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Insulation (airspace) layer for under Reflectix

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#51 Sarkikos

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Posted 14 December 2018 - 10:07 PM

This has been my experience too. I use the cheap black plastic shield that I got with the scope. The one from Celestron that fits 6 and 8. I have imaged well below freezing when everything around me has a good frost. On the outside of the dew shield frost. On the inside a few crystals, but nothing on the corrector plate. I was imagining near zenith and had no dewheaters. So in my case cheap plastic dew shields do work. Maybe not as well as others but better than nothing.

 

Chris

Much better than nothing.  Nothing would get me a dewy corrector in about a half-hour.  Much better keeps the corrector dew-free all night.

 

Mike


Edited by Sarkikos, 14 December 2018 - 10:14 PM.


#52 Sarkikos

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Posted 14 December 2018 - 10:08 PM

Keep it going.  Nothing worthy of a locked thread … yet. 

 

:grin:

Mike


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#53 Sarkikos

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Posted 14 December 2018 - 10:13 PM

What amazes me is the degree to which anecdotal evidence rules over empirical evidence on topics like this! bigshock.gif

 

Everyone thinks their own experience is empirical evidence and everyone else's experience is merely anecdotal.  And thus was born post-Modernism ...

 

grin.gif

Mike


Edited by Sarkikos, 14 December 2018 - 10:16 PM.

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#54 Paradoxdb3

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Posted 14 December 2018 - 10:17 PM

popcorn.gif


I know, right! This topic is far from closed, in my opinion, and given my own personal experience, I am not yet sold on the idea of an insulated tube. However, as I stated on a Facebook group, we either have to conclude that the people claiming that insulating works are all either mistaken, or have shares in the Reflectix company, OR are actually seeing an improvement. I, however, have not. My tests are far from over, however.
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#55 Sarkikos

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Posted 14 December 2018 - 10:25 PM

I know, right! This topic is far from closed, in my opinion, and given my own personal experience, I am not yet sold on the idea of an insulated tube. However, as I stated on a Facebook group, we either have to conclude that the people claiming that insulating works are all either mistaken, or have shares in the Reflectix company, OR are actually seeing an improvement. I, however, have not. My tests are far from over, however.

But don't you know?  Your experience is only anecdotal.  It doesn't rise to the level of empirical evidence.  wink.gif 

 

Mike


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#56 charlesgeiger

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Posted 14 December 2018 - 10:35 PM

I agree with WadeH237 regarding both the baffle tube and the mirror cooling. I believe these items are far more important than the outside main tube. I would suggest as an experiment that one should cool the primary mirror including the baffle.  I also agree as would most that an ambient scope (whole tube assembly and optics) will provide the best images.

I agree that a long dew shield made of insulating material (I used a fiber container that has the same diameter as my C14 and added a collar outside ring glued to the main fiber tube which fits over the corrector cell).  I have sealed the fiber tube with sealer and flat black paint and it is still good after 30+ years.  The fiber tube is about 3/16" thick and about 2 feet in length that I got from work all those years back.  

I used to set up my scope about 8' from the front of my truck.  If we got heavy dew, I would start my truck engine, lift the hood for a few minutes and point the tube in the direction of the engine.  Within a few minutes all dew would be gone and I would be good for the rest of the night.  And no, there was no contamination to be found on the corrector as the tube and optics were away from any engine residue.  No exhaust or oil scum only heat radiating from engine.

For those of us who live in dew prone/major temp. changes during day and night, I would suggest that insulation is not the only way to go.  I do have a cat cooler (old C14) and that is a good ventilator.  I am thinking that (as with CCD cameras) that a cooling coil system placed near the mirror and cell which can cool the mirror and cell (and thus the baffle tube) down to several degrees below ambient might be a good thing to experiment with on a cat.  Or one could add the cooling fans along with direct contact on rear cell those electronic gadgets that cool the metal. 

I can say that most of the problems I have had regarding image quality were directly related to seeing a plume coming off the baffle tube seen just inside and outside of focus.  And, of course, seeing has to be descent.

Charlie



#57 Paradoxdb3

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Posted 14 December 2018 - 10:46 PM

But don't you know? Your experience is only anecdotal. It doesn't rise to the level of empirical evidence. wink.gif

Mike


I value empirical evidence, and actually would love to do a "Mythbusters" style test. Two identical Scopes side by side, same targets, same conditions, same cooling time (say, fifteen minutes). Take raw video of a defocused star and see if the images vary from insulated scope to non insulated. The closest I have to compare is my 7" SkyWatcher Mak-Cass and a 6" Orion Mak-Cass. Not really a fair test.

But I truly believe that my experience would prove nothing, even IF I were getting amazing results. Was I getting amazing results due to good seeing? Low humidity? Comparing one night to the next, even if all else stayed the same, still seems flawed, even if the results were consistent. I think what I'd need as empirical evidence would be to take video without insulation. Record results over three or more days. Then insulate, and take video. Record results for another three or more days. Then repeat at least a few more times and make predictions based on all the observations. If the predictions are accurate, then we have a case. But relying on human experience alone is quite flawed.
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#58 precaud

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Posted 14 December 2018 - 11:53 PM

As the OP of this thread, I'd like it to not get too far afield from the original intent... the efficacy of using an insulating (airspace) layer under the Reflectix. And how/when to use it. So please don't drag it into dueling theoretical territory that will get it locked.

 

My guiding principles are different than John Hayes' "radiant heat loss" model. I am looking at it more in terms of the convective model; an object made of metal and glass immersed in a large, cold, gaseous medium. And I'm concerned not only with thermals, but also with the morphing mirror figure as temps fall. I'm not that concerned with dew, as I rarely encounter it here.

 

I do totally agree with John's statement that "The bottom line is that you want a thermally stable system", to which I would add the observation, that is a statement of the "ideal", which may or may not be achievable. Especially when the dominant aspect of this system, the atmosphere, is "unstable" most of the time. And what we're sussing out is, under at least two different main conditions, how best to stabilize the scope, specifically the glass parts of it encased inside the metal parts, within that unstable environment.

 

I've made the insulated jacket for the OTA. Next is to make the rear cover and dewshield. I hope to make some headway on it manana. Then I can test it out.


Edited by precaud, 14 December 2018 - 11:54 PM.

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#59 jhayes_tucson

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Posted 15 December 2018 - 02:13 AM

As the OP of this thread, I'd like it to not get too far afield from the original intent... the efficacy of using an insulating (airspace) layer under the Reflectix. And how/when to use it. So please don't drag it into dueling theoretical territory that will get it locked.

 

My guiding principles are different than John Hayes' "radiant heat loss" model. I am looking at it more in terms of the convective model; an object made of metal and glass immersed in a large, cold, gaseous medium. And I'm concerned not only with thermals, but also with the morphing mirror figure as temps fall. I'm not that concerned with dew, as I rarely encounter it here.

 

I do totally agree with John's statement that "The bottom line is that you want a thermally stable system", to which I would add the observation, that is a statement of the "ideal", which may or may not be achievable. Especially when the dominant aspect of this system, the atmosphere, is "unstable" most of the time. And what we're sussing out is, under at least two different main conditions, how best to stabilize the scope, specifically the glass parts of it encased inside the metal parts, within that unstable environment.

 

I've made the insulated jacket for the OTA. Next is to make the rear cover and dewshield. I hope to make some headway on it manana. Then I can test it out.

John,

I don't know where you are in north-central NM but at DSW, I often see the temperature/dew point spread fall to within a couple of degrees during the winter.  During this time of year in the observatory, I often measure RH values above 80% and that's well within the "trouble zone" for dew or frost.  (I often see the same thing during the monsoon season as well.)  My scope has dewed up and frosted over a number of times before I got my anti-dew system tuned up.  So, "rarely" is not the word that I would choose to describe the frequency of dew conditions out there but that certainly depends a bit on the season.  Regardless, without proper protection, I learned the hard way that dew (or frost) can be a real problem whenever it occurs so I consider it a big deal--even in north-central NM.

 

As for your optics, most SCTs have center mounted borosilicate primaries so it would take a pretty big temperature gradient to warp the mirror.  Maybe you could get some minor rib print through with enough of a temperature shift, but I doubt that mirror figure distortion is much of an issue for most systems under most "normal conditions."  The bigger concern is thermal convection coming off of the mirrors or the baffle tube.  As long as the flow is in the light path, it's a problem and the only way to eliminate it is to achieve thermal equilibrium.

