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Insulation (airspace) layer for under Reflectix

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#101 choward94002

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Posted 18 December 2018 - 10:43 AM

First up, I think you should read my Post #96 reply to your Post #95 for some clarification of my earlier descriptions etcs where I think you have misinterpreted some aspects. smile.gif

 

Done, thx!  There's an adage that when you're passing along info to someone else only 50% will "stick", 25% will get misinterpreted and the other 25% will just get missed ... so if you really want to someone to understand something you repeat it a few times to increase the 50% that "sticks" ... your post #96 a case in point ... smile.gif

 

<"I also wanted to verify the results that Kokatha had gotten a few years back when he first described his cooling process and indeed based my peltier coolers on his peltiers that he had put onto his C14 (although I use considerable larger heat sinks).  Those experiments were successful at verifying experimentally what I had conjectured and what you've stated a number of times when this comes up, "shiny is better" ... they also established that Kokatha's peltiers really could cool a cell the size of a C14 in a reasonably rapid period of time...">

 

<"Kokatha's work a few years ago had always intrigued me as it doesn't rely on active air circulation but rather passive thermal changes...">

 

Just a couple of clarifications here also - my peltiers were on the C11 & this scope actually had "active air circulation" to copy the terminology...I actually had brass (for good heat conductivity/transfer) cowlings on the underside of where the peltiers were positioned on the outside of the rear-casing: my "active" cooling relied upon the peltiers actually cooling the aluminium casing specifically at those positions & the adjacent brass cowlings a logical place to position said cowlings...

 

But on each brass cowling was a central 12volt fan - the cowlings were spaced an extremely minimal distance from the interior of the rear-casing (only a couple of mm) to force the air that was "dragged" in under them to be cooled most effectively & then expelled towards the primary (the fans "sucking" so to speak) & circulating cold air onto the back of the primary.

 

A couple of piks speaks a thousand words: here's one brass internal cowling as well as one external heatsink on the "hot" side of the peltier - these also possessed cowlings to maximise airflow & heat expulsion via each fan...& to blow the hot air out & away from the scope: one very old graph from the C11 also btw, in those days I utilised digital & bulb as well as iR thermometers to verify the accuracy of any/all readings & it tells me it was created in December 2009 so that explains the pretty high starting temperatures: Saturn reached opposition in March 2010 with Jove in September - the switch to ice etc after we purchased the C14 was most likely to do with Jupiter reaching opposition in the Summer period from 2012 onwards...some of this is becoming a bit shaky in my memory..! lol.gif

 

But it does indicate it took about 3 hours in that test to bring the primary down to ambient air temperature, starting at a pretty high 42°C which I imagine was one of the more severe tests at that time - salted ice can handle those starting temperatures much more easily & in less time now, but obviously one tries to keep the scope & its' primary nowhere near this hot for quickest cooling - unless it is one of those stinking hot nights in Summer! lol.gif.

 

This will be really helpful when I do my glass block thermal test (which I have no problem drilling holes for internal temperature probes in!) ... if I get a similar kind of curve I know that I'm in the ball park!

 

Please appreciate that this graph from the C11 with "active" peltier (TEC) cooling bears very little resemblance to those with the salted ice bags on the C14 nowadays - I post it out of interest but am still hopeful we can find something from recent times with the salted ice & C14. fingerscrossed.gif

 

Totally understandable, your salted ice mix has a TON of thermal potential you can dump into the mirror cell, much much more than I can reasonably generate with my peltiers ... so that's the right answer!  You're also using fans on the heatsinks for the hot side of the peltiers, where i don't want anything to vibrate when the peltiers come on in the middle of a shoot so I'm using passive ... your graph will be most helpful when I'm looking at my data, mainly at the point where the mirror overcomes the thermal inertia at about 1:45 into the experiment for you ...

 

What is an interesting wrinkle is your use back then of an active fan on the cold side of the peltier attachment underneath the primary, speeding up the convective heat transfer from "hot" mirror to "cooled" air while now (as I understand it, correct me if I've missed it, 50% rule) you don't use that in your salted ice mixture, you're relying on thermodynamics to get the convection currents going.  That would make sense; the temperature differential between a "hot" mirror and a "cold" peltier side would only be about 5C or so at any one time which would take awhile to get something going, but the salted ice differential would be huge, easily 40-50C so it would be hurricane time for the airmass between the two ...

 

Again, really good stuff there!  You're saving me a ton of time and really letting me focus on some specific areas of research here!


Edited by choward94002, 18 December 2018 - 10:50 AM.


#102 jhayes_tucson

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Posted 18 December 2018 - 04:27 PM

No worries, editorialize away! smile.gif

 

You're quite correct that I'm heading into the weeds here and not so much as making experiments than observing effects ... although to be fair, I'm building upon experiments done about three years ago here [https://www.cloudyni...pe-temperature/] as well as some other experiments with the effect of dew shields and work done here [https://www.cloudynights.com/topic/296302-temperature-focus-shift/] as well as all of the discussion on focus and temperature between you and freestar8n over the last few years as well as the work earlier this month by corsica and dickerson [https://www.cloudyni...-3#entry8985035] so I'm not completely ignorant of the prior art (after all, the starting point for any experimenter is the local library!) ...

 

My first experiment was simply "what is the best material for reducing radiative cooling from an OTA tube" and my premise was to follow the emissivity; black substance, white substance, Reflectix, ReynoldsWrap, Mylar (no budget for gold foil).  I was expecting to be able to see some skin temperature changes (that's why I chose to use metal trash cans rather than sonotubes) and expected to see some thermal air currents along the way.  I also wanted to verify the results that Kokatha had gotten a few years back when he first described his cooling process and indeed based my peltier coolers on his peltiers that he had put onto his C14 (although I use considerable larger heat sinks).  Those experiments were successful at verifying experimentally what I had conjectured and what you've stated a number of times when this comes up, "shiny is better" ... they also established that Kokatha's peltiers really could cool a cell the size of a C14 in a reasonably rapid period of time and gave me some actual numbers for how long that took and by how much as well as some insights into how the airmass inside the tube was actually behaving under the cooling of the thermal mirror mass and how the skin affected the airmass when it was being radiatively cooled ...

 

My next experiment is simply going to verify that a dew shield of the size calculated by some prior work here [https://www.cloudyni...691-dew-shield/] is valid (scaled to the size of a C14) as well as come up with some actual measurements of the effect of the dew shield rather than the observed "makes dew happen less often" ... I am expecting to see that the skin temperature of the corrector plate under the influence of a .58 factor dewshield, a 1.41 factor and a 1.81 factor dew shield will follow the same kind of linear relationship that the equations here [https://www.cloudynights.com/topic/437691-dew-shield/#entry5653518] posit ... if they do then I can verify that relationship and use a 1.81 factor dew shield in my further experiments (larger than that becomes too much of a sail, even behind my windscreen ... I'd like to go even shorter in fact, and hopefully my numbers will give me a curve I can use) ...

 

Once I have that information the next experiment will be to verify the heat transfer from cooled mirror cell to primary mirror that Kokatha alluded to by needing to "rest" the mirror; I think that's due to the mirror reaching thermal equilibrium after being cooled by the air that is being cooled by the mirror cell, but I want to make sure experimentally.  In his post a few years ago Kokatha mentioned wanting to place a therocouple inside of the primary mirror assembly but being too afraid of doing that (even though he mentioned he was a jeweler) ... using my glass block as an analogue I have no similar compunctions about drilling the stuffing out of the block and putting thermocouples everywhere!  I am expecting to see that the glass block reaches thermal equilibrium with the outside air relatively quickly but more importantly establish what the hysteresis is between a change in the skin temperature where the glass is getting cooled by the air surrounding it and when the glass block reaches equilibrium with that change ... seconds/ minutes/ hours/ days?  There are thermal transfer equations that have given me a rough idea of when that is, Kokatha's observation of needing to let the mirror "relax" for an hour before use agrees with those numbers, I want to experimentally get some better numbers ... 

 

And you're correct, testing on a garbage can is nice but I don't image using a 32gal WasteKing ... I need to use a real C14 to do that.  Luckily (???) I have a C14 I've been working on for a few weeks that got molded to death by it's prior owner who didn't know that storing a scope in an unheated storage shed over a few seasons of winter/ summer is a "bad idea" [re: my earlier soapbox mode rant, there's nothing sadder than seeing a C14 ravaged by mold, maybe except one that was dropped on the ground like a sack of potatoes] so that scope and it's contents are essentially scrap.  I am planning on recoating the primary and secondary and turning it into a really powerful terrestrial observation scope, but prior to that I can do pretty much whatever I want to it so using it as a test scope once I've got some numbers from my can experiments is a logical next step ...

 

Finally I'll get around to the main purpose for all of this stuff ... finding a way to achieve and maintain a balance between the desire to have a thermally stable OTA (no thermals, no tube expansion/ contraction) and my no-dew/ no-frost "hot tube" design without a) needing to run my heating pads quite so hot and inducing the thermals that you showed me how to detect in images (and that I'm now finding in about every light I take) AND b) keeping the tube expansion as low as possible based on your work in May of this year regarding tube expansion and focus shifts (since I now take 20+ minute L channel lights and don't have the ability to do the realtime focus adjustment that you can) by keeping the tube skin temperature as constant as possible and the contraction/ expansion minimized for as long as possible.  Now that I'm [trying!] to get L channel data at .14"/pixel with my ASI 183 every little thermal bubble and tube expansion hurts and I really need to figure out how to cut those down without sacrificing the "hot tube" functionality.  Kokatha's work a few years ago had always intrigued me as it doesn't rely on active air circulation but rather passive thermal changes, but now that I've got a real need to get thermals and focus shifts due to tube expansion/ contraction under control I don't have any excuse but to get out and do the experiments ...

 

So, there *is* a method to my madness, I'm not just flailing about wildly seeing what falls out when I whack the Pinata ... and in the spirit of Kokatha's discussing his work on cooling and your discussions on focal zone changes from mirror and tube expansion/ contraction and other studies I've read about on CN affecting the focal zone and baffle tube thermals I figured I'd share what I'm learning in my own experiments ...

 

Ok...good enough.  I looked through the threads that you referenced and I don't see much in the way of theory but I do see some incorrect stuff.  For example, Glen's post about the effect of a dew shield gets the concept right but the numbers wrong.  It turns out that the effect of a dew shield on heat exchange with the sky varies as the square of the sine of the angle of the opening as viewed from the front surface.  However, to determine the rate of heat exchange you have to add the net contributions from the sky, the dew shield itself, and from convective exchange with the surrounding air so this is a very non-linear effect.  The article that I'm working on will go through all this stuff to show where it comes from and how it works.  I normally don't like to release results before I'm finished but I've done enough to verify these numbers and I think that they are good, so I'll share them with you.  You'll have to wait for the derivation but as you can see, none of this stuff is linear.

