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Insulating my SW 180 Mak.

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#26 yellobeard

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Posted 29 August 2018 - 01:26 PM

Very nice to see how this thread is evolving!

#27 elwaine

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Posted 29 August 2018 - 01:27 PM

Usquebae, a few things to keep in mind with insulating jackets:

 

1. Make sure you use enough insulation for your environment. E.g., if you use Reflectix, depending on your environment, you may be fine with just a single wrap of the material... or you may need to double up on the insulation. Better still, a thin foam blanket- the thin white sheets of foam used to cushion objects during shipping - wrapped directly onto your OTA and then followed by a single wrap of Reflectix on top of the foam sheet will increase the R value significantly... probably more that using a double wrap of Reflectix without the underlying foam. (That suggestion comes from information found on the Reflectix website.)

 

There have been a few people who reported that insulation jackets did not work for them. I suspect they failed to use sufficient insulation, but they provided no details.

 

2. If you also insulate your dew shield, chances are you will not have to resort to active heating to prevent dew from forming on the corrector. That means no need for heating strips, controller or batteries to prevent dew buildup. 

 

3. In my experience, Tempest fans do work. But they work to lessen cool down time. They do not eliminate the need to wait until thermal equilibrium has been reached. They just lessen the time one has to wait before using the scope. With insulation jackets, the object is not to achieve thermal equilibrium between the ambient temperature and the volume of air inside the OTA. The object is to maintain the thermal equilibrium that already exists within the OTA before bringing the scope from a warm house into a colder environment. The jacket greatly retards convective and radiative cooling of the OTA, thus preventing thermals from arising inside the tube. So the scope is ready to use as soon as it’s brought outside (providing the jacket is constructed properly: see #1 above). 

 

There are two additional benefits of an insulation jacket over the Tempest fans: fans pull moist air and dust inside the OTA, and fans require wires and batteries. 


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#28 yellobeard

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Posted 29 August 2018 - 01:30 PM

Now that is a post that should be pinned elwaine!!
I could not have said it better myself!

Edited by yellobeard, 29 August 2018 - 01:31 PM.

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#29 Joe1950

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Posted 29 August 2018 - 01:48 PM

So, would I be correct in assuming... the same very slow temperature change within the tube that would minimize air convection and thermals, would significantly slow the effect a quickly changing temperature would have on the primary - that being to alter its figure during temperature change?


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#30 aruckle

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Posted 29 August 2018 - 02:05 PM

     This thread appears to be mostly for Telescope visual use.    It would seem, using the insulation on telescopes for astro photography would help also.  In this case the need for refocusing the telescope should increase the amount of time between the need to refocus.

 

aruckle 


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#31 astrobeast

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Posted 29 August 2018 - 03:08 PM

 

 

There have been a few people who reported that insulation jackets did not work for them. I suspect they failed to use sufficient insulation, but they provided no details.

 

 

Over the years I have seen posts and pics of users SCT's wrapped in similar materials, usually posted from cold climates in the winter. They have spoken to how it helped, but I never recall anyone saying that eliminated the need for fans, etc. But not details.

 

Maybe as the poster suggested, they didn't use enough, or completely insulted the tube.

 

R



#32 elwaine

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Posted 29 August 2018 - 03:35 PM

Over the years I have seen posts and pics of users SCT's wrapped in similar materials, usually posted from cold climates in the winter. They have spoken to how it helped, but I never recall anyone saying that eliminated the need for fans, etc.

I think you miss the point. Fans aid in cool down, with the goal being to get the air inside the OTA to be the same temperature as the surrounding ambient air. The reason that is important in uninsulated closed telescope systems is because the metal tube radiates heat into the atmosphere until such time as the temperature inside the tube equals the temperature outside the tube. But even then, with a metal tube, radiation can continue and result in a tube temperature that is actually colder than ambient. 

 

In environments with rapidly changing temperatures, some of those closed tubes never catch up with the large temperature deltas... even with small fans. 