 

Finally, the idea of only considering convection might be meaningful when the scope is not in thermal equilibrium.  I totally agree that when you take a warm scope out into the cold outdoors, you will get a lot of convection as the scope cools and that will cause a lot of problems with thermals in the optical path.  It sounds like we agree that the only way to fix that stuff is to get the scope stabilized--and in my view, it's best to get to that point as quickly as possible.  However, once the scope is in equilibrium, radiative cooling is what causes problems with temperature gradients within the tube and with dew and/or frost--and that's what I'm much more concerned about.  You simply cannot discuss dew prevention without considering radiative cooling, which is at the heart of the issue.

 

John


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#60 Kokatha man

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Posted 15 December 2018 - 02:32 AM

Everyone thinks their own experience is empirical evidence and everyone else's experience is merely anecdotal.  And thus was born post-Modernism ...

 

grin.gif

Mike

Au contraire Mike - without getting ott, empirical at its' most loose definition requires some tangible evidence, ie data, to validate the assertion.

 

There is nothing apart from "it works for me" in the insulative approach to affirm that view...one is quite entitled to ask the question as to how one might arrive at that appraisal...have they compared insulation to mirror/ambient equilibrium - or even perhaps a bout of meditation before using their scopes...although that could conceivably be deemed as a cooling devotee's  sneaky suggestion..? (such as me! wink.gif )

 

Someone else was going to have a woollen vest knitted for their SCT in another of these threads - & was criticised re specific aspects of that approach - but how do we really know if it isn't the answer to every problem here?

 

...are they forcing their optics to perform at very high magnifications/focal lengths?

 

...regardless, what evidence is there beyond their appraisals: "I know what I say is correct, you'll just have to take my word on that!"

 

"Circumstantial" or "anecdotal" evidence might have some relevance in certain situations...but despite wide-spread belief in UFO's, or the importance of the Kardashians etc to a modern, critical world-view; I'd meekly suggest that we be wary of such "facts" or populist opinions - in my experience AA'ers are no less immune to irrational beliefs: I understand that "flat-earthers" - of which there are very many on the planet, received a fillip earlier this year when, to quote Wikipedia: <"Mike Hughes, a daredevil and flat earth conspiracy theorist, used a homebuilt manned-rocket in an attempt to see for himself if the Earth is flat on March 24, 2018. His rocket made of scrap metal was estimated to cost $20,000, and using a mobile home as a custom launchpad managed to climb 1875 feet with Mike inside and ended with a hard landing but with parachutes successfully deploying. The amateur rocketeer was not seriously injured and remains firm in his Flat Earth beliefs. He claims that real evidence will come with "larger rockets".>

 

But I'm wavering under the onslaught of the faithful & all the evidence I've researched since reading about Mike: if Mr Hughes can flip $20G to take such a risk, he must be onto something!!! waytogo.gif  - I think I'm beginning to understand the temerity of rude questioning like: "you want me to actually record images of a star's diffraction patterns etc: heavens above, even do something like a continuous series of high resolution images to confirm that my assertions bear some witness..?!?" bigshock.gif

 

But then again...thinking1.gif - I think I see the light (or rather the edge of the World etc) - why don't I just give up: NASA et al got it all wrong: the HST wasn't really producing bad images, they just didn't want anyone to see the truth so now they merely relay info from their store of propaganda to keep us all fooled, like the Moon landings - ignore my disparaging views on UFO'ers, Kim & the FE Society...you can't fight "faith" & I'm a fool to try...I knew there was a reason to refrain from these topics, which seem to go from strength to strength - like the hordes that constantly descend upon my front door trying to convert me regularly.

 

I certainly don't want to be complicit in having these sorts of threads locked - it's the right of freedom loving folks to believe anything they like...& I hereby promise to withdraw from all such topics/threads..!angel2.gif BeatingADeadHorse.gif BeatingADeadHorse.gif 


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#61 Sarkikos

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Posted 15 December 2018 - 07:50 AM

Au contraire Mike - without getting ott, empirical at its' most loose definition requires some tangible evidence, ie data, to validate the assertion.

Raw experience is empirical evidence.  Problems arise in the interpretation.  A UFO is not necessarily an extraterrestrial entity.  It is merely an unidentified flying object.

 

Mike



#62 eklf

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Posted 15 December 2018 - 08:25 AM

As the OP of this thread, I'd like it to not get too far afield from the original intent... the efficacy of using an insulating (airspace) layer under the Reflectix. 

Why would there be a need for "airspace" under the reflectix? 

 

From what I read reflectix needs airspace on the otherside - outer cooler side- in presence of which it looses heat at about 4%. The reason it needs airspace is that in absence of that - that is- if it is in contact with some material then the heat transfer is no longer radiative but conductive - and with a R value of 1.1 - reflectix is a pretty good conducter.  But if there is no contact with any other material other than air, then most of the heat is retained within the reflectix (heat loss, then, being 4%).

 

 

 

Now, having insulation beneath the reflectix would have some benefit - but I would have to say its minimal.  Lets say its 50% effective, then the total heat loss through radiation from the insulation + reflectix would be 2%, rather than 4% with reflectix alone.

 

But no practical experience here, only theory.  And the theory is based on my interpretation of reflectix literature.  So ample possibility for flaws.  



#63 erin

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Posted 15 December 2018 - 08:35 AM

As the OP of this thread, I'd like it to not get too far afield from the original intent... the efficacy of using an insulating (airspace) layer under the Reflectix. And how/when to use it. So please don't drag it into dueling theoretical territory that will get it locked.

 

My guiding principles are different than John Hayes' "radiant heat loss" model. I am looking at it more in terms of the convective model; an object made of metal and glass immersed in a large, cold, gaseous medium. And I'm concerned not only with thermals, but also with the morphing mirror figure as temps fall. I'm not that concerned with dew, as I rarely encounter it here.

 

I do totally agree with John's statement that "The bottom line is that you want a thermally stable system", to which I would add the observation, that is a statement of the "ideal", which may or may not be achievable. Especially when the dominant aspect of this system, the atmosphere, is "unstable" most of the time. And what we're sussing out is, under at least two different main conditions, how best to stabilize the scope, specifically the glass parts of it encased inside the metal parts, within that unstable environment.

 

I've made the insulated jacket for the OTA. Next is to make the rear cover and dewshield. I hope to make some headway on it manana. Then I can test it out.

Hi John-I am looking forward to your report. I am going to try it out on my sct.


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#64 precaud

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Posted 15 December 2018 - 11:26 AM

Hi John,

 

I don't know where you are in north-central NM

 

Santa Fe, NM

 

but at DSW, I often see the temperature/dew point spread fall to within a couple of degrees during the winter.  During this time of year in the observatory, I often measure RH values above 80% and that's well within the "trouble zone" for dew or frost.  (I often see the same thing during the monsoon season as well.)  My scope has dewed up and frosted over a number of times before I got my anti-dew system tuned up.  So, "rarely" is not the word that I would choose to describe the frequency of dew conditions out there but that certainly depends a bit on the season.  Regardless, without proper protection, I learned the hard way that dew (or frost) can be a real problem whenever it occurs so I consider it a big deal--even in north-central NM.

 

 

I agree, and I didn't mean to minimize in in general. I've seen it too (just last week), but not often. The differences may be circumstantial. I'm not viewing from an observatory. High RH in a high-LP environment really takes a toll on transparency. Since this is a hobby for me, if I don't like the conditions, I pack up and go inside.
 

    As for your optics, most SCTs have center mounted borosilicate primaries so it would take a pretty big temperature gradient to warp the mirror.  Maybe you could get some minor rib print through with enough of a temperature shift, but I doubt that mirror figure distortion is much of an issue for most systems under most "normal conditions."  The bigger concern is thermal convection coming off of the mirrors or the baffle tube.  As long as the flow is in the light path, it's a problem and the only way to eliminate it is to achieve thermal equilibrium.

 

 

That may be true in general, I can't comment on it. My scope is a 7" MCT (Skywatcher 180) and I'm reporting on behavior I am encountering. On night one, using no insulation, both thermals and mirror figure were impacted until late in the evening when the scope acclimated and views were sharp again. On night 2, using the OTA insulation only, thermals were much improved from the get-go but the mirror figure never corrected. I've described this in another thread and won't repeat it here.

 

Finally, the idea of only considering convection might be meaningful when the scope is not in thermal equilibrium.

 

Which will be the case every time I set it up in the winter, and is the problem I'm trying to solve... or improve. And again, I take exception to this notion that "achieving thermal equilibrium" is a single event... that the scope reaches it and stays there for the rest of the session.

