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Questions About Insulating SCT's with Reflectix

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#151 choward94002

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Posted 03 December 2018 - 10:03 AM

choward94002 wrote: "so if the goal is "no thermals" then a sealed tube, no matter how well mixed, simply won't work …"

 

Really?  Have YOU actually tried it?  Or are your views just 'theory'?

Been "trying" it for almost two years nearly every night with 11 different scopes in two different locations ... so for me I would say the answer is a guarded "why yes, yes I have ..."

 

Now that John told me what to look for a week or so ago regarding thermal artifacts in imaging and going back through my TB's of archived lights, I can indeed see thermal artifacts in my wintertime images (when the heaters are running full tilt and the corrector plate is getting mighty cold) becoming less noticeable in the spring/ fall (when the heaters come on sporadically and I'm only dealing with radiative cooling to space) and disappearing in the summertime (when the heaters don't come on and the OTA stays at essentially the same temperature as it was at dusk) ... the effects are small and I hadn't noticed them in the past (sigh) but they are there ... so for my specific locations(s), using my individual setups, I would say that my observations are a practical reality for me.  Your reality, of course, may vary ... smile.gif

 

It's important to note that I do *NOT* do visual, I don't look at planets, I don't use any kind of magnification (unless I'm collimating), I do long(er) duration imaging of 10-20min where on the whole the effect of tube thermals are subsumed into the "seeing" of the evening (which is why I never saw the artifacts in the past, they just aren't noticeable unless you know what to look for).  I don't do globulars or clusters of any kind so I don't have the double splitting/ duplication issues that John has (although now that I know what to look for I can see the effect in some galaxies) so again, the limited thermals aren't a factor for me ... my needing to refocus every 15min, or getting dust/ pollen onto my primary over the span of a season, or suddenly dewing up my CCD cover glass because I'm running my cooler as low as I reasonably can, those are much more of a factor and things I've run into in the past ... and again, your mileage and reality will likely vary ... smile.gif


Edited by choward94002, 03 December 2018 - 10:17 AM.


#152 Peter Besenbruch

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Posted 03 December 2018 - 02:45 PM

As for insulation - that again is all over the map in terms of what people have been saying.  When EdgeHD came out - people complained that the vents were too small to be effective.  Later add-on fans came along - and people said they worked great.  Now people are saying you should seal up the scope - and fans are bad - or maybe they're good - I think it depends on how they're used.  Yet in an adjacent thread on EdgeHD vs. XLT, the big problem with EdgeHD isn't the vents - but the field correcting elements - which seal the scope and prevent it from equilibrating properly.  And the claim is backed by the usual expert assessments and side by side comparisons - none of which ever yield objective data in the form of diffraction limited images or something.

No-one has said that fans are bad. The question is whether they are need when insulating the tube. John Hayes is arguing that they are needed in his experience with large SCTs. There was also the issue raised about insulated Maksutovs vs insulated SCTs. Is the extra thickness of the Maksutov meniscus a boon to those who would insulate?

 



#153 freestar8n

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Posted 03 December 2018 - 03:52 PM

Hmmmm…  That's a rather long, detailed way of saying that the answer to the question I asked you is 'NO'.  So, when you DO have the time, situation, $18, or other motivation to actually try it for yourself, let me know.  Until then, you're theorizing.

I have not tried insulation - but I have indeed posted images of the Airy pattern that compare measurement of a star in-situ and compare it with theory - and it is essentially perfect.  I have also posted numerous thermal studies and so forth.  I am way ahead of the insulation folks in terms of posting actual results to demonstrate that my approach works well.  And there is nothing contrarian about my approach - whereas insulation goes against over 100 years of progress in professional observatory design, where ventilation is critical.

 

That doesn't mean insulation is bad or doesn't work.  But I have never seen compelling results to show it is a good idea - and the various interpretations of why it should work are fairly simplified.  My main point is that the results are backed by anecdotal evidence and subjective assessments.  If you take such evidence as proof you will find that most everything works and is backed by someone who tried it and liked the results.

 

Frank



#154 freestar8n

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Posted 03 December 2018 - 04:00 PM

No-one has said that fans are bad. The question is whether they are need when insulating the tube. John Hayes is arguing that they are needed in his experience with large SCTs. There was also the issue raised about insulated Maksutovs vs insulated SCTs. Is the extra thickness of the Maksutov meniscus a boon to those who would insulate?

I haven't kept track of it carefully but I thought in this thread some people say to leave fans on and others say not.

 

The problem with visually assessing an improvement with fans on is that it induces high speed turbulence that may appear to the eye has having been eliminated - but only because it is so fast the eye can't track it.  But it could nonetheless provide a blur to the image.  It should show in a view of the Airy pattern, though, if there is any high speed turbulence remaining.

 

In my thermal study years ago that I have linked to multiple times, I was trying to improve my planetary imaging - so I installed powerful fans and left them running all night in a C11 - and I saw virtually no improvement - with them on or off.  There were no plumes evident or anything - but my planetary views retained atmospheric distortion that I concluded must be somewhere above the telescope.  That was a carbon fiber C11 - and all my efforts to improve things with ventilation led to a negative result - despite measurements that showed it was well equilibrated.

 

The claims about Maksutov meniscus have been made for years, but I don't know anything to back them.  And even a carbon fiber tube is controversial.  Some people say they are great for reduced expansion - and others say they are bad because they act as insulators (!).  This stuff just goes round and round.

 

Frank


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#155 Bean614

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Posted 03 December 2018 - 05:39 PM

Freestar 8N wrote:  "..the results are backed by anecdotal evidence and subjective assessments".

 

 

Really????  99.9999% of ALL Comments, Reviews, Comparisons, and Impressions found on the Cloudy Nights Forums ARE anecdotal!  Are we NOT to believe ANY of them????

 

And what of the many who just like to see their 'words' online, and start their posts with  "While I haven't used any of the scopes the Original Poster is inquiring about, I think that...….yada, yada, yada".  How fair is THAT to newbies looking for advice from ACTUAL users of the products in question?

