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Insulation (airspace) layer for under Reflectix

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#76 choward94002

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Posted 15 December 2018 - 08:18 PM

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

 

Thanks, I really appreciate your and others feedback!

 

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

 

The surface temperatures were measured by the sensor being bedded into thermal contact grease [http://www.ti.com/product/LMT70] "gooped" onto the glass/ can skin, same as I use between the peltier "hot side" and the heat sink [https://www.amazon.com/ARCTIC-Freezer-Xtreme-Twin-Tower-Heatsink/dp/B001X017G2/ref=sr_1_16?ie=UTF8&qid=1544920160&sr=8-16&keywords=cpu+cooling+tower+heat+pipe] .. the peltier "cool side" uses a thermal conductive pad [https://www.amazon.com/Arctic-4237-Efficient-Conductivity-Handling/dp/B00UYTTLI4/ref=sr_1_3?ie=UTF8&qid=1544920267&sr=8-3&keywords=thermal+conductive+pad] between it and the can bottom (same that I use for cooling my D5100) ... the air sensors were simply free floating ... what would be a better way to get the temperatures?  I wasn't able to find any references to a "black sensor", does that refer to the coloration it should have or is there a part I need to find?

 

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

 

I placed the cans about 10ft apart from each other in the middle of an open area with about 100ft from the nearest trees, with the horizon being no less than 45 deg altitude from any can there ... I was also careful to place the wind screens far enough away so that I still had a 55 deg horizon from the can to the top of the windscreen for just that reason.  I don't image at altitudes less than 60deg due to airmass, so for me that was an acceptable "cone".  Note that the cans didn't have any kind of dew shield, just a flat sheet lying across the top ... dew shield testing is my next step, already in process ...

 

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

 

Yep, I use this sensor already to gather temperature and humidity levels [https://www.mouser.c...hdc1080-sensor/] to feed into the RaspPi to do the dewpoint calcuations already both inside the OTA as well as next to the corrector plate in the OTA ... looking back at my wall of text I (apparently) didn't mention that I put in this kind of sensor outside the can (2x), inside next to the plate glass 2" away from the top and 2" away from the side (4x) as well as next to the high precision temp sensors next to the weight/ thermal mass (4x) this to track the humidity ... would there be other places to put this for the experiment? 

 

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

 

Sure, I've got that data for inside OTA and outside can temps, RH's inside the can next to the glass and outside the can during the runs of the experiments!

 

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

 

(smile) You can repurpose trash cans like I did and be the new talk of the neighborhood!

 

Looking at my data, I am not seeing drops on my plate glass that I can attribute to the radiative cooling of more than about 2-3C (the outside glass sensor tracks almost precisely with the ambient air temperature, and the inside glass sensor tracks very closely with the interior OTA air temperature less about a 20% differential (so, if the outside air temp is 5C and the interior OTA temp is 10C, the outside glass sensor will read 5C and the inside glass sensor will read 8.5C which makes sense as glass is a thermal insulator) ... you probably has a smaller differential because you're running tempest fans to help stabilize the internal/ external air temps ...

 

Later this week I'm going to be running some more rigorous experiments with the cans to experiment with dew shields ... first, the can interiors right now are 'kinda shiny inside and that's not how a real OTA is inside so I've put down some coats of Rustoleum High Temp black paint on the interior of the cans to simulate the black interior of an OTA (even though I know that's going to dramatically increase the internal radiative cooling from inside the OTA through the glass).  Second I've made four sets of dew shield mockups with some Reflectix at various lengths (I bought the 100ft roll, so I have a TON of the stuff here) with the shields at various lengths (I've cut some for 6", 8" and 10" ... that's about as long as I want to go without it becoming a sail) and have spray-adhesived Reynolds Wrap to two of the sets ... they will be velcro'ed on to the plate glass surface but should approximate a "dew shield".  I've also spray-adhesived some black felt to two of the sets (one of the Reynolds wrap and one of the bare ones) so I can test to see how a black interior affects things ...

 

John

One of the things I'm excited about trying is the focusing ... you write a nice treatise several years ago here about the effect of temperature changes in the mirror on the nominal focus zone, and as I recall you found that very VERY slight changes in the mirror temperature would result in noticeable changes in the focus zone ... so the discovery of this "protective inversion layer" redarding mirror temperature changes is quite fascinating for me.  I have no way to duplicate your testing regiment you used back then, so I'm going to be taking pictures of airy disks of target stars throughout the night with both a "protected" and an "unprotected" OTA and see how the focus changes for them ...



#77 choward94002

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Posted 15 December 2018 - 08:43 PM

 

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

 

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

 

... I'll go with the super-developed, intuitive mind at work ... :)

 

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

 

That's a really good idea, thanks for that!  Gasket on the corrector, gasket on the secondary should fix capillary ingress and give me a vapor barrier to boot, woohoo!

 

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

 

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

 

After you made your comments a few weeks back in a different thread I went back and researched your postings over the years here on CN and came across your idea for that experiment, it's what gave me the idea to mock up my garbage cans in the first place ... so again, thanks for that too!

 

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

 

It's easy to get a sheet of 1/4" float, much harder to get a block of 1" float (although I could probably use some glass block down at Home Depot) that would be interesting to put a sensor on it and see how that changes through the experiment ... the "strings" that you mention were just there to hold the temperature sensors, I had no way to detecting air currents (and had better not have air currents in the closed system anyway) ...

 

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

 

I thought about that when I was putting together my experiment ... on the one hand a mirror has thermal mass, but on the other hand it's nowhere near as conductive as aluminum so once it eventually gets heated/ cooled then it basically will stay heated/ cooled.  That's likely one reason that the inversion layer surrounding the mirror lasts so long ... even though the surrounding cell is going up and down in relation to outside ambient temperature, the mirror temperature itself stays relatively fixed.  There also isn't really a good conductive attachment point from the mirror to the cell (the baffle tube is phenolic plastic and the only metal is the focusing arm) so it's mainly being heated/ cooled by the surrounding air which has been heated/ cooled by the mirror cell ... in the end I decided to address this issue with a later experiment, for now I just wanted to see if I could minimize radiative cooling from the OTA tube and OTA skin cooling (look for a "frosty scope" pix from a few days ago to see what that looks like ...)

 

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

 

When I get to the "test the focus" phase I'm going to be setting my focus at the beginning of the experiment and take Airy disk pix during the duration of the evening; I anticipate seeing the disks getting worse and worse as the temperature changes the nominal focus zone as John Hayes wrote about a few years ago, I'm hoping that I can correlate those changes to the temperatures of the airmass immediately surrounding the mirror standin/ glass block, as I now know from my experiments that it's easiest to keep that airmass at a fixed temperature by using the peltiers and a Reynolds Wrap OTA cover ...

 

Enough from me for now... wink.gif

 

Keep the comments/ ideas coming, so far you've given me a) cooled mirror cell [brilliant!], trash can experiment genesis [brilliant] and neoprene corrector plate gaskets [brilliant!] ... as always, much thanks!