 

I totally agree that when you take a warm scope out into the cold outdoors, you will get a lot of convection as the scope cools and that will cause a lot of problems with thermals in the optical path.  It sounds like we agree that the only way to fix that stuff is to get the scope stabilized--and in my view, it's best to get to that point as quickly as possible.  However, once the scope is in equilibrium, radiative cooling is what causes problems with temperature gradients within the tube and with dew and/or frost--and that's what I'm much more concerned about.  You simply cannot discuss dew prevention without considering radiative cooling, which is at the heart of the issue.

 

 

I think it's great that you're pursuing that angle, and I am following that thread with interest.

 

I'm going at it from a different angle, some of which is inclusive of yours; there is much overlap in the solutions for each approach.

 

One aspect of my concept is; that "ambient" for the mirror is different than "ambient" for the OTA/corrector. For the OTA, the atmosphere is ambient. But for the mirror, the OTA and its internal space is "ambient". During "phase 2" (observing while outside temps are falling at 4-6º per hour), the difference is significant in my mind. Modeling the system as an electronic circuit (which we did with great success in solar's early days), the OTA is directly connected to the air mass, is highly convective, and releases its heat quickly. But from the mirror's POV, the OTA appears as a series resistance (dissipative) followed by a capacitive (storage and delay) element in the circuit. Nothing reaches the mirror without going through the OTA. And in response, the mirror release its heat more slowly into the OTA. The difference between these "heat release" curves is what causes the thermals, and changes the mirror figure., until they settle and coincide.

 

And so my theory is: the solution is to bring these two "heat release" curves closer together. Smooth out and slow down the OTA's heat release, so that it better tracks what the mirror is capable of. How much insulation is required to do this, and where (and when) it is placed, is what I'm sussing out.

 

 

Why would there be a need for "airspace" under the reflectix? 

 

Because Reflectix by itself is a poor insulator, about the same as a single pane of glass. I'm pretty sure this is spelled out in the Reflectix literature.

 

I have plenty of experience with the stuff, and I can verify that. I experimented with its use 6-7 years ago to insulate air intake and outtake pipes leading to/from a solar heater I built. One layer is useless. Two, little better. Three, useful but barely. But use one layer to enclose an airspace filled with fibrous material (a good insulator), and it magnifies the R value of the insulating material. The difference is significant. And that's what we went with; a 2-inch layer of fiberglass completely covered with a layer of Reflectix. Works great.



#65 eklf

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Posted 15 December 2018 - 12:19 PM

--------

 

 

 

Because Reflectix by itself is a poor insulator, about the same as a single pane of glass. I'm pretty sure this is spelled out in the Reflectix literature.

 

-------------

Since the use of reflextix in this application is radiant barrier, its insulation property (quantified by the R  value - very poor indeed) is not relevant.  What is relevant is its emmisivity, which at 4%, is what provides the benefit in this application.  And the literature says that an enclosed air space is not necessary for the radiant barrier function.

 

 "Radiant barriers by definition do not require an enclosed air space to provide benefit."

 

https://www.reflecti...sked-questions/

 

To be clear, not suggesting that one should not use an interior enclosed air space.  It seems to have worked for you, so go for it.  Simply saying that literature says there is little benefit.  


Edited by eklf, 15 December 2018 - 12:20 PM.


#66 precaud

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Posted 15 December 2018 - 12:28 PM

Since the use of reflextix in this application is radiant barrier, its insulation property (quantified by the R  value - very poor indeed) is not relevant.  What is relevant is its emmisivity, which at 4%, is what provides the benefit in this application.  And the literature says that an enclosed air space is not necessary for the radiant barrier function.

 

 "Radiant barriers by definition do not require an enclosed air space to provide benefit."

 

https://www.reflecti...sked-questions/

 

To be clear, not suggesting that one should not use an interior enclosed air space.  It seems to have worked for you, so go for it.  Simply saying that literature says there is little benefit.  

 

But if you have read this thread, you can see that I am not primarily concerned with its performance as a radiant barrier. And nothing I am doing stops it from functioning as such. So lets not get hung up in pedantics.



#67 Jaimo!

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Posted 15 December 2018 - 12:51 PM

Everyone thinks their own experience is empirical evidence and everyone else's experience is merely anecdotal.  And thus was born post-Modernism ...

 

grin.gif

Mike

I have been following this thread and Mike's statement pretty much sums up a majority of the experiences of using Reflectix...

 

However, no one has suggested using the scientific method to determine if Reflectix makes a real difference or if it is a placebo effect.

 

I have an extra 127mm Synta Mak (mine is celestron, but the Orion and Skywatcher are the same) sitting in my office that I am not using any time soon; it is an extremely common, widely available Maksutov.  If someone else reading this thread, and lives within a few hours of NYC, and also has a 127mm Synta, I would be willing to lend my scope out for research.  I also have the ADM dual vixen mount, so you could put both scopes on the same mount...  We need to do a head to head controlled comparison of a wrapped vs. naked scope.  After that we could then begin looking deeper into variables which will effect the views in two identical scopes (I say identical, but we all know every scope is slightly different).   If any one is interested, shoot me a PM and we can set up a study.

 

Jaimo!


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#68 Cpk133

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Posted 15 December 2018 - 03:35 PM

There's just no substitute for getting a heavy compound OTA out early and letting it cool.  If I was going to construct a diy insulative jacket I'd make it with Velcro so I could put the OTA out early for the big delta T, then put it on to mitigate radiative heat transfer.  Since I'm lazy, my removable insulative jacket is a mostly white Williams and Sonoma kitchen towel that gets draped over the top of the OTA once things have cooled down.  


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#69 choward94002

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Posted 15 December 2018 - 04:06 PM

(smile) Yep, hard to find a topic more active/ heated [pun intended] than ones with cooling and/ or Reflectix ...

 

@Kokatha:   Not sure where you got the idea that your observing style is being discounted or ignored, but at least for me that's very far from the truth: the idea of cooling the mirror cell/ thermal mass to below observing temperature to more quickly achieve and hold John's "thermally stable system" was an eye-opener for me, and I'm even now working on ways to get my mirror cell to be thermally equivalent to the OTA airspace (but I'm using peltier coolers with passive heat sinks rather than bags of ice) ... so, for tube currents "you 'da man, man!"

 

@jhayes:  "Thermally stable system" is a good napkin summary of what's needed to minimize tube currents ... great!

 

As to the "empirical versus anecdotal" thing ... well ... I really don't need to empirically experience my sliding around on an icy road before ending up upside down into a ditch to understand the need for road chains for my car in the winter, I'm quite happy to learn from other people's anecdotal stories to convince me to buy a set.  I don't need to empirically have my engine cut out due to a clogged fuel filter to understand the need to religiously check my gas lines, I'm quite happy to learn from what happened to other people who weren't so proactive ... and I don't need to run out of air at 150fsw to understand the importance of soaking my regulators right after my last dive so, yes, the saying "experience is the best teacher but only a fool learns from no other" comes to mind ...

 

<soapbox mode on>

 

It's also interesting that the issue of dew/ frost on the corrector seems to be so downplayed by so many of the informed and experienced folks here ... IMHO, allowing the formation of dew on the corrector plate is the second single most destructive thing you can do to your OTA other than dropping it on the ground like a sack of potatoes.  It's like discussing the importance of air quality with a scuba diver who then just pumps any old mixture into the tank, or the importance of gas quality with a pilot who goes and just dumps any old blend into the tank that they happen upon  ... the implications of bad air, or bad gas to either of those people should override any other consideration ...