 

A properly insulated closed tube seeks to maintain its temperature rather than to get the air inside the tube to equal that of the ambient air. Anyone who uses fans in combination with an insulating jacket doesn’t understand the principle of using insulation. Using fans with an insulation jacket would only prolong the process of reaching thermal equilibrium between the air inside and outside the tube. So use fans - with the drawbacks I listed in post #27, or use an insulation jacket - whose benefits are also stated in post #27 - but don’t combine the two. 


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#33 elwaine

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Posted 29 August 2018 - 03:41 PM

So, would I be correct in assuming... the same very slow temperature change within the tube that would minimize air convection and thermals, would significantly slow the effect a quickly changing temperature would have on the primary - that being to alter its figure during temperature change?

Correct.

 

 

     This thread appears to be mostly for Telescope visual use.    It would seem, using the insulation on telescopes for astro photography would help also.  In this case the need for refocusing the telescope should increase the amount of time between the need to refocus.

 

it should be a big help. (I have not yet tried doing AP with my insulated scope, so I’m not speaking from experience. But I can’t see why it shouldn’t be a benefit when imaging.)


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#34 yellobeard

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Posted 29 August 2018 - 04:29 PM

Ok, it seems that many of you are deep enough into this matter to listen to a theory I already have in my mind for two years, but didn't yet put to the test:

Imagine your house: When it is cold outside, you put on the heater.. Now when your house is not properly insulated, two things happen, 1: you need to put a lot of energy in the heating, 2: You never feel cozy because of the cold drafts that are everywhere..
Now when your house is properly insulated, the cold drafts are history, and you need much less energy to heat it.

Now project this to the closed telescopes we're talking about here: Properly insulated, there are virtually no internal drafts (turbulence!) But still the scope cools down very slowly. Well, I think that the next step would be, the very gently heating of the inside of the scope, just enough to stop the already very slow cooling. When I, or one of you, manages to very gently put some warmth in a properly insulated scope, in such way that there is no internal turbulence, then we would have permanently dealt with foggy schmidt correctors and stuff..

I still did not find the time to test this out, also because, being a visual observer, I don't need it in my situation.
But especially for astophotography, it would be a great new insight, if no focal shift would occur because of the termal stabillity.

I think that very little power is needed to stabilize the internal temperature in mid-size, properly insulated scopes.
The brainstorming needs to be about the question of how to very evenly warm up that tube under the insulation layer, so that there are no differences of significance inside that scope.

Your thoughts on this matter are welcome of course..

Edited by yellobeard, 29 August 2018 - 04:32 PM.

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#35 GUS.K

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Posted 29 August 2018 - 04:45 PM

Just a note regarding the insulation  I used, it’s not reflectix as that isn’t available in Australia, but a product called Thermospan  aircell, which has a foam insert between two layers of foil. It’s used in walls and under timber flooring.I’m not sure what the difference is between this and reflectix. I was skeptical about the claims regarding the use of insulation on scopes, but  thought I’d try it for myself and in my case(after one try) it seems to work, but will see how it goes after a few more sessions.

I also don’t get the huge temp variations as you do in the U.S or Europe, the other night was as bad as it get’s in my location.



#36 Joe1950

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Posted 29 August 2018 - 04:45 PM

Interesting concept, YB.

 

In reading the info on the Reflectix site, it seems they highly recommend to have an air space of some sort between the insulation and what it is insulating; the telescope tube in this case.

 

They speak about a closed cell foam layer, or using thin strips of the Reflectix itself to establish the free air space.

 

In the latter situation, perhaps a flow of warmed air, between the insulation and the telescope tube would achieve the desired goal. It would be necessary to position the spacers to have a continuous path so that the slightly warm air could reach all parts of the OTA.

 

Additionally and ideally, if the air could be thermostatically controlled with a sensor inside the confines of the telescope tube, all the better.