 

1)  The first plot shows the expected temperature drop of a glass surface under a full (hemispherical) sky as a function of the ambient air temperature and the relative humidity.  The maximum drop at near zero relative humidity is around 7C, but that's not very relevant with respect to dew formation because you'll never get dew at near-zero RH.

 

2)  The second plot shows the temperature drop of a glass surface under a full (hemispherical) sky with respect to the dew point.  Anything below zero is where dew will form.  As you can see, the risk of dew at any temperature doesn't start until the RH is around 60%.  In my article I'll also address how this relates to the frost point but that's for later.

 

3)  Third plot shows the effect of dew shield length on the temperature drop on the front surface as a function of the length of the dew shield for a C14 in conditions with RH = 80% and the dew shield at the air temperature.  As you can see, making the shield longer decreases the temperature drop.  You can also see that you reach a point of diminishing returns at roughly 1.5x the diameter of the primary.  This data probably doesn't quite represent what goes on under the sky because the dew shield will also cool a degree or two below the ambient air temperature.  My model can easily handle that but I've presented only the base result here--without any conclusions.

 

4)  The more important number relates to the effect of the dew shield on the temperature of the front element with respect to the dew point and that's shown in the last plot.  Again, this is for a C14 at 80% HR with the dew shield at the ambient air temperature.

 

Ultimately, this is the stuff that you need from the theory of heat exchange in order to verify the numbers. I want to emphasize that there are a few approximations and assumptions embedded in these calculations and that they are moderately complicated so it requires some measurements to know how closely they predict the actual temperatures.  For example, the sky emittance is assumed at the zenith under a perfectly clear sky, and these numbers are for the expected values at sea level.  The correction values are generally relatively small for off-zenith angles or at different elevations but my model can handle all of that stuff.  Still the sky emittence itself is determined from measurements and those values seem to vary with the author, year, and location—so that introduces another uncertainty.  Ultimately, if you want to measure the effects of a dew shield and you want to compare it to theory, you need to record the air temperature, the RH, the temperature of the dew shield, the sky clarity, the height of any horizon that is above the view of the dew shield, and the altitude of the site.  And, even then, the goal is to see if you can measure numbers close to the what the theory predicts in a way that verifies the overall behavior of the system.  It’s unrealistic to expect exact agreement but it should still be possible to verify the overall behavior—and the numbers should be “reasonably” close.

 

BTW, some of this stuff does relate to the possibility of getting dew or frost inside the OTA, which actually does relate to the question of wrapping the OTA in Reflextix; but, that discussion would require too much background so I'm not going to go into that level of detail here.

 

Have fun with your measurements and I look forward to seeing what you come up with...

 

John

Attached Thumbnails

  • Temp Drop-No shield vs T and HR.jpg
  • Temp wrt DP vs T and HR.jpg
  • Effect of DS Length on Temp Drop.jpg
  • Effect on DS Length on temp wrt DP.jpg

Edited by jhayes_tucson, 18 December 2018 - 05:04 PM.


#103 choward94002

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Posted 18 December 2018 - 05:20 PM

Ok...good enough.  I looked through the threads that you referenced and I don't see much in the way of theory but I do see some incorrect stuff. 

 

< lot's of really good stuff >

 

Have fun with your measurements and I look forward to seeing what you come up with...

 

John

Thanks John, I *really* appreciate your sharing this info with me, it will help me to not only design my experiment better but also tells me what to look for and what to ignore ... and as I now understand it's important to get some good humidity numbers inside and outside the OTA so I can better correlate it once I get my raw data sets ... so, again, I really appreciate it!

 

... and since you're here ... :)

 

In my second to last paragraph I mentioned my "end game" for all this for me ... see if I can get not only a "hot tube" that doesn't dew/ frost but also get something that has more controllable focus shifts and even minimal thermals ...

 

In the work you presented about CFZ's in May of this year [https://www.cloudyni...fz temperature] you related focus shifts to tube shrinkage (which makes perfect sense, as the tube is going to be expanding/ contracting and throwing off the distance from primary to secondary) as well as earlier [https://www.cloudynights.com/topic/613360-celestron-edge-main-mirror-slop/] which is all good stuff and directly related to the need to have a thermally stable system as well as a thermally inert system like Kokatha does his imaging wth (it would actually be interesting to see how his rig performs if he were to paint the OTA exterior in Rustoleum HiTemp black, the lowest emissivity paint I could find ASTM info for to INCREASE the radiant cooling of his tube and retard the warming and expansion of his OTA tube from the ambient outside air even more ...).  I looked at a number of previous threads regarding tube expansion as well as literature for aluminum tube expansion in the oil and gas industry and although they have nice anecdotal info I couldn't find any hard numbers about the expansion/ contraction characteristics themselves ... for instance,

 

- If the OTA tube in the non-carbon C14 and C11's were a simple unconstrained tube then it would be easy to calculate the movement due to temperature; however, the OTA *is* a constrained system due to the dovetail bar (and sometimes two bars) people have.  Since I don't shear off the dovetail bar bolts everytime it gets cold, I would assume that what's happening is that because the tube is constrained longitudinally it deforms laterally ... it gets fatter, not longer.  That would potentially throw off the CFZ numbers you posted ... so do you know if those measurements were taken with a constrained or unconstrained OTA tube?

 

- One of the areas that you and freestar8n sparred about was the application of the dew heater throwing off the expansion/ contraction numbers, and for good reason: if an unconstrained tube exposed to a constant cooling temperature contracts at a fixed rate then it's easy to see when it would exceed the CFZ bounds that Dickerson's measurements on the Meade shows and that would gibe well with your graph showing that for a SCT at 10" the CFZ is exceeded every 8min or so ... cool.  If we *ADD* heat to the tube, however, we are interfering with that contraction by adding an expansion element.  One thing we don't know is the conduction of heat through the aluminum from the point of application through the tube, so it's quite possible that the tube near the heater is expanding and the tube near the base is contracting, but the next effect should be less of an overall contraction than a system where there was no dew heater ... do you know of any folks who have experimented/ seen the results of CFZ changes with and without a dew heater?

 

- One of the things I'm going to try with my sacrificial C14 is to coat the outside of the OTA with this [https://www.amazon.c...rds=copper foil] to see if I can minimize those hot/ cold zones in the OTA tube (since the OTA tube is not made to be thermally conductive, that's just a circumstance of it being made of aluminum) ... you've got a lot more experience with older C14 and C11 tubes (before the carbon fiber things), do you know anything about the tube composition?  

 

Basically, I want a thermally stable tube ... but that can be at any temperature, it just needs to be a stable one.  Kokatha has chosen his stable temperature to be at a number below what the ambient is going is, and he removes heat from the system using his ice to get to that and then thermal inertia to maintain it during the relatively short period of his observing.  I want to do the same but with a higher stable temperature, 5-10C above the dewpoint as the night goes on, so I will need to ADD heat to the system with my heating pads as well as remove it with the peltier's as the night goes on and the outside ambient temperature changes.  If that's going to be successful I need to make sure the tube itself has as little thermal inertial as possible, hence the coating in copper foil to get any thermal variations in the tube (such as from my heating pads, or from the cooling mirror cell) distributed as rapidly as possible.  Dickerson's numbers showed a shift in the CFZ resulted from a temperature swing of 6-7C which is easily detected with my high resolution thermocouples and with strategically placed peltier's and heating pads around the tube and a copper skin I should be able to keep the OTA tube at a constant temperature (i.e. no change to the CFZ) at least during the duration of my 20min L channel exposure ...

 

Thoughts?



#104 charlesgeiger

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Posted 18 December 2018 - 05:23 PM

For JHayes

Thank you regarding responding back.  I hope to look at property near Bend in the early part of 2019.  

Sincerely,

Charlie



#105 freestar8n

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Posted 18 December 2018 - 06:00 PM

- One of the areas that you and freestar8n sparred about was the application of the dew heater throwing off the expansion/ contraction numbers, and for good reason: if an unconstrained tube exposed to a constant cooling temperature contracts at a fixed rate then it's easy to see when it would exceed the CFZ bounds that Dickerson's measurements on the Meade shows and that would gibe well with your graph showing that for a SCT at 10" the CFZ is exceeded every 8min or so ... cool.  If we *ADD* heat to the tube, however, we are interfering with that contraction by adding an expansion element.  One thing we don't know is the conduction of heat through the aluminum from the point of application through the tube, so it's quite possible that the tube near the heater is expanding and the tube near the base is contracting, but the next effect should be less of an overall contraction than a system where there was no dew heater ... do you know of any folks who have experimented/ seen the results of CFZ changes with and without a dew heater?


My main point in those discussions was that modeling an OTA as a monolithic entity slaved to "ambient" temperature greatly oversimplified what is a complex system driven largely by radiative cooling - along with possibly a dew strap.  I'm baffled that all of a sudden radiative cooling is important to model - since that was my whole point.  Now when you put a dew strap near the corrector - it is even more complicated.  But it clearly works for me and others - since I post diffraction limited views of a nearly perfect Airy pattern of a star overhead - while others with similar approaches post excellent views of planets.
 
As these discussions continue regarding insulation, it's refreshing for me to look at this S&T write up from 2006:
 
https://www.skyandte...aling-with-dew/
 
It describes in simple and accurate terms how dew forms and the role of radiative cooling - and points out something I have emphasized - which is simply to screen the telescope with an umbrella if you can.  It also points out the ideal place for a dew strap - at or below the objective or corrector.
 
 

 

"Even on nights when dewing is not noticeable," Houston wrote, "the star images seem better with the dew heaters on than without them!" This may be because, contrary to what you might think, gentle heating, such as with dew heaters, keeps a telescope close to the temperature of the surrounding air, minimizing poor "seeing" caused by air-temperature differences near your optics. After all, the whole idea is to stop the telescope from growing colder than the air, so dew heaters are a great option to consider.

 
On the topic of dew shields and this thread - it's important to note that the dew shield can be very thin and it will do its job well - and the color and insulation doesn't matter unless it actually frosts on the inside.  The dew shield may cool and end up a bit below ambient - but as long as it is blocking the cold sky it will do its job by greatly reducing the amount of exposed sky for radiation loss.  It doesn't hurt to insulate it - but the thin black velcro ones such as I use are perfectly fine - especially when combined with a dew strap at low power.
 
If you have an insulated telescope and the front of it is cold - it will encourage convection within the tube - which is bad.  And if you heat the dew shield you will have a chimney effect directly in view of the scope - which is also bad.
 