 

Dew on the corrector plate will not only ruin your imaging session while it's on there, make your corrector plate dirtier from the surface tension of the water catching and holding dirt and pollen particles that would otherwise bounce off the glass but more insidiously as soon as it touches the edges of the plate (either on the sides or where the secondary is attached) it will get into the OTA interior as water from the capillary action in the gaps between the plate and the tube/ secondary holder sucking in as much water as your plate will hold (and if you're imaging in a near vertical orientation and have turned your corrector plate into a water bowl, that's a lot a water).  Even if you're removing it as fast as it develops with the dew heater or hair dryer the water vapor from the humidity differential between the ambient air and the OTA interior air will seep through those same gaps, plus during the day the mixing with the warmer more humid air inside your house with the OTA interior air when you take off your imaging train ... any and all of those things will increase the humidity of the air inside your OTA which will lead inevitably to "bad things" .. such as

 

- Formation of aluminum and iron oxides on exposed parts, usually threads which then become much harder to take off

- Absoption of moisture into the phenolic baffle tube and focuser grease, not only giving mold a growth medium but making the focuser more prone to binding, and inevitably

- Once the humidity level inside your OTA hits 70% and hold that for five consecutive days you'll get the formation of optical mold [https://www.ncbi.nlm...les/PMC1705860/] which is probably the single biggest killer of mirrored surfaces other than scratches.  A simple perusal of threads here on CN will reveal the dozens of people who are trying to recover from a mold infestation on the primary and/or secondary (you can't, the damage is permanent, time to recoat) ... and if you also preuse the threads you'll find the people who run into this problem are those same people that have kept the OTA's outside in an observatory or under a cover or under a deck where over time the OTA just keeps picking up water from the ambient air and concentrating it in the OTA until it hits that magic 70% number and the inevitable happens. 

 

IMHO, for remote sites especially where the OTA is essentially sitting there unattended day after day slowly turning into a terrarium humidity management inside of the OTA should be focus number one for the owner, above any consideration for thermals or anything else ... and seeing dew develop on the corrector in any form should fill them with the same dread as a pilot would have from seeing little bits of debris on his fuel filter or the scuba diver would get seeing the regulator primary filter with little bits of black "soot" on it ... it's a sign that something seriously wrong is happening that needs to be addressed before anything else.

 

And, humidity management isn't that hard to do if you have physical access to the OTA ... FarPoint makes visual back dessication caps [https://farpointastr...ps-accessories/] that can be recharged with microwave rechargeable packets from Amazon [https://www.amazon.com/Wisedry-Reactivated-Indicating-Rechargeable-Dehumidifiers/dp/B01MPYB16J/ref=pd_sim_328_2?_encoding=UTF8&pd_rd_i=B01MPYB16J&pd_rd_r=af6932a3-0088-11e9-90c0-f9307a5a46b8&pd_rd_w=1eOSU&pd_rd_wg=whZox&pf_rd_p=18bb0b78-4200-49b9-ac91-f141d61a1780&pf_rd_r=Z5V6KYM2BKMH5TY4FVTS&psc=1&refRID=Z5V6KYM2BKMH5TY4FVTS].  If you're remote and can't swap out the imaging train then that's not a problem ... get an OAG with a fat guidescope tube like the Celestron OAG (not one of the TOAG soda-straws), get an M42 to 2" adapter and put the dessicant cap on the guide stalk ... it's got direct access to the OTA interior air, isn't in the way of the optical path and you can recharge it every few weeks as long as you keep the dew from wicking into the OTA chamber in the first place! (no dessicant can keep up with the quart or so of water that will pump into the OTA from that, see the "why is my OTA full of water" thread from a few weeks ago)

 

Or, you can ignore the problems that humidity in the OTA interior can cause ... and I'll be right there to purchase your OTA for parts when the primary is corroded beyond use, the focuser won't slide cleanly, the corrector plate screws are hopelessly jammed, the secondary can't be unscrewed anymore and you're another person posting on CN about "how to clean mold from inside of an OTA" ...

 

<soapbox mode off>

 

OK, on to what I've found so far ... one of the "nice" things of having a farm of scopes on a hill at my high mountain mostly-remote dark site and a pair of scopes at my crappy foggy less-remote coastal site is that I can vary conditions between the otherwise identical scopes, aim them at the same targets and do an "apples to apples" comparison.  Due to my "hot tube" configurations I don't have dew problems (and don't want to possibly impair that ability with my changes) and with my RaspPi climate control computer managing the temp inside of the scope (to keep me away from dewpoint and monitor the overall humidity to keep things below 60%) it's pretty easy to hook up some more thermocouple and humidity sensors and chart those.  I've also been taking airy photo's of stars in my testing (in response to other folks talking about how detect and grade tube currents) to judge optical differences using StarBlinker (which I like better than PI's blink routine because SB can use FITS files natively and it's faster) ... here's what I've found so far ...

 

- Kokatha's cooling of the primary mirror cell prior to observing fits right into John's "thermally stable system" and really makes a difference.  I can't exactly put bags of ice onto my mostly-remote sites scopes but using four 12v 5a peltier's with computer CPU passive heat sinks works nicely (and, makes the OTA look like a radial aircraft engine with REALLY turns heads!) ... previously the mirror cell lagged behind the OTA interior temperature by as much as 10C as the sun went down and didn't hit equilibrium until much later and even then never really got less than 2C from the OTA interior temperature, now I can get the two in sync and it improved the airy disk patterns noticeably ... so, cool the mirror cell!  There is also a difference in orienting the OTA nose down when I'm cooling the cell ... I don't know if Kokatha discovered this experimentally or it was a happy circumstance of keeping the ice bags from falling off the mirror cell, but I now have a 45min cooling stage during the twilight just after the sun has set with the OTA pointing now down to cool the mirror cell down as low as possible (without hitting dewpoint of the interior OTA airspace) ...

 

- As jhayes noted in one of his thread responses, folks use Reflectix to limit radiative cooling because the surface has a low emissivity; aluminum foil or mylar were suggested to be better, but Reflectix is more durable.  I went with that ball and did some experiments to measure radiative heat loss with a standin for my OTA ... a 32gal metal trash can [https://www.amazon.c...ds=aluminum can]. 

 

Four metal trash cans were used, with plate glass used as a lid to simulate my corrector plate and a 22lb counterweight in the bottom to give me a thermal mass like the mirror cell (not the mirror, the mirror cell).  The cans were allowed to conductively acclimate to the ambient air through the metal (simulating my OTA tubes), and they were all placed behind my semi-permeable wind barrier I use with my OTA's to minimize any kind of wind chill effects.  One can was then spray painted with three layers of Rustoleum high temperature black paint, one was painted with three layers of Rustoleum appliance white, one was covered with Reflectix and one was covered with [material under test].  The can was fully sealed, as I espouse the "hot tube" approach and in line with my humidity management protocol ... l don't let outside air circulate with inside air so it that's a no-go for you then stop reading ....  I got some 2'x2' 1/4" float glass sheets for the can lid ends, put some window seal foam around the can lid ends to get a nice tight seal with the glass and put the trash cans pointed straight up to simulate radiative cooling into space (actually the upper atmosphere black body temperature, not space) with the can resting on 2x4's shelf 12" off the ground to limit conductive cooling with the ground.  I kept the cans under a painters tarp during the day (simulating an observatory or TG365 cover) to let the cans generally acclimate but not get heated up by the sun during the day and started getting my readings after astronomical twilight, they were all placed about 10ft apart and in an open area so there was no shading going on ...

 

Measurements were recorded on my RaspPi thermal management system, which normally is responsible for keeping my hot tubes "hot" but which also very happily records thermal and humidity sensor values to within a .5C accuracy [http://www.farnell.c...ets/2345098.pdf].  I spinkled those liberally throughout the cans: humidity on the can outside and can glass inside (since you will get higher humidity where it's warmer and warm air will rise) and temperature sensors on the can "skin" outside (8x), skin between the can surface and the Reflectix/ "substance under test" for those cans (8x), skin inside the can (8x), glass plate inside (4x), plate outside (4x), counterweight inside (2x) and the can skin underneath the weight outside (2x). For the air temperature readings I brought out the "big dogs" [http://www.ti.com/product/LMT70] which are accurate to .05C within the anticipated temperature range ... those were placed about 1" away from the can skin (8x) next to the skin sensors, about 2" above the bottom of the can next to the weight (4x), about 2" below and 2" inside of the glass plate (4x) and a string made to hang in the center of the can (4x, one 1" above the weight, one 1" below the glass and the other two spaced equally in the center).  The "big dog" sensors were more than able to detect thermal convection currents).  The entire sensor array was sampled every 30sec from astronomical twilight until astronomical dawn with outlier values discarded, results between similar sensors normalized and allowed me to build a very accurate thermal map of what was happening in the system as the night wore on [did I ever mention that I make a living building digital control circuits and firmware for companies in the Bay Area?  Maybe not ...] ...