 

Comments from Actual users, and their impressions and misgivings about a product are FAR more useful than remarks from those who have to tell their 'social media' friends every time they sneeze.  Yes, those who have used reflectix HAVE posted their comments, and a good many of these folks have been on CN for MANY years, are well respected, and do NOT just post things for the heck of it.  

 

Then there is 'yellobeard', who started this whole thing in these Fora and DID back up his theories with scientific evidence.  Just look it up.    For the rest of us, for whom Physics equations are a bit daunting, we're left to share our results, nothing more and nothing less.

 

It costs $18 Frank, give it a try.


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#156 Sarkikos

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Posted 03 December 2018 - 05:42 PM

I haven't kept track of it carefully but I thought in this thread some people say to leave fans on and others say not.

Yes, you are correct.  Some say that fans should still be run when the OTA is covered by Reflectix … allowing openings for the fans, of course.  Others say that fans should not be run when the OTA is covered by Reflectix.

 

Mike


Edited by Sarkikos, 03 December 2018 - 05:42 PM.


#157 Jaimo!

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Posted 03 December 2018 - 06:52 PM

Gentlemen, 

Before we get carried away and this thread gets locked, like other Reflectix threads in the past, let me remind you of the main principles of the TOS:

 

The Basics

The guiding principles of your participation on Cloudy Nights are:

Play Nice

Share

Be Polite

Be Honest

Be Tolerant

Respect other members

Respect and cooperate with the Administrators and Moderators who have unselfishly volunteered their time to keep this board a useful resource.

Cloudy Nights is a family friendly forum. We have members under the age of 14. We expect you to behave accordingly.

The Golden Rule applies here.

If you cannot abide by the above guidelines for civil conduct, then this is NOT the place for you.

 

 

Please resume the discussion, this is a very interesting thread for many people, but please abide by the above.

Jaimo!


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#158 Bean614

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Posted 03 December 2018 - 10:26 PM

Yes, you are correct.  Some say that fans should still be run when the OTA is covered by Reflectix … allowing openings for the fans, of course.  Others say that fans should not be run when the OTA is covered by Reflectix.

 

Mike

Mike, if the object of the Reflectix is to retain heat, and the object of fans is to cool, why on earth would one use both?  Is there a logical reason for it, beside the fact that 'some say' one should?



#159 jhayes_tucson

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Posted 03 December 2018 - 11:35 PM

Mike, if the object of the Reflectix is to retain heat, and the object of fans is to cool, why on earth would one use both?  Is there a logical reason for it, beside the fact that 'some say' one should?

Check out post #28 here:

https://www.cloudyni...dvantage/page-2

 

John


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#160 Ron359

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Posted 04 December 2018 - 02:22 AM

Mike, if the object of the Reflectix is to retain heat, and the object of fans is to cool, why on earth would one use both?  Is there a logical reason for it, beside the fact that 'some say' one should?

The logic is obvious if you don't make the wrong assumption that fans are for cooling.  

 

The furnace in your house puts out heat and in forced air systems, or with ceiling fans, their purpose is to redistribute the heat, not cool your house.  Insulation is used to retain that heat!  Have you ever observed over the roof of house on cold night?  Thermals or "bad seeing" is all you see despite the insulation which only slows the release of heat from your warm house if insulation wasn't there at all.  

 

The point of fans in a newtonian telescope is also to redistribute the heat away from the mirror surface by breaking up the thermal currents caused by radiative transfer from warm mirror surface to the air above it.  The same applies for SCTs or Maks.  The fans 'let the warmer air in the tube out faster than radiative cooling by itself does.  As I've pointed out in previous threads where here on CNs  this seems to be a terribly misunderstood science.  Thermodynamics tells you when you open a window on a cold day, you're NOT letting cold air in, you're letting the heat out.  Energy transfer is always from "warm" to ""cold"or higher energy to lower energy.  Dry ice (solid CO2) and liquid oxygen are made in series of steps that remove energy, (or heat),  even down to near zero where all atomic motion stops.  Simple enough in concept but takes a lot of hardware to accomplish,like that big pump and coils of 'coolant' in your freezer or refrigerator that 'suck' the heat out of inside the insulated box.  The radiative transfer of heat from inside the tube to outside air creates convection which is turbulent and results in 'bad tube seeing' just like it does in the atmosphere.  Thats why the best seeing is over warm oceans of stable non-turbulent air, equal pressure and laminar flow, or the vacuum of space.  The vacuum of space is where your reflectix would really work by reflecting the 'heat' of light away from the tube, as the layers of mylar do on spacecraft.   The fans in a tube more quickly push the bad warmer air out which is replaced with ambient air from outside unless you're tube is totally closed.  If the tube really were completely closed you'd start drawing a vacuum and the tube would collapse or deform from 14.7 psi atmospheric pressure just like the grade school science coke can experiment.  

 

 Fans may also make the air flow in the tube more laminar as it heads to the openings and is accelerated out but basically breakup pockets of warmer air until all is at ambient temp of the outside.  If that causes your corrector to dew up faster its simply cause you local weather condition has a dew point humidity near the ambient temp.  In all these endless threads of debate I've noticed that many of the evangelists of Reflectix mainly seem to only visually observe with smaller Mak or SCTs in the 6" or smaller range.  The apparent effects of fans and 'insulation' may be completely different to the visual observer who uses a small 6" or smaller scope from a C-14 with 4 meters of focal length because the volume of air inside the tube goes up by the cube of the volume inside.   Everything in a small 6" scope is going to retain far less heat and generate small thermals just by fact of far less mass alone that has to reach ambient temp.  

 

There are plenty of top rate planetary imagers in the planetary-solar imaging forum who go to great lengths, such as ice packs on their tubes, to get their SCT 11s and 14s to ambient temp. and never mention Reflectix.  But few in these threads seem to read or follow what the really good SCT planetary imagers do to get their sub arc second details on every night of good seeing. Instead, in thread after thread I only read anecdotal arguments against well known principles of physics with "common sense" and a cheap fix.  And I thought astronomy and those who practice it, was all about science.  Was I wrong.  