As for temperature drift - that will also depend on the thermal environment of the scope - and I have posted numerous studies and examples based on direct measurements that show the focus drift in sct's is greatly exaggerated.  If it were as bad as some claim then film photography over decades would have shown the problem.  And as I described, for deep sky imaging the "CFZ" based on quarter-wave defocus is not the relevant criterion for defocus tolerance - and instead it is how much the fwhm swells with defocus - which in general is much more relaxed.
 
So although the OP may view the use of insulation as "settled down" - for many of us it remains an unproven and odd approach - compared to doing all you can to equilibrate the system as much as possible - and then apply a low power dew strap to help compensate for the cooling near the front/top of the scope and avoid both dew and convection.
 
Frank


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#106 Kokatha man

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Posted 18 December 2018 - 06:29 PM

<"Kokatha has chosen his stable temperature to be at a number below what the ambient is going is, and he removes heat from the system using his ice to get to that and then thermal inertia to maintain it during the relatively short period of his observing.">

 

 

I applaud your efforts & attempts to examine every aspect that you can quantify & qualify bro, but just a little further clarification

wink.gif - my actual "aim" is to have the primary at or as near as possible to the outside air temperature as I can...that's why I stop cooling (by removing the ice etc) when the gauges indicate the primary is about 2°C below the outside/ambient: by the time I factor in 30-40 minutes "relaxation" the primary & ambient are usually within 0.5°C of each other & the defocused star pattern's diffraction rings will be the best that the seeing will allow, ie not severely distorted etc due to uneven temperatures throughout the primary...you can actually see this "relaxing" occur if you plug in your imaging train etc & target a star. 

 

An interesting video could be made by doing said on a night of excellent seeing. You'd of course have to cut or simply shoot "takes" from the entire timespan of the relaxation period & speed it up - & a night of good seeing would present the best opportunities: for most of the last few months we've been lucky to have had clear skies, let alone clear skies & decent seeing conditions! frown.gif And we'd be unlikely to waste those on something we already appreciate if planetary imaging was viable! lol.gif

 

Oh! - the other clarification: I'm not sure if 5 or even up to 8 hours of imaging on any night we do so qualifies for <"the relatively short period of his observing.">

 

This year with Jove, Saturn & Mars strung out across the sky there have been plenty of marathon sessions, although tbh we have imaged Saturn less frequently than usual, but even so Jove & Mars have still consumed 4 or 5 hours most nights we get the right conditions... wink.gif

 

Reading Frank's (freestar8n) reply whilst I type, I can't but agree with the thrust of most of his last paragraph: my concern is with dew-heaters, although I have to say that I avoid them mainly from what I see when we do have to defog the corrector: on those nights of good seeing where the planets reveal their finest details it's easy to observe the image settling down & clarifying of the planet's image as the heat blown onto the corrector glass from my hair-dryer dissipates.

 

It's one where I think I rather logically conclude that the corrector has returned to the temperature is was at previously, although I have not made any of the plethora of measuring-point temperature logs I did years ago with the C11...the short hiatus to clear the fog isn't an issue for us as said previously & I am concerned from my experience therein that any constant heating (& likely air currents) would display an analogous effect to the latter moments of that of the hair-dryer - although in this instance I have to confess that "it works fine our way" is part of my position here & see no reason to introduce that variable to a successful practice. smile.gif

 



#107 jhayes_tucson

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Posted 18 December 2018 - 06:31 PM

Thanks John, I *really* appreciate your sharing this info with me, it will help me to not only design my experiment better but also tells me what to look for and what to ignore ... and as I now understand it's important to get some good humidity numbers inside and outside the OTA so I can better correlate it once I get my raw data sets ... so, again, I really appreciate it!

 

... and since you're here ... smile.gif

 

In my second to last paragraph I mentioned my "end game" for all this for me ... see if I can get not only a "hot tube" that doesn't dew/ frost but also get something that has more controllable focus shifts and even minimal thermals ...

 

In the work you presented about CFZ's in May of this year [https://www.cloudyni...fz temperature] you related focus shifts to tube shrinkage (which makes perfect sense, as the tube is going to be expanding/ contracting and throwing off the distance from primary to secondary) as well as earlier [https://www.cloudynights.com/topic/613360-celestron-edge-main-mirror-slop/] which is all good stuff and directly related to the need to have a thermally stable system as well as a thermally inert system like Kokatha does his imaging wth (it would actually be interesting to see how his rig performs if he were to paint the OTA exterior in Rustoleum HiTemp black, the lowest emissivity paint I could find ASTM info for to INCREASE the radiant cooling of his tube and retard the warming and expansion of his OTA tube from the ambient outside air even more ...).  I looked at a number of previous threads regarding tube expansion as well as literature for aluminum tube expansion in the oil and gas industry and although they have nice anecdotal info I couldn't find any hard numbers about the expansion/ contraction characteristics themselves ... for instance,

 

Kirchoff's law states that a for a system in thermal equilibrium, emissivity is equal to absorptivity.  This is simply energy conservation.  Lowering the emissivity means painting the tube white--not black.  Black has a high absorptivity and hence it also has a high emissivity.  Painting the tube black will indeed lower it's temperature due to radiative cooling, but in my view, that's not what you want to do (as I've said numerous times.)  I won't repeat all the reasons here.

 

- If the OTA tube in the non-carbon C14 and C11's were a simple unconstrained tube then it would be easy to calculate the movement due to temperature; however, the OTA *is* a constrained system due to the dovetail bar (and sometimes two bars) people have.  Since I don't shear off the dovetail bar bolts everytime it gets cold, I would assume that what's happening is that because the tube is constrained longitudinally it deforms laterally ... it gets fatter, not longer.  That would potentially throw off the CFZ numbers you posted ... so do you know if those measurements were taken with a constrained or unconstrained OTA tube?

 

An aluminum tube with one, two, or twenty aluminum dovetails bolted to it won't make any difference.  Aluminum is aluminum and it all reacts to temperature change in the same way so there won't be any differential flexure or stress induced.  If the tube is carbon fiber, the dovetail should be mounted with a flexure to avoid differential expansion--and this is the way my aluminum dovetail is mounted over my CF tube.

 

- One of the areas that you and freestar8n sparred about was the application of the dew heater throwing off the expansion/ contraction numbers, and for good reason: if an unconstrained tube exposed to a constant cooling temperature contracts at a fixed rate then it's easy to see when it would exceed the CFZ bounds that Dickerson's measurements on the Meade shows and that would gibe well with your graph showing that for a SCT at 10" the CFZ is exceeded every 8min or so ... cool.  If we *ADD* heat to the tube, however, we are interfering with that contraction by adding an expansion element.  One thing we don't know is the conduction of heat through the aluminum from the point of application through the tube, so it's quite possible that the tube near the heater is expanding and the tube near the base is contracting, but the next effect should be less of an overall contraction than a system where there was no dew heater ... do you know of any folks who have experimented/ seen the results of CFZ changes with and without a dew heater?

 

What!  Me sparring with Frank?  I can't image such a thing.  lol.gif lol.gif lol.gif   

 

As I said during that discussion, if you add just the right amount of heat so that the tube remains at a constant temperature, there won't be any focus shift; but, that's not what my study was about.  I personally don't like the idea of adding heat to the tube but I know that a lot of folks do it.  The answer is to use an auto-focus system and then it's mostly a moot point.  I say "mostly" because not all auto-focus systems hold focus in real time so you have to make sure that the re-focusing interval is enough to handle the effects of any local temperature changes--heated tube or not.

 

- One of the things I'm going to try with my sacrificial C14 is to coat the outside of the OTA with this [https://www.amazon.c...rds=copper foil] to see if I can minimize those hot/ cold zones in the OTA tube (since the OTA tube is not made to be thermally conductive, that's just a circumstance of it being made of aluminum) ... you've got a lot more experience with older C14 and C11 tubes (before the carbon fiber things), do you know anything about the tube composition?  

 

All of the current Celestron tubes are made of Aluminum.  If it's an aluminum tube and it's in equilibrium, the only hot/cold zones will be determined by differential radiative cooling.

 

Basically, I want a thermally stable tube ... but that can be at any temperature, it just needs to be a stable one.  Kokatha has chosen his stable temperature to be at a number below what the ambient is going is, and he removes heat from the system using his ice to get to that and then thermal inertia to maintain it during the relatively short period of his observing.  I want to do the same but with a higher stable temperature, 5-10C above the dewpoint as the night goes on, so I will need to ADD heat to the system with my heating pads as well as remove it with the peltier's as the night goes on and the outside ambient temperature changes.  If that's going to be successful I need to make sure the tube itself has as little thermal inertial as possible, hence the coating in copper foil to get any thermal variations in the tube (such as from my heating pads, or from the cooling mirror cell) distributed as rapidly as possible.  Dickerson's numbers showed a shift in the CFZ resulted from a temperature swing of 6-7C which is easily detected with my high resolution thermocouples and with strategically placed peltier's and heating pads around the tube and a copper skin I should be able to keep the OTA tube at a constant temperature (i.e. no change to the CFZ) at least during the duration of my 20min L channel exposure ...

 

As I've said, I'm not a fan of heating the OTA.  That's a path that leads to thermal convection, which under many circumstances can lead to objectionable tube currents.  I don't know if you've done the calculation, but it takes about 9.7W of power to raise the temperature of the outside of a C14 corrector plate by 1C relative to the inside of the plate by pure conduction.  To achieve the 10C difference that you mentioned, you'll need to conduct nearly 100W of heat!  That's a lot.  Of course that 10C delta is only needed when the RH is very close to 100% so the 100W number represents the worst case scenario; but, that's when you are most likely to get dew so that's what you need to plan on providing as a maximum.

 

Thoughts?

 

See above...

John


Edited by jhayes_tucson, 18 December 2018 - 06:38 PM.


#108 choward94002

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Posted 18 December 2018 - 06:41 PM

Hey Frank, great to get your input into this, you've forgotten more about scopes and stuff in the last month than I've learned since, well, forever!  Great to hear your insights, thx!

 

My main point in those discussions was that modeling an OTA as a monolithic entity slaved to "ambient" temperature greatly oversimplified what is a complex system driven largely by radiative cooling - along with possibly a dew strap.  I'm baffled that all of a sudden radiative cooling is important to model - since that was my whole point.  Now when you put a dew strap near the corrector - it is even more complicated.  But it clearly works for me and others - since I post diffraction limited views of a nearly perfect Airy pattern of a star overhead - while others with similar approaches post excellent views of planets.

 

Agreed, it's a VERY complex calculation, which is why I'm still at the "gathering data and looking for correlations" phase and any additional information or insights is greatly appreciated!
 