#78 Sarkikos

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Posted 15 December 2018 - 09:27 PM

 

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

 

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

Please don't call me "post modern." shocked.gif  My comment was meant to be a semi-serious criticism of the distinction between empirical evidence and anecdotal experience, as well as a jab at post-Modernism.  I detest post-Modern philosophy.  It is a self-hating hot mess of nihilism which enshrines ad hominem fallacies as the key to understanding history, literature, society and the universe.  Post-Modern philosphers tend to see all knowledge as biased and anecdotal.  Except, of course, their own deconstruction and criticism of the knowledge posited by others.  Strange how it works out that way, isn't it?  thinking1.gif

 

If anything, I like to approach my own experiences phenomologically.  I accept them as experiences.  If I'm not certain about the cause(s) of a particular phenomenon, I suspend judgment.  You won't find me assuming that a strange light in the sky is an ET, or a glow in the dark is a ghost.  Though I have seen and heard a few things that others thought were ETs and ghosts when I told them about it.

 

I have no opinion one way or another about post-Modern art.   Art is a matter of taste.  Knowledge shouldn't be.  grin.gif

 

Mike


Edited by Sarkikos, 15 December 2018 - 09:39 PM.


#79 Kokatha man

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Posted 15 December 2018 - 11:35 PM

Well, I had to reply simply to assuage your (possible) notion that I was calling you "post-modern" in any of your comments etc Mike! lol.gif shameonyou.gif

 

...the "that" was not referring to you, it was aimed at the gaggle of characters & their beliefs I (humorously) mentioned, which they either subscribe to or are complicit in...more pointedly their wholesale appropriation of some concept etc in a kind of "de rigeur" fervour...along with my distaste for such, I also detest "reality" television...substitution of gravity or substance with mindless distraction...etc, etc... blahblah.gif  

 

I actually eschew most philosophies that, whether by intent or ideas "categorise" - but it certainly seems as if aspects of p-m really stick in your craw...& I can appreciate that! wink.gif

 

As to p-m philosophy specifically in art...well, I won't say anything about that here! lol.gif

 

But knowledge shouldn't be about "taste" regardless of taste's broadest interpretation - that postings was a rather muddy humorous poke about "taste" tbh. smile.gif

 

Trying to now keep (vaguely) within the parameters of this thread's subject matter, I'd like to respond to Howard's most recent posting, but will do so later: I'm off to the coffee-shop with Pat to grab one & "sit & watch the river go by" just around the corner. smile.gif


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#80 jhayes_tucson

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Posted 16 December 2018 - 02:46 AM

One of the things I'm excited about trying is the focusing ... you write a nice treatise several years ago here about the effect of temperature changes in the mirror on the nominal focus zone, and as I recall you found that very VERY slight changes in the mirror temperature would result in noticeable changes in the focus zone ... so the discovery of this "protective inversion layer" redarding mirror temperature changes is quite fascinating for me.  I have no way to duplicate your testing regiment you used back then, so I'm going to be taking pictures of airy disks of target stars throughout the night with both a "protected" and an "unprotected" OTA and see how the focus changes for them ...

 

First, a couple of comments about your experiment:

 

1)  Just understand that a ~45 degree horizon is like a short dew shield.  I can handle that in my model but realize that that kind of horizon can actually help lower the possibility of dew!

 

2)  Thermal grease is fine, but it would be better if you paint the sensor black and put it in thermal grease against the glass.  The thermocouples that I use are shiny and that's really no good so I paint them flat black.  Remember that your pieces of glass is opaque and black at 10 microns!   You are trying to measure radiative cooling so painting your sensor black makes sure that the thermometer sees the same thing as the glass under the sky.

 

3)  You are trying to measure relatively small temperature differences so to get good results, you need to calibrate all of your sensors so that they read the same when they are at the same temperature--over the expected temperature range of the measurement.  This is pretty easy to do and it's important so that you can put error bars on your results.

 

4)  Finally, my thermal stability study didn't assume any change in the shape of the mirrors.  It was all about the first order changes in OTA dimensions with respect to temperature.  Yes, I did show that if you use borosilicate glass (which accounts for the dimensional changes in the first order properties) the results agree almost perfectly with a simple, first order model with the assumption that the mirrors have a zero expansion coefficient.  The experiment that you have in mind will be difficult to do properly.  You'll need to outfit the OTA with a lot of thermocouples to correlate temperature change with focus shift but if you do it right, it will agree with the numbers that I posted.

 

John



#81 Kokatha man

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Posted 16 December 2018 - 04:33 AM

...back again - the river (Onkaparinga) & the coffee were both nice, as always. waytogo.gif

 

The neoprene gasket is pretty thin - I went there a few years back with the idea of getting the thinnest material available but decided upon something a little thicker...thinnest was just too difficult to handle: they cut them on a type of CNC pantograph while you wait.

 

It certainly stops water ingress when I wash my corrector on the outside - spray bottle of distilled water liberally applied...leave a couple of minutes to loosen anything...another very liberal spray/sluicing...then careful radial cotton swab strokes with very saturated Dr Clay formula...another uber-sluicing with the spray bottle of distilled water & then air-nozzle blow-dry...always with a high quality water/oil trap inline from the compressor & not while it is pumping.

 

When I blow out around the outer edges of the retaining ring no moisture gets under & inside the corrector like it used to do before I added the gasket.

 

All this with the scope on mount & pointing down at a steep angle...I haven't experienced any issues around the secondary housing seal btw...the metal retainer ring/corrector plate lip on the ota stuck to that face of the gasket after a couple of removals & replacement of the corrector, so now I don't have to worry about re-positioning a flimsy component anymore when I do take the corrector off...

 

Btw, I didn't really think the string would flap in the ota Howard bigshock.gif lol.gif  - those would've been some air currents for that, but when you spoke of "thermals" I automatically think of ensuing air currents...

 

I haven't thought anymore about any effect (if any) the primary might have on the overall equation, in particular your observation on temperature differences directly between the inside & outside of the  black ota - mine's black btw...several different reference sources make the point that: <"If the temperature differences are small and there are no winds or drafts, transfer of heat to/from air/glass will be by conduction. Heat transfer by radiation will be trivial.">

 

I'll have to cogitate on this aspect to see if I can devise any way I might "incorporate" -or more likely "invent" lol.gif this into some sort of rationale! But I'd best be careful...I jump on others when they do that, the old hypocrite I am! blush.gif   

 

On the matter of focus shifts, I can appreciate temperature playing some role in the whole equation - but to my mind (backed by continuous data) this is quite trivial in comparison to what I'll define as "atmospherics."

 

Pat (my co-pilot) keeps a log of all focus positions during all/every imaging sessions, so I'm speaking with a wealth of data (hers! wink.gif ) over many years now: the "general rule of thumb" is that focus shifts inwards as each planet rises & then outwards as it begins its descent each night...having an accurate digital readout makes this data easy to collate & if I can be so bold, I think perhaps my strongest forte is knowing where optimum focus occurs at any single point in time...