Edited by Ron359, 04 December 2018 - 02:24 AM.

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#161 luxo II

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Posted 04 December 2018 - 03:45 AM

Ron, only the insides of a scope only have to be at thermal equilibrium to stop tube currents.

 

One way to achieve that is obviously to cool it all down to the ambient exterior temp - and for those with open tubes (newtonians, classical cassegrains) this is the only solution.

 

But where the OTA is closed there is no need for the interior to be at the same temperature as the exterior ambient temp, and worse, by doing that, the creator will probably cool further and dew over. So people start adding heaters, which was a firm clue that cooling idea was not necessary in the first place.

 

As long as the gradient across the corrector to the exterior is only a few degrees this is sufficient to stop it dewing and not enough to cause thermals externally.


Edited by luxo II, 04 December 2018 - 03:47 AM.

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#162 Kokatha man

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Posted 04 December 2018 - 06:22 AM

I have not tried insulation - but I have indeed posted images of the Airy pattern that compare measurement of a star in-situ and compare it with theory - and it is essentially perfect.  I have also posted numerous thermal studies and so forth.  I am way ahead of the insulation folks in terms of posting actual results to demonstrate that my approach works well.  And there is nothing contrarian about my approach - whereas insulation goes against over 100 years of progress in professional observatory design, where ventilation is critical.

 

That doesn't mean insulation is bad or doesn't work.  But I have never seen compelling results to show it is a good idea - and the various interpretations of why it should work are fairly simplified.  My main point is that the results are backed by anecdotal evidence and subjective assessments.  If you take such evidence as proof you will find that most everything works and is backed by someone who tried it and liked the results.

 

Frank

 

 

There are plenty of top rate planetary imagers in the planetary-solar imaging forum who go to great lengths, such as ice packs on their tubes, to get their SCT 11s and 14s to ambient temp. and never mention Reflectix.  But few in these threads seem to read or follow what the really good SCT planetary imagers do to get their sub arc second details on every night of good seeing. Instead, in thread after thread I only read anecdotal arguments against well known principles of physics with "common sense" and a cheap fix.  And I thought astronomy and those who practice it, was all about science.  Was I wrong.  

Good grief - I was not going to make any comments in this thread lol.gif because as I see it there is a large dose of the proverbial "faith" in many things that AA'ers prescribe to...

 

I freely admit I'm driven by practicality...if it appears to work with sufficient testing it gets the nod...if not - the flick!

 

With my old C11 I did insulate the entire scope...& I thought the pattern-making of the rear-casting assembly & the creation of said insulative cap would've just about qualified me as a bespoke tailor! smile.gif

 

Not this seemingly marvellous Reflectix...but a double layer of 5mm thick natural cork, which I reckon provided pretty good insulative qualities...the ota was also sheathed in the same btw.

 

I collimate on a star before every imaging session & have what I consider to be a very intimate awareness of the collimating process in all sorts of seeing etc, but also the characteristics of many of the effects of various parameters on star pattern appearances...both in & out of focus. 

 

Our results are well-known in the Solar System Imaging forum & elsewhere...some of our achievments can be read in my signature & a visit to our website should attest to the quality & resolution we regularly achieve on just about every planetary target. (well, with the exception of Venus & Mercury!)

 

Insulation per se was of almost no benefit, whether that was imaging with whatever the scope's ambient temperature was within - or aiming for a specific value...we could switch on a rather sophisticated cooling system I derived myself using multiple TEC's with cowled heatsinks & fans blowing the heated air away from the scope's rear...internal fans also inside the rear casing  & under the primary mirror with brass cold-plates acting as additional internal cooling cowls...

 

I had multiple temperature sensors on the primary, at the front of the corrector, inside the ota & also at several positions outside the scope itself...I must have logged literally hundreds of hours with data, graphs & also diffraction ring patterns & of course the planetary results themselves.

 

Without making this post one of my (often longer usually!) tomes, we are now using a C14 - & "yes" I think I have refrained completely from posting in any of those threads that go along the lines of "are C14's any good" - "has anyone ever found a C14 that has decent optics" - "why are all SCT's ****" etc, etc... rofl2.gif

 

What do we believe..?!?

 

If an SCT (or in particular a C14) can have the mirror cooled to air/ambient temperature you will notice the most stable set of diffraction rings or Airy Disk that seeing allows...once collimated to the best degree possible the results will be outstanding. (the nexus between seeing, primary at ambient & collimation quality implicit here...)

 

We don't try to retard the primary's temperature changes once we achieve the desired value...at certain times of the year the temperature literally "drops like a stone" after Sunset & whilst I admit we don't get the freezing temperatures of many Northern folks, we have often had temperatures of -4°C to -6°C in Winter in the Oz bush, where the scope ends up covered in a sheath of ice.

 

We also regularly encounter a mirror at 40°C+ at this time of the year before Sunset & with our practice need to lower it to around 8°C or less in time for the start of an imaging session.

 

I'm not overly interested in whether deep space/the sky has a temperature of -60° or whatever - I want the primary mirror to be about 2°C below the predicted temperature for the start of the imaging session knowing that only in exceptional circumstances will the primary not track the ambient air temp over the course of 4 to 8 hours...

 

All corrector plate defogging is done with a hair-dryer...I've seen what dew heaters do at 10,000mm focal length/image scale & it's not too much better than what a quick blast of the hair-dryer does...except the effects of the hair dryer settle down with a minute or 2..!

 

A long dew shield when imaging Neptune or Uranus in conditions where fogging occurs fairly rapidly is very helpful for the 500" to 800" durations of these captures btw.