 

 

Or, have you ever seen those small rubber cushion dots that have an adhesive on one side, are about 1/4" in diameter and maybe 1/8" thick?

 

If you had enough of them and placed them perhaps every 2" on the inside of your insulation, that would establish a free air space and protect the scope from scratching.

 

Flow the warmed air easily in the space and wala!

 

 

These are cork and I was referring to rubber, but the same idea. Even doing this without warm air would make the insulation work better according to the Reflectix web site.

 

 

Here you are. 100 for under $2. A couple sheets would do it.

 

These are actually what I had in mind. Don't have to wait for overseas shipping and still reasonable.


Edited by Joe1950, 29 August 2018 - 05:04 PM.

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#37 yellobeard

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Posted 29 August 2018 - 05:02 PM

Yes Joe, that is a great idea! Warm air can be very evenly distributed!

"Saved in my mind for further concideration!!"

Edit: Thanks for the links Joe! I can already see the possebillities!

Edited by yellobeard, 29 August 2018 - 05:04 PM.

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#38 Joe1950

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Posted 29 August 2018 - 05:08 PM

Wonderful. Thank you for all the great information on this excellent concept.



#39 Rock22

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Posted 29 August 2018 - 05:40 PM

The double layer of insulation has been good for both visual and astrophotography sessions on my 180mm mak this summer. I’m at 34-deg N. I haven’t tried imaging in the fall or winter yet, but will see if the temp differentials are related to any significant focus shift. Not sure I will have a long enough session to determine this, but it sounds like a good test for me to try.

As for the layer of air between the insulation and the tube, I’ve not had an issue on a scope of my size. Not sure about larger cats, though. Maybe it’s because my layers of insulation are wrapped loosely enough so there is already a layer of air there, yet snug enough to not move around.

Just glad I happened on the thread on insulation a while back. It likely saved me from spending more money and having to modify my scope. I don’t want to drill holes in my telescope!
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#40 elwaine

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Posted 29 August 2018 - 07:02 PM

Interesting concept, YB.

 

In reading the info on the Reflectix site, it seems they highly recommend to have an air space of some sort between the insulation and what it is insulating; the telescope tube in this case.

 

They speak about a closed cell foam layer, or using thin strips of the Reflectix itself to establish the free air space.

 

In the latter situation, perhaps a flow of warmed air, between the insulation and the telescope tube would achieve the desired goal. It would be necessary to position the spacers to have a continuous path so that the slightly warm air could reach all parts of the OTA.

 

Additionally and ideally, if the air could be thermostatically controlled with a sensor inside the confines of the telescope tube, all the better.

 

 

Or, have you ever seen those small rubber cushion dots that have an adhesive on one side, are about 1/4" in diameter and maybe 1/8" thick?

 

If you had enough of them and placed them perhaps every 2" on the inside of your insulation, that would establish a free air space and protect the scope from scratching.

 

Flow the warmed air easily in the space and wala!

 

 

These are cork and I was referring to rubber, but the same idea. Even doing this without warm air would make the insulation work better according to the Reflectix web site.

 

 

Here you are. 100 for under $2. A couple sheets would do it.

 

These are actually what I had in mind. Don't have to wait for overseas shipping and still reasonable.

 

That’s the concept I was referring to. 

 

According to the info on the Reflectix website, if the outer layer of aluminum on Reflectix is in direct contact with the OTA, it acts as a convector, rather than a radiator of heat. That’s why they recommend an air space between the metal OTA and Reflectix. 

 

I just got a new (to me) TEC 7” Mak. I used rubber foam to make a jacket for my TEC 6” Mak; but for the TEC 7 I plan to use a thin (2mm) sheet of polyethylene foam (the kind used as a packing material) that will contact the metal OTA, and then wrap Prodex (similar to Reflectix, but with 2 to 3 times the R value) around the OTA. That way there will be a continuous air/foam layer between the Prodex and the OTA. It’s a simpler solution to using strips of cork, etc..  