 

I have made commentary quite often about focus indicators at very long f/ls for folks in the SSI forum: eg the finest filament tendrils of festoons on Jove...simply the "blackest" appearance of the Cassini Division on Saturn...NP brightening on Uranus & the edge sharpness & overall brightness of Neptune &, when seeing is uber-high quality, storm spots can be discerned in the onscreen feed...etc, etc.

 

As long as the seeing cooperates, 2 steps of the DRO varies the focus from "just off" to "razor sharp" - this equates to a 1/2 micron movement of the focuser...

 

Without doing too much investigation as to precisely "why" I have accepted that this is something to do with atmospheric diffraction...I don't believe temperature plays very much at all in this because there are plenty of occasions when the "general rule of thumb" doesn't transpire - & focus remains static over long periods during the night despite falling temperatures in the order of 10°C or more...both our DRO's have focuser temperature readouts I should add, with the newest one possessing both an internal & external probe & of course we are regularly checking the primary & outside air temps also...

 

And there are plenty of other anomalies re focus shifting which I also throw into that (admittedly) rather vague "atmospherics" influences - here is a link to an image on our website where you can see an horrific change of over 500 steps (125 microns) over the course of 26 minutes re the red filter captures!

http://momilika.net/...-35_rgb_dpm.png

 

These occasions are not rare but thankfully not uber-regular: seeing is undoubtedly a factor there although it does not necessarily mean it is such that hi-res results cannot be evinced.

 

Anyway, enough for now...apologies if some of this is "old hat" or if I sound a bit didactic but it isn't meant to sound patronising...it's just that this stuff really is a constant "in our face" aspect of our imaging where I am as intense & scrutinising as any characteristics I adopted when I was at my most creative. (artistically-speaking) bigshock.gif lol.gif bounce.gif



#82 choward94002

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Posted 16 December 2018 - 02:02 PM

First, a couple of comments about your experiment:

 

Great, I really appreciate your taking the time to help make my experiment more useful/ meaningful!

 

1)  Just understand that a ~45 degree horizon is like a short dew shield.  I can handle that in my model but realize that that kind of horizon can actually help lower the possibility of dew!

 

That's true, I'm essentially masking off that area of the sky and in actuality I'm currently masking off more as a portion of that arc is also occluded by the wind screens on one side at 55 degrees ... 

 

2)  Thermal grease is fine, but it would be better if you paint the sensor black and put it in thermal grease against the glass.  The thermocouples that I use are shiny and that's really no good so I paint them flat black.  Remember that your pieces of glass is opaque and black at 10 microns!   You are trying to measure radiative cooling so painting your sensor black makes sure that the thermometer sees the same thing as the glass under the sky.

 

Got it, I'll hit them with the same black Rustoleum High Temp paint, thx!

 

3)  You are trying to measure relatively small temperature differences so to get good results, you need to calibrate all of your sensors so that they read the same when they are at the same temperature--over the expected temperature range of the measurement.  This is pretty easy to do and it's important so that you can put error bars on your results.

 

Yep here also ... I checked all of the temp sensors early on by putting them into my dark refrigerator and letting them acclimate for about 15min and trashing any sensor that was more than 5% off from the average for the low precision sensors, 1% off for the high precision ones but that was only at one temperature, you're correct that I should check them at both ends of the range ... thx again!

 

4)  Finally, my thermal stability study didn't assume any change in the shape of the mirrors.  It was all about the first order changes in OTA dimensions with respect to temperature.  Yes, I did show that if you use borosilicate glass (which accounts for the dimensional changes in the first order properties) the results agree almost perfectly with a simple, first order model with the assumption that the mirrors have a zero expansion coefficient.  The experiment that you have in mind will be difficult to do properly.  You'll need to outfit the OTA with a lot of thermocouples to correlate temperature change with focus shift but if you do it right, it will agree with the numbers that I posted.

 

(smile) I don't have THAT many thermocouples!  My thinking on the focusing once I get to it was to essentially replicate what Kokatha does every night observationally ... focus on a know object and see if stays "looking better" over time and by how much.  I think that by getting the FWHM (maybe even an airy disk) of several known stars and turning that into an average quality metric (to account for stray atmospherics on one star) I should be able to track focus changes over time due to your mirror expansion/ contraction work.  Since I can train multiple scopes on the same object at the same time, by keeping the focus position constant (I use FeatherTouch/ MicroTouch focusers and focus through FocusMax) once it's gotten to proper focus on the scopes the only cause for focus changes would be from internal temperature changes of the mirror, as all scopes would see the same changes due to airmass and atmospherics.  The temperature of the mirror (should) be driven by the temperature of the surrounding cold air inversion layer ... it has a lot of thermal inertia, but the changes you were talking about were are at the micron level so I should see *some* changes.  Empirically we already know we have to refocus when we have temperature swings, so we know those changes occur.  

 

So, the goal of the inversion layer/ focus experiments when I do them will be threefold:

 

- Does the existence of a cold "inversion layer" (defined as an airmass at the same temperature as the mirror cell/ mirror after they have been cooled to a setpoint of 5C lower than the forecast low for the evening) alter the focal change over time?  If I compare a scope that does not have an inversion layer to a scope that does have an inversion layer, will I have observable differences in the change in FWHM over time between the scopes (hopefully even an airy disk change)?  If so, what is the slope of those changes?

- What is the effect of different angles of tilt on the OTA to maintaining this inversion layer?  I don't always image directly up at the zenith :) so the vertical position of the OTA will change (I don't image lower than 50deg, so that gives me a lower bound) and like a cup of water the inversion layer should slosh around as the OTA goes off of vertical.  Obviously at 90deg (horizontal) the inversion layer is unmaintainable, is it maintainable at my lowest 50deg position?  What is the level of peltier cooling needed to maintain it at the various angles of tilt and how long does it take to effect a change? (That's important for programming my thermal control RaspPi for the peltier's)

- What will be the effect on the inversion layer and focus once I start powering up the "hot tube" system?  That should be interesting ... the OTA is a closed system, so the only source of convective mixing is from the skin currents off the tube and the glass ... after the results from my dew shield experiments are in and coupled with the results from my Reynolds Wrap experiment I should have almost no convective mixing in the OTA, so like a bad afternoon in LA I should have two distinct non mixing thermal regions in the OTA, giving me (hopefully) an environment that's above dewpoint internally and above dewpoint on the corrector plate ("hot tube") yet have the mirror nestled in a Kokatha cold layer minimizing mirror expansion and refocusing ...