 

After all the experimenting & faffing around with all sorts of ideas & apparatus where our sole objective is to achieve the highest resolution on a regular basis, in whatever the seeing etc dishes up anytime over the last decade or so, we are convinced that our method of attaining ambience between primary & air clearly delivers the best outcomes.

 

I'll humbly suggest our results over that time make this point not only indisputable...but are clearly also a tangible/evidence-based proof of the "workability" of said...

 

Nowadays it is the simplest modus operandi for effecting this objective: a double-bagging (for leak safety) of 5Kgm of crushed ice mixed with 1Kgm of common table salt...placed on the rear-casing of the C14 with Moonlite focuser in situ & ice evenly arranged in the bag...pointing face down on the mount with a doona wrapped around ice bag & ota...

 

We wait till our gauges display the primary temperature is around -2°C below what is predicted for when we begin imaging, take off the ice & doona etc...point the scope straight up skyward with corrector uncovered & wait about 1/2 an hour or so for the primary to "relax" & its' temperature to become uniform throughout et -c then set the rest of the cables & gear etc up & start the collimation process...collimation is always done "in camera" at the sorts of f/l's I have mentioned.

 

I'll finish by saying that "YMMV" but we want what we get...& get what we want, weather permitting - everyone is free to believe what they like & think differently but for us the only thing that matters is "primary temperature = ambient air" for maximum outcomes that can be quantified! wink.gif


Edited by Kokatha man, 04 December 2018 - 06:25 AM.

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#163 Sarkikos

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Posted 04 December 2018 - 06:23 AM

Mike, if the object of the Reflectix is to retain heat, and the object of fans is to cool, why on earth would one use both?  Is there a logical reason for it, beside the fact that 'some say' one should?

I was only stating the fact that some say fans should be run when the OTA is covered by Reflectix, while others say they shouldn't.  This is true.  My answer was a direct response to freestar8n's question.  I have no settled opinions about it.  I just want to find out as much as I can about the general subject of this thread - Insulating SCT's with Reflectix - before experimenting with Reflectix myself.

 

Mike


Edited by Sarkikos, 04 December 2018 - 06:25 AM.

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#164 Jimmy462

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Posted 04 December 2018 - 08:25 AM

Good grief -

 

>snip<

 

I'm not overly interested in whether deep space/the sky has a temperature of -60° or whatever - I want the primary mirror to be about 2°C below the predicted temperature for the start of the imaging session knowing that only in exceptional circumstances will the primary not track the ambient air temp over the course of 4 to 8 hours...

 

All corrector plate defogging is done with a hair-dryer...I've seen what dew heaters do at 10,000mm focal length/image scale & it's not too much better than what a quick blast of the hair-dryer does...except the effects of the hair dryer settle down with a minute or 2..!

 

A long dew shield when imaging Neptune or Uranus in conditions where fogging occurs fairly rapidly is very helpful for the 500" to 800" durations of these captures btw.

 

After all the experimenting & faffing around with all sorts of ideas & apparatus where our sole objective is to achieve the highest resolution on a regular basis, in whatever the seeing etc dishes up anytime over the last decade or so, we are convinced that our method of attaining ambience between primary & air clearly delivers the best outcomes.

 

I'll humbly suggest our results over that time make this point not only indisputable...but are clearly also a tangible/evidence-based proof of the "workability" of said...

 

Nowadays it is the simplest modus operandi for effecting this objective: a double-bagging (for leak safety) of 5Kgm of crushed ice mixed with 1Kgm of common table salt...placed on the rear-casing of the C14 with Moonlite focuser in situ & ice evenly arranged in the bag...pointing face down on the mount with a doona wrapped around ice bag & ota...

 

We wait till our gauges display the primary temperature is around -2°C below what is predicted for when we begin imaging, take off the ice & doona etc...point the scope straight up skyward with corrector uncovered & wait about 1/2 an hour or so for the primary to "relax" & its' temperature to become uniform throughout et -c then set the rest of the cables & gear etc up & start the collimation process...collimation is always done "in camera" at the sorts of f/l's I have mentioned.

 

I'll finish by saying that "YMMV" but we want what we get...& get what we want, weather permitting - everyone is free to believe what they like & think differently but for us the only thing that matters is "primary temperature = ambient air" for maximum outcomes that can be quantified! wink.gif

Hi Kokatha man,

 

Just a quick question...are you encountering (and, therefore, having to contend with) any internal dewing problems when bringing the primary down to temperatures "around -2°C below what is predicted for when we begin imaging"? If so, I'd be curious to hear about your work-arounds and/or procedures. Thanks in advance.

 

:)

Jimmy G

 

P.S. I'm a big fan of your work and generosity over on the Solar System Imaging & Processing board!



#165 choward94002

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Posted 04 December 2018 - 11:52 AM

What do we believe..?!?

 

If an SCT (or in particular a C14) can have the mirror cooled to air/ambient temperature you will notice the most stable set of diffraction rings or Airy Disk that seeing allows...once collimated to the best degree possible the results will be outstanding. (the nexus between seeing, primary at ambient & collimation quality implicit here...)

 

...

 

I'm not overly interested in whether deep space/the sky has a temperature of -60° or whatever - I want the primary mirror to be about 2°C below the predicted temperature for the start of the imaging session knowing that only in exceptional circumstances will the primary not track the ambient air temp over the course of 4 to 8 hours...

 

...

 

A long dew shield when imaging Neptune or Uranus in conditions where fogging occurs fairly rapidly is very helpful for the 500" to 800" durations of these captures btw.

 

After all the experimenting & faffing around with all sorts of ideas & apparatus where our sole objective is to achieve the highest resolution on a regular basis, in whatever the seeing etc dishes up anytime over the last decade or so, we are convinced that our method of attaining ambience between primary & air clearly delivers the best outcomes.

 

...

 

We wait till our gauges display the primary temperature is around -2°C below what is predicted for when we begin imaging, take off the ice & doona etc...point the scope straight up skyward with corrector uncovered & wait about 1/2 an hour or so for the primary to "relax" & its' temperature to become uniform throughout et -c then set the rest of the cables & gear etc up & start the collimation process...collimation is always done "in camera" at the sorts of f/l's I have mentioned.