On the topic of dew shields and this thread - it's important to note that the dew shield can be very thin and it will do its job well - and the color and insulation doesn't matter unless it actually frosts on the inside.  The dew shield may cool and end up a bit below ambient - but as long as it is blocking the cold sky it will do its job by greatly reducing the amount of exposed sky for radiation loss.  It doesn't hurt to insulate it - but the thin black velcro ones such as I use are perfectly fine - especially when combined with a dew strap at low power.
 

Also agreed, I think as long it's strong enough to not flop around it should be good.  I also want to do an experiment with a perforated dew shield; I lose some shielding, yes, but if I can end up with a longer length due to less area to get blown around that might be a net win ...

As for temperature drift - that will also depend on the thermal environment of the scope - and I have posted numerous studies and examples based on direct measurements that show the focus drift in sct's is greatly exaggerated.  If it were as bad as some claim then film photography over decades would have shown the problem.  And as I described, for deep sky imaging the "CFZ" based on quarter-wave defocus is not the relevant criterion for defocus tolerance - and instead it is how much the fwhm swells with defocus - which in general is much more relaxed.
 

If you could point me to some links on this that would be great!  I think I've found about all of your postings here on CN about focus drift, any spots I might have missed?  Also, I was planning on using FWHM's of known stars at known positions within 15deg of zenith to quantitatively gauge the focus quality rather than airy disks (since those don't lend themselves to being a calculated number as much as an FWHM), any thoughts on how to measure that in a repeatable way?

 

Thanks again!

 



#109 freestar8n

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Posted 18 December 2018 - 06:54 PM

Hey Frank, great to get your input into this, you've forgotten more about scopes and stuff in the last month than I've learned since, well, forever!  Great to hear your insights, thx!

Thanks!

 

There is another point that has come up - and that is with regard to empirical verification.  To me this is challenging in an amateur context because it is easy to make measurements of various kinds - but it is hard to make a well-designed experiment that draws a clear conclusion from such measurements - because getting a good measurement may be very hard to do.

 

One place where this comes up is the seemingly trivial concept of measuring "ambient" air temperature.  That is actually very hard to do properly either in direct sunlight or with a cold sky overhead - and you can read about various shielding approaches that are used in order to get a good reading.

 

The only thing you can be sure about a temperature measurement is that you are measuring the temp. of the thermometer itself - but it may be very different from the air temperature if it is radiatively coupled to the sun or the cold sky.  The thermometer needs to be properly shielded - or you may think that the ota is much warmer than the "air" - when in fact the thermometer itself is radiatively cooled below the air temp.

 

That's why I like to go by final results when possible - and in this case it refers to diffraction limited imaging results that unambiguously show the 'scope is working as well as it can.

 

Kokatha Man is part of a highly competitive group of imagers doing all they can to get good images of the planets - and they have honed techniques over the years - trying different things.  So if he has an approach that is backed by good planetary images - that to me is very strong empirical evidence for a sound approach worth emulating.  That doesn't mean other approaches won't work well - but it is strong evidence of one thing that works.  His approach is also consistent with both common sense and professional techniques - where the whole goal is to get the system close to ambient as quickly as possible - often involving ice on the mirror or equivalent.

 

I was surprised he likes hot air guns - but the explanation that the corrector has low thermal mass - combined with his good results by that approach - make me believe it's a viable way to deal with dew and still have good views.  But I'm content with a dew strap at low power - in combination with a short-ish dew shield.  The combo has worked well in southern NY, Dublin Ireland, and now Melbourne Australia - based on diffraction limited results.

 

Frank



#110 Kokatha man

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Posted 18 December 2018 - 08:38 PM

<"I was surprised he likes hot air guns - but the explanation that the corrector has low thermal mass - combined with his good results by that approach - make me believe it's a viable way to deal with dew and still have good views.  But I'm content with a dew strap at low power - in combination with a short-ish dew shield.  The combo has worked well in southern NY, Dublin Ireland, and now Melbourne Australia - based on diffraction limited results.">

 

Struth - getting good results in Melbourne is pretty good! lol.gif

 

Not really, JohnK images the planets from inner Melbourne: my very limited DSO imaging tells me aspects such as seeing are nowhere near as demanding at (much) shorter f/l's...but that of course doesn't mean there aren't aspects of it that are any less demanding...some more so from my small indulgences there! waytogo.gif 

 

A hair-dryer set on "medium"* (as opposed to a glue-gun or paint stripper type that we'd term "hot air guns") quickly removes the fog...& I agree about the thermal mass rationale: with Uranus & Neptune - especially when fogging is prevalent - where you capture for 8 to 10 minutes, a good dew shield comes into its' own...

 

Howard, I've brought up the files for focus shifts on our current capture laptop - a quick look shows Saturn data going back to the start of February 2016, probably similar for the rest of the planets...Pat notes the DRO figures for each capture but only places the times on some nights, but times could be derived from the actual capture files themselves.

 

A lot of cross-referencing involved then...& some sessions were indeed short, others very long: probably the best data to present (certainly in graphic presentation) would be those nights where many successive captures were taken...still a lot of work but with the skies lately it might be something I could compile over the Xmas period. wink.gif 

 

* A very old one nowadays - looks like something out of the 1950's or early 60's - & I've had to rewire it a few times but it's an "old friend" to me lol.gif - about 800W or even less on "medium" so it isn't like many of the new 3kW "scorchers" you could probably use as paint strippers also nowadays..! bigshock.gif mrevil.gif 



#111 choward94002

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Posted 19 December 2018 - 11:41 AM

OK, quick update, a caveat and a question ... so I did some experiments with the thermal properties of a C14 tube ...

 

... and as many readers will note, a LOT of this is work that Frank and John and a lot of others did and commented on at length in May of this year and others in years before, and indeed both Frank and John have written entire papers on the subject; I'm not rehashing what they have done, or duplicating the same work, I'm instead trying to ensure that my test system is properly setup and instrumented to be able to follow on and take advantage of prior work.  My results (so far) seem pretty common sense and tracks what others have found in the past: that's not a surprise to me, what would be surprising would be if I got different numbers which would indicate a flaw in my methodology or procedure ... so, again, not duplicating or challenging prior work, making sure I'm traveling down the same road in the same kind of car ...

 

As Frank suggested earlier I need to be sure I'm measuring the right thing, so I used an angle grinder/ dremel to grind out some divots in my sacrificial C14 tube and bedded one of my high precision (.01C) temperature probes inside of some thermal paste in the divot, then covered that with a layer of silicone caulk and then a strip of my copper foil ... that should ensure that the thermocouple is measuring tube temperature and not ambient or radiative cooling of the thermocouple.  The thermocouple was already in a black plastic packaging, so I didn't need to paint it as John suggested ... the sensors were bedded every 4in from top to bottom, plus a few more spinkled around the corrector holder and mirror cell since they have more thermal mass) four per "layer".  I also instrumented the corrector plate around the periphery and around the secondary holder inside the OTA by bedding the thermocouple in thermal paste, covering that with a blister of silicone caulk and then covering that with my copper foil as well as putting a square of copper foil on the opposite side of sensor glass (that's a total of 65 sensors used just for this, DigiKey loves me and the tube looks like it's in the hospital on life support!).  These will come in handy once I start doing my focus tests ... 

 

I wanted to see how long it would take the tube to change temperature and equalize once I started to apply heat; I use two of these [https://www.amazon.c...e?ie=UTF8&psc=1] to apply heat about 2" from the tube top normally, sitting half on and half off the corrector plate holder and they are pretty powerful, able to change the temperature of a C14 corrector plate by about .15 - .2C per minute to keep up with a rapidly cooling night temperature ...

 

I did my experiment at room temperature, 20C so I didn't have to deal with ambient air cooling/ heating and powered up the heaters with the tube in a vertical position (since I image within a 30deg arc of vertical).  Once the heat was applied the glass heated up as expected, reaching the target temperature of 30C ("hot tube"), 10C above the 20C initial after about an hour.  I repeated the experiment four more times and normalized the results, what I found was that the tube followed the expected properties for an aluminum tube pretty well [https://www.engineer...s_of_metals.htm], with the temperature delta migrating as down the tube at about .05C per inch per minute from the point source (slightly slower than predicted, actually) ...

 

What this means is that although the CFZ for a C14 has been found by John and Dickerson to vary quite a bit as the temperature changes (and now that we know it's a tube of aluminum we can actually calculate that overall change in length and by inference change in CFZ per degree C for a C14 or C11 tube pretty accurately, as John did here [https://www.cloudyni...d/#entry8570101]) the application of local heating (or cooling) doesn't change the tube temperature by enough to make a big difference; once I hit my 30C target temperature the calculated thermal expansion of the entire tube (by calculating from the delta's between the probe layers) was only about 75 microns .. the key insight being that with so much thermal mass in a C14 tube you need a MASSIVE amount of point heating/ cooling before you will need to refocus ...

 

This lines up with various people like Kokatha using hair dryers on the corrector plate not complaining about needing to refocus; between the themal inertia of the glass the and thermal inertia of the tube I could probably run a heat gun on the glass and not have to change my focus (thermals introduced as the plate heats up are a different story, of course) ...

 

So, the takeaway (for me) from this experiment is a) don't worry about goofing up your focus if you're blasting away dew/ frost from the plate with a hair dryer and b) apply dew heater heat in short intense bursts and not continuously so that the tube itself doesn't have a chance to start to expand ...

 

My question: does this line up with what people have found in the field?  Do people notice changes in focus after blasting with a hair dryer (not thermal eddies, changes in focus)?


Edited by choward94002, 19 December 2018 - 11:59 AM.


#112 jhayes_tucson

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Posted 19 December 2018 - 04:21 PM

...

What this means is that although the CFZ for a C14 has been found by John and Dickerson to vary quite a bit as the temperature changes (and now that we know it's a tube of aluminum we can actually calculate that overall change in length and by inference change in CFZ per degree C for a C14 or C11 tube pretty accurately, as John did here [https://www.cloudyni...d/#entry8570101]) the application of local heating (or cooling) doesn't change the tube temperature by enough to make a big difference; once I hit my 30C target temperature the calculated thermal expansion of the entire tube (by calculating from the delta's between the probe layers) was only about 75 microns .. the key insight being that with so much thermal mass in a C14 tube you need a MASSIVE amount of point heating/ cooling before you will need to refocus ...

 

This lines up with various people like Kokatha using hair dryers on the corrector plate not complaining about needing to refocus; between the themal inertia of the glass the and thermal inertia of the tube I could probably run a heat gun on the glass and not have to change my focus (thermals introduced as the plate heats up are a different story, of course) ...

 

So, the takeaway (for me) from this experiment is a) don't worry about goofing up your focus if you're blasting away dew/ frost from the plate with a hair dryer and b) apply dew heater heat in short intense bursts and not continuously so that the tube itself doesn't have a chance to start to expand ...

