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Using an insulation layer under Reflectix (part deux)

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#1 precaud

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Posted 10 January 2019 - 10:01 PM

I have decided to start a new thread for the continuation of this little experiment, as the first thread got overrun by other interests.

 

The goal of this project is to have my new 7" Mak producing sharp, detailed views from start to finish during the hours I typically observing in the winter months, roughly 7pm to 10pm.

 

The challenge: When skies are clear, outside temperatures during (and before) this time period are always falling quickly. In stock form, the scope's mirror loses its otherwise-excellent optical figure in response to the falling temps. I'll explore using Reflectix with an insulating layer to minimize this deterioration.

 

Conditions consistent for all observations: The scope is stored in an unheated room, whose temperature in the mid-afternoon is roughly in the 48ºF range. It is moved from there to its mount on the outdoor observing pad at 4pm. Setup/alignment is done at 6:45pm, and observing starts at about 7pm. Humidity has been relatively high lately in the evening, in the 65-75% range. Temperature excursion has been typical for this time of year: peaks at 3pm in the low-mid-40's; 38º at 4pm; 30º when observing starts at 7pm; low-20's when observing ends at 10pm.

 

The first observation was with the scope in stock form (no dew shield). I saw that tube currents were not a major issue, but the optical figure deteriorated and the images were soft. A star test showed asymmetry on either side of focus, which is not the case when the scope is thermally stable. This lack of detail persisted until the very end of the session, when the thermal acclimation apparently caught up and the excellent optical figure returned.

 

On the second evening, I wrapped the cylindrical part of the OTA in a single layer of Reflectix before starting to observe (still no dewshield). On this evening, the optical figure was never restored, the images were soft through the entire session. Star tests confirmed that correction never occurred.

 

I have 10" metal tube and 8" Sonotube dobs. Their cooling systems are identical (an enclosed rear fan). I have noticed over time that the Sonotube dob not only has less tube current issues, but its primary mirror maintains its figure better during the transitional hours. It is reasonable to think that the insulative value of the Sonotobe vs sheet metal plays a role in this.

 

Applying this to the behavior of the Mak, it suggests that perhaps the OTA is cooling the outer edge of the mirror faster than the middle, which leads to its deformation, until it equalizes.

 

When taking a scope from a "warm inside" to a "cold outside" to observe, there are two different phases that it goes through. And each have to be dealt with differently.

Phase One is the major cooldown that happens when the scope is first moved out into the Great Outdoors. This is the biggest adjustment and there's nothing subtle about it. The scope's mass is puny compared to the cold atmosphere and objects around it, so equilibrium is completely on the Outdoor's terms, and the scope gives up its stored heat via every available means; radiation, convection, and conduction. Insulating the scope would only slow this process down. So best to just take it outside and let it "acclimate".

 

Phase Two basically begins when you want to start using the scope. Here things get more subtle. We can no longer talk about the scope as "one thing"; we have to deal with its component parts and their different thermal properties, mostly made of glass or metal. The challenge in Phase Two is to get the various parts of the scope to cool down at the same gradual rate, while immersed in a falling-temperature environment. This is where insulation might be useful, to "tune" and/or "smooth out" the various parts of the scope's response to temperature transitions.

 

As most of us know by now, Reflectix is a decent radiant barrier but a poor insulator, about the same R value as a single pane of glass. The solid plastic provides a conductive path to whatever it is in contact with. But when used to cover or enclose a good insulation material, it enhances that material's R value by stopping the air from infiltrating and cooling it (vapor barrier). Good insulators are generally "fibrous tangles" made of non-conductive material.

 

In numerous threads on CN, it has been said that the primary goal is to maintain the telescope mirror "at ambient". So, what is "ambient"? For most scopes, "ambient" means the temperature of the air that the scope is immersed in. This POV falls flat when applied to a closed optical system, such as the Mak-Cass I'm using. The mirror is enclosed and is never directly exposed to the atmosphere. Air temperature variations only reach it indirectly through the corrector plate, the OTA body, and the backplate/cell assembly. They also cool the air inside the OTA. For an enclosed mirror, "ambient" is a complex mix determined by its proximity to the OTA and the back plate/cell assembly, which are typically metal, and predominantly by radiative cooling, and secondarily by the air temp inside the OTA.