 

Very interesting!  OK, to summarize from the wall of text ... (and good work solar imaging too!)

 

- The primary mirror needs to be the same temperature as the ambient air temperature/ desired imaging temperature ... so, if it's expected to get down to 10C at the time you're going to be using the OTA then you'll cool the mirror down to that point less 2C  and then it's time to take off the ice bag  ... so at this point you've got a thermal sink/ mirror cell at 2C below (or more) of the outside air temperature.  As the thermal sink/ mirror has been cooling the interior air of the OTA has also been cooling and doing the convection thing, with cooler air from the thermal sink/ mirror at the top of the airmass column dropping down to the bottom of the airmass column inside the OTA.  You're basically not only cooling the thermal sink/ mirror to the target temperature but also getting the airmass internal to the OTA cooled to that temperature as well over time ...

 

- The OTA is then rotated to have the corrector plate skyward (which will keep the colder OTA air at the bottom where the cell is, warmer air will rise to the corrector plate and stay there, as you don't mention anything about using fans) and then you mention waiting for it to "relax" and the temperature to become uniform, I'm assuming that you're talking about the internal air temperature to be uniform.  This implies that you're letting the corrector plate (which is cooling down due to conduction with the outside air as well as from radiative cooling from the OTA) cool any warmer air in the OTA, which would no longer be in contact with the (cooler) thermal sink/ mirror now at the bottom of the OTA ...

 

- You mention that you use a longer dew shield, I assume that's to prevent radiative cooling through the axis of the OTA cooling the airmass at the top of the OTA through the corrector and stating convection currents inside the tube ...

 

So, it appears that when you start your imaging session then ...

 

- The thermal sink/ mirror is at or slightly below the ambient air temperature at the time you're doing your imaging (so if the weather report said it would be 15C then you wait until it's 15C before you start doing your stuff, having already cooled the thermal sink/ mirror to 13C or so

- The interior OTA airmass has already been cooled to 13C or so due to the cooling of the thermal sink/ mirror, and the corrector plate and any residual heat in the OTA interior have been cooled due to exposure to sky as well as convection during the thermal sink/ OTA cooling

 

So, you're basically at thermal equalibrium with the outside air AT THAT TIME; the thermal mass/ mirror has been cooled to the outside air temp, the OTA airmass has been cooled to that point, the corrector plate has been cooled to that point, there aren't any thermals to see because there are aren't any thermal differentials to generate them ... nice!!!  You didn't mention what happens over time as the ambient air gets colder and the OTA starts to radiatively cool, however (since nights rarely stop at a setpoint and radiative cooling always wants to do that) ... once that ambient temperature drops much below the temperature target or the OTA tube/ corrector plate drops below that value then your thermal mass/ mirror and interior airmass will no longer be in equilibrium with the outside air temperatures and we will start to develop tube currents ... and we're back to the endless thread(s) about dealing with that ...

 

In the northern land of dew and ice and rapid cooling/ fog banks, however, this will inevitably lead to the OTA being just as covered in dew as the patio furniture you left out for the evening ... and you mentioned that in your post, how your OTA's will become "sheathed in ice" as soon as it drops below the dewpoint due to radiative cooling and then the condensation games begin ... OTA's left out for the evening can't really do that, so we're back to the endless thread(s) about cooling and such ...

 

The implication from your post is that you arrive at a site/ open the observatory doors, start cooling, do your stuff for a comparatively short period of time (since the target temperature window you've been cooling for is relatively short) then either break everything down and go home for the evening or if you're in an observatory close the dome doors, both of which which lets you avoid both the patio furniture phase and the inevitable tube currents ...

 

So, my takeaway from this so far (other than needing a more elegant way to cool down the thermal mass/ mirror, I would prefer to use peltier coolers than ice bags because I'm doing this at a remote site and flipping a switch to cool down the thermal mass/ mirror is a lot easier than getting a plane ticket every night) is to try to get not just the OTA airmass to a set value but also to match the thermal sink/ mirror to that value, which I hadn't considered before.  Peltier coolers are pretty cheap on Amazon and can be pretty easily integrated into my RaspPi heater controller, I can add another temperature sensor to the mirror cell to see how that's doing (you stated you cool down the mirror cell and that also cools the primary mirror, I'm not sure how that works since there isn't a solid conductive path from mirror cell to the primary mirror other than the baffle tube and some grease but that's likely due to the convective cooling from the interior airmass off of the mirror cell interior) ...

 

Once I get stuff wired up I'll try to keep all three components (OTA airmass, corrector plate and thermal sink/ mirror) to the same "hot tube" temperature and see if that diminishes my interior tube currents, now that I know what to look for imaging the BeeHive should detect those in short order and I can compare before/ after results with that ... nice!

 

Really good post, well worth the time spent digging through the walls of text to get that nugget of insight about the thermal sink/ mirror!  Thanks again for your contribution!


Edited by choward94002, 04 December 2018 - 11:55 AM.

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#166 Sarkikos

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Posted 04 December 2018 - 01:14 PM

I don't image.  Strictly visual here.  I don't intend to save anything for posterity.  Posterity can come to the dark site and look through my telescope if they want.  lol.gif

 

How do the requirements for decent visual compare to imaging? 

 

Having to refocus every few minutes for planets was never a big deal for me, as long as I have sharp and steady images between the refocusing.  Same for double stars.  I never felt the need for electronic focusing either.  I have a steady hand.

 

Requirements for deep sky visual should be less strict than planetary, since we are observing faint fuzzies with dark adapted eyes.  Dark adapted eyes lose much of their ability to discern detail, color and contrast.

 

So for me, some setup that is better but not necessarily perfect would be in order.

 

Also, I don't like the idea of placing bags of ice around my telescope.  That would be more hassle than I want to deal with.  Reflectix is looking better and better.