 

My question: does this line up with what people have found in the field?  Do people notice changes in focus after blasting with a hair dryer (not thermal eddies, changes in focus)?

 

Hmm...let's straighten a few thing out here:

 

1) The CFZ is determined only by the focal ratio of the system and it does not vary with temperature.  (For anyone tempted to nit-pik, the F/# does vary by a small amount as the spacing changes but this is a higher order effect that is insignificant for the purpose of this discussion.)  There are different ways to define the CFZ (called the depth of focus in optics) but it's close enough to use 2.5 (F/#)^2 (in microns), which is about 291 microns for a F/10.8 system.

 

2) Your 75 micron length change for a 0.59m long aluminum tube undergoing a 10C temperature change isn't right.   Can you explain in more detail how you arrived at that number?  Was this for a tube in equilibrium?  Were you heating only part of it?   A 0.59m long aluminum tube in equilibrium will change length by 13.92 microns per degree C.  So if you induce a uniform temperature change of +10C, the tube will grow by 139.2 microns.

 

3)  The change in focus for a 75 micron change in length of the tube is huge!   For any Cassegrain type configuration, the shift in focus will be approximately equal to the length change of the tube times the square of the optical magnification.  A C14 has an optical magnification of roughly 5 so the change in focus due to a spacing change of 75 microns will be about 1.875 mm or 6.4 times the CFZ.  That's not a small focus shift.

 

4)  The amount of change in length of the tube depends only on the temperature change--not on its thermal mass.  The thermal mass (or heat capacity) is related to the specific heat, which is how much energy is required to induce a 1C change per mass of the material.  Aluminum and glass have very similar specific heat values (0.90 vs 0.84 J/g-K) and since there is a lot more glass (by mass) in the system that's where most of the heat is stored.  I think that what you may be referring to is the the thermal diffusivity of the material, which relates to the heat transfer rate through a unit thickness and unit area of material per degree C of temperature difference.  It is a measure of how well a material conducts heat.  Aluminum is a very good heat conductor (with a thermal conductivity of 237 W/m-K it's not quite half that of copper) so it will change temperature relatively quickly as the ambient temperature changes.  On the other hand, glass has a thermal conductivity of only 0.78 W/m-K so it will take a long time to change temperature--as we all know.

 

 

John


Edited by jhayes_tucson, 19 December 2018 - 04:35 PM.


#113 choward94002

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Posted 19 December 2018 - 04:57 PM

Hmm...let's straighten a few thing out here:

 

Absolutely!  If I've misinterpreted things please set me straight!

 

1) The CFZ is determined only by the focal ratio of the system and it does not vary with temperature.  (For anyone tempted to nit-pik, the F/# does vary by a small amount as the spacing changes but this is a higher order effect that is insignificant for the purpose of this discussion.)  There are different ways to define the CFZ (called the depth of focus in optics) but it's close enough to use 2.5 (F/#)^2 (in microns), which is about 291 microns for a F/10.8 system.

 

As understand things, the CFZ is set by the distance between the primary and the secondary plus or minus a value dependent upon the airy disk diameter at a specific seeing; you had a very nice picture in the May thread [https://www.cloudyni...p/#entry8496954]  that showed that, as well as formula to compute the change in that distance as a function of change in temperature [https://www.cloudyni...p/#entry8497752] ... which implies that it does vary by temperature as the tube expands or contracts and changes the distance between primary and secondary.  There was also a graph by Dickerson on a Meade in thread a few months prior that showed the change in CFZ also as a function of temperature as well as a discussion by Frank in 2016, by Jerry Lodriguess earlier in an essay on his website and even by Jon Rista about the importance of the CFZ and how it relates to temperature ... did I err?

 

2) Your 75 micron length change for a 0.59m long aluminum tube undergoing a 10C temperature change isn't right.   Can you explain in more detail how you arrived at that number?  Was this for a tube in equilibrium?  Were you heating only part of it?   A 0.59m long aluminum tube in equilibrium will change length by 13.92 microns per degree C.  So if you induce a uniform temperature change of +10C, the tube will grow by 139.2 microns.

 

Recall that in my experiment I'm only heating the end of the tube (para 4) and only applied the point heat until the glass next to the heat source hit the target temperature (para 5), *not* until the tube hit equilibrium; that was the whole point of the experiment, to find out how much of the tube would "heat up" before the glass hit the target temperature.  That's why I asked about the composition of the tube, to see what the anticipated thermal conduction of it would be ...

 

3)  The change in focus for a 75 micron change in length of the tube is huge!   For any Cassegrain type configuration, the shift in focus will be approximately equal to the length change of the tube times the square of the optical magnification.  A C14 has an optical magnification of roughly 5 so the change in focus due to a spacing change of 75 microns will be about 1.875 mm or 6.4 times the CFZ.  That's not a small focus shift.

 

Hmm ... from the Dickerson graph [https://www.cloudyni...-3#entry8985035] I show a CFZ "spread" of 345 microns using a Meade 10, from threads that Frank and Jon had over the years like this [https://www.cloudyni...s/#entry7206543] discussing vCurves it also appeared that the CFZ had a "spread" of about 300 microns for the C14 ... so what would you consider the CFZ "spread" to be for a non-edge C14?

 

4)  The amount of change in length of the tube depends only on the temperature change--not on its thermal mass.  The thermal mass (or heat capacity) is related to the specific heat, which is how much energy is required to induce a 1C change per mass of the material.  Aluminum and glass have very similar specific heat values (0.90 vs 0.84 J/g-K) and since there is a lot more glass (by mass) in the system that's where most of the heat is stored.  I think that what you may be referring to is the the thermal diffusivity of the material, which relates to the heat transfer rate through a unit thickness and unit area of material per degree C of temperature difference.  It is a measure of how well a material conducts heat.  Aluminum is a very good heat conductor (with a thermal conductivity of 237 W/m-K it's not quite half that of copper) so it will change temperature relatively quickly as the ambient temperature changes.  On the other hand, glass has a thermal conductivity of only 0.78 W/m-K so it will take a long time to change temperature--as we all know.

 

Yep, I think I'm using the wrong terminology but we're describing the same effect ... different materials conduct heat at different rates (thats why copper is a good heatsink material and not glass block) ... what I was referring to as the "thermal mass" should have been more properly described as "more of it" ... a 2 ton block of copper will heat up a lot slower than a 6" cube of glass simply because there's more of it to heat up ... so basically what I was interesting find out was how much of the aluminum tube would heat up by the time that the glass plate hit it's target temperature (answer: not too much) and if that increase would be uniform (answer: it's not) ...

 

Great feedback, it's really appreciated!

 

 

John


Edited by choward94002, 19 December 2018 - 05:16 PM.


#114 freestar8n

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Posted 19 December 2018 - 05:30 PM

My question: does this line up with what people have found in the field?  Do people notice changes in focus after blasting with a hair dryer (not thermal eddies, changes in focus)?


Hi-

Here is a recent summary of focus drift in sct's - where all my measurements are based on an sct that is outside all day under a tg365 cover - in a courtyard that blocks the horizon up to 20 or 30 degrees.  And there is a dew strap at low power just below the corrector - with a short dew shield in front.

 

https://www.cloudyni...tar-no-problem/

 

It contains links to other studies of focus and temperature drift in sct's that I have presented.

 

For deep sky imaging you are limited by seeing in how often you need to focus - and CFZ plays no role since it assumes diffraction limited.  It would be very different for planetary imaging - but people don't tend to use autofocus for that - and instead they are constantly tweaking focus by eye and studying the video - I think.  So "time between focus" doesn't really matter because it is always being tweaked.

 

For my ambient temperature measurement I was using a bme280 under a plastic milk carton shield with holes in it.  It's not perfect but it's much better than having the sensor directly exposed to the sky.

 

The tube measurements are based on ds18b20 sensors taped onto the tube with some thermal paste.  Those should provide good measurements of the tube because they are in direct contact.

 

The sky IR temperature is with a Melexis sensor and should not be taken literally - but the sky is very cold when clear - and the temperature decreases as the sky becomes more transparent.

 

For this system I don't have any sensors inside the tube - unlike years ago when I went inside a C11 and added sensors.

 

The main takeaways from all this are:  1)  With a tg365 cover removed near sunset, the scope equilibrates rapidly but the tube will slightly lag ambient - and the mirror will lag the tube - presumably.  2)  With a dew heater the front of the tube - and the secondary - will be slightly warmer than the middle of the tube - but that is a stable situation in terms of preventing convection.  And all these differences are small.  3)  The resulting thermal drift over time would resulting in extremely small bloat in stacked star images - and if I have 2.3" fwhm in hyperstar images - for that evening I can go 0.66 hours without refocusing and the stack will only bloat 3.3%.  If the drift is steady I can slightly offset the initial focus and go 1.3 hours between focus - again with only 3.3% bloat.

 

The overall system has components at different temperatures and those temperatures will involve a mixture of ambient temperature, sky temperature, sky exposure, and dew strap power.  The fact that it still has good seeing and low turbulence is verified by images of Airy patterns and planets that I have posted separately.  My deep sky images go down to the low 1" from Melbourne - but that is less confirmation than actual diffraction limited results derived from video.

 

Frank



#115 choward94002

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Posted 19 December 2018 - 05:41 PM

Hi-

Here is a recent summary of focus drift in sct's - where all my measurements are based on an sct that is outside all day under a tg365 cover - in a courtyard that blocks the horizon up to 20 or 30 degrees.  And there is a dew strap at low power just below the corrector - with a short dew shield in front.

 

https://www.cloudyni...tar-no-problem/

 

It contains links to other studies of focus and temperature drift in sct's that I have presented.

 

Perfect, thank you!  I've read some of those but more is always good!

 

For deep sky imaging you are limited by seeing in how often you need to focus - and CFZ plays no role since it assumes diffraction limited.  It would be very different for planetary imaging - but people don't tend to use autofocus for that - and instead they are constantly tweaking focus by eye and studying the video - I think.  So "time between focus" doesn't really matter because it is always being tweaked.

 

Got it, good insight!

 

For my ambient temperature measurement I was using a bme280 under a plastic milk carton shield with holes in it.  It's not perfect but it's much better than having the sensor directly exposed to the sky.

 

The tube measurements are based on ds18b20 sensors taped onto the tube with some thermal paste.  Those should provide good measurements of the tube because they are in direct contact.

 

The sky IR temperature is with a Melexis sensor and should not be taken literally - but the sky is very cold when clear - and the temperature decreases as the sky becomes more transparent.

 

For this system I don't have any sensors inside the tube - unlike years ago when I went inside a C11 and added sensors.