 

In this experiment for my "hot tube" approach "cold is bad" ... the colder a can gets compared to it's buddies means it's losing more heat due to radiative cooling (since all other heat transfer methods are identical) ... and once the internal skin temperature drops below the OTA internal air ambient next to the skin you've got a thermal gradient between that airspace and the central airspace, you're no longer in thermal equilibrium and you'll get some convection going on ... remember, in this experiment the interior airspace is sealed so you will expect to see the outside air cooling below that airspace first, then conductive cooling through the metal can sides and glass would cool down the air next to the sides and you would start to see a convection current starting in the center, as time went on that airspace would continue to circulate moderated by the big weight thermal mass in the bottom which would tend to heat cooler air descending from the sides ... but again, this effect will be identical between all four cans so the only variable in the system is the radiative cooling happening through the can sides.  It's also important to note that when I say "the temperature dropped XC per hour" that's the rate AFTER I have subtracted the drop due to outside air conduction and other kinds of cooling so that the only rate left is that directly attributable to radiative cooling, which is what I'm testing ... obviously the cans went through a lot more temperature change throughout the night than a few degrees!

 

... so, after a little over a week of running so far here's what I've found (and more runs are ongoing) ...

 

- The black can was the "worse" performer, the skin temp got about 2-2.5C colder per hour than the outside ambient air temperature drops (and would transfer that cooling to the internal air side space), so the skin temp went below the interior ambient side temperature after just 2hr (and a convective thermal gradient started, with about a .75C temperature difference between skin and center column).  The skin dropped even further, below the outside ambient temperature after about 5hr (which puts it in danger of dew forming if fog moves in, see the "frosty scope" picture posted a few days ago) and indeed was getting really close to the ambient still air dewpoint on some evenings. 

 

Notes: If I ever need to collect water in the desert at night I'm just going to get a (black) tire, put it out in the open, attach a string to the bottom and I'll be drinking by morning.  Oy ...

 

- The white can was noticeably better, the skin temp dropped about 1-1.5C per hour, dropping below the interior air ambient temperature after about 3hr, below the outside ambient temperature after about 6hr but same result ... a thermal gradient with the skin started a convective circulation in the can. 

 

Notes: One of my C14's has an exterior white color, good to know this ...

 

- The Reflectix covered can was definitely better, it only dropped about .75C per hour and remained thermally stable with the interior ambient temperature until about 6hr into the experiment and never dropped below the outside ambient air for the evening ... so if I wanted to maintain a thermally stable system this would be a good way to help with it ...

 

Notes:  One thing I discovered was that it is a PAIN to deal with taking on and off the Reflectix ... and since I'm just using it as a cover it occurred to me that I don't care if it has gaps so I ended up getting some velcro stick rolls and putting down some fabric side tabs on the can, hook side tabs on the Reflectix and building four "panels" for the can.  When I want to apply it I just line up the tabs, when I want to remove it I just pull it away ... there's a small 1/8" gap between panels and obviously there's an air gap between the Reflectix and the can but after I measured got some numbers comparing a can using the tabs with a can with the Reflectix tightly wound up I didn't find any significant difference ... so that answers the OP's question if having an airspace between the OTA and the Reflectix makes a difference: no.

 

- My "substance of the day" can went through a variety of materials with very low emissivity values, specifically mylar and aluminum foil.  Durability is important, as is resistance to wind and tearing so I ended up with Reynolds Wrap heavy duty cooling aluminum foil (shiny side out) attached to the Reflectix with 3M drywall spray adhesive since the Mylar would still tear no matter how I attached it to the Reflectix backing.  In tests this gave me an extremely low .1C per hour temperature drop for the skin and it never dropped below the interior ambient sidewall temperature ... so this performed the best of the group! 

 

Notes: I decided to use the Reflectix as a backing material for the "substance of the day" because I had a bunch of it and had already made up some velcro panels, but any sufficiently stiff material would work: plastic, aluminum, construction paper, anything.  The function of the backing material isn't insulation, it's to hold the low emissivity material in place throughout the night ...

 

I then did some experiments working to replicate Kokatha's work, specifically seeing what would happen if I cooled the thermal mass/ weight to below the anticipated outside ambient air temperature using my peltiers.  The results were quite surprising to me (at least initially) ... from my readings, in all four test cases the area around the thermal mass stayed at the thermal cooling setpoint temperature for almost the entire evening, which means that if that was a mirror then thermal expansion expansion would have been much reduced and you'd need to focus much less frequently.  I believe what is happening is that when the thermal mass is cooler than the interior ambient air it cools the air next to it and sets up an inversion layer, with the even cooler air surrounding the thermal mass in the bottom of the can/ OTA keeping the thermal mass insulated from any convection currents happening ... "cooled" ambient air which is descending from the sides drops, hits that "even cooler" air layer surrounding the thermal mass and bounces right off, setting up a thermal circulation that never even gets close to the mirror/ thermal mass (and you could see this from the sensors surrounding the thermal mass, barely any movement while the central and side air temperatures are going up and up, I saw a 8C differential in the "black" can which was huge!  The secondary is expanding, of course, but not the primary which means in a "live fire" test I should only need to do minimal temperature refocusing if I precool the mirror cell ... amazing!  I found that if I cooled the thermal mass/ weight to about 5C below the forecast low temp for the evening I would *never* get any convection currents around the mirror for the entire evening, even using the "black" can. 

 

Whew, quite a wall of text, gone to sleep yet? :)  More tests are underway, I want to characterize the effects of a "dew shield" on the system (my cans don't have "dew shields" yet) and I want to see how far I can "tilt" the cans before I lose my protective inversion cooling layer and I need to use the peltiers to regenerate it.  I also want to see how the inversion layer is affected by the active heaters for my "hot tube" with an aim at having a "cold bottom" and a "hot tube" in one system ... for me, for the "hot tube" approach, here's my current plan of action for my OTA's:

 

- Have Reflectix panels with Aluminum foil attached on the outside, velcro tab attached around the OTA tube and extending 8" up from the end with the inside of that area flocked with black velvet fabric

- After the sun sets but before astronomical twilight have the OTA assume a nose down orientation and crank up the peltier's to drop the mirror cell temperature to 5C below the anticipated low for the evening

- After astronomical twilight, as the scope does it's slewing and tracking and such have the thermal control computer maintain not only the "hot tube" internal temperature to keep above the calculated interior dewpoint as well as the calculated exterior dewpoint for the corrector glass but now also control the peltiers to keep the mirror cell temperature at the "5C below low" setpoint ... 

- Add my high precision temperature sensors to the area around my primary mirror to give information if I'm losing my protective inversion layer

 

So, back to my workshop and on with the festivities on this thread!  Clear skies and don't get it locked!  


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#70 eklf

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Posted 15 December 2018 - 04:08 PM

But if you have read this thread, you can see that I am not primarily concerned with its performance as a radiant barrier. And nothing I am doing stops it from functioning as such. So lets not get hung up in pedantics.

Having read the thread I am assuming the main intent is "the efficacy of using an insulating (airspace) layer under the Reflectix" as stated in post #58.

 

If you are not primarily concerned with its performance as a radiant barrier, and agree that reflectrix is a poor insulator, then really no airspace or insulation under the reflectix is going to make it a better insulator. In other words, pedantically speaking, ditch the reflectix for your intended purpose as it does not bring anything to the table other than the radiant barrier property.

 

For those who do intend to use it as a radiant barrier, an interior space should make little difference - most of the benefit will be from reflectrix itself only,


Edited by eklf, 15 December 2018 - 04:26 PM.


#71 precaud

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Posted 15 December 2018 - 05:41 PM

If you are not primarily concerned with its performance as a radiant barrier, and agree that reflectrix is a poor insulator, then really no airspace or insulation under the reflectix is going to make it a better insulator. In other words, pedantically speaking, ditch the reflectix for your intended purpose as it does not bring anything to the table other than the radiant barrier property.
 
Ummm... no... don't ditch the Reflectix. Just because "radiant barrier" isn't my "primary" concern doesn't mean I need or want to ditch it. There's no need to frame it in such extreme terms. We can have both:
And a closed, insulated airspace is a MUCH better insulator than a leaky or open one. Air currents which can't comingle are (obviously) better insulated from each other.

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#72 jhayes_tucson

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Posted 15 December 2018 - 06:15 PM

(smile) Yep, hard to find a topic more active/ heated [pun intended] than ones with cooling and/ or Reflectix ...

 

@Kokatha:   Not sure where you got the idea that your observing style is being discounted or ignored, but at least for me that's very far from the truth: the idea of cooling the mirror cell/ thermal mass to below observing temperature to more quickly achieve and hold John's "thermally stable system" was an eye-opener for me, and I'm even now working on ways to get my mirror cell to be thermally equivalent to the OTA airspace (but I'm using peltier coolers with passive heat sinks rather than bags of ice) ... so, for tube currents "you 'da man, man!"