 

Mike


Edited by Sarkikos, 04 December 2018 - 01:24 PM.


#167 Jeff B

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Posted 04 December 2018 - 01:19 PM

All I know is what I've seen, but I can speculate with the best.

 

What I've experienced with Reflectix is a considerable improvement of visual usability when taken from my warm basement to the cold outdoors for both my TEC 7 mak and C11 when using the stuff.  Temperature deltas can easily be over 40 degrees F.  Both scopes are completely double wrapped, including dew shields and rear cells.   

 

The differences in visual performance has bee striking.  With the C11, the scope is usable immediately and continually where as sans wrap, it would be complete mush for a good two hours.  With the wrap, the high power out of focus diffraction rings close to focus are sharp and stable with one artifact.  At focus, I can actually see the airy disk plainly, again with one artifact, external seeing permitting.  Solar system images are fairly sharp, "stable" with focus easy to achieve and keep.  I've experienced no dewing off the corrector until  ~4 hours later and then a little hair dryer heat is all that's needed to clear it for a good half hour.  Without the wrap the images are fuzz balls in or out of focus.  The internal thermal machinations and instabilities are easy to see in the out of focus images.  It takes a good hour and a half to really settle out to the point where the high power out of focus rings close to focus sharpen up and become stable and the in focus airy disk discernible with good solar system object sharpness.  But then, I'm typically chasing off dew every 10 minutes or so.  And I still get my "artifact". 

 

Now the C11 does have a powerful forced air cooling system consisting of three filtered fans impinging on the primary mounted on the back casting.    Sans wrap, this does "speed things up" for the C11 considerably and it comes on line much quicker, in about half the time.  But I still fight dew, but the artifact is typically gone.  I've not tried the fans with the wrap.  

 

The artifact that I mentioned earlier is the same for both scopes with and without the wrap, and appears as a narrow, defined, piece of pie shaped notch or wedge in the out of focus diffraction pattern and a small divot or bite out of the airy disk at focus (assuming the seeing actually lets me see it).   I'm convinced this is a plume of some sort from the main baffle, and/or the secondary baffle.  It goes away or is greatly reduced with time with both wrapped scopes, in about an hour.  The fact that its effect is greatly reduced with the C11's ventilation system leads me to speculate it is indeed a plume.  

 

So, for me and my typical set up and visual observing scenarios, the wrap is a definite big step up over a stock, non-ventilated scope in useful observing time for both scopes and the difference is dramatic...visually.  Is this anecdotal evidence?  Yes.  But it's still evidence.  Not as precise as recorded videos or images as it is related rather than directly recorded, but it's still evidence.   

 

I agree with John that this is a good start, or half solution.  But I'll take it!  And now I want to address the plume.  

 

I can engage in speculation and arm chair theory with the best.  But why bother when my C11 does have those cooling fans.  So I'm just going to try them out.  Their inlets are sealed off right now but it would be easy enough to just turn them on and see what happens.  Ditto with the boundary layer fan in the TEC 7.  Who knows what may happen?  Maybe nothing.  Maybe something.  Maybe something like busting up that plume.  But it's easy enough to try out.  With the C11, I can modulate the incoming air simply by placing smaller apertures over the fan inlets, or, more preferably, doubling up on the filter media....or both.  Then seeing what happens.  If nothing else happens, I'll take my half solution.  If I get another quarter, I'll take that too.  I will say, I'm loath to introduce outside air into the C11 and will avoid it if I can.  I get tired of having to take the thing apart for cleaning and yes, even with good filters, stuff still does get inside.  So If I can make it work somehow while keeping the tube sealed, I much prefer doing that.

 

Jeff


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#168 Sarkikos

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Posted 04 December 2018 - 01:29 PM

The artifact that I mentioned earlier is the same for both scopes with and without the wrap, and appears as a narrow, defined, piece of pie shaped notch or wedge in the out of focus diffraction pattern and a small divot or bite out of the airy disk at focus (assuming the seeing actually lets me see it).   I'm convinced this is a plume of some sort from the main baffle, and/or the secondary baffle.  It goes away or is greatly reduced with time with both wrapped scopes, in about an hour.  The fact that its effect is greatly reduced with the C11's ventilation system leads me to speculate it is indeed a plume.  

Sounds like tube currents.  I see the same thing in my Cats before they acclimate, even in my C90.  Typically the artifact is at the top of the diffraction pattern, indicating rising thermals.  It look like a little trailing piece added or a little piece cut out.  In focus, it can make many stars like as if they were tight doubles.  Confusing if you are looking for double stars.

 

http://aberrator.ast...be_current.html

 

Mike


Edited by Sarkikos, 04 December 2018 - 01:33 PM.


#169 WadeH237

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Posted 04 December 2018 - 01:51 PM

How do the requirements for decent visual compare to imaging?

I have many scopes and I do both visual and imaging.

 

The only scope where I've experimented with Reflectix is on my C14, and only for visual use.  I would further qualify it and say that the biggest difference I've seen is with planets, close double stars, and things like going after the central star in M57.

 

I simply haven't needed it for my smaller scopes and for my 14" dob.  The C14 is the only one where there is a significant performance difference early in the evening vs. late at night - at least in my use case where the scope is outside under cover all day.

 

After reading the whole thread, I can easily believe that it's not a complete solution and that the best answer is to get all parts of the scope acclimated.  I suspect that the issue with the C14 in the early evening is a heat plume coming out of the baffle tube - and the Reflectix seems to kill it.  And that's good enough for me right now.

 

I am considering everything that I read here for consideration of changes to my imaging setup when I use the C14, but I image through it rarely enough (and don't do planetary imaging), that this would be a future project.


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#170 Ron359

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Posted 04 December 2018 - 02:26 PM

All I know is what I've seen

 

Now the C11 does have a powerful forced air cooling system consisting of three filtered fans impinging on the primary mounted on the back casting.    Sans wrap, this does "speed things up" for the C11 considerably and it comes on line much quicker, in about half the time.  But I still fight dew, but the artifact is typically gone.  I've not tried the fans with the wrap.  