 

The main takeaways from all this are:  1)  With a tg365 cover removed near sunset, the scope equilibrates rapidly but the tube will slightly lag ambient - and the mirror will lag the tube - presumably.  2)  With a dew heater the front of the tube - and the secondary - will be slightly warmer than the middle of the tube - but that is a stable situation in terms of preventing convection.  And all these differences are small.  3)  The resulting thermal drift over time would resulting in extremely small bloat in stacked star images - and if I have 2.3" fwhm in hyperstar images - for that evening I can go 0.66 hours without refocusing and the stack will only bloat 3.3%.  If the drift is steady I can slightly offset the initial focus and go 1.3 hours between focus - again with only 3.3% bloat.

 

The overall system has components at different temperatures and those temperatures will involve a mixture of ambient temperature, sky temperature, sky exposure, and dew strap power.  The fact that it still has good seeing and low turbulence is verified by images of Airy patterns and planets that I have posted separately.  My deep sky images go down to the low 1" from Melbourne - but that is less confirmation than actual diffraction limited results derived from video.

 

Very good!  Aside from staring at Airy disk photo's to gauge focus quality, is there a metric that can be measured and recorded like HFD or FWHD values to also gauge focus shift and quality?

 

Thanks for the feedback, much appreciated and very valuable!

Frank



#116 jhayes_tucson

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Posted 19 December 2018 - 05:42 PM

 

Hmm...let's straighten a few thing out here:

 

Absolutely!  If I've misinterpreted things please set me straight!

 

1) The CFZ is determined only by the focal ratio of the system and it does not vary with temperature.  (For anyone tempted to nit-pik, the F/# does vary by a small amount as the spacing changes but this is a higher order effect that is insignificant for the purpose of this discussion.)  There are different ways to define the CFZ (called the depth of focus in optics) but it's close enough to use 2.5 (F/#)^2 (in microns), which is about 291 microns for a F/10.8 system.

 

As understand things, the CFZ is set by the distance between the primary and the secondary plus or minus a value dependent upon the airy disk diameter at a specific seeing; you had a very nice picture in the May thread [https://www.cloudyni...p/#entry8496954]  that showed that, as well as formula to compute the change in that distance as a function of change in temperature [https://www.cloudyni...p/#entry8497752] ... which implies that it does vary by temperature as the tube expands or contracts and changes the distance between primary and secondary.  There was also a graph by Dickerson on a Meade in thread a few months prior that showed the change in CFZ also as a function of temperature as well as a discussion by Frank in 2016, by Jerry Lodriguess earlier in an essay on his website and even by Jon Rista about the importance of the CFZ and how it relates to temperature ... did I err?

 

Yes, you are confusing the thermal sensitivity values that I posted for various telescopes with the depth of focus.  The depth of focus does not vary with temperature and depends only on the F/#.  The thermal sensitive shows how much of a change in temperature is required to move the focus position by 1/2 the CFZ for any given telescope.  The CFZ is not changing; it's the focus point that changes with temperature.

 

2) Your 75 micron length change for a 0.59m long aluminum tube undergoing a 10C temperature change isn't right.   Can you explain in more detail how you arrived at that number?  Was this for a tube in equilibrium?  Were you heating only part of it?   A 0.59m long aluminum tube in equilibrium will change length by 13.92 microns per degree C.  So if you induce a uniform temperature change of +10C, the tube will grow by 139.2 microns.

 

Recall that in my experiment I'm only heating the end of the tube (para 4) and only applied the point heat until the glass next to the heat source hit the target temperature (para 5), *not* until the tube hit equilibrium; that was the whole point of the experiment, to find out how much of the tube would "heat up" before the glass hit the target temperature.  That's why I asked about the composition of the tube, to see what the anticipated thermal conduction of it would be ...

 

OK, but I'm not sure what that tells you.  If all you care about is getting the temperature of the inside of the glass to a particular temperature, the temperature distribution over the tube will depend on the ambient air temperature.  That's all ok but what does it mean?  Finally, I still don't understand how you came up with the 75 micron number.

 

 

3)  The change in focus for a 75 micron change in length of the tube is huge!   For any Cassegrain type configuration, the shift in focus will be approximately equal to the length change of the tube times the square of the optical magnification.  A C14 has an optical magnification of roughly 5 so the change in focus due to a spacing change of 75 microns will be about 1.875 mm or 6.4 times the CFZ.  That's not a small focus shift.

 

Hmm ... from the Dickerson graph [https://www.cloudyni...-3#entry8985035] I show a CFZ "spread" of 345 microns using a Meade 10, from threads that Frank and Jon had over the years like this [https://www.cloudyni...s/#entry7206543] discussing vCurves it also appeared that the CFZ had a "spread" of about 300 microns for the C14 ... so what would you consider the CFZ "spread" to be for a non-edge C14?

 

​The depth of focus (CFZ) in microns is roughly 2.5 times the focal ratio squared.  So take the focal ratio, square it and multiply by 2.5. For a F/10.8 system, the CFZ = 291 microns.  However, the change in focus for a C14 with a 0.59 m long aluminum tube that grows (or shrinks) by 75 microns will be roughly 1.875 mm.  That's 6.4x the CFZ!   At that level the telescope will be severely out of focus!

 

4)  The amount of change in length of the tube depends only on the temperature change--not on its thermal mass.  The thermal mass (or heat capacity) is related to the specific heat, which is how much energy is required to induce a 1C change per mass of the material.  Aluminum and glass have very similar specific heat values (0.90 vs 0.84 J/g-K) and since there is a lot more glass (by mass) in the system that's where most of the heat is stored.  I think that what you may be referring to is the the thermal diffusivity of the material, which relates to the heat transfer rate through a unit thickness and unit area of material per degree C of temperature difference.  It is a measure of how well a material conducts heat.  Aluminum is a very good heat conductor (with a thermal conductivity of 237 W/m-K it's not quite half that of copper) so it will change temperature relatively quickly as the ambient temperature changes.  On the other hand, glass has a thermal conductivity of only 0.78 W/m-K so it will take a long time to change temperature--as we all know.

 

Yep, I think I'm using the wrong terminology but we're describing the same effect ... different materials conduct heat at different rates (thats why copper is a good heatsink material and not glass block) ... what I was referring to as the "thermal mass" should have been more properly described as "more of it" ... a 2 ton block of copper will heat up a lot slower than a 6" cube of glass simply because there's more of it to heat up ... so basically what I was interesting find out was how much of the aluminum tube would heat up by the time that the glass plate hit it's target temperature (answer: not too much) and if that increase would be uniform (answer: it's not) ...

 

Ah, ok.  That's not really right.  You should adjust the amount of heat so that the glass stays at the target temperature for, say 30-40 minutes--and then measure the tube temperatures.  Aluminum is a good conductor but you want to make this measurement in equilibrium so that you aren't recording transient values.  BTW, in my view, this is not a good way to operate a telescope but I'll address that issue later.  Right now, I'm just trying to help you to make a valid measurement and to help you to better understand your results.

 

Great feedback, it's really appreciated!

 

 

John

 

Hopefully my responses in blue will come through...

John

 


Edited by jhayes_tucson, 19 December 2018 - 05:43 PM.


#117 freestar8n

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Posted 19 December 2018 - 06:07 PM

 

 

 

Very good!  Aside from staring at Airy disk photo's to gauge focus quality, is there a metric that can be measured and recorded like HFD or FWHD values to also gauge focus shift and quality?

 

Thanks for the feedback, much appreciated and very valuable!

Frank

 

The Airy patterns and focus curves are showing two different things.  If you want to claim that insulation works well to provide stable seeing, then I would want to see evidence that the scope is performing at the diffraction limit.  Getting a good image of the Airy pattern of a star requires not just focus to be good - but the aberrations and tube currents need to be under control.  And you need the same for good planetary images.  So for me that is the difference between an anecdotal "this worked great" vs. an actual result that shows things are working well.  Temperature measurements by themselves don't tell you things are working well - but they provide good info when combined with a good result.

 

Any measurement of focus drift for me requires good evidence you are measuring focus precisely and in a repeatable way.  A multi-star focus curve is the best way I know to do this.  If you can get a nice curve, calculate best focus, and then repeat the curve and have the resulting curve well centered - it is a self-validating way to show your measure of focus is meaningful and repeatable.  In my case it also lets me calculate error bars on the resulting focus value - as shown in my plot of focus drift.

 

I have seen these direct measurements and indication of good performance for systems that have been thermally equilibrated - but I'm not aware of similar results for approaches based on insulation.

 

Frank


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#118 choward94002

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Posted 19 December 2018 - 09:06 PM

 

 

Hmm...let's straighten a few thing out here:

 

Absolutely!  If I've misinterpreted things please set me straight!

 

1) The CFZ is determined only by the focal ratio of the system and it does not vary with temperature.  (For anyone tempted to nit-pik, the F/# does vary by a small amount as the spacing changes but this is a higher order effect that is insignificant for the purpose of this discussion.)  There are different ways to define the CFZ (called the depth of focus in optics) but it's close enough to use 2.5 (F/#)^2 (in microns), which is about 291 microns for a F/10.8 system.

 

As understand things, the CFZ is set by the distance between the primary and the secondary plus or minus a value dependent upon the airy disk diameter at a specific seeing; you had a very nice picture in the May thread [https://www.cloudyni...p/#entry8496954]  that showed that, as well as formula to compute the change in that distance as a function of change in temperature [https://www.cloudyni...p/#entry8497752] ... which implies that it does vary by temperature as the tube expands or contracts and changes the distance between primary and secondary.  There was also a graph by Dickerson on a Meade in thread a few months prior that showed the change in CFZ also as a function of temperature as well as a discussion by Frank in 2016, by Jerry Lodriguess earlier in an essay on his website and even by Jon Rista about the importance of the CFZ and how it relates to temperature ... did I err?

 

Yes, you are confusing the thermal sensitivity values that I posted for various telescopes with the depth of focus.  The depth of focus does not vary with temperature and depends only on the F/#.  The thermal sensitive shows how much of a change in temperature is required to move the focus position by 1/2 the CFZ for any given telescope.  The CFZ is not changing; it's the focus point that changes with temperature.

 

Got it ... so when the temperature changes the length of the tube it doesn't change the CFZ (which is a function of the optics), it changes where that CFZ is physically located ... thanks for helping me to understand that! (Remember the 50% rule, that was the 25% that I misinterpreted) ... given that, your formula is good for calculating the change in CFZ physical location, yes?

 

2) Your 75 micron length change for a 0.59m long aluminum tube undergoing a 10C temperature change isn't right.   Can you explain in more detail how you arrived at that number?  Was this for a tube in equilibrium?  Were you heating only part of it?   A 0.59m long aluminum tube in equilibrium will change length by 13.92 microns per degree C.  So if you induce a uniform temperature change of +10C, the tube will grow by 139.2 microns.