 

@jhayes:  "Thermally stable system" is a good napkin summary of what's needed to minimize tube currents ... great!

 

As to the "empirical versus anecdotal" thing ... well ... I really don't need to empirically experience my sliding around on an icy road before ending up upside down into a ditch to understand the need for road chains for my car in the winter, I'm quite happy to learn from other people's anecdotal stories to convince me to buy a set.  I don't need to empirically have my engine cut out due to a clogged fuel filter to understand the need to religiously check my gas lines, I'm quite happy to learn from what happened to other people who weren't so proactive ... and I don't need to run out of air at 150fsw to understand the importance of soaking my regulators right after my last dive so, yes, the saying "experience is the best teacher but only a fool learns from no other" comes to mind ...

 

<soapbox mode on>

 

It's also interesting that the issue of dew/ frost on the corrector seems to be so downplayed by so many of the informed and experienced folks here ... IMHO, allowing the formation of dew on the corrector plate is the second single most destructive thing you can do to your OTA other than dropping it on the ground like a sack of potatoes.  It's like discussing the importance of air quality with a scuba diver who then just pumps any old mixture into the tank, or the importance of gas quality with a pilot who goes and just dumps any old blend into the tank that they happen upon  ... the implications of bad air, or bad gas to either of those people should override any other consideration ...

 

Dew on the corrector plate will not only ruin your imaging session while it's on there, make your corrector plate dirtier from the surface tension of the water catching and holding dirt and pollen particles that would otherwise bounce off the glass but more insidiously as soon as it touches the edges of the plate (either on the sides or where the secondary is attached) it will get into the OTA interior as water from the capillary action in the gaps between the plate and the tube/ secondary holder sucking in as much water as your plate will hold (and if you're imaging in a near vertical orientation and have turned your corrector plate into a water bowl, that's a lot a water).  Even if you're removing it as fast as it develops with the dew heater or hair dryer the water vapor from the humidity differential between the ambient air and the OTA interior air will seep through those same gaps, plus during the day the mixing with the warmer more humid air inside your house with the OTA interior air when you take off your imaging train ... any and all of those things will increase the humidity of the air inside your OTA which will lead inevitably to "bad things" .. such as

 

- Formation of aluminum and iron oxides on exposed parts, usually threads which then become much harder to take off

- Absoption of moisture into the phenolic baffle tube and focuser grease, not only giving mold a growth medium but making the focuser more prone to binding, and inevitably

- Once the humidity level inside your OTA hits 70% and hold that for five consecutive days you'll get the formation of optical mold [https://www.ncbi.nlm...les/PMC1705860/] which is probably the single biggest killer of mirrored surfaces other than scratches.  A simple perusal of threads here on CN will reveal the dozens of people who are trying to recover from a mold infestation on the primary and/or secondary (you can't, the damage is permanent, time to recoat) ... and if you also preuse the threads you'll find the people who run into this problem are those same people that have kept the OTA's outside in an observatory or under a cover or under a deck where over time the OTA just keeps picking up water from the ambient air and concentrating it in the OTA until it hits that magic 70% number and the inevitable happens. 

 

IMHO, for remote sites especially where the OTA is essentially sitting there unattended day after day slowly turning into a terrarium humidity management inside of the OTA should be focus number one for the owner, above any consideration for thermals or anything else ... and seeing dew develop on the corrector in any form should fill them with the same dread as a pilot would have from seeing little bits of debris on his fuel filter or the scuba diver would get seeing the regulator primary filter with little bits of black "soot" on it ... it's a sign that something seriously wrong is happening that needs to be addressed before anything else.

 

And, humidity management isn't that hard to do if you have physical access to the OTA ... FarPoint makes visual back dessication caps [https://farpointastr...ps-accessories/] that can be recharged with microwave rechargeable packets from Amazon [https://www.amazon.com/Wisedry-Reactivated-Indicating-Rechargeable-Dehumidifiers/dp/B01MPYB16J/ref=pd_sim_328_2?_encoding=UTF8&pd_rd_i=B01MPYB16J&pd_rd_r=af6932a3-0088-11e9-90c0-f9307a5a46b8&pd_rd_w=1eOSU&pd_rd_wg=whZox&pf_rd_p=18bb0b78-4200-49b9-ac91-f141d61a1780&pf_rd_r=Z5V6KYM2BKMH5TY4FVTS&psc=1&refRID=Z5V6KYM2BKMH5TY4FVTS].  If you're remote and can't swap out the imaging train then that's not a problem ... get an OAG with a fat guidescope tube like the Celestron OAG (not one of the TOAG soda-straws), get an M42 to 2" adapter and put the dessicant cap on the guide stalk ... it's got direct access to the OTA interior air, isn't in the way of the optical path and you can recharge it every few weeks as long as you keep the dew from wicking into the OTA chamber in the first place! (no dessicant can keep up with the quart or so of water that will pump into the OTA from that, see the "why is my OTA full of water" thread from a few weeks ago)

 

Or, you can ignore the problems that humidity in the OTA interior can cause ... and I'll be right there to purchase your OTA for parts when the primary is corroded beyond use, the focuser won't slide cleanly, the corrector plate screws are hopelessly jammed, the secondary can't be unscrewed anymore and you're another person posting on CN about "how to clean mold from inside of an OTA" ...

 

<soapbox mode off>

 

OK, on to what I've found so far ... one of the "nice" things of having a farm of scopes on a hill at my high mountain mostly-remote dark site and a pair of scopes at my crappy foggy less-remote coastal site is that I can vary conditions between the otherwise identical scopes, aim them at the same targets and do an "apples to apples" comparison.  Due to my "hot tube" configurations I don't have dew problems (and don't want to possibly impair that ability with my changes) and with my RaspPi climate control computer managing the temp inside of the scope (to keep me away from dewpoint and monitor the overall humidity to keep things below 60%) it's pretty easy to hook up some more thermocouple and humidity sensors and chart those.  I've also been taking airy photo's of stars in my testing (in response to other folks talking about how detect and grade tube currents) to judge optical differences using StarBlinker (which I like better than PI's blink routine because SB can use FITS files natively and it's faster) ... here's what I've found so far ...

 

- Kokatha's cooling of the primary mirror cell prior to observing fits right into John's "thermally stable system" and really makes a difference.  I can't exactly put bags of ice onto my mostly-remote sites scopes but using four 12v 5a peltier's with computer CPU passive heat sinks works nicely (and, makes the OTA look like a radial aircraft engine with REALLY turns heads!) ... previously the mirror cell lagged behind the OTA interior temperature by as much as 10C as the sun went down and didn't hit equilibrium until much later and even then never really got less than 2C from the OTA interior temperature, now I can get the two in sync and it improved the airy disk patterns noticeably ... so, cool the mirror cell!  There is also a difference in orienting the OTA nose down when I'm cooling the cell ... I don't know if Kokatha discovered this experimentally or it was a happy circumstance of keeping the ice bags from falling off the mirror cell, but I now have a 45min cooling stage during the twilight just after the sun has set with the OTA pointing now down to cool the mirror cell down as low as possible (without hitting dewpoint of the interior OTA airspace) ...

 

- As jhayes noted in one of his thread responses, folks use Reflectix to limit radiative cooling because the surface has a low emissivity; aluminum foil or mylar were suggested to be better, but Reflectix is more durable.  I went with that ball and did some experiments to measure radiative heat loss with a standin for my OTA ... a 32gal metal trash can [https://www.amazon.c...ds=aluminum can]. 

 

Four metal trash cans were used, with plate glass used as a lid to simulate my corrector plate and a 22lb counterweight in the bottom to give me a thermal mass like the mirror cell (not the mirror, the mirror cell).  The cans were allowed to conductively acclimate to the ambient air through the metal (simulating my OTA tubes), and they were all placed behind my semi-permeable wind barrier I use with my OTA's to minimize any kind of wind chill effects.  One can was then spray painted with three layers of Rustoleum high temperature black paint, one was painted with three layers of Rustoleum appliance white, one was covered with Reflectix and one was covered with [material under test].  The can was fully sealed, as I espouse the "hot tube" approach and in line with my humidity management protocol ... l don't let outside air circulate with inside air so it that's a no-go for you then stop reading ....  I got some 2'x2' 1/4" float glass sheets for the can lid ends, put some window seal foam around the can lid ends to get a nice tight seal with the glass and put the trash cans pointed straight up to simulate radiative cooling into space (actually the upper atmosphere black body temperature, not space) with the can resting on 2x4's shelf 12" off the ground to limit conductive cooling with the ground.  I kept the cans under a painters tarp during the day (simulating an observatory or TG365 cover) to let the cans generally acclimate but not get heated up by the sun during the day and started getting my readings after astronomical twilight, they were all placed about 10ft apart and in an open area so there was no shading going on ...