 

The artifact that I mentioned earlier is the same for both scopes with and without the wrap, and appears as a narrow, defined, piece of pie shaped notch or wedge in the out of focus diffraction pattern and a small divot or bite out of the airy disk at focus (assuming the seeing actually lets me see it).   I'm convinced this is a plume of some sort from the main baffle, and/or the secondary baffle. 

Jeff,   I suggest you could think about the mystery 'artifact' not as a plume of heat coming off the baffle but perhaps its outside ambient air entering the tube and getting into the light path from outside.  There has to be openings that lets outside air gets into the tube so you may be seeing this entry from the focuser hole up the baffle or fan holes or some other opening in the tube.  Celestron or makers may have 'built in' pressure equalization holes so that their telescopes don't 'explode' when 'sealed' with an end cap and shipped by air cargo or go in trucks to high altitude locations like mountains.  Where I live at over 7000 ft. alt., we see ballooned potato chip bags, exploding pop cans and all sort of sealed containers shipped from sea level expand and/or blow their contents all the time when opened quickly or dropped.   

 

Of course the 'artifact' disappears when all the inside air is at ambient with the outside and all differential thermals or flow from outside stop.  I would ignore the formation of dew in the process unless you're going to record the dew point temp and humidity every time you 'do a test'.  Stick to the visual or video observation of the in and out of focus image.   Dew point is an external variable of the climate outside your telescope you can't control, not an effect from the thermal mass of mirror etc., fans or internal air of the telescope.  After you figure out if insulation and fans together work faster than no insulation and fans or vice versa, then figure out how best to deal with dew that forms no matter on telescopes or lawn furniture.   



#171 Sarkikos

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Posted 04 December 2018 - 02:36 PM

As long as the optics are kept above the dew point, they will not experience dew.  I've had good success keeping the optics clear with dew shields and warming strips, even at very dew dark sites.  I take along a portable dryer just in case.  But if I have to use it to clear the optics, I see that as a fail.  

 

If the outside surface of the OTA tube gets dewy, it's messy, but it really doesn't matter.

 

Mike


Edited by Sarkikos, 04 December 2018 - 02:37 PM.


#172 Jeff B

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Posted 04 December 2018 - 04:14 PM

Actually, image "c" from post 21 above, looks rather like what I get.  However, what I actually see, is much less pronounced and more of a narrow "V" shape than what's depicted in the figure.  

 

I assume that's suppose to depict some sort of plume from either the primary and/or secondary baffle.

 

Jeff



#173 Kokatha man

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Posted 04 December 2018 - 08:21 PM

Ok - I'll try & address a few things from posts directed at my comments...but as we'd say down here Howard (?) - "fair suck of the sauce bottle" when you comment about my "wall of text" - your post was pretty long also..! smile.gif

 

Jimmy G: - occasionally we experience internal dewing issues...this year we've probably gone out well over a hundred nights & perhaps encountered said on 2 or 3 of those nights...

 

The worst (very occasionally) is fogging of the secondary because it can be difficult to detect at times & even harder to actually see by looking into the scope's front - if this occurs I pull out the imaging train, stick the neck of the hair dryer (a nice fit) into the 2" focuser opening & give it a 10 second blast or so, turn the scope facing directly up & the warmed air rises & defogs the secondary...same with fogging on the inside of the corrector plate but longer hair-drying from the front surface will also remove this underside fogging...it takes longer & then longer still to cool down naturally.

 

Remember that dew/fog usually occurs on the coldest surfaces: should the primary itself actually fog up (rarer but not unknown) you shove the hair dryer up the focuser spout wink.gif but with the scope pointing downwards such that the introduced warm air rises up to the primary & very quickly dissipates the fogging.

 

I have never had to do any of the internal defoggings more than once in a night btw - once internal fog (wherever that might be) is dissipated it does not come back again that session...

 

Howard: please excuse the haphazard way  I approach some of your questions - we are heading out later today & whilst I'm normally happy to rabbit on & on ad infinitum I am in a bit of a rush atm & find it easy to pick out some comments you made in this way. (happy to answer/elaborate further tomorrow our time)

 

No observatory - we image outdoors, quite often travelling 200 to 600km to one of our main preferred imaging sites. We usually image for 4 or 5 hours at a time & often much longer in a single night.

 

That "predicted" temperature (about -2°C of that) is usually the prediction a half hour or so into our session on the actual planet itself

 

This difference is usually consumed by then although perhaps the primary's temp might still be 0.5° below ambient air temp...temperatures within +/- 1°C are usually fine & lower primary temps seem to affect the star patterns less than plus amounts...

 

The "relaxing" takes into account how the primary cools (an insulator as far a mediums go) - it is clear from the star patterns that there are major distortions in the figure & my presumption is that it takes a bit of time for the primary to achieve some sort of temp uniformity across & within its mass - you can see this happening slowly before your eyes on the screen in a star's diffraction rings if you plug in soon after taking the ice bag off - usually a 40 minute wait suffices.

 

<"You didn't mention what happens over time as the ambient air gets colder and the OTA starts to radiatively cool">

 

Actually I did in para. 4 under "what do we believe."

 

Slightly dependant upon any planet's elevation (& we have been blessed with decent elevations for some of them lately!) we rarely find that primary & ambient don't track each other very accurately: my take is that the dropping air temp (black ally tube ota btw) & (likely) radiative cooling effect via the corrector allows this to happen: it is the "kick-start" removing of the temperature lag via the ice that allows this temperature tracking to occur imo where there is rarely much difference over very long sessions...

 

 We might image right through the night in the Winter-time & it is often the case where both primary & ambient gauges are displaying -2°C to -4°C at the finish then...either precisely the same or within 0.1°C or 0.2°C at most difference.