 

Recall that in my experiment I'm only heating the end of the tube (para 4) and only applied the point heat until the glass next to the heat source hit the target temperature (para 5), *not* until the tube hit equilibrium; that was the whole point of the experiment, to find out how much of the tube would "heat up" before the glass hit the target temperature.  That's why I asked about the composition of the tube, to see what the anticipated thermal conduction of it would be ...

 

OK, but I'm not sure what that tells you.  If all you care about is getting the temperature of the inside of the glass to a particular temperature, the temperature distribution over the tube will depend on the ambient air temperature.  That's all ok but what does it mean?  Finally, I still don't understand how you came up with the 75 micron number.

 

Cool, maybe (hopefully) there's a flaw in my methodology that you can help with!  Basically we know the rate of thermal expansion/ contraction for aluminum, and since it's a tube we can calculate that pretty easily (and you did that in May).  Since you told me the tube is all aluminum (homogeneous) then we know that assuming the temperature change is the same for all sections of the tube then each section will move by a proportional amount; if the length of the tube is 1000cm and the formula says it will expand 1cm per 10C then we can infer that every 10cm of tube will expand 10mm per 10C, or 1mm per 1C.  That's assuming a perfectly even temperature change across all sections with no loss of heat to the system.

 

Now let's change the experiment a bit and apply heat only to one 10cm section, and suppose that each section warms at a rate of 1C per minute ... so, at one minute the section being heated has warmed 1C, all of the other sections are unaffected ... at two minutes the heated section is up to 2C, the one next to it is at 1C, all the rest are unaffected ... after 5 minutes the progression of warming is 5C, 4C, 3C, 2C and 1C with the other five sections unaffected.  Let's take our measurements at this point, at 5 minutes ... the section that has heated by 5C will have expanded (1mm per 1C times 5C) 5mm, the section next to it will be at (1mm per 1C times 4C) 4mm, etc. for a total aggregate movement of the tube after 5min of point heating of 5+4+3+2+1 = 15mm

 

I don't have any equipment capable of measuring micron distances, but I *do* have high precision thermocouples (+/- .01C)... so as I described I segmented the tube into five equal 4" sections (starting about an inch away from the mirror cell and the corrector cell) and placed thermocouples that were bedded into the tube material and insulated from both ambient air as well as radiative cooling at equal distances around the tube (12, 3, 6 and 9 o'clock).  I also used either an angle grinder or a dremel to bed thermocouples into the mirror cell/ corrector material about an inch away from the junction between it and the tube, as described. 

 

For the experiment as described I acclimated the tube to room temperature 20C, placed the tube in a nose up orientation and applied a heat source to the corrector plate section at positions 3 and 9 o'clock capable of heating the corrector plate section by .15-.2C per minute and recorded all of the thermocouple readings every 30sec.  As expected from thermal conduction the corrector cell section warmed up first at 3 and 9 o'clock where it was being warmed, soon after the first tube section warmed at 3 and 9 o'clock and the corrector section 12 and 6 o'clock warmed, etc. and the glass also began to warm by a much smaller degree.

 

At the same time I monitored the interior skin temperature of the corrector plate at four locations around the secondary holder as described to ensure I was reading glass skin temperatures and not ambient air temperatures.  The experiment ended when the plate temperature next to the secondary holder reached the target 10C above ambient temperature, at which point I had temperature readings for all five 4" tube sections, the corrector cell section and the mirror cell section.  All section readings (12, 3, 6 and 9 o'clock) were summed and divided by four to get an average temperature (you stated that tube expansion is unconstrained, which means even if the 6 o'clock section is warmer than the 9 o'clock section it will be constrained by the cooler 9 o'clock section and the net expansion would be the sum of the two sections divided by two).  This gave me temperature differential between the six physical sections of tube.

 

Once I had those differentials I simply applied your formula to compute the expansion movement of that section and summed them; the result was 74.98mm, rounded up to 75mm (so, that's where I got the number).  It was interesting for me to note that the warming was only detectable down to the 4th section, or slightly midway down the tube, at the end of the experiment

 

3)  The change in focus for a 75 micron change in length of the tube is huge!   For any Cassegrain type configuration, the shift in focus will be approximately equal to the length change of the tube times the square of the optical magnification.  A C14 has an optical magnification of roughly 5 so the change in focus due to a spacing change of 75 microns will be about 1.875 mm or 6.4 times the CFZ.  That's not a small focus shift.

 

Hmm ... from the Dickerson graph [https://www.cloudyni...-3#entry8985035] I show a CFZ "spread" of 345 microns using a Meade 10, from threads that Frank and Jon had over the years like this [https://www.cloudyni...s/#entry7206543] discussing vCurves it also appeared that the CFZ had a "spread" of about 300 microns for the C14 ... so what would you consider the CFZ "spread" to be for a non-edge C14?

 

​The depth of focus (CFZ) in microns is roughly 2.5 times the focal ratio squared.  So take the focal ratio, square it and multiply by 2.5. For a F/10.8 system, the CFZ = 291 microns.  However, the change in focus for a C14 with a 0.59 m long aluminum tube that grows (or shrinks) by 75 microns will be roughly 1.875 mm.  That's 6.4x the CFZ!   At that level the telescope will be severely out of focus!

 

Hmm ... OK, great to know that! That means that the maximum allowable expansion for the tube to remain in the CFZ would be (291microns / 25 = 11) microns, yes?  

 

4)  The amount of change in length of the tube depends only on the temperature change--not on its thermal mass.  The thermal mass (or heat capacity) is related to the specific heat, which is how much energy is required to induce a 1C change per mass of the material.  Aluminum and glass have very similar specific heat values (0.90 vs 0.84 J/g-K) and since there is a lot more glass (by mass) in the system that's where most of the heat is stored.  I think that what you may be referring to is the the thermal diffusivity of the material, which relates to the heat transfer rate through a unit thickness and unit area of material per degree C of temperature difference.  It is a measure of how well a material conducts heat.  Aluminum is a very good heat conductor (with a thermal conductivity of 237 W/m-K it's not quite half that of copper) so it will change temperature relatively quickly as the ambient temperature changes.  On the other hand, glass has a thermal conductivity of only 0.78 W/m-K so it will take a long time to change temperature--as we all know.

 

Yep, I think I'm using the wrong terminology but we're describing the same effect ... different materials conduct heat at different rates (thats why copper is a good heatsink material and not glass block) ... what I was referring to as the "thermal mass" should have been more properly described as "more of it" ... a 2 ton block of copper will heat up a lot slower than a 6" cube of glass simply because there's more of it to heat up ... so basically what I was interesting find out was how much of the aluminum tube would heat up by the time that the glass plate hit it's target temperature (answer: not too much) and if that increase would be uniform (answer: it's not) ...

 

Ah, ok.  That's not really right.  You should adjust the amount of heat so that the glass stays at the target temperature for, say 30-40 minutes--and then measure the tube temperatures.  Aluminum is a good conductor but you want to make this measurement in equilibrium so that you aren't recording transient values.  BTW, in my view, this is not a good way to operate a telescope but I'll address that issue later.  Right now, I'm just trying to help you to make a valid measurement and to help you to better understand your results.

 

Hmm ... once the glass got to the target temperature it would start to reach equilibrium with the aluminum tube, so essentially I would be warming the entire OTA up to the target temperature over time but that wasn't the experiment: I wanted to see how much the tube would be calculated to expand once the glass got to the target temperature ... I'll be repeating the experiment another few times to verify the results, then start doing the experiment with the OTA having a lower starting temperature and see what changes ...

 

And yes, I know that this isn't a good way to operate a telescope because this will cause thermals but remember my end goals ... if I can't stop dew and frost from collecting on the corrector (or even worse, inside the OTA) then no amount of focus adjustments will make any difference, if I can't get the focus shifts under control for my 20min window then getting the thermals controlled won't make any difference ... one step at a time ... smile.gif 

 

As always, great feedback and suggestions!

 

 

John

 

Hopefully my responses in blue will come through...

John

 

Addendum: Looking at the Foresight Innovations SharpLock site as well as John's Sharplock graphs in May looking for info on tube expansion characteristics to verify my numbers as being in the ballpark ...

 

RCT10_focusShift-1024x403.png

 

Something odd ... for a total temperature shift of 3C the focuser made an adjustment of 200 microns to compensate for the tube expansion of 3C.  My target temperature of 10C was only able to affect about 1/3 of the mass of the tube (halfway down the tube, nothing on the primary or mirror cell) which implies that it would have hit thermal equilibrium at 3C ... which from the graphs would have required a 200micron adjustment for the tube expansion.  At the time of my experiment end only a bit over 1/3 of the tube had been affected which resulted in a calculated 75micron movement ... but with all of the thermal energy in the system as the tube came to equilibrium at the new 3C level some parts would have expanded but others would have contracted as they cooled and the system should have equalized at a total of 75microns length, not the 200microns shown in the graph for a 3C movement ...

 

post-211853-0-93613000-1525969371_thumb.jpg

 

... and looking at John's calculation for a C14, he shows that for each 1C change there is a movement of almost 24microns per meter, or 14.1microns for the length of the C14 ... so if we plug in the 3C from the ForeSight graph we should have a total change of 42.28microns instead of the 200microns that they list ... ForeSight has listed their formula for expansion here [https://www.cloudyni...t/#entry7175410] with a formula of 22 microns/ meter C which matches John's 24 microns/ meter C (since the tube diameter isn't a factor) ...

 

If John's calculation and the Foresight calculation is correct that there is a 42-44micron shift for a 3C change then my calculated 75micron number should have actually been 14-16microns  ...

 

 

Hmm ...


Edited by choward94002, 19 December 2018 - 11:31 PM.


#119 jhayes_tucson

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Posted 20 December 2018 - 12:09 AM

[original post not copied by CN]

 

 

 

Please forgive me, but I believe that you might be in over your head with this stuff, but let me try to clarify a few things.

 

1)  I can't make any sense out of this statement:  "Since...the tube is all aluminum (homogeneous) then we know that assuming the temperature change is the same for all sections of the tube then each section will move by a proportional amount; if the length of the tube is 1000cm and the formula says it will expand 1cm per 10C then we can infer that every 10cm of tube will expand 10mm per 10C, or 1mm per 1C.  That's assuming a perfectly even temperature change across all sections with no loss of heat to the system."