 

Measurements were recorded on my RaspPi thermal management system, which normally is responsible for keeping my hot tubes "hot" but which also very happily records thermal and humidity sensor values to within a .5C accuracy [http://www.farnell.c...ets/2345098.pdf].  I spinkled those liberally throughout the cans: humidity on the can outside and can glass inside (since you will get higher humidity where it's warmer and warm air will rise) and temperature sensors on the can "skin" outside (8x), skin between the can surface and the Reflectix/ "substance under test" for those cans (8x), skin inside the can (8x), glass plate inside (4x), plate outside (4x), counterweight inside (2x) and the can skin underneath the weight outside (2x). For the air temperature readings I brought out the "big dogs" [http://www.ti.com/product/LMT70] which are accurate to .05C within the anticipated temperature range ... those were placed about 1" away from the can skin (8x) next to the skin sensors, about 2" above the bottom of the can next to the weight (4x), about 2" below and 2" inside of the glass plate (4x) and a string made to hang in the center of the can (4x, one 1" above the weight, one 1" below the glass and the other two spaced equally in the center).  The "big dog" sensors were more than able to detect thermal convection currents).  The entire sensor array was sampled every 30sec from astronomical twilight until astronomical dawn with outlier values discarded, results between similar sensors normalized and allowed me to build a very accurate thermal map of what was happening in the system as the night wore on [did I ever mention that I make a living building digital control circuits and firmware for companies in the Bay Area?  Maybe not ...] ...

 

In this experiment for my "hot tube" approach "cold is bad" ... the colder a can gets compared to it's buddies means it's losing more heat due to radiative cooling (since all other heat transfer methods are identical) ... and once the internal skin temperature drops below the OTA internal air ambient next to the skin you've got a thermal gradient between that airspace and the central airspace, you're no longer in thermal equilibrium and you'll get some convection going on ... remember, in this experiment the interior airspace is sealed so you will expect to see the outside air cooling below that airspace first, then conductive cooling through the metal can sides and glass would cool down the air next to the sides and you would start to see a convection current starting in the center, as time went on that airspace would continue to circulate moderated by the big weight thermal mass in the bottom which would tend to heat cooler air descending from the sides ... but again, this effect will be identical between all four cans so the only variable in the system is the radiative cooling happening through the can sides.  It's also important to note that when I say "the temperature dropped XC per hour" that's the rate AFTER I have subtracted the drop due to outside air conduction and other kinds of cooling so that the only rate left is that directly attributable to radiative cooling, which is what I'm testing ... obviously the cans went through a lot more temperature change throughout the night than a few degrees!

 

... so, after a little over a week of running so far here's what I've found (and more runs are ongoing) ...

 

- The black can was the "worse" performer, the skin temp got about 2-2.5C colder per hour than the outside ambient air temperature drops (and would transfer that cooling to the internal air side space), so the skin temp went below the interior ambient side temperature after just 2hr (and a convective thermal gradient started, with about a .75C temperature difference between skin and center column).  The skin dropped even further, below the outside ambient temperature after about 5hr (which puts it in danger of dew forming if fog moves in, see the "frosty scope" picture posted a few days ago) and indeed was getting really close to the ambient still air dewpoint on some evenings. 

 

Notes: If I ever need to collect water in the desert at night I'm just going to get a (black) tire, put it out in the open, attach a string to the bottom and I'll be drinking by morning.  Oy ...

 

- The white can was noticeably better, the skin temp dropped about 1-1.5C per hour, dropping below the interior air ambient temperature after about 3hr, below the outside ambient temperature after about 6hr but same result ... a thermal gradient with the skin started a convective circulation in the can. 

 

Notes: One of my C14's has an exterior white color, good to know this ...

 

- The Reflectix covered can was definitely better, it only dropped about .75C per hour and remained thermally stable with the interior ambient temperature until about 6hr into the experiment and never dropped below the outside ambient air for the evening ... so if I wanted to maintain a thermally stable system this would be a good way to help with it ...

 

Notes:  One thing I discovered was that it is a PAIN to deal with taking on and off the Reflectix ... and since I'm just using it as a cover it occurred to me that I don't care if it has gaps so I ended up getting some velcro stick rolls and putting down some fabric side tabs on the can, hook side tabs on the Reflectix and building four "panels" for the can.  When I want to apply it I just line up the tabs, when I want to remove it I just pull it away ... there's a small 1/8" gap between panels and obviously there's an air gap between the Reflectix and the can but after I measured got some numbers comparing a can using the tabs with a can with the Reflectix tightly wound up I didn't find any significant difference ... so that answers the OP's question if having an airspace between the OTA and the Reflectix makes a difference: no.

 

- My "substance of the day" can went through a variety of materials with very low emissivity values, specifically mylar and aluminum foil.  Durability is important, as is resistance to wind and tearing so I ended up with Reynolds Wrap heavy duty cooling aluminum foil (shiny side out) attached to the Reflectix with 3M drywall spray adhesive since the Mylar would still tear no matter how I attached it to the Reflectix backing.  In tests this gave me an extremely low .1C per hour temperature drop for the skin and it never dropped below the interior ambient sidewall temperature ... so this performed the best of the group! 

 

Notes: I decided to use the Reflectix as a backing material for the "substance of the day" because I had a bunch of it and had already made up some velcro panels, but any sufficiently stiff material would work: plastic, aluminum, construction paper, anything.  The function of the backing material isn't insulation, it's to hold the low emissivity material in place throughout the night ...

 

I then did some experiments working to replicate Kokatha's work, specifically seeing what would happen if I cooled the thermal mass/ weight to below the anticipated outside ambient air temperature using my peltiers.  The results were quite surprising to me (at least initially) ... from my readings, in all four test cases the area around the thermal mass stayed at the thermal cooling setpoint temperature for almost the entire evening, which means that if that was a mirror then thermal expansion expansion would have been much reduced and you'd need to focus much less frequently.  I believe what is happening is that when the thermal mass is cooler than the interior ambient air it cools the air next to it and sets up an inversion layer, with the even cooler air surrounding the thermal mass in the bottom of the can/ OTA keeping the thermal mass insulated from any convection currents happening ... "cooled" ambient air which is descending from the sides drops, hits that "even cooler" air layer surrounding the thermal mass and bounces right off, setting up a thermal circulation that never even gets close to the mirror/ thermal mass (and you could see this from the sensors surrounding the thermal mass, barely any movement while the central and side air temperatures are going up and up, I saw a 8C differential in the "black" can which was huge!  The secondary is expanding, of course, but not the primary which means in a "live fire" test I should only need to do minimal temperature refocusing if I precool the mirror cell ... amazing!  I found that if I cooled the thermal mass/ weight to about 5C below the forecast low temp for the evening I would *never* get any convection currents around the mirror for the entire evening, even using the "black" can. 

 

Whew, quite a wall of text, gone to sleep yet? smile.gif  More tests are underway, I want to characterize the effects of a "dew shield" on the system (my cans don't have "dew shields" yet) and I want to see how far I can "tilt" the cans before I lose my protective inversion cooling layer and I need to use the peltiers to regenerate it.  I also want to see how the inversion layer is affected by the active heaters for my "hot tube" with an aim at having a "cold bottom" and a "hot tube" in one system ... for me, for the "hot tube" approach, here's my current plan of action for my OTA's:

 

- Have Reflectix panels with Aluminum foil attached on the outside, velcro tab attached around the OTA tube and extending 8" up from the end with the inside of that area flocked with black velvet fabric

- After the sun sets but before astronomical twilight have the OTA assume a nose down orientation and crank up the peltier's to drop the mirror cell temperature to 5C below the anticipated low for the evening

- After astronomical twilight, as the scope does it's slewing and tracking and such have the thermal control computer maintain not only the "hot tube" internal temperature to keep above the calculated interior dewpoint as well as the calculated exterior dewpoint for the corrector glass but now also control the peltiers to keep the mirror cell temperature at the "5C below low" setpoint ... 

- Add my high precision temperature sensors to the area around my primary mirror to give information if I'm losing my protective inversion layer

 

So, back to my workshop and on with the festivities on this thread!  Clear skies and don't get it locked!  