 

Mulling all of this over, it seems to me that what is needed is to present temperature changes to the mirror more uniformly over its entire surface. Which means, lessening the influence of the OTA due to its outsized impact on the mirror edge. I'll try accomplishing this by insulating the cylindrical portion of the OTA, and allow the backplate/cell assembly to dominate the temperature transfer to the mirror.

 

So that is the reasoning framework behind this approach. Lessen the influence of the OTA, and allow the back plate to be the dominant "heat exchanger" to the outside world. It presents temp changes to the mirror across its whole surface much more uniformly than the OTA does.


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#2 precaud

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Posted 10 January 2019 - 10:04 PM

The jacket I made is a three-piece affair. The entire cylindrical surface is insulated with 1/2" poly batting, the kind sewn into jackets, sold at your local fabric shop. It is glued to the Reflectix with 3M "Super 77" spray adhesive. Velcro is glued to the Reflectix for closure.

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#3 precaud

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Posted 10 January 2019 - 10:06 PM

The dew shield has black felt glued to it with the #77, and a 1/2" x 1/2" felt strip that provides a solid material to better secure the shield to the OTA. The sheet felt is a low-density type which has low reflectivity at grazing angles of incidence. Denser felts are worse in this respect.

 

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#4 precaud

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Posted 10 January 2019 - 10:07 PM

The backplate cover is a single layer of Reflectix only. Its main purpose is to cover the ends of the insulated jacket and enclose the insulation. It attaches to the OTA cover by Velcro.

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#5 precaud

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Posted 10 January 2019 - 10:10 PM

Here's what it looks like installed on the scope. Pretty nice.

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#6 precaud

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Posted 10 January 2019 - 10:12 PM

Conditions were good enough for observing on Tuesday. Seeing was maybe 3.5 of 5; surface winds wre calm but the edge of a storm passing to our north stirring up some winds aloft. With 70%+ humidity and freezing temps, tranaparency was not great. Not conditions for high magnification views; 150-160X was about tops.

 

The OTA was brought from the storage room out to the pad as usual at 4pm to cool. At 6:45, the insulated jacket was installed and the setup was prepped and mount aligned. Observing began shortly after 7pm.

 

The last calibration star was Capella, so I used it for a star test. I was pleased to see that the rings were symmetrical on both sides of focus. From prior experience, this is a good sign... In focus, the star was nicely round, sharp, excellent.

 

Exploring M35 and M37, the clusters were sharp, pinpoint, beautiful!

 

I swung the scope over to M42 and observed with a 2" 40mm plossl (68X). I was amazed to see E solidly and F fading in and out, with direct vision. I don't think I've ever seem them at such a low magnification. Changing to a 34mm SWA (79X) F was now solid and stable. I loved the 3D sense of the entire view.

 

I continued observing various objects until 10pm, checking the star test whenever a bright star was nearby. Ring symmetry held true all night. As did pinpoint stars, within the sky's limits that night.

 

I'm very pleased with the results. The scope performed superbly from the get-go. Does it prove my theory/thesis? Not conclusively. Who cares? It solved the problem. Just like others in more temperate climates have experienced success from covering their OTA's, adding the insulation layer to the Reflectix on the OTA body extends its usefulness into lower temperature conditions.


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#7 Brett Waller

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Posted 10 January 2019 - 10:28 PM

I'm confused. Why is the reflectix on the outside?  If the goal is to keep the OTA from losing heat, I think the reflectix should be on the inside with the insulation outside of that.  At first glance, I would think the arrangement as you have would be best for solar observing.  Flip them around and give it a try.

 

Brett



#8 Cali

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Posted 10 January 2019 - 10:30 PM

I've been swamped with little time to work on my insulating layer but here goes:

 

1). Darice 2mm Foamie Roll, 36-Inch by 60-Inch, Black

 

2). Reflectix

 

3). Darice 2mm Foamie Roll, 36-Inch by 60-Inch, Black, again.

 

4). A protective "decorative" vinyl layer. 

 

Each layer will be adhered with this.

 

 

 

- Cal


Edited by Cali, 11 January 2019 - 02:15 PM.

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#9 Rock22

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Posted 10 January 2019 - 10:41 PM

Great to hear your results were worth your hard work in making the insulation jacket! Two layers of Reflectix works for me, and I have it covered like yours on the dew shield and back plate, but my temp changes are nowhere near yours.