 

First though I need to get the cans to model more closely to my OTA's ... I've already painted the interior to a flat black and modeling it off of a C14 calculated the volume of the primary mirror and of the mirror cell.  I've upped the amount of thermal mass with some barbell weights (coupled with a thermal conduction pad) and picked up some 12x12 glass blocks and cut them down to the same volume with my tile cutter so those two masses are the same (yes, iron isn't aluminum and green float isn't the same as borosilicate but they are pretty comparable for thermal conductivity).  I've also placed the glass block on a sheet of styrofoam to thermally isolate it from the weight; in a real OTA the mirror is also thermally isolated (the focus arm connection is just too small and has a grease barrier in the focuser to isolate that) so like in an OTA it will be cooled entirely from the airmass around the weight/ mirror cell.  The actual airspace in the can is quite a bit larger than that of my OTA so to model the C14 airspace in the can I've gotten more styrofoam sheet and used that to shim around the weights/ block such that the air volume surrounding them is correct by volume and shape to a C14 up to about 18" above the bottom of the can (more than that I'll run into blocking the can interior wall which I have a suspicion will make a difference in the dewshield experiments ... yet glass is opaque to infrared so I shouldn't get radiative cooling off of the can interior but I'm going to run an experiment with two cans wrapped in Reynolds Wrap, one with the black interior and one with Reynolds Wrap for the interior and see if there is a difference.  If the glass really is opaque then there shouldn't be any, but if I'm radiatively cooling through the glass then I'll see it).  I've also made a "baffle tube" analogue in the center using some thinwall PVC pipe going up above the level of the styrofoam ... and will be festooning all of that with more high precision temperature sensors (luckily the RaspPi can handle 256 sensors, I'm going to be close to hitting that mark when I'm done I'm afraid!) ... that should get me closer to a C14 OTA for my next experiments ...

 

... which is first understanding the thermal relationship of the mirror/ glass blank and the Kokatha cold layer around it, then do my dewshield experiments with dewshields of different sizes (scaled to the diameter of the can versus the C14 diameters ... I may also try that with a C11 diameter) to find an optimal height and interior/ exterior material for that ... so those experiments happen later this week ...

 

You had also earlier mentioned checking the humidity levels, likely because an airmass with water vapor in it has to overcome the heat capacity of the water vapor before it makes a change ... so, dry air gets colder faster than moist air.  Good catch, but remember the OTA and the cans are a closed system (I even used weatherstripping on the can lid to seal the glass plates) so I had better not have humidity changes!  I think I'm also safe with the humidity differences of the can interiors because I did my opening and closing of them in the same afternoon so they should all of the airspaces should have the same thermal capacity ... but again, good call and I'll start monitoring humidity levels with my humidity/ temp sensors in the future ...

 

John

Thanks again for your suggestions! :)



#83 choward94002

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Posted 16 December 2018 - 02:32 PM

...back again - the river (Onkaparinga) & the coffee were both nice, as always. waytogo.gif

 

Most excellent!  Always be able to take a step back from things and just relax ...

 

The neoprene gasket is pretty thin - I went there a few years back with the idea of getting the thinnest material available but decided upon something a little thicker...thinnest was just too difficult to handle: they cut them on a type of CNC pantograph while you wait.

 

It certainly stops water ingress when I wash my corrector on the outside - spray bottle of distilled water liberally applied...leave a couple of minutes to loosen anything...another very liberal spray/sluicing...then careful radial cotton swab strokes with very saturated Dr Clay formula...another uber-sluicing with the spray bottle of distilled water & then air-nozzle blow-dry...always with a high quality water/oil trap inline from the compressor & not while it is pumping.

 

When I blow out around the outer edges of the retaining ring no moisture gets under & inside the corrector like it used to do before I added the gasket.

 

All this with the scope on mount & pointing down at a steep angle...I haven't experienced any issues around the secondary housing seal btw...the metal retainer ring/corrector plate lip on the ota stuck to that face of the gasket after a couple of removals & replacement of the corrector, so now I don't have to worry about re-positioning a flimsy component anymore when I do take the corrector off...

 

I like the neoprene a LOT ... it solves a lot of problems for me; water ingress from capillary action not only from dew (preventable) but also from snow/ rain storms (my scopes are at a mainly-remote site out in the open behind windscreens but otherwise exposed to the elements ... I have an alert setup through another RaspPi getting data from a SkyAlert to rotate the OTA's to a nose-down orientation if there's precipitation/ increase in humidity above 80%, but there are occasional "dry" snow flurries that come in without triggering that and my OTA's get dusted.  By having the neoprene there I'm not going to sweat those flurries as much.  In addition I have to replace the dessicant packs on those OTA's about once every three weeks just due to water vapor exchange through those gaps which is a major hassle ... the neoprene should cut down that exchange dramatically, meaning maybe I only have to replace the dessicant packs every few months ... oh, did I mention that I like the neoprene a LOT!  Remind me to buy you a pint for that! smile.gif

 

Btw, I didn't really think the string would flap in the ota Howard bigshock.gif lol.gif  - those would've been some air currents for that, but when you spoke of "thermals" I automatically think of ensuing air currents...

(smile) That made me think of the sensors jangling like wind chimes ... but folks who run tempest fans or the insertable OTA fans probably DO have currents like that!

 

I haven't thought anymore about any effect (if any) the primary might have on the overall equation, in particular your observation on temperature differences directly between the inside & outside of the  black ota - mine's black btw...several different reference sources make the point that: <"If the temperature differences are small and there are no winds or drafts, transfer of heat to/from air/glass will be by conduction. Heat transfer by radiation will be trivial.">

 

One of my next experiments is going to be to determine just what the relationship is between the mirror cell, the inversion layer airspace (which I'm calling the "Kokatha layer" now, just 'cause it's easier to type) and the mirror mass itself.  The mirror mass on the C14 is thermally isolated from the mirror cell (it's potted onto an aluminum "star" which then rides on a phenolic tube recall ... and the grease between the "star" and tube is thermally isolated with grease.  The focus arm connection is also thermally isolated from the mirror cell by the focuser grease) so really it's just getting cooled down by the airmass heat exchange ...

 

Pat (my co-pilot) keeps a log of all focus positions during all/every imaging sessions, so I'm speaking with a wealth of data (hers! wink.gif ) over many years now: the "general rule of thumb" is that focus shifts inwards as each planet rises & then outwards as it begins its descent each night...having an accurate digital readout makes this data easy to collate & if I can be so bold, I think perhaps my strongest forte is knowing where optimum focus occurs at any single point in time...

 

I have made commentary quite often about focus indicators at very long f/ls for folks in the SSI forum: eg the finest filament tendrils of festoons on Jove...simply the "blackest" appearance of the Cassini Division on Saturn...NP brightening on Uranus & the edge sharpness & overall brightness of Neptune &, when seeing is uber-high quality, storm spots can be discerned in the onscreen feed...etc, etc.

 

As long as the seeing cooperates, 2 steps of the DRO varies the focus from "just off" to "razor sharp" - this equates to a 1/2 micron movement of the focuser...

 

Without doing too much investigation as to precisely "why" I have accepted that this is something to do with atmospheric diffraction...I don't believe temperature plays very much at all in this because there are plenty of occasions when the "general rule of thumb" doesn't transpire - & focus remains static over long periods during the night despite falling temperatures in the order of 10°C or more...both our DRO's have focuser temperature readouts I should add, with the newest one possessing both an internal & external probe & of course we are regularly checking the primary & outside air temps also...