 

I dispensed with the other gauges I formally had on different sections of the scope & now only utilise the one hanging off the rear that gives me the air/ambient & the one affixed to the rear of the primary itself about 2/3 the way out from the centre...it is of course insulated from reading anything but the glass directly underneath it - when I bought my "Non-Edge" C14 one of the things Celestron did was to use the same rear casting for both models, so getting to the back of the primary was simple via one vent: the C11 had to be completely disassembled btw...& I have both vents now taped shut.

 

At one stage (I did say I've tried a lot of ploys! wink.gif ) I had my own fans operating via the rear vents & also made a "super blower" from a 12 volt auto aircon turbine fan pumping air via flexible tubing to manifolds I fabricated over both rear vents - these both worked quite well - but they caused a lot of problems dragging moisture into the scope such that fogging occurred much more often, & heavily... frown.gif

 

As said "ice 'er is nicer...& much quicker!" lol.gif

 

Down here we either get the ice sheathing, or so much water is dripping off the ota it is almost unbelievable...the only "danger-spot" is the front of the corrector plate usually - a very thin plastic transparent sheet placed over the corrector & held with elastic does stop very heavy condensation on it when we stop imaging one planet & have to wait several hours for the next one to rise sufficiently - if we use a thin opaque "cap" instead it retards the ability of the scope's primary to track the falling air temp in the interim, the plastic is much better & once you remove it the (presumed) radiative cooling via the corrector quick redresses any temperature imbalance.

 

Some dew still forms on the corrector in these circumstances but nowhere near the amount from a completely uncovered corrector that quickly turns into large amounts of water with hair dryer application - it may take 10 minutes or more to dry it then & leaves lots of ugly watermarks on the c/p. frown.gif Hence the thin sheet when leaving it outside pointing upwards for a few hours in the middle of the night when we have to...

 

The gauges themselves may have errors but what they don't have are any differences, which is the important thing...sit the scope in a temp-controlled aircon'd room...& over the course of a day or night both gauges end up reading identically...

 

<"I'm not sure how that works since there isn't a solid conductive path from mirror cell to the primary mirror other than the baffle tube and some grease but that's likely due to the convective cooling from the interior airmass off of the mirror cell interior)">

 

My rationale also...semi-enveloping the primary in a frigid casing is quick...salting the ice makes it much quicker still - TECs work but nowhere near the rate of the ice but I understand your situation. wink.gif

 

More than enough from this old fella for now, but as said I'm happy to elaborate further if asked. :)


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#174 speedster

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Posted 05 December 2018 - 12:24 AM

Kokatha man, great info!  Particularly when empirical data is scarce.  Do you happen to remember about what dry bulb and humidity is when you get internal condensation?

 

Time for some numbers.  This may give everyone a better feel for this.  First, this thread started off about dew and became thermals.  There will always be a thermal if any of the interior surfaces of the OTA are at a different temp.  Fortunately, these get imperceptible as everything comes near to equilibrium.  Fans cause faster equilibrium, insulation causes slower equilibrium.  The fan issue has too many variables but the insulation is easy if we make some assumptions:

 

8" SCT

72 degree indoor temp going out into 35 degree ambient.

1/4" corrector plate

1/16" aluminum tube

3/16" aluminum back

 

No point arguing about these assumptions as you will see that the end result is not terribly sensitive to them.

 

Heat loss rates:

 

Stock SCT = 258 Btuh

With Reflectix covering all the tube = 227 Btuh

1" polyurethane foam covering all the tube = 194 Btuh

 

To see how many Btu's we have to lose to get to equilibrium, let's just consider the OTA to be 15 pounds of aluminum.  Yes, much of it is glass but glass is within about 5% of the same specific heat.

 

So, we need to lose 112 Btu to get to ambient.

 

The rate of heat loss is not static and approaches zero as we get near equilibrium but we ignore that too.  It's not a linear function and, to me, not worth figuring for this illustration.

 

Then,

Stock SCT takes 26 minutes to reach equilibrium.

With Reflectix takes 30 minutes.

1" PU foam takes 34 minutes.

 

Remember, we are not in steady state conditions and actual times will be significantly longer but still reasonably proportional. 

 

Insulation really is a moot point.  You WILL reach equilibrium.  Insulated, it will take you a few minutes longer which is a few more minutes of tube currents and a few more minutes until you get dew (if you are going to hit the dew point) and a few more minutes before you can observe without the effect of air currents.

 

If you want to delay equalization a bit, then insulate.  If you want to speed up equalization a bit, use a fan.  Fans circulating ambient air can shave a good deal of time off this.  Fans circulating interior air of a closed OTA might shave a few seconds off, if that much.  I suspect, if you look at how fast you can transfer heat to the circulating air, you'll find it doesn't take many CFM before you hit very diminished returns.

 

This stuff is not always intuitive and, in terms of radiation wavelength, we are dealing with near IR instead of the visual spectrum.  That shiny white tube is 90% reflective in the visual spectrum but only 10% in IR.  Glass is nearly transparent in the visual but nearly opaque in the IR.  Putting on a blanket makes perfect sense because we do it every night and it works but, we are trying to AVOID equilibrium whereas, with our scopes, we are trying to REACH equilibrium.


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#175 Sarkikos

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Posted 05 December 2018 - 07:03 AM

Actually, image "c" from post 21 above, looks rather like what I get.  However, what I actually see, is much less pronounced and more of a narrow "V" shape than what's depicted in the figure.  

 

I assume that's suppose to depict some sort of plume from either the primary and/or secondary baffle.

 

Jeff

Looks like a diffraction pattern caused by rising thermals inside the OTA:  tube currents.  I've seen the same pattern in my C90 many times.  It's difficult to pinpoint the exact source.  If someone can differentiate the exact source by looking at the diffraction pattern, let me know.

 

This isn't ambient air entering the OTA.  The C90 is a closed tube.  It is tube currents.  Warm air rising in the tube.  

 

Mike


Edited by Sarkikos, 05 December 2018 - 07:13 AM.



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