I sincerely believe every observer needs to do what you did - put the effort into finding what insulation or temp control works for your scope and observing conditions.

I so enjoy using my 180mm mak, and insulation was a very big contributor to that.

Happy observing!
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#10 precaud

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Posted 10 January 2019 - 10:45 PM

I'm confused. Why is the reflectix on the outside?

 

It is a radiant barrier (read recent threads on radiant heat loss), and it encloses the insulation to make it more effective.

 

If the goal is to keep the OTA from losing heat, I think the reflectix should be on the inside with the insulation outside of that.

 

That's normal backward thinking about insulation and radiant/vapor barriers. The insulated air space needs to be enclosed between the Reflectix and the object being insulated, so that cooler air doesn't intermingle wthrough the insulation and kill its R value. Not to mention, having the batting exposed would be very impractical...

 

  At first glance, I would think the arrangement as you have would be best for solar observing.  Flip them around and give it a try.

 

No thanks... you give it a try...


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#11 Brett Waller

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Posted 10 January 2019 - 10:50 PM

I understand fully the reflectix is a radiant barrier, and that is my point. The radiation (heat) is coming from the inside, and you have the radiant barrier on the outside. The physics of heat loss suggests you would get better performance with it the other way around.

 

I'm sorry you aren't as open to suggestions as it at first appeared. Your loss ….

 

Brett



#12 erin

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Posted 10 January 2019 - 10:51 PM

Thanks for posting your results. Looks good. waytogo.gif  

 

How do you know when phase 1is “done”? I took out my sct tonight to try and get a sense of how long cooldown takes. I know it is probably an hour minimum given the temp swing here-about 50 degrees. I was also going by whether or not tube currents were present, but that may not be a good indicator since there is still a temp difference between the glass and the metal.



#13 precaud

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Posted 10 January 2019 - 11:01 PM

I understand fully the reflectix is a radiant barrier, and that is my point. The radiation (heat) is coming from the inside,

No... it's not radiant heat we're concerned about. It's the radiant cooling coming from space... and from the ground... the object is not to "contain heat" but to "slow/control cooling".

 

Besides, as I said, the insulated enclosure would be entirely lost with the insulation on the outside, lowering its effectiveness hugely.


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#14 precaud

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Posted 10 January 2019 - 11:08 PM

Thanks for posting your results. Looks good. waytogo.gif

 

How do you know when phase 1is “done”? I took out my sct tonight to try and get a sense of how long cooldown takes. I know it is probably an hour minimum given the temp swing here-about 50 degrees. I was also going by whether or not tube currents were present, but that may not be a good indicator since there is still a temp difference between the glass and the metal.

 

Thanks Erin. it sounds like you are taking your scope out from normal indoor temperatures (say 70ºF) to cool? If so, that is a tough transition to make, and it would take a couple hours, I'd guess. The wider the temperature range, the longer the "Phase 1" period needed for cooldown. Is there anywhere you can put it, an unheated space like a porch or garage, for a while before taking it out?



#15 precaud

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Posted 10 January 2019 - 11:14 PM

Great to hear your results were worth your hard work in making the insulation jacket! Two layers of Reflectix works for me, and I have it covered like yours on the dew shield and back plate, but my temp changes are nowhere near yours.

I sincerely believe every observer needs to do what you did - put the effort into finding what insulation or temp control works for your scope and observing conditions.

I so enjoy using my 180mm mak, and insulation was a very big contributor to that.

Happy observing!

 

I completely agree with that POV, Rock. It's very location-dependent. Two layers of Reflectix works for you in temperate CA, as it does for Peter in HI. It may well be that 1/4" of batting would be fine in many places, perhaps even here. Those further north may need something more extreme. In all cases though, I would use only a single layer at the rear.

 

And yes, this is a very impressive scope, indeed! 


Edited by precaud, 10 January 2019 - 11:23 PM.


#16 erin

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Posted 10 January 2019 - 11:15 PM

Yes, exactly.

 

No unheated space in my condo, but I can let it cool in an Orion scope bag outside on my deck. 



#17 precaud

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Posted 10 January 2019 - 11:26 PM

Yes, exactly.

 

No unheated space in my condo, but I can let it cool in an Orion scope bag outside on my deck. 