 

That's REALLY useful info to know ... and I would agree that since you're viewing through such a large arc you're hitting a lot of atmospheric changes ... diffraction, airmass sizes, cloud strata, etc.  I only image in an arc 50deg elevation so I'm much less affected by those vagarities, but again your info here will be really useful when I'm trying to interpret my focus experiment results ... so I owe you another pint!

 

And there are plenty of other anomalies re focus shifting which I also throw into that (admittedly) rather vague "atmospherics" influences - here is a link to an image on our website where you can see an horrific change of over 500 steps (125 microns) over the course of 26 minutes re the red filter captures!

http://momilika.net/...-35_rgb_dpm.png

 

These occasions are not rare but thankfully not uber-regular: seeing is undoubtedly a factor there although it does not necessarily mean it is such that hi-res results cannot be evinced.

 

Anyway, enough for now...apologies if some of this is "old hat" or if I sound a bit didactic but it isn't meant to sound patronising...it's just that this stuff really is a constant "in our face" aspect of our imaging where I am as intense & scrutinising as any characteristics I adopted when I was at my most creative. (artistically-speaking) bigshock.gif lol.gif bounce.gif

 

On the contrary, while all of this info might be "old hat" to you it's all new and fascinating for me, and I'm absorbing this new info like dry sponge ... keep it coming!  I am of the feeling that in any system (imaging, visual seeing, diving, flying, whatever) there are elements that we can't control (the weather, satellites, etc.) and elements that we CAN control (tracking, guiding, collimation, focusing, etc.) and we should try our darndest to understand and adjust those things we CAN control and hope we don't get too many things that we CAN'T control.  By better understanding how you're doing what you're doing I can better understand how to control the quality of what I'm doing which to my mind makes perfect sense ... why wouldn't I want to improve something that I CAN control?  So, keep it up!  The movie is really quite revealing too, oy!  That kind of focus shift would convert my nice beautiful blue supergiant clusters in M33 into little bluish blobs, or the nice crisp dust lanes in the Cigar galaxy into little worm squiggles ...


Edited by choward94002, 16 December 2018 - 02:34 PM.


#84 Kokatha man

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Posted 16 December 2018 - 06:18 PM

...just a quickie - those focus shifts on ascent & descent (the "general rule of thumb") are only over a relatively short period of any planet's nightly course: our usual practice is to only image a specific target from approx. 1.5 hours before, till no more than that time after it reaches culmination/transit.

 

For some strange reason it is often the case that ascending is better than descending for outcomes...although I wouldn't say that pattern has enough statistical frequency to include it in any of our "general rules of thumb." ;)



#85 choward94002

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Posted 17 December 2018 - 12:08 AM

...just a quickie - those focus shifts on ascent & descent (the "general rule of thumb") are only over a relatively short period of any planet's nightly course: our usual practice is to only image a specific target from approx. 1.5 hours before, till no more than that time after it reaches culmination/transit.

 

For some strange reason it is often the case that ascending is better than descending for outcomes...although I wouldn't say that pattern has enough statistical frequency to include it in any of our "general rules of thumb." wink.gif

 Hmm ... OK, so your "culmination/ transit" is when the object is at zenith, yes?  So you would be imaging at +/- 25deg off zenith, yes?  That would make sense, airmass starts to become an issue at +/- 30deg from vertical ... looking at a planetarium from Australia, it looks like the ecliptic spends most of it's time right at 70-80deg so your scopes are always basically pointing in a 65deg cone from the horizon (or pretty close to it), yes?

 

Doesn't help to explain the "better going up than down", but that is another datapoint for my experiments on the inversion layer coming up ...

 

Thx!



#86 charlesgeiger

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Posted 17 December 2018 - 12:50 AM

This post has been educational and obviously, very much in the vein of theory and experimentation.  The main things I get from all this are that the optics and tube must be in equilibrium with the ambient temperature, there has to be consideration for dew prevention while monitoring the relative humidity and temperature for dew point while not risking deforming the corrector or other optics, and that insulation might be a good thing on your tube assembly depending on your real environment with consideration for temperature delta.

It seems the bottom line for high contrast detailed planetary imaging is having perfect collimation, perfect focusing, good guiding, and excellent seeing.  Seeing that there are short periods of time when the exposure series takes place, with a camera with large pixels or pixels that match your setup and fast frame rates with high resolution, using the corresponding barlows or powermates needed, that most of the picture data is taken in rapid sessions maybe a few minutes between sessions.

However we do all these things must be based on empirical evidence we ourselves can replicate.

It does seem these posts have expanded greatly over the original poster's questions but then the subject does lead us in many directions.

A couple questions I still have which are peripheral are:

*The neoprene gasket mentioned fits between the corrector and the retaining ring...does one also need a gasket for the back of the corrector where it sits in the cell?  Again, what thickness of these gaskets...similar to the cork originally used?

*If using the ice/salt pack on the back of the mirror cell, do you use some kind of ring on the back of the cell with the tube pointed down to catch the ice in?  Do you use plain ice packs on the mirror cell and do you attach them in some way such as using velcro straps?

*John Hayes; do you have a C14 in Bend, OR and also another in Tucson or New Mexico?  I live in Portland and am retired and thinking to move to Bend for dark sky and good transparency...I would assume that if you are operating your scope out of state you don't think the sky is adequate in Bend area...unsteady sky?  Just interested in your thinking...I have an old C14.

Thanks for all the ideas and inspiration from so many.

Sincerely,

Charlie



#87 choward94002

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Posted 17 December 2018 - 01:34 AM

 

*The neoprene gasket mentioned fits between the corrector and the retaining ring...does one also need a gasket for the back of the corrector where it sits in the cell?  Again, what thickness of these gaskets...similar to the cork originally used?

 

Putting a gasket on the back of the corrector plate as well as the retaining ring side makes sense, doubles your protection ... seeing as my chances of getting a fabricator to cut me a gasket here are zero, here's what I'm ordering up [https://www.grainger...ber-Sheet-2UMY6] and will cut it into short segments to go around the plate ... note that I'll be using the adhesive on the tube/ retaining ring side of the plate so there's nothing on the glass ...

 

 



#88 Kokatha man

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Posted 17 December 2018 - 02:37 AM

Just got home after a long drive sawing up Norfolk Pine material - & I'm a bit like the walking dead atm after a few days of very hard physical labour...I am 69! gramps.gif

 

So with more than a bit of laziness, I'll let you work that out Howard - we're at 35°S & yes, I meant the zenith. (of the particular planet, not as in "directly overhead wink.gif )

 

F'rinstance, at 2:17am in mid-July (UT 15:47) Mars would culminate/reach its highest elevation at 79° - 1.5 hours earlier it is at 69° & 68° 1.5 hours later...that's the arc we would've imaged in. My head says a 43° arc but I'm tired... lol.gif

 

We are very favourably placed currently, but I should state that this focus shift encompasses a lot of widely-variable situations from years gone by with Mars well below 40° & Jupiter similarly-placed.