Edited by elwaine, 29 August 2018 - 07:03 PM.

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#41 Joe1950

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Posted 29 August 2018 - 07:17 PM

That will work great.

 

I have a roll of the Reflectix with the aluminum on both sides, so I'll probably try one layer, with the polyethylene foam under it. that should work for my 127mm Mak.


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#42 Mike Mc

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Posted 29 August 2018 - 09:07 PM

 I made an insulating blanket for my 8SE with a single layer of Reflectix sized to cover the aluminum straight sides of the tube. I also glued a layer of black felt to go between the Reflectix and the scope to provide added insulation. My motive was primarily to aid my dew shield in protecting my objective from fogging on cold nights. I had found that a dew shield alone only protected for a short while ( hour max) on those nights. Dew is not normally an issue in North Texas, but it can be a problem on cold nights with no air movement. Since adding the insulating blanket, I  have had no dew form on my objective, even on nights when I'm having to keep my eyepiece case closed to keep it's insides from getting wet.  So it has definitely helped in that regard. 

 

I just recently replaced the 8SE with an edge 8HD and am wondering if I should tape over the vents provided on the back cover. I haven't been using the blanket when dew is not a threat, but I might start using it all the time to see if i have good results with reducing eddy currents. Gotta admit I get funny looks from my club mates when I put that blanket on, though.


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#43 cturek

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Posted 29 August 2018 - 09:53 PM

Recently, I purchased a roll of this:

 

https://www.amazon.c...1?ie=UTF8&psc=1

 

I was going to try a wrap on my 150 Mak. It has the benefit of no foil layer contacting the tube, but does not have an outer foil layer, just closed cell foam rubber similar to wetsuit material. Any opinions on the overall insulating value of this material vs the Reflectix when applied to the metal OTA?


Edited by cturek, 29 August 2018 - 10:05 PM.


#44 elwaine

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Posted 30 August 2018 - 07:28 AM

Recently, I purchased a roll of this:

 

https://www.amazon.c...1?ie=UTF8&psc=1

 

I was going to try a wrap on my 150 Mak. It has the benefit of no foil layer contacting the tube, but does not have an outer foil layer, just closed cell foam rubber similar to wetsuit material. Any opinions on the overall insulating value of this material vs the Reflectix when applied to the metal OTA?

 

It should work. How light weight is the neoprene? I used closed cell rubber foam, made from a 1/2” thick camping mat. It worked fine and it weighed only a few ounces. See post #9


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#45 Axunator

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Posted 31 August 2018 - 02:51 AM

Just a note regarding the insulation I used, it’s not reflectix as that isn’t available in Australia, but a product called Thermospan aircell, which has a foam insert between two layers of foil. It’s used in walls and under timber flooring.I’m not sure what the difference is between this and reflectix. I was skeptical about the claims regarding the use of insulation on scopes, but thought I’d try it for myself and in my case(after one try) it seems to work, but will see how it goes after a few more sessions.
I also don’t get the huge temp variations as you do in the U.S or Europe, the other night was as bad as it get’s in my location.


We have similar stuff here in Finland (I have not found US-type Reflectix anywhere here). I think it’s very obvious that it’s even more effective insulator than even double-layered Reflectix due to foam layer in the middle.

As you can imagine, temp deltas between inside and outside can be really extreme in wintertime here in the Arctic. My impressions so far are, that insulating my C9.25 with that stuff is really effective, although I haven’t tried it in the heart of the coldest winter yet.

FWIW, I do run a dew heater under it at low power, because late summer nights are often extremely humid, and in winter nights we have problems with frost. Required battery power is not a problem because I anyway use that scope on tracking mount.
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#46 happylimpet

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Posted 31 August 2018 - 04:27 AM

I can understand how this eliminates internal thermally driven currents, and what a benefit that is. I also understand how it reduces overall cool-down, thermal contraction and focus shift etc. And how it reduces corrector fogging. Quite a boon!