 

That should do it. Give it a couple hours or more.

 

The main reason I put mine out on the pad at 4pm is because that's when the sun no longer shines there...



#18 erin

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Posted 10 January 2019 - 11:34 PM

Will do!



#19 luxo II

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Posted 11 January 2019 - 05:45 AM

3 hours cooling ? That’s ridiculous - of course it should be steady by then.

Try observing 30 minutes after it’s taken outside and see how it fares - here the scope is ready to go by the time I’ve assembled and aligned the mount, no waiting. I usualy start out at 120x and go straight to 330X, more if the seeing is good.

That was the real point we found here - basically no internal tube currents visible with any of the MK91, M715 or two 8” SCTs - all could be used at high power immediately.

We used a layer of 3mm foam and an outer layer of 6mm coreflute, works well enough here. While the temps get down to 0 Celsius, maybe -5C, Reflectix I agree may be better in the US winter.

The second effect - prolonging the time for the scope to dew over is an entirely separate issue.

There’s a third effect too - all the maks and SCTs showed a slight improvement in contrast as a result of the dewcap reducing stray skylight entering the OTA

Edited by luxo II, 11 January 2019 - 05:55 AM.


#20 erin

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Posted 11 January 2019 - 07:59 AM

I had my scope on the mount last night while it cooled so I could align the finder, and after about 45 minutes, a defocused star showed some tube currents, so it will need more time than that. I am asking because this is my first winter with an sct, so it’s new territory for me.



#21 Peter Besenbruch

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Posted 11 January 2019 - 01:36 PM

Here's what it looks like installed on the scope. Pretty nice.

You may have trouble getting sharp images, if your bottom photo is any indication. ;) Nice job.

 

Next, try taking out the scope with the insulation on, and setting up right when you want to observe. My insulation isn't as extreme. I use a double layer of Reflectix over the tube, and a single layer over the sides of the back casting. The back area with the diagonal and focus knob is clear. Then again, my conditions aren't as extreme. Still we are freezing here, as temps plummet to 64°F. Yes, that's plenty of differential to cause tube currents.



#22 precaud

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Posted 11 January 2019 - 02:46 PM

You may have trouble getting sharp images, if your bottom photo is any indication. wink.gif Nice job.

 

But in the manual it says to keep the corrector covered at all times...   ;)

 

Next, try taking out the scope with the insulation on, and setting up right when you want to observe.

 

Not sure what the point of this would be?

 

My insulation isn't as extreme. I use a double layer of Reflectix over the tube, and a single layer over the sides of the back casting. The back area with the diagonal and focus knob is clear.

 

 

I may enlarge the hole around the diagonal on mine, to make it easier to install.

 

Then again, my conditions aren't as extreme. Still we are freezing here, as temps plummet to 64°F.

 

Sounds like a crisis! Thank gawd for 911...



#23 TG

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Posted 11 January 2019 - 08:21 PM

I don't understand why Reflectix is a poor insulator. It's two layers of Mylar separated by an air layer which is a poor conductor. What an I missing?

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#24 precaud

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Posted 11 January 2019 - 08:54 PM

I don't understand why Reflectix is a poor insulator. It's two layers of Mylar separated by an air layer which is a poor conductor. What an I missing?

 

Actually, its two layers of metallized mylar attached to a sheet of small bubble wrap. It's highly conductive.

 

From their website:

 

"Different types of insulation products can reduce the heat transferred by conduction, convection and radiation to varying degrees. As a result, each provides a different thermal performance and corresponding “R” values. The primary function of reflective insulation is to reduce radiant heat transfer across open spaces. The low emittance metal surface of reflective insulation blocks up to 96% of the radiation, and therefore, a significant part of the heat transfer."

 

Note the "across open spaces" part. That is a clear statement that the stuff is less effective when in direct contact with a surface that it is providing a radiant barrier for.



#25 Peter Besenbruch

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Posted 12 January 2019 - 10:11 PM

Quote

    Next, try taking out the scope with the insulation on, and setting up right when you want to observe.

 

Not sure what the point of this would be?

Because if it worked straight out of the storage area, you would not have to attach and detach the insulation every viewing session (and you might have more dew resistance to boot). The ideal is a scope you just set up and view with, and which doesn't need advance prep.




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