 

Charlie - you're correct about the wandering from the OP's original focus but I hope he'll indulge me & anyone else: I am hoping to collate a series of piks showing various cooling devices I've trialled over the years as well as some of Pat's specific figures for focus shift - it is an exercise because the focus stats are on the laptop we use to capture with & the other stuff is on a collection of external  HD's stacked here in my studio & I have to plug each one in & go through the files to find what I want...

 

I'll definitely start a new thread for that but hopefully enough people are finding what we all post as worthwhile in a broader sense! smile.gif

 

The only thing I'd say about the parameters is that the "good guiding" one is not necessary...I like the planet to move around on the imaging ROI a bit & because we usually do a lot of our imaging in the field we have "plonk & go" PA where a Wixey (digital inclinometer) & an old surveyor's compass provide all the alignment I want/need.

 

The neoprene gasket: Howard, 1/64" is 0.4mm, considerably more than what I used but I don't see why it should cause any issues. My desire for thinness was to not influence the ability of the corrector as a whole to sit squarely in the ota assemble, ie being able to be compressed unevenly. 0.4mm shouldn't be an issue imo...

 

Charles, it depends upon the setup of your SCT - my Chinese-era C14 has rear vents the same as the Edge. (no mirror locks although I have engineered said & no corrector lens etc as the Edge has)

 

My corrector glass drops into a stepped lip/flange in the ota...there are a series of nylon-tipped spigots (adjustable via allen-key headed screws from the outside of the ota) that centre the glass in this position...

 

The retainer ring itself sits flat on top of the glass & screws into the first (outside) flange - it has a continuous thin layer of cork on the underside of said retainer originally meant to contact the corrector glass. 

 

I was going to scrape this cork off the retainer underside but decided it did no harm or interference if left there...so any/all of the glass's interior face is "water-proofed."

 

Hopefully this (very quick) drawing will assist - I did it without any reference to the scope & it is NOT to scale & the stepped black section of the drawing should actually be drawn longer, that is higher/wider insomuch as it it all part of that (roughly) 3" wide cast flange assembly at the front of the scope that the (thinner) ally ota affixes to also. I hope that is decipherable..! lol.gif

 

We use large orange garbage/trash bags, one inside the other to safeguard against leaks: the scope is usually on the mount facing straight down to the ground.

 

The ice is smashed a bit more in the bag to make sure no large lumps have formed...emptied into one bag, the salt added & mixed in (washing-up gloves are mandatory here!) & then this bag is placed inside the other bag & both are tied off a bit above the ice level.

 

I place a handkerchief-sized piece of cloth over the Moonlite focuser & gently lower the ice bag down onto the scope's rear...carefully ensure the salted ice is arranged relatively uniformly around & on the scope's rear casing (in fact if you tie the bags off in the right spot nearly all of the ice spreads out around the focuser itself to settle in a circular or donut-shaped "pad" around the rear casing.

 

The temperature gauge displays air temp as well as primary mirror temperature - the mirror sensor lead passes through one vent & onto the primary back, the sensor itself being affixed to the mirror back & insulated from the inside air via a  large cap over it filled with silicone. As I said previously, the vents are actually taped shut...

 

The gauge is usually slung over one of the finder adjusting screws to keep it clear of the scope etc when cooling - the scope is also usually wrapped in a doona & pegged close...

 

Hope this clarifies things - boy I need a nap now! lol.gif

 

lol.gif Edit: gotta have a chuckle - I've edited this quick sketch 'cos I went & pulled all the covers off the C14 in the car to check just then...turns out that except for the casting extending further out from the corrector plate everything was pretty right (which I did say earlier in this post wink.gif ) but also (very!) roughly to scale! 

 

Neoprene-Corrector-GasketEdit.jpg

 

 

 

 


Edited by Kokatha man, 17 December 2018 - 05:23 AM.

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#89 choward94002

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Posted 17 December 2018 - 10:48 AM

Good stuff Kokatha, as always!  And as much data as you are willing to provide about temps and focus info will help me greatly to correlate stuff that I will be finding as well as the results from John's focus zone work a few years ago ... and the gasket info is also very helpful, I'm excited to get my sheet in the mail and start cutting!  I have a C14 I'm working on which got molded over when it was stored outside in an unheated shed for a number of years, so I have a perfect test subject for this ... ;)

 

I'm really interested in this right now because I'm currently trying to get L channel data on a setup with a .14"/pixel scale ... and at that level little tiny focus changes really whack things ... and when I'm doing 20min exposures per light those focus changes are enough that was bright and crisp at the beginning of the exposure is now soft and mushy at the end ... so I can go shorter exposures so things stay nice and sharp, but then I don't get enough signal to swamp the read noise ... if I go long enough exposures that I can extract a signal then I'm back to the soft and mushy ending.   If however I can stabilize the focus shifts during the duration of the exposure then I can run as long as guiding will allow and I'd be a very happy camper!  But, i digress ...

 

You've mentioned a few times that you add salt to the water, got it, that lowers the solution temperature from 0C to about -10C ... I also understand that you're cooling the cell down to the forecast low when you're observing less 5C (so if the low is going to be 5C then you're going to be shooting for 0C) ... and you've also mentioned a few times that your OTA is sealed, so there's no air exchange with the outside, got that too ... and I get that you've placed your sensors on the mirror as well as the air temp, cool.  My question is, as you recall how quickly does the mirror start showing a temperature change once you've plopped on the ice, and does it hit a plateau at some point during the cooling? 

 

I ask because the C14 mirror seems to be basically thermally isolated from the mirror cell which just leaves air convection to "do the deed" ... and in a sealed system that process takes awhile to get going; on my "cans" with admittedly a much larger air space it took at least a few hours to get a measurable convection current going.  With your OTA oriented nose down you're basically putting the cold element at the warmest point of the airmass so that should get things circulating faster but I'd still like to know how long it is before you start seeing movement on the mirror sensor?

 

Finally, the mirror itself has quite a bit of thermal inertia and you need to cool the whole thing to get good results ... which means even though the outer surfaces of the mirror hit one temperature, it will take a bit longer before the interior of the mirror gets there ... hence a "plateau".  Once your mirror sensor hits the target temperature, can you immediately "swivel and go" or do you wait for an additional amount of time before it's swivel time?

 

Thx much, this is all really good stuff and quite insightful for how to design my experiments here!



#90 cytan299

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Posted 17 December 2018 - 12:58 PM

Hi Choward,

   Thanks for the experiment and the great work documenting it.

 

If my reading comprehension is not way off left field, the results show that we should:

  1. Use reflectix.
  2. Even better: use Mylar which is better than reflectix.

Is this the correct interpretation of your results?

 

cytan



#91 choward94002

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Posted 17 December 2018 - 01:21 PM

My experiments so far are for a very specific question in a very specific circumstance: what will cause the least amount of radiative cooling from the skin of the OTA.  That is important to some, unimportant to others ... what I have found is what was expected, that you want to use a substance with a very low emissivity (i.e. really shiny).  Gold foil would be probably the best, but isn't too durable and attracts the wrong crowd in your neighborhood.  People have used Reflectix because it's shiny, isn't too expensive and is quite durable ...

 

Mylar would likely give better results (to a point, diminishing returns and all) but I could never find a way to keep it in one piece; the mylar I could find was actually made of two layers, a shiny layer and a plastic layer which was it's backing and it kept delaminating on me.  I eventually settled on using Reynolds Wrap spray-adhesived to the Reflectix ... it held up really well and gave me better results than the Reflectix ...

 

So, if you are concerned with reducing the amount of radiative cooling from your OTA, I found it best to use Reflectix as a backing material for Reynolds Wrap shiny side out ... I'm going to be doing experiments on Dew shields later this week to verify some numbers that were posted by another member here a few years ago for dew shield ideal lengths [https://www.cloudyni...d/#entry5653518] as well as seeing if extending the ReynoldsWrap up the length of the shield and covering the inside of the shield with a black flocking material make any difference in the corrector plate skin temperature change over time ...



#92 charlesgeiger

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Posted 17 December 2018 - 04:39 PM

Kokatha man and Choward,

Thank you both for your contributions and clarifications!  I have learned much.

And I thank JHayes too for all his imput.  I hope JHayes will respond about Bend observing conditions!

Sincerely,

Charlie



#93 Kokatha man

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Posted 17 December 2018 - 07:37 PM

...I'll keep posting here because the OP did ask in his original post  "any more clever ideas" which I'll take to encompass attaining optimum situations for SCT operation, although imo they (possibly) contradict aspects of his intended approach...whether they are clever I'll leave up to others! lol.gif

 

A single word from him will make me desist here of course. smile.gif

 

Post #14 here gives a brief summary of what to expect Howard re temperature drop rates: https://www.cloudyni...areless-claims/

 

No "plateau" as such although obviously the temperature cannot fall below that of the cooling mixture ultimately...suffice to state that dependant upon humidity/water vapour the rear casting becomes sheathed in a thick skin of ice during many of the coolings...

 

I've also realised that quite a lot of the more recent temperature fall logs were made on our mobile phones...& even they were quite awhile back, but I hope to find a few sets of data somewhere. fingerscrossed.gif

 

I will start a new thread with that & other images however... wink.gif

 

Just a note Howard - if the anticipated low at the start of an imaging session was predicted to be 5°C then we would pull the ice off at 3°C, point the scope upright & allow for about 1/2 hour before starting. (or attempting to, depending upon the seeing) This 1/2 hour is specifically to allow the primary to "relax" & attain temperature uniformity throughout - a defocused star displays all sorts of distortions to its' diffraction rings if you try to operate immediately or too soon after removing the ice.

 

As to the sketch in my last post, I checked in semi-darkness for the edited sketch alterations - I "think" there are little "seats" on the underside of that bottom lip for the glass, but that might be incorrect - but the nylon spigots themselves are a wonderful improvement to later Celestrons - they firmly locate the corrector in the ota & can be adjusted for optical axial centricity - my old C11 relied on cardboard spacers positioned around the glass's edge to do this! smirk.gif

 

 



#94 precaud

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Posted 17 December 2018 - 08:52 PM

A single word from him will make me desist here of course. smile.gif
 
I don't know why you wouldn't want to have a separate thread for what you're doing...


#95 choward94002

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Posted 17 December 2018 - 09:43 PM

Hey Kokatha!  REALLY helpful link to that thread about your timings ... it gives me an idea of what might be happening and where to place my sensors (not that I am trying to prove a hypothesis, simply trying to gather data) ... based on what you've been telling me, here's what I think is happening ...

 

- You start out with the OTA interior and the outside air at ambient in the afternoon, mix up your water and salt and head out.  As you get to your destination, tube ambient is probably slightly higher than outside ambient ... that's the equivalent of someone with an observatory opening the dome/ roof for the night's festivities

- You mix up your solution and put the OTA into a nose-down configuration, then put the ice directly onto the cell for 2+ hr ... that will cool the cell down to your low temp setpoint, as it does it cools the interior OTA air ... but not the mirror, since it's thermally isolated.  The warmer air in the OTA rises to meet the cold mirror cell, which then chills that air and it descends to the nose/ correctory plate.  That does two things; it sets up a convection current between the cold air descending from the cold mirror cell at the "top" and the cold air on the "bottom" cools down the corrector plate.  Note the position of the baffle tube; since it's inverted it will have a dead air space so no convection currents inside the baffle tube

- After 2+ hr, your temperature probe on the surface of the primary mirror (which is actually reading the air temperature at that spot and not the mirror temperature, I'l wager) hits your target point; at this point all of the air in the OTA is as cold as or near the corrector plate on the "bottom" colder than your setpoint temperature, likely by 2-4C

- You invert the tube to it's viewing position within 20deg of vertical and "let the mirror relax" for a bit ... what I think is now happening is the supercooled air that had been at the bottom of the tube chilling down the corrector plate is now surrounding the primary mirror and cooling it down.  Two new, smaller convection systems are started with the primary giving off heat to the supercooled air which then rises to the corrector plate and is again chilled down as well as residual warm air in the baffle tube rises and is replaced by cool air ... eventually the primary mirror hits thermal equilibrium with the supercooled air (likely heating it up by some amount) and that system stops, the baffle tube interior hits equilibrium with the interior air mass and that system stops and we have thermal stability ... the primary is cold, the air around it is just as cold, the interior of the baffle tube is the same temperature, the corrector plate and the secondary, everything is now at the same temperature which is probably within 3C of your initial setpoint temperature.  There isn't anything to add heat to the system at this point except for conductive cooling with the outside ambient air which you have described as "dropping like a stone once it gets dark" and radiative cooling will remove heat from the system, not add it ...

- While you're using the system the primary continues to be surrounded by the coolest layer of air, inhibiting the focus shifts from thermal expansion that JohnH has documented ...eventually the outside ambient warms up the tube skin enough to start some skin thermals but by then you're good.  Given this, I would surmise that not only do you NOT want to use insulation or a radiant barrier (as you've said you tried and abandoned) but indeed want to encourage that radiative cooling since the colder the tube gets the longer the interior OTA temperature will remain constant ... maybe hit it with some Rustoleum HiTemp Black! smile.gif

 

Like I said, you've given me lot's of empirical observations to go with, now I need to see if this sequence holds water or falls flat and try to characterize it ... but actually you know all of this already, don't you ... because it was discussed at length in this thread several years ago [https://www.cloudyni...pe-temperature/] where you were even nice enough to discuss how you experimented with peltier coolers instead of ice bags and even took a nice pic of your scope too!  That's where you also mentioned cocooning the OTA in another bag after putting on the ice bag to further cool down the tube skin beyond what simple ambient exposure would be able to do which I think is also helping with your focus shifts ... in addition to what John documented for mirror thermal changes, there is also documentation for tube length changes affecting focus [https://www.cloudyni...re-focus-shift/] which would definitely be diminished if the OTA tube was already at the target temperature before the session started rather than slowly contract as the evening wore on ...


Edited by choward94002, 17 December 2018 - 09:53 PM.


#96 Kokatha man

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Posted 17 December 2018 - 10:13 PM

Hi - quick response 'cos we're about to head out...Pat has scoured her mobile phone & only has a few notes on the cooling stats but very little & some without times! shocked.gif  Judging by their dates (pre 2015) I'd say they were purely ice-bags without salt.

 

You mention a thread - I'm pretty sure I've posted figures or a graph at least once on CN...are you referring to that when you say above <"REALLY helpful link to that thread about your timings"> 

 

I hope you are...we're drawing blanks here because a lot was written down on notepads that have been thrown out etc, but I'll keep searching! smile.gif

 

I "think" you are confusing the order of things a bit also - the ice is mixed with the salt in the bag immediately before it is placed atop the C14's rear casing...

 

<"your temperature probe on the surface of the primary mirror (which is actually reading the air temperature at that spot and not the mirror temperature">

 

"No" - I don't think so brother...if it was I'd be very disappointed & my entire rationale would need to be re-thought! wink.gif

 

The sensor itself has an adhesive surface which is the sensor's actual "sensing" surface - this is placed face down on the primary & then a plastic cap maybe 1.5" in diameter & 1/2" tall is way over-filled with neutral cure silicone...this is then quickly up-ended over the sensor on the primary where there is a slight "nick" that I cut in the edge to allow for the thin lead to pass through so that the cap fits flat upon the mirror.

 

This is left to cure - the effect being that the sensor is thermally isolated from the air inside & also from any of the primary glass nearby - I know this to be true because I used the same arrangement on the C11 & used 3 sensors in the initial stage, 1 of them un-insulated adhered to the primary back & another "dangling" in the airspace - all within close distance of each other btw...& the temperatures were quite different between them. I have no reason to suspect that the C14 reacts any differently. wink.gif

 

I'm confident the isolated/insulated sensor probe gives a fairly good indication of the overall glass's interior temperature allowing for variations in such an insulator...why the "relaxation" time is important btw.

 

I have no issue with the cooling convection you've espoused above...incidentally the corrector glass itself is of very small significance in the overall scheme in that if dew/fogging of the corrector occurs during anytime in the night a quick blast of the hair-dryer (30" - 60") removes said...I flap a thick magazine in front of it for maybe a minute (it helps keep me active over long imaging sessions!) & then watch the planet onscreen until it settles down & returns to normal for us to recommence imaging: a 3 or 4 minute hiatus at most.

 

I'll re-read your post again later but these are salient points I thought I'd post before we head out.

 

Regards, Mo.



#97 jhayes_tucson

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Posted 17 December 2018 - 10:42 PM

You had also earlier mentioned checking the humidity levels, likely because an airmass with water vapor in it has to overcome the heat capacity of the water vapor before it makes a change ... so, dry air gets colder faster than moist air.  Good catch, but remember the OTA and the cans are a closed system (I even used weatherstripping on the can lid to seal the glass plates) so I had better not have humidity changes!  I think I'm also safe with the humidity differences of the can interiors because I did my opening and closing of them in the same afternoon so they should all of the airspaces should have the same thermal capacity ... but again, good call and I'll start monitoring humidity levels with my humidity/ temp sensors in the future ...

 

Actually, the reason that I suggested checking RH values is to know the sky temperature, which strongly varies with the ambient RH level.  If you want to understand radiative cooling, the sky temperature is very important.  Convective heat exchange may vary a bit with humidity but that’s not something that I’ve come across or researched—and I suspect that it’s a relatively minor effect.  I’m not so sure that heat capacity of the internal air mass has much if anything to do with what you are tying to measure, but without a solid theory, it’s hard for me to tell.  

 

Please forgive me but I have to editorialize a little.  You can certainly make measurements but I’m used to seeing a clearly stated theory that makes measurable predictions.  The purpose of the experiment is to make careful measurements that can be used to either verify or refute the predictions.  If the results disagree with the theory, it fails.  This process requires carefully designed expiments that eliminate variables so that you can reduce uncertainty in the results.  From what I can tell, you are merely looking for certain “effects” based on some generalized contentions so it might be a good idea to write down what defines success or failure of your measurements—particularly if you are just trying to characterize vague notions of what works best.  It’s also a good idea to make sure that you don’t make measurements on a system that is so different from the system of interest that the results don’t mean much.

 

John



#98 jhayes_tucson

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Posted 17 December 2018 - 11:01 PM

...

*John Hayes; do you have a C14 in Bend, OR and also another in Tucson or New Mexico?  I live in Portland and am retired and thinking to move to Bend for dark sky and good transparency...I would assume that if you are operating your scope out of state you don't think the sky is adequate in Bend area...unsteady sky?  Just interested in your thinking...I have an old C14.
Thanks for all the ideas and inspiration from so many.
Sincerely,
Charlie

Charlie,

I ran an XLT and a C14 Edge system for about 4 years in Bend.  The Edge has been at DSW for over a year.  The sky in Bend is superb in the Fall (better than DSW) but it stinks in the winter.  AP is possible in Bend but sometimes you have to be patient.  It’s definitely way better than Portland but the Chamber of Commerce reports of 300 clear days a year in Bend are more than a bit exaggerated.

 

John



#99 choward94002

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Posted 17 December 2018 - 11:32 PM

Actually, the reason that I suggested checking RH values is to know the sky temperature, which strongly varies with the ambient RH level.  If you want to understand radiative cooling, the sky temperature is very important.  Convective heat exchange may vary a bit with humidity but that’s not something that I’ve come across or researched—and I suspect that it’s a relatively minor effect.  I’m not so sure that heat capacity of the internal air mass has much if anything to do with what you are tying to measure, but without a solid theory, it’s hard for me to tell.  

 

Please forgive me but I have to editorialize a little.  You can certainly make measurements but I’m used to seeing a clearly stated theory that makes measurable predictions.  The purpose of the experiment is to make careful measurements that can be used to either verify or refute the predictions.  If the results disagree with the theory, it fails.  This process requires carefully designed expiments that eliminate variables so that you can reduce uncertainty in the results.  From what I can tell, you are merely looking for certain “effects” based on some generalized contentions so it might be a good idea to write down what defines success or failure of your measurements—particularly if you are just trying to characterize vague notions of what works best.  It’s also a good idea to make sure that you don’t make measurements on a system that is so different from the system of interest that the results don’t mean much.

 

John

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 ...


Edited by choward94002, 18 December 2018 - 01:35 AM.


#100 Kokatha man

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Posted 18 December 2018 - 03:41 AM

Hi again Howard (it is Howard I'm presuming?!? lol.gif )

 

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

 

<"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.

 

4Brass-InternalFanShroud.jpg

 

5FinishedExtHsnkCowling.jpg

 

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

 

C11-Mirror-Temp-Graph.jpg

 

 

 

 


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