 

The equation for simple linear thermal expansion is simply dx = c * L * dT, where dx = change in length, c = the coefficient of expansion, L = the length of the part, and dT = the change in temperature.   When you introduce heat at one end of the tube, you have to introduce a model for the temperature distribution so that you can do an integration to determine the total change in length due to the non-uniform temperature distribution.  That temperature model can be based on your measurements, on a heat exchange model, or both.  Either way, the change in any incremental length is determined by the linear behavior of the material with respect to temperature as shown in the above equation.   What formula are you using that says a 1000 cm tube of aluminum will expand by 1 cm per 10C?  The coefficient of thermal expansion for aluminum is 23.6 um/m-C.  So a 10 m tube will expand by 236 um per degree C and 2360 microns (2.36 mm) per 10C; but, why are we talking about a 10 m tube?  As I've explained, the C14 tube is roughly 0.59m long and it will change length by 13.92 microns per degree C with a uniform temperature change. Perhaps you are confusing mm with microns but I can only read what is written so it's hard to sort out.  

 

Finally, based on your last comment concerning heat loss, it doesn't look like you understand the concept of heat transfer.  The amount of heat contained within a system is far less important than the rate of heat transfer.  Any system in equilibrium has zero net heat loss but it may still have significant heat transfer, which is what determines temperature differences.  Heat transfer is the important thing!

 

2)  "Hmm ... OK, great to know that! That means that the maximum allowable expansion for the tube to remain in the CFZ would be (291microns / 25 = 11) microns, yes?"  Normally, we talk about the tolerance about the perfect focus point so the way that I would say this is that the tube has to maintain it's length to within approximately +/- 5.8 microns for the system to remain in focus.  I say approximately because the formula that I gave you off the top of my head loses accuracy as the system gets faster.  It is however, plenty good enough for a F/10.8 system.

 

3)  The data that you reproduced from the IFI site is from a carbon fiber, truss mounted RC telescope.  This telescope does not have an aluminum tube.  Gaston and I have discussed this data at great length and there are a lot of issues with this telescope that don't apply to the C14 that you are trying to measure.  I can tell you more about it but it's best that you learn the basics first and forget about this data for now.  It simply does not apply to what you are doing.

 

My suggestion is that before you go any further, you should first read up on:

1)  The thermal properties of materials,  [I don't have a recommendation for a reference off the top of my head]

2)  Heat exchange.  Here are some good references:

 

Read and understand all three chapters here and you'll be in really good shape.  These are superb!

http://cecs.wright.e...htchapter01.pdf

http://cecs.wright.e...htchapter02.pdf

http://cecs.wright.e...htchapter03.pdf

 

These are class notes derived from the above reference (I'd like to know what book this is from!):

http://cecs.wright.e...tureslides.html

 

Read this material first and then we can discuss this stuff without having to rehash all the basics.

 

John

 

 

EDIT:  I just realized that the folks at Wright State have been kind enough to put all of the materials for their heat transfer course on line.  This stuff is excellent!   So if you really want to understand heat transfer, here's the reading, the homework, the lecture notes, handouts, answers to the problems...the whole thing.  You can find it here:  http://cecs.wright.e...attransfer.html.  This is the starting point for understanding how to compute temperature drops due to radiative cooling and the data that I presented above.  It's also where you start to compare the different methods of keeping the front element free of dew.  If I still had access to a PhaseCam, it would be very interesting to look at tube currents directly using different heating methods.  Unfortunately, since 4D has been sold to Nanometrics, that opportunity has pretty much evaporated.


Edited by jhayes_tucson, 20 December 2018 - 09:54 AM.


#120 choward94002

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Posted 20 December 2018 - 10:01 AM



Please forgive me, but I believe that you might be in over your head with this stuff, but let me try to clarify a few things.

 

No offense taken, hasn't been the first time I've heard that exact statement and won't be the last time either ... but that just means there's more to learn and understand smile.gif

 

1)  I can't make any sense out of this statement:  "Since...the tube is all aluminum (homogeneous) then we know that assuming the temperature change is the same for all sections of the tube then each section will move by a proportional amount; if the length of the tube is 1000cm and the formula says it will expand 1cm per 10C then we can infer that every 10cm of tube will expand 10mm per 10C, or 1mm per 1C.  That's assuming a perfectly even temperature change across all sections with no loss of heat to the system."

 

That was just an example to keep the math simple and hopefully get the concept across of what I was trying to observe (hysteresis the thermal expansion and how much expansion before I hit my target point at the end) ... obviously aluminum doesn't behave that way ... maybe taffy ... smile.gif

 

The coefficient of thermal expansion for aluminum is 23.6 um/m-C.  So a 10 m tube will expand by 236 um per degree C and 2360 microns (2.36 mm) per 10C; but, why are we talking about a 10 m tube?  As I've explained, the C14 tube is roughly 0.59m long and it will change length by 13.92 microns per degree C with a uniform temperature change. 

 

Yep, that's the number that you and ForeSight calculated and I take as accurate ... I want to make sure that my C14 tube doesn't have any other characteristics that might throw that off (like a non-homogeneous composition or a thermal barrier somewhere) but I'm using that number as the "right one" for delta over time.  I'd like to get some empirical data to match that, but that might not be possible ... 

 

Finally, based on your last comment concerning heat loss, it doesn't look like you understand the concept of heat transfer.  The amount of heat contained within a system is far less important than the rate of heat transfer.  Any system in equilibrium has zero net heat loss but it may still have significant heat transfer, which is what determines temperature differences.  Heat transfer is the important thing!

 

Yes and no; I think I've been unclear as to what my experiment is measuring ... I'm not calculating the change for the entire system due to temperature (you and a lot of others have done that many, many times in the past, really well characterized by now), rather I'm calculating the rate of change as the system reaches thermal equilibrium ... so, when I apply heat to one end it isn't instantly distributed across the entire tube mass, it takes some time for the heat to travel across the tube and reach equilibrium.  By monitoring the temperature at discrete intervals down the tube I can gain a sense for how that happens ... is it an instantaneous slow change across the entire tube, like a tube made of copper? [no]  Is the change rapid, moving like a wave in a few minutes? [no, by the time the point hit the target temperature only about a third of the tube had changed by a measurable amount that was outside of the sensor error bars]

 

2)  "Hmm ... OK, great to know that! That means that the maximum allowable expansion for the tube to remain in the CFZ would be (291microns / 25 = 11) microns, yes?"  Normally, we talk about the tolerance about the perfect focus point so the way that I would say this is that the tube has to maintain it's length to within approximately +/- 5.8 microns for the system to remain in focus.  I say approximately because the formula that I gave you off the top of my head loses accuracy as the system gets faster.  It is however, plenty good enough for a F/10.8 system.

 

Hmm ... in the discussion that the ForeSight folks had on CN, they stated they were working with a CFZ of 143 microns due to seeing giving them some "wiggle room" ... so, my calculation had been that if the zone is 143 microns across and I started to "move the zone" (depth of focus, as you corrected me) then I would see what the Dickerson graph showed, that I can change the length of the tube by up to 70 microns without leaving that 143 micron CFZ and needing to refocus which is a lot more than your 5.8 microns you stated ... what am I missing?

 

I also wasn't aware that the expansion rate was nonlinear ... the observed rate of expansion over time is nonlinear (as each section expands it pushes the expanding section next to it) but the actual expansion rate seemed linear ... I'll research that more, thx!

 

3)  The data that you reproduced from the IFI site is from a carbon fiber, truss mounted RC telescope.  This telescope does not have an aluminum tube.  Gaston and I have discussed this data at great length and there are a lot of issues with this telescope that don't apply to the C14 that you are trying to measure.  I can tell you more about it but it's best that you learn the basics first and forget about this data for now.  It simply does not apply to what you are doing.

 

Ah, didn't know that!  (See, that's why it's important to have a legend on your graph!) ... OK, data forgotten about ... smile.gif

 

My suggestion is that before you go any further, you should first read up on:

1)  The thermal properties of materials,  [I don't have a recommendation for a reference off the top of my head]

2)  Heat exchange.  Here are some good references:

 

Read and understand all three chapters here and you'll be in really good shape.  These are superb!

http://cecs.wright.e...htchapter01.pdf

http://cecs.wright.e...htchapter02.pdf

http://cecs.wright.e...htchapter03.pdf

 

These are class notes derived from the above reference (I'd like to know what book this is from!):

http://cecs.wright.e...tureslides.html

 

Read this material first and then we can discuss this stuff without having to rehash all the basics.

 

Great materials, thanks!  My plan is to automate my testing cycle so that I can get about 50 or experiment datasets and do some scatterplots of the output, hopefully that will let me get some idea about the error in the measurements to understand why my numbers are 6x off from what the equations predict ... so these are excellent materials to get familiar with as that grinds away! smile.gif

 

John


Edited by choward94002, 20 December 2018 - 10:06 AM.


#121 jhayes_tucson

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Posted 20 December 2018 - 11:05 AM

"Hmm ... in the discussion that the ForeSight folks had on CN, they stated they were working with a CFZ of 143 microns due to seeing giving them some "wiggle room" ... so, my calculation had been that if the zone is 143 microns across and I started to "move the zone" (depth of focus, as you corrected me) then I would see what the Dickerson graph showed, that I can change the length of the tube by up to 70 microns without leaving that 143 micron CFZ and needing to refocus which is a lot more than your 5.8 microns you stated ... what am I missing?"

 

What you are missing is what I keep telling you.  Let me break it down:  The change in the tube length merely changes the spacing between the elements but in a Cassegrain configuration, that doesn't move the focal point by the same distance.  The location of the focal point moves by the square of the secondary magnification factor times the change in spacing.  So if the tube length changes by 'x', the focal point moves by roughly 25x (in a Celestron, since the optical magnification is about 5x.)  Changing the spacing (i.e. the tube length) by 5.820 microns moves the focal point by about 145.5 microns.  That's half the depth of focus (CFZ) for a F/10.8 system.  Changing the tube length by 70 microns will shift the focal plane by 1,750 microns = 1.75 mm!  That is WAY outside the CFZ tolerance and the image will be completely out of focus.

 

Seeing can increase the size of the CFZ; however, under good conditions (~ 1" seeing) using the diffraction based CFZ value is still a good number to use for tolerancing.

 

John

 

 

 

 

PS  This is one reason (there are others) that a refractor may be less sensitive to thermal changes.  The change in tube dimensions due to temperature change directly translates into how far the image moves.  In a refractor, it's the sensitivity of the refractive index in the objective elements that is more important in determining the thermal stability of the system.



#122 choward94002

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Posted 20 December 2018 - 11:26 AM

Ah ... (face palm) ... you're right, the tube length change will be magnified by the magnification of the system, so that's where you get the 11 micron "tube max length movement" number (which would actually put me right at the edge of the CFZ) ... oy ...

 

That's way beyond my ability to measure using my thermocouple approach, so I'll have to defer that until I get to the focus testing experiments, for now I'll just focus on characterizing the thermal changes in the OTA as I apply point heat to the nose ...

 

Thanks for clarifying that! :)


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