 

That's an interesting experiment that I'd try myself if it weren't almost continuously cloudy here in Bend right now.  Here are a few questions and comments:

 

1)  Have you measured the temperature difference between the front element and the ambient air temperature of the glass itself?  You'll need a black temperature sensor to do that.

 

2)  How open is your horizon?  The results of your experiment can be significantly impacted if you have a lot of trees and buildings around.  You'll get the best results if the horizon is unrestricted.

 

3)  Are you recording the ambient temperature and relative humidity?  The humidity makes a very big difference in what you'll measure.  You'll get the biggest drop with the lowest humidity.

 

4)  If you tell me the air temp (C ), the relative humidity (%) and the measured temp of the glass after a few hours under a clear sky, we can compare what you measure with what my model predicts.  I can do the same thing for an unheated dew shield if you give me the diameter and length.  It's best if you wrap the dew shield in Reflectix to keep it as close to the ambient air temperature as possible.

 

I have the ability to measure the ambient air temperature and the corrector temperature on my scope out at DSW.  My model predicts a temperature drop at the front surface of the corrector of -2.3C and I two nights ago (when it was really clear) I measured a drop of -2.01C.  That's with a 381mm long dew shield on a C14.  My two sensors have not been carefully calibrated so that level of agreement is well within the accuracy of my measurement.  I've made the measurement informally a number of times and the values range from about -1.8C to -2.5C.  When I have some time, I'll have to get more methodical so I can get a good average value.  It's hard to do everything I want to do since the scope isn't in my backyard.

 

John


Edited by jhayes_tucson, 15 December 2018 - 06:15 PM.


#73 WadeH237

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Posted 15 December 2018 - 06:34 PM

With all due respect SCT telescopes aren't uber complex systems & are considerably more robust than many might imagine! (especially when it comes to the shock-resistance of the glass components!) 

I may not have been clear.

 

I agree that SCTs are not complex systems.  It's the specific behavior of the currents inside the tube that are complex.  And when I say it's not trivial to model, I'm talking about mathematically.  There's been some discussion about how to even measure the sky temperature to do so.  It's not as simple as it seems.

 

Finally, I have no argument whatsoever with the results that you are able to get imaging.  I was quite clear in my response that I believe that the way to get the best performance that the scope can deliver it to get it to reach temperature equilibrium.

 

What seems to cause all the commotion is that there may be a middle ground that benefits some of us.  For example, I tend to observe out in the field, where I don't have access to quantities of ice or other means to rapidly cool the telescope to a specific target temperature.  And earlier this year, we had a nice parade of planets, but at my latitude they were all low in the sky.  Also, Jupiter set fairly early, followed by Saturn.

 

If I took the attitude that "cooling the scope to exactly match ambient temperature is the only way to reach top performance, and if I can't do that, I will do nothing", I would have had terrible planetary viewing.  I don't have to dispute the science and I don't have to dispute your results to say that using a Reflectix wrap significantly improved the performance of my telescope when the objects I wanted to view were available for viewing.  I've had my C14 for 15 years, and I know what it's capable of when Jupiter and Saturn are favorably placed between 2am and 4am.  I know how it works at the sites I use, in the temperatures that I see, and on the targets that I'm viewing.  The is zero doubt whatsoever that I got improved results with it, no matter what anyone else says.

 

Perfect?  Certainly not.  Better than doing nothing?  Absolutely.

 

I am actually pretty interested in understanding why I got the results that I did this year.  So far, I don't know that I've seen a complete explanation.



#74 Kokatha man

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Posted 15 December 2018 - 06:55 PM

I think some of my last post was misinterpreted - but that's nothing new...I misinterpret myself regularly! rofl2.gif

 

I think I'm overly-pedantic a lot of the time so I can understand others making that interpretation of my own prose: the Kim, Mike Hughes & UFO et al devotees have not all "borne witness" but, like my door-knockers, find it no impediment to their unwavering faiths.

 

As a pro artist I'd personally very much encompass that as "post modern" Mike - but much more widespread as "perceived" rather than "received" wisdom...in actuality it wouldn't matter one whit whether objects flew, floated, fell or fabricated themselves into view for the Lucky Ones who've actually experienced their epiphanies - especially those who can describe said aliens or have had "ahem" things done to them as reported in the "news" link of another CN thread (first post) in the "Off Topic" forum: https://www.cloudyni...west-Australia/

 

...but there I go again, obfuscating! foreheadslap.gif

 

<"I don't know if Kokatha discovered this experimentally or it was a happy circumstance of keeping the ice bags from falling off the mirror cell">

 

Funny about that Howard (?) - I reckon it was my super-developed, intuitive mind at work there..!?!

 

Actually, it was when my original idea of standing the ota face-up in the ice-bucket kept on causing the darn thing to eventually flood with water on hot nights, luckily before the salt addition phase! rofl2.gif

 

<"It's also interesting that the issue of dew/ frost on the corrector seems to be so downplayed by so many of the informed and experienced folks here ... IMHO, allowing the formation of dew on the corrector plate is the second single most destructive thing...">

 

On a (slightly) more serious note - & I think I've covered the issue of dew both internally & externally comprehensively - I wash my corrector face perhaps a dozen times a year on the outside & perhaps 2 or even 3 times on the interior...the primary about every 18 months with a "huff & clean" of the secondary when I wash the interior of the corrector. I'm fussy & believe a clear corrector without light inhibition/scatter etc is a bonus, however small.

 

I also installed a neoprene gasket under my corrector stopping water ingress...anyone can get these made to any thickness by taking their corrector retaining ring as the pattern to a gasket maker - for a few bucks with spares.

 

My rear vents have been taped shut now for years btw. (I bought the Chinese-made C14 whence they had the sense to standardise the casings of Edges & non-Edges)

 

I like the experimentation you're carrying out Howard, I did mention in this thread or one of the other "Reflectix" ones that I have no problems (99% of the time anyway) with the primary tracking the ambient & left my own notions as to why open, ie <"radiative/convection etc">

 

I'd like to see your experiment include a large slab of glass equivalent to the primary (appreciating this is probably something very difficult to do) in your trash cans: I say this in relation to your observed temperature differential between outside skin & internal air therein: your black can displaying a lower temperature than the ambient/outside air which you either deduce would cause internal air currents due to temp. differentials - or are those "strings" actually some sort of air current detectors & sway in said currents..?!? confused1.gif  

 

My interest in the effects of the primary's large mass of glass in any equations has a couple of factors - I myself was initially concerned when I started out all those years ago with the ability to effectively cool a material that is inherently such a good insulator: our practise clearly demonstrated that surrounding the back & sides with a super-chilled metal jacket (ie, the C14's rear-casing) created a very effective means of cooling said glass slab...pure irrational speculation atm but I wonder whether there is some form of "moderation" going on inside the ota with its' presence..? confused1.gif

 

When we are imaging for very lengthy periods over the night it is usually Neptune & Uranus that get left till last, anywhere up to 4am or so...as said, the primary usually tracks the outside air temperature very well but it is (nearly) always our practise to re-collimate for either of the Ice Giants. (not so for the very few Neptune captures this apparition - because it has been relative near Mars in the latter months.)

 

But as I've said repeatedly, the defocused/focused star is our guide to the "health" of our system & I see nothing most of the time when we re-examine these to indicate any imbalances, such as tube currents which are very easy to detect - although collimation tweaking is sometimes needed... (if I have not mentioned it previously, this is always an "in camera" practise with the self-same imaging setup)

 

Enough from me for now... wink.gif

 

 

 



#75 Kokatha man

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Posted 15 December 2018 - 07:11 PM

ps: Wade, in my Post #49 I tried to emphasize there was no inherent criticism per se of your belief/practice re your own outcomes...I can completely accept the conclusion that what you did was "better than nothing" - my next post completely obfuscated it for many I think! flowerred.gif

 

In my mind there are quite a few situations where we do not need cooling at all - or like on 5th December I was distracted & let the mirror fall way below ambient when probably ice was an unnecessary option...the fan on the rear casing - with scope pointing upwards - was probably well-sufficient that night when the night temperature dropped very slowly...I should confess that on the odd occasions it isn't necessary it can be hard to adapt to that approach..! rofl2.gif

 

There are times & places where ambient is automatically attained - my main contention is that for optimum outcomes over the (normal) wide temperature variations "active" cooling is the pre-requisite & getting your primary mirror stabilised to the outside air/ambient temperature is the way to go...




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