 

This is a very good idea!

 

However, as a Newtonian owner, it seems to me that while you have hugely improved the above potential problems, you're now in a situation very similar to that of most Newtonian owners, namely of having a warm-ish optical surface (mirror for us, corrector for you) at the bottom of a tube.

 

So you must still be getting the boundary layer effects that plague us for high power work.  Admittedly they will be halved, as your light is passing through this boundary layer only once, whereas for us it does so twice! So maybe this is sufficient to make it negligible.

 

Anyway, just wanted to point out that you are making a boundary layer problem by this approach of keeping the scope 'warm'. But if its not a real problem, then brilliant!!!

 

Nice work.



#47 elwaine

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Posted 31 August 2018 - 09:38 AM

With proper insulation, there should be no boundary layer.  Initially, the temperature of the mirror, the corrector and the air inside the closed tube are all equal. Then, as the entire system s l o w l y cools, it does so uniformly, and avoids the formation of boundary layers. Or, if one forms in front of, or just beneath the corrector, it is as unnoticeable as when one uses a dew heater (which is adjusted properly to the point at which it just prevents dew from forming). 

 
The only way I know of to avoid boundary layers from forming in open tube systems is by the use of fans... or converting the tube to a truss.


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#48 Bean614

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Posted 31 August 2018 - 08:46 PM

Happylimpet wrote- "you're now in a situation very similar to that of most Newtonian owners, namely of having a warm-ish optical surface (mirror for us, corrector for you) at the bottom of a tube".  

  "mirror for us, corrector for you"...????  Actually, no.

An MCT/SCT is(!) a Reflector, and has the Primary Mirror on the bottom, same as a Newtonian Reflector. The Corrector Plate is on the Top!



#49 yellobeard

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Posted 01 September 2018 - 12:56 AM

The only way I know of to avoid boundary layers from forming in open tube systems is by the use of fans... or converting the tube to a truss.

Well, then I can tell you about another way:

Back in 1992, when the believe still was that you needed thick glass blanks to make 'optically stable' mirrors, I finished my 8" Newtonian, which was (and still is) optically perfect under autocollimation test!
But in 99.9835% of the cases, in the field, it was easily overpowered on sharpness by my earlier home build 6" newt!
Where my 6" has a mirror thickness of about 0.7 inch, the 8" has 1.9...
It was in a time when there was no talk yet about boundary layers and stuff..

Doing some star testing with the 8", I quickly found my 'warm air coming from the primary' problem..
And one night, at 2am, I suddenly woke up with the solution: Water cooling! A thin closed chamber at the back of the primary, two hoses, an aquarium pump with water and ice cubes, made my thick 8" mirror cool down to ambient temperature in some 20 minutes! And all turbulence and boundary layers absolutely gone, with no fan needed!
I never saw such sharp images with a 8" of any kind of scope! Super APO quality..
In 1995, I build my 31cm (12") water cooled Newt, which is well known in my country for its overwhelmingly sharp and stable image quality. It still has the very heavy 2.3" thick primary mirror, but It still is used with pleasure, on a heavy mount in a garden observatory, by my friend Bertus.

..

Edited by yellobeard, 01 September 2018 - 01:00 AM.

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#50 Traveler

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Posted 01 September 2018 - 01:28 AM

With proper insulation, there should be no boundary layer.  Initially, the temperature of the mirror, the corrector and the air inside the closed tube are all equal. Then, as the entire system s l o w l y cools, it does so uniformly, and avoids the formation of boundary layers. Or, if one forms in front of, or just beneath the corrector, it is as unnoticeable as when one uses a dew heater (which is adjusted properly to the point at which it just prevents dew from forming). 

 
The only way I know of to avoid boundary layers from forming in open tube systems is by the use of fans... or converting the tube to a truss.

 

This "slowly cooling", was that what Celestron had in mind when they brought their carbon SCT tubes on the market?




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