to keep warm.  They had no clue what thermal physics were, but yet, they had common sense to see that stopping air flow keeps heat in. Common sense that you seem to be discouraging, unless of course, one has a "technical education".  You hijacked the original question and turned the thread into a "superior technical" thread.  Do you remember the OP?  Here, I'll help...

 

"I observe in areas that are prone to prolonged heavy dew/frost.  If there is no dew/frost, that would be the exception.  Unless I put the dew shield on the scope, and wrap warming strips around the OTA behind the corrector, around the eyepiece, on the Telrad and around the finderscope objective and eyepiece, turn on the batteries and keep them on, dew/frost will form on the optics, guaranteed.  It's not really a question of if dew/frost will form, but when it will form:  sooner or later.  The warming strips DO prevent dew/frost.

 

So if I wrap the OTA in Reflectix, will I still need the warming strips to prevent dew/frost?  If there is a choice between no thermals and no dew/frost, I'd rather do without the dew/frost.  The effect of dew/frost on the corrector is a lot worse for observing than thermals in the tube.

 

Mike

"



#98 Rock22

Rock22

    Ranger 4

  • *****
  • Posts: 338
  • Joined: 28 Aug 2017
  • Loc: Diamond Bar, California

Posted 28 November 2018 - 07:04 PM

The OP is trying to understand something that is not intuitive, and I appreciate John for demonstrating that it is in fact not a simple explanation that can answer the OP’s questions about the rationale behind using insulation a certain way.

John’s info is certainly valuable for those who need more precise adjustments for more advanced and complex imaging and observations, but it gives me more knowledge about what I can do and what I can’t with my current gear.

I also think common sense is useful, but such sense must be informed, validated, and shared. Without the first two qualifiers, it’s common nonsense. Too much of that on our college campuses as it is.

The OP seems to be working this out in his mind and trying things out, and the info John gives is helpful not merely to the OP, but to the community here. I give kudos to the OP for trying to wrap all this around his brain and asking questions that many of us might have. If there is one bit of common sense relevant to me about insulating with Reflectix it’s this: if it ain’t broke, don’t fix it. Insulating the tube and dew shield with Reflectix just as others have said, works for my mak in keeping the views good from the get go and keeping dew off my corrector from where I observe.

Edited by Rock22, 28 November 2018 - 07:09 PM.

  • Skywatchr likes this

#99 Skywatchr

Skywatchr

    Mercury-Atlas

  • *****
  • Posts: 2843
  • Joined: 03 Jun 2006
  • Loc: North-Central Pa.

Posted 28 November 2018 - 08:22 PM

The OP is trying to understand something that is not intuitive, and I appreciate John for demonstrating that it is in fact not a simple explanation that can answer the OP’s questions about the rationale behind using insulation a certain way.

John’s info is certainly valuable for those who need more precise adjustments for more advanced and complex imaging and observations, but it gives me more knowledge about what I can do and what I can’t with my current gear.

I also think common sense is useful, but such sense must be informed, validated, and shared. Without the first two qualifiers, it’s common nonsense. Too much of that on our college campuses as it is.

The OP seems to be working this out in his mind and trying things out, and the info John gives is helpful not merely to the OP, but to the community here. I give kudos to the OP for trying to wrap all this around his brain and asking questions that many of us might have. If there is one bit of common sense relevant to me about insulating with Reflectix it’s this: if it ain’t broke, don’t fix it. Insulating the tube and dew shield with Reflectix just as others have said, works for my mak in keeping the views good from the get go and keeping dew off my corrector from where I observe.

Oh, I can agree with that.  However the OP didn't ask for an in-depth scientific, peer-reviewed explanation of the intricacies of thermal dynamics.  Mike asked a simple question for which I, among others, gave a straight forward simple answer.  This thread wasn't at all about "the science", it was simply a "yes" or "no" answer with a reason for the "yes" (or no).  Hey, if you guys want to know about the "science", or try and see who is "smarter", read all the other threads on here about it.  There is no need to hijack every thread and turn it into yet another one of the other many threads when the OP just asked a simple question.  John's knowledge is a plus to these forums.  But he sometimes goes too deep making a mountain out of a mole hill. And sometimes belittles someone in the process...  I think it's time to wrap this one up (pun intended) tongue2.gif

 

Clear Skies!


  • Rock22 likes this

#100 bikerdib

bikerdib

    Viking 1

  • *****
  • Posts: 740
  • Joined: 30 Apr 2014
  • Loc: Southeast Texas

Posted 28 November 2018 - 08:54 PM

When I first started using an SCT (a 9.25" that I owned previous to the one I now own), I set up long before I intnded to use it so it could acclimate.  Well, we all know how well that works.  Then I bought a CatCooler which significantly helped shorten the acclimation time.  A few years later I bought a 14" XLT and a longer tube for the CatCooler.  It did work but as those of us that have a large SCT know, a large SCT takes a REALLY long time to acclimate.  So, I added the Tempest fans since my 14" was a later version that had the rear vents.  In the 14" I usually ran both the CatCooler and the Tempest fans at first to speed up the acclimation process then the Tempest fans for a while while observing.  Another upgrade brought in the 14" Edge.  I transfered over the Tempest fans but could no longer use the CatCooler in the Edge.  That slowed the process down once again.

 

Then I found a thread about wrapping the tube.  I did both my 9.25" and my 14" Edge.  I still use the Tempest fans in the 14" and the CatCooler in the 9.25" to help acclimate before starting to view but the wrap definately helps with stabilising currents.  As previously stated, I think it helps by slowng the temperature variants between the tube and the outside air which calms the currents enough to prevent larger thermals inside the tube.  All I know is, it helps speed up the aclimation of the scope.  BTW, what I do is stand the scope upright on the corrector end while I run the fans to acclimate.  Since heat rises, the fans seem more effective at removing the warm air from the tube at the rear where the fans are located.  It even seems to help the 9.25 running the CatCooler to have the OTA standing upright.


  • Skywatchr likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: catadioptric, DIY, equipment, SCT



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics