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Does SCT insulation with reflectix make tube vents a *disadvantage*?

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#26 Dimperev

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Posted 28 November 2018 - 12:32 AM

My observation experience led to the following configuration for the C11EDGE for visualization.

 

1. Inside the telescope tube there are 2 fans 50x50 mm installed. On the back cast base behind the main mirror.

2. One fan draws air in from the gap between the OTA wall and the main mirror.

3. The second fan blows air into the gap between the OTA wall and the main mirror.

4. Fans are always on. This ensures the convection of air inside the OTA pipe, even up to the correction plate.

5. A bag of selikogel is attached to the grille of one of the holes of the OTA. This prevents dew and frost inside the OTA.

6. Both air vents are tightly sealed outside.

7. Installed a short shield dew, about 10-12 inches.

8. On the shield of dew above the corrector installed 1 strip of the heater (up to 10W). It heats only the dew shield for radiation exchange with the corrector.

    I almost never use a heater. On wet nights I switch on 2-3W mode, but this is probably not necessary.

9. The OTA pipe and the dew shield are covered with 2 layers of insulator (analogue of Reflectix).

10. The back surface and part of the OTA under the mounting plate are covered with 1 layer of insulator (analogue of Reflectix).

 

This is a stable configuration.


Edited by Dimperev, 28 November 2018 - 12:34 AM.

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#27 jhayes_tucson

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Posted 28 November 2018 - 01:52 AM

I can't claim experience with the insulated scopes, but I have some logic problems with the insulated dewcap.  If the end of the cap is open to the air (of course) how would insulating the outer surface make any difference at all? The inner surface would immediately assume the ambient temp of the air freely entering.

If I had a house with a huge bay window, and insulated the walls of the bay portion, then left the huge window wide open, what would be the benefit of the insulation?

The insulation of the scope tube makes perfect sense. I can understand that no problem.

Insulating an open dewcap just seems like a waste of time.

 

You are forgetting about radiative cooling.  If you have a black dew shield, it will stabilize at a temperature below the air temperature due to radiative heat loss to the sky and it will be much less effective at shielding the front element from heat loss to the sky.  By ensuring the shield is at the same temperature as the air (or even warmer,) you greatly reduce the amount of radiative cooling on the front surface of glass, which reduces the possibility of dew.

 

John



#28 jhayes_tucson

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Posted 28 November 2018 - 02:47 AM

Why would the vents or fans IMPROVE anything, if using insulation, since they would COOL the inside of the tube, which is NOT desired!?  Reflectix RETAINS the internal heat of the tube, as it is when indoors, and ever so slowly radiates the heat away, once it is brought Outdoors.  To have Reflectix AND fans is extremely counter productive, and defeats the purpose!  If some are asking if fans AND Reflectix can work, they are not understanding the issue.

  Perhaps I can help.  Since their emergence as a very popular, commercial scope, SCT's have been know, in colder climates, as 'warm weather scopes'.  The reason being that when the temperature differential between where they were stored inside a home, and the ambient outside air temps (Delta) was great, there were Air Currents created inside the tube.  This is especially noticeable on 8" and larger versions.  Sometimes, with a large delta, or rapidly falling outside temps, the inside NEVER catches up to outside ambient!  And the views are horrible---mushy, thermal spikes, etc. And this is true EVEN if you put the scope out to 'cool' hours before viewing---the tube never catches up to outside ambient. 

     What Insulation does, when applied INSIDE where it is warm, is retain the heat for a long, long time inside the tube.  Thus, NO air currents are created, and seeing/views are sharp the moment it is taken outside and viewed through.

   So asking if anyone has tried it WITH fans is not a good question IF you understand the science of the problem, and the science of using Insulation.  

 

Well, I think that I understand heat transfer pretty well so let me take a crack at it.  It looks to me like you are forgetting about achieving a thermally stable system, which is the best way to insure optimum imaging performance.  Optimum imaging performance comes from low wavefront errors and it doesn't take much to introduce 1/4 wave errors due to very small thermal variations in the air.  Yes, insulating the heck out of a system before taking it from a warm house into the cold outside under a clear, dark sky will slow cooling of the system but that won't eliminate thermal problems.  First off, the front corrector has to be open to the air and to the sky so it will cool fairly rapidly.  Heat conducted through the corrector due to the warm temperature of the inside of the scope will cause convection in the light path and the more heat that you transfer the worse the problem.  Remember that a 1/4 wave error in the wavefront is very easy to introduce.  Second, the temperature of the inside of the corrector plate will fall fairly quickly to be at a considerably colder temperature than the air inside the tube, which can lead to stable thermal gradients in the air or thermal convection (depending on the temperatures.)  I've personally seen double imaging from a stable thermal gradient in a C14.  Yes, the effects may be less than what you will get without any insulation, but they will last a long time because the majority of heat transfer will have to occur through the corrector plate (and perhaps the rear cell as well if it isn't covered.)  If you really want to keep heat from leaking out of the tube, you need a double-pane corrector plate!  BTW, that's why air-spaced refractors with a wrapped tube hold heat so well.

 

What you want for the best imaging is a thermally stable system.  Reflectix is effective at eliminating radiative heat loss at the tube.  It also greatly lowers convective loss at the tube but that's not good for achieving thermal stability.  Adding fans to convectively cool the inside of the tube will remove baffle currents, eliminate stable temperature gradients, and more quickly bring the inside of the tube to temperature equilibrium with the outside air, which is what you want.  And it doesn't take very much air flow to make it effective.  Tempest fans are very tiny and yet they work extremely well to dissipate tube currents and more quickly stabilize the system.  I run them all the time on my Reflectix covered C14 Edge and the difference in performance with the fans running is night and day.  In my view, Reflectix is not a very good solution for achieving thermal stability without the use of fans.  

 

I would agree that Reflectix is a great solution if mediocre imaging is preferable to poor imaging when you first bring a scope out into a cold night but without fans, it's half a solution.

 

John


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#29 Dimperev

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Posted 28 November 2018 - 06:23 AM

Additionally, I can report that for a typical decrease in temperature over night by 4-7 degrees, the temperature inside my C11 is always 1-3 degrees higher. I try to keep this temperature difference. This does not degrade the image quality when the fans are running, but it almost eliminates the problem of dew. So I save the charge of my battery and almost never use the dew belt heated.


Edited by Dimperev, 28 November 2018 - 06:27 AM.

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#30 Conaxian

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Posted 28 November 2018 - 11:03 PM

You are forgetting about radiative cooling.  If you have a black dew shield, it will stabilize at a temperature below the air temperature due to radiative heat loss to the sky and it will be much less effective at shielding the front element from heat loss to the sky.  By ensuring the shield is at the same temperature as the air (or even warmer,) you greatly reduce the amount of radiative cooling on the front surface of glass, which reduces the possibility of dew.

 

John

Since I have the reflectix and since it took about 10 minutes to do, I already put one layer on the dewcap. My problem now is the el stinko weather pattern we suffer with in these parts.  It was supposed to be clear today but no.. it is socked in and 20F, with clouds for tomorrow and rain for Friday.  I may have to move to Arizona to try it out. 

Another poster reminded me how easy it is to try, so I thought, 'why not?'

 

I wonder, is this insulation needed at all in warmer weather? One fellow on Oahu has his wrapped, and they enjoy a very mild climate. Compared to the arctic tundra I inhabit.



#31 DMach

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Posted 29 November 2018 - 04:34 AM

Since I have the reflectix and since it took about 10 minutes to do, I already put one layer on the dewcap. My problem now is the el stinko weather pattern we suffer with in these parts.  It was supposed to be clear today but no.. it is socked in and 20F, with clouds for tomorrow and rain for Friday.  I may have to move to Arizona to try it out. 

Another poster reminded me how easy it is to try, so I thought, 'why not?'

 

I wonder, is this insulation needed at all in warmer weather? One fellow on Oahu has his wrapped, and they enjoy a very mild climate. Compared to the arctic tundra I inhabit.

The thought has definitely occurred ... but I figure it can't hurt.

 

For me though it'll be more important as part of my portable/traveller setup.



#32 Peter Besenbruch

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Posted 30 November 2018 - 03:41 PM

I wonder, is this insulation needed at all in warmer weather? One fellow on Oahu has his wrapped, and they enjoy a very mild climate. Compared to the arctic tundra I inhabit.

I frequently got tube currents before insulating. Jupiter's moons produced a star like point at all focus points, that wasn't really the moon. It's what got me to try insulating. Jupiter's moons are fine, now.



#33 Jeff B

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Posted 01 December 2018 - 12:56 AM

One big issue is the temperature delta between the mirrors and internal air.  Rapidly exchanging the air inside the tube with ambient will result in the the mirrors being warmer than the internal air.  They will then shed heat and plumes can result, just like in a newt.  The bigger the temperature delta between the internal air and the mirror, the more heat that will be shed from the mirror.  So, it's best to have the mirrors at or very close to the internal air temperature to mitigate heat shed from the mirrors.  Insulating the tube and back plate considerably slows the heat loss to ambient with a sealed system.  Yes, there is loss via the corrector and that will cool off the interior but that will occur at a much slower rate with the insulated tube.  But as John points out, the back of the corrector will cool quickly to below the internal temperature of the internal air.  Fortunately, the correctors don't have much optical "power" so any distortions resulting from changes in the corrector's figure with temperature are small.  Again, as John points out, the real problem will be stable thermal gradients in the air and especially on the back of the plate.  That cool air will fall and settle in the tube, creating other gradients.   And that has been my experience as well with my C11 and TEC 7.  But the Reflectix stuff really does help nicely from a usable visual perspective and really mitigates or pushes way out the time it takes the corrector to dew up.  

 

Now the arm chair speculation.  To me, it then makes sense to mix the internal air up enough to break up stable temperature gradients.  So, do this mixing with well placed internal fans.  Yes, it will cool the internal air volume quicker with the increased convection cooling via the corrector (and yes, a little bit via the tube even with the reflectix) but it would break up that cold air layer on the back of the corrector and keep pools of cooler air from forming elsewhere in the tube. 

 

However, this cooling could lead you right back a bit towards the very issue to avoid in the first place which is the mirror shedding heat to the internal air because the interior air temperature drops too quickly.   

 

So add heat to the tube's interior to replace, at least in part, the heat lost when mixing the interior air around  

 

Maybe a heater strip wrapped around the tube's exterior, under the Reflectix?   And, BTW, blow ambient air over the corrector front face to bust up any pesky stable gradients there too. 

 

This way you can directly control, manage or modulate a uniform thermal state inside the tube while also minimizing performance robbing stable temperature gradients and plumes and pools(hot or cold).  An added bonus is that there would be no need for a dew shield as there will be no more dew. 

 

Sounds like a project brewing for me or others maybe?

 

Jeff


Edited by Jeff B, 01 December 2018 - 01:03 AM.


#34 jhayes_tucson

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Posted 01 December 2018 - 01:26 AM

One big issue is the temperature delta between the mirrors and internal air.  Rapidly exchanging the air inside the tube with ambient will result in the the mirrors being warmer than the internal air.  They will then shed heat and plumes can result, just like in a newt.  The bigger the temperature delta between the internal air and the mirror, the more heat that will be shed from the mirror.  So, it's best to have the mirrors at or very close to the internal air temperature to mitigate heat shed from the mirrors.  Insulating the tube and back plate considerably slows the heat loss to ambient with a sealed system.  Yes, there is loss via the corrector and that will cool off the interior but that will occur at a much slower rate with the insulated tube.  But as John points out, the back of the corrector will cool quickly to below the internal temperature of the internal air.  Fortunately, the correctors don't have much optical "power" so any distortions resulting from changes in the corrector's figure with temperature are small.  Again, as John points out, the real problem will be stable thermal gradients in the air and especially on the back of the plate.  That cool air will fall and settle in the tube, creating other gradients.   And that has been my experience as well with my C11 and TEC 7.  But the Reflectix stuff really does help nicely from a usable visual perspective and really mitigates or pushes way out the time it takes the corrector to dew up.  

 

Now the arm chair speculation.  To me, it then makes sense to mix the internal air up enough to break up stable temperature gradients.  So, do this mixing with well placed internal fans.  Yes, it will cool the internal air volume quicker with the increased convection cooling via the corrector (and yes, a little bit via the tube even with the reflectix) but it would break up that cold air layer on the back of the corrector and keep pools of cooler air from forming elsewhere in the tube. 

 

However, this cooling could lead you right back a bit towards the very issue to avoid in the first place which is the mirror shedding heat to the internal air because the interior air temperature drops too quickly.   

 

So add heat to the tube's interior to replace, at least in part, the heat lost when mixing the interior air around  

 

Maybe a heater strip wrapped around the tube's exterior, under the Reflectix?   And, BTW, blow ambient air over the corrector front face to bust up any pesky stable gradients there too. 

 

This way you can directly control, manage or modulate a uniform thermal state inside the tube while also minimizing performance robbing stable temperature gradients and plumes and pools(hot or cold).  An added bonus is that there would be no need for a dew shield as there will be no more dew. 

 

Sounds like a project brewing for me or others maybe?

 

Jeff

 

Jeff,

One thing to remember is that forced convection cooling is much more efficient than natural convective cooling.  Tempest fans don't provide a huge amount of air movement but it's enough to speed things up and to break up the currents that come from the components as they cool.  You can easily see the difference if you look at the currents in a defocused star image.  Let them develop with the fans off and then watch what happens when you turn on the fans.

 

I personally think that it's folly to try to figure out how to produce good imaging from a scope that's come directly from a warm house (or garage) directly into a cold night.  I think that it's better to figure out how to get a scope stabilized as quickly as possible.  Of course, that time will vary considerably with the size of the scope.  A C6 clearly won't need as much time as a C14.  Before I took my scope (a C14 Edge) out to DSW, I would roll it out just after the sun went down so that it could cool off for about 2 hours before I was ready to image.  With the Tempest fans running, that usually worked out to be about right.  I found that the thermal wrap and fans helped to keep thing really stable as the night wore on.  It's about maintaining a stable system and I never viewed the thermal wrap as a way to eliminate the cool-down period.  I should mention that I kept my scope in a building that had a thermostat set at 36F just to keep things from freezing.  Regardless, it had a lot of thermal mass so it was always quite a bit warmer than the night air outside.

 

John


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#35 Jeff B

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Posted 01 December 2018 - 12:34 PM

Jeff,

One thing to remember is that forced convection cooling is much more efficient than natural convective cooling.  Tempest fans don't provide a huge amount of air movement but it's enough to speed things up and to break up the currents that come from the components as they cool.  You can easily see the difference if you look at the currents in a defocused star image.  Let them develop with the fans off and then watch what happens when you turn on the fans.

 

I personally think that it's folly to try to figure out how to produce good imaging from a scope that's come directly from a warm house (or garage) directly into a cold night.  I think that it's better to figure out how to get a scope stabilized as quickly as possible.  Of course, that time will vary considerably with the size of the scope.  A C6 clearly won't need as much time as a C14.  Before I took my scope (a C14 Edge) out to DSW, I would roll it out just after the sun went down so that it could cool off for about 2 hours before I was ready to image.  With the Tempest fans running, that usually worked out to be about right.  I found that the thermal wrap and fans helped to keep thing really stable as the night wore on.  It's about maintaining a stable system and I never viewed the thermal wrap as a way to eliminate the cool-down period.  I should mention that I kept my scope in a building that had a thermostat set at 36F just to keep things from freezing.  Regardless, it had a lot of thermal mass so it was always quite a bit warmer than the night air outside.

 

John

Agreed!  To me it's thermal management of the interior (both air and glass) and there could be a couple valid of paths and associated tool sets to doing it.  I view the wrap as a tool.  

 

Traditional thinking seems to be having a very high exchange rate for the air inside the tube as well as air blowing on the back of the primary and maybe across the primary face to break up boundary layers, just like in a newt.  And it works.  My original cooling scheme for the C11 did just that with a large amount of ambient air impinging on the back of the primary ( but no boundary layer fan).  It would  indeed "come on line"  much faster than doing nothing after taking it out of my warm basement to a 30+ degree F lower ambient temperature (yes I too found it best to put the thing out a couple of hours sooner but sometimes I could just not do that).   However, I found practical disadvantages with that approach such as fine stuff entering the OTA, internal dewing once inside, and chronic corrector dewing.   

 

I find the idea of managing a big CAT's internal environment interesting and it just might be possible to do it while keeping it sealed, even after taking it from a warm house .  When I say "manage", I mean slow the cooling down in a controlled, stable manner, which is the flip of dropping its internal temperature very quickly and then stabilizing it   Or some sort of hybrid, like your C14, with its very large internal thermal mass, in which the low flow of the vent fans is just about right combined with the thermal blanket.  The impingement fans are sealed off on my C11 but I can certainly experiment with turning them on to mix the internal air.

 

Do you have any residual issues with dew formation?  The lack of my typical chronic dewing and the much improved performance of the scope, which was immediately had with the insulation, are big attractions for me.

 

Jeff



#36 geothomas

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Posted 23 February 2019 - 06:48 PM

Do you take the Vixen plates off of the scope, wrap the scope in reflectix all the way around and then put the vixen plates back on or do you just wrap reflectix around the scope up to but not over the vixen plate and just leave the part of the scope under the vixen plate as is?



#37 gnowellsct

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Posted 23 February 2019 - 11:47 PM

Thing to remember is that after dark the ground is always going to be warmer than the air and the universe is almost absolute zero. Thus the OTA is being heated from the ground's radiation and losing heat to the universe by radiation. Also aluminum is a great heat sink and a lousy insulator.

I was thinking about this today and although it is true broadly speaking, it doesn't get to the particulars.  I actually don't have much of an issue cooling my c14.  In fact the real problem is heating it.  It cools so well that it is always cooling below ambient and getting dew.

 

You might say well, OK, then you have a weird thing going on  where the mirror is hot and the corrector plate is cold.  But I don't think that's it either.

 

I think these scopes are actually fairly efficient at cooling down.  So the issue is what would slow them down from cooling down.  I'm trying to figure out why in the days when I wrapped the C14 in insulation it didn't really appreciably change things.

 

I come down to idea that when one observes on grass 

 

(a) the ground is in fact warmer but

(b) the grass is losing heat very rapidly due to radiative heat loss and convection cooling.  The latter comes from the release of moisture into the sky. It is easy to see in my part of the world after a hot day that grass surfaces are ten to fifteen degrees cooler than the asphalt driveway and street.  

 

The convective cooling of the grass means that it is releasing a stream of moisture around the scope.  Ground side of the OTA is warmer than sky side (I have measured this) but on the other hand the flow moisture would create and enhance a convective cooling effect.  

 

In any case it always remains that the main temperature issue I deal with in my part of the world is the tendency of the scope to cool and in particular the tendency of the corrector plate to cool faster than anything else.  I'm not sure the reflectix is going to do anything positive except delay the cooling of the scope to approximately air ambient.

 

I have in the past done IR temperature readings on the OTA and corrector plate but I've never really it systematically over several hours.  I have the tool so it's probably time for me to try again.  

 

Year after year I read here about all the problems people have with cooling.  I have had a hot scope under some circumstances: when I left it in a car during the day, for example, I had a real bad first hour but the scope quickly began to steady out (on grass).   I didn't have to wait all night for the views to get good, as I read here. 

 

Greg N  



#38 mr1337

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Posted 05 April 2019 - 10:11 PM

Do you take the Vixen plates off of the scope, wrap the scope in reflectix all the way around and then put the vixen plates back on or do you just wrap reflectix around the scope up to but not over the vixen plate and just leave the part of the scope under the vixen plate as is?

I found on my EdgeHD 8" that the insulation will actually fit underneath the vixen plate. As long as it isn't longer than the painted metal part of the OTA you can shimmy it in there between the two. The trick I found is to keep the leading edge completely paralell to the dovetail and slide it back and forth along the OTA as you push it under the dovetail. Once enough gets through to grab it's easy to finish pulling it through. On the other side where the seam is I just use some velcro so I can peel back the insulation until the tube cools down, then I can fasten it back if I want to slow the effects of rapidly dropping temperatures. Still need to get some Tempest fans though...




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