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Questions About Insulating SCT's with Reflectix

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#101 bikerdib

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Posted 28 November 2018 - 10:18 PM

BTW, concerning the dew issue, I have aluminum dew shields for both of my SCT scopes and used to have a dew strap for the 14".  But I went to Hobby Lobby and bought some sheet craft foam in black that is about 1/8" thick.  I measured and rolled some dew shield extensions that simply slide over the aluminum ones.  This has been enough to prevent dewing even here where I am with average night time humidity that ranges from 80° all the way up to 100°

 

I eventually sold the dew strap and controller.


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#102 jhayes_tucson

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Posted 29 November 2018 - 12:05 AM

I really don't care what you "don't mind".  You don't need a degree in thermodynamics, statistical mechanics, and kinetic theory to understand how to put a blanket on a telescope, professor John.  Reflectix is a product that was developed for the construction, and packing industry, not for amateur Astronomy.  Though many who possess common sense have learned how to adapt it even without your superior intellect.  I didn't "actively discourage" anything, except perhaps, over-thinking. Interestingly, the cave dwellers knew enough to put a "door" on a cave, and to wrap themselves in an animal's pelt with the fur to keep warm.  They had no clue what thermal physics were, but yet, they had common sense to see that stopping air flow keeps heat in. Common sense that you seem to be discouraging, unless of course, one has a "technical education".  You hijacked the original question and turned the thread into a "superior technical" thread.  Do you remember the OP?  Here, I'll help...

 

"I observe in areas that are prone to prolonged heavy dew/frost.  If there is no dew/frost, that would be the exception.  Unless I put the dew shield on the scope, and wrap warming strips around the OTA behind the corrector, around the eyepiece, on the Telrad and around the finderscope objective and eyepiece, turn on the batteries and keep them on, dew/frost will form on the optics, guaranteed.  It's not really a question of if dew/frost will form, but when it will form:  sooner or later.  The warming strips DO prevent dew/frost.

 

So if I wrap the OTA in Reflectix, will I still need the warming strips to prevent dew/frost?  If there is a choice between no thermals and no dew/frost, I'd rather do without the dew/frost.  The effect of dew/frost on the corrector is a lot worse for observing than thermals in the tube.

 

Mike

"

 

Huh?  I did not personally attack, belittle, or mock you.  I did take direct issue with your post that directly discouraged understanding the basics in favor of what you think is common sense and with the incorrect information that you are spreading.  Instead of responding with facts, you've come back mocking me in a personal attack, which in my view, requires a direct response.  The fact that you can't make a connection between the question that the OP asked and the explanations that I've provided shows that perhaps the discussion has gone over your head; but, don't project your reaction on everyone else.  It doesn't mean that other participants either don't get it, don't care, or don't want to learn something.  I certainly have not provided an "in-depth scientific, peer-reviewed explanation of the intricacies of thermal dynamics [sic]".  What I have pointed out is that radiative heat transfer plays a significant and often under-appreciated role in why you want to wrap an OTA in Reflectix in the first place.  That's a very basic principle of heat transfer ( https://en.wikipedia...i/Heat_transfer).  Since you think that explaining very basic principles is "making a mountain out of a mole hill" and that basic principles don't rise to the level of basic common sense, perhaps you should back away from the keyboard and sign off.  It would lower your blood pressure, you could do what you want to do without having to worry about how it does or doesn't work, and it would let the rest of us try to understand some of this stuff better with more signal and less noise. 

 

 

"Professor John"  lol.gif lol.gif lol.gif


Edited by jhayes_tucson, 29 November 2018 - 12:33 PM.

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#103 luxo II

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Posted 29 November 2018 - 03:12 AM

I’ll suggest “much less, is more” to both of you.
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#104 Dynan

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Posted 29 November 2018 - 10:19 AM

And they frivolously say Americans can't get along nowadays...


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#105 Sarkikos

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Posted 29 November 2018 - 10:51 AM

And they frivolously say Americans can't get along nowadays...

So nothing has changed ...

 

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Mike


Edited by Sarkikos, 29 November 2018 - 10:52 AM.


#106 eklf

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Posted 29 November 2018 - 12:42 PM

Its better to have access to technical information and not read it rather than to need it and not have it. 

 

I, personally, prefer to read it even if I dont need it.


Edited by eklf, 29 November 2018 - 12:45 PM.

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#107 choward94002

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Posted 29 November 2018 - 05:02 PM

Its better to have access to technical information and not read it rather than to need it and not have it. 

 

I, personally, prefer to read it even if I dont need it.

Agreed ... this is a very complex hobby with lot's of things that demand precision, attention to detail and an in depth knowledge of how things work and why, and that only gets worse as you progress in the hobby.  Anyone can point a camera at the sky and click the shutter, but as you add CCD's and mounts and tracking and guiding and stacking and postprocessing and automation that becomes more true ... any less and you're just wasting your time and your money. 

 

Those that fail to realize that will suffer from frustration, wasted evenings and eventually put their stuff on AstroMart or CN classified ... I choose not to suffer that fate, and welcome as many "walls of text" by people who know more than I do as they can churn out ...

 

So, ... teach on, "Professor John"! :)


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#108 Endymion

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Posted 30 November 2018 - 02:36 AM

I am enjoying the technical discussion of conductive/convective and radiative heat transfer as it pertains to my SCT and getting better views.   Let's continue the technical discussion by all means!  I just bought a roll of reflectix and am looking forward to some fun in experimenting with it.

 

John, if -30C wasn't the temperature of the sky, did you find a number for what it actually is?

 

Thanks,

John


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#109 Jimmy462

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Posted 30 November 2018 - 09:03 AM

I observe doubles with a 10” f/20 Maksutov which has a big, heavy corrector. I have the scope permanently mounted in a RoR observatory which is not too far from ambient temperature at any time (max 5-7 degrees) since our winters are not as harsh as in other countries and summers are warm not hot, there is no need for air conditioning, etc.
I used to need to leave the roof open at least 1 hour before observing if I was to get any stable images during the nights of most significant change in temperature (spring, autumn). Since I am looking at very close doubles (<5 arc-sec separation), I need good observing conditions.  Atmospheric seeing is the main determinant factor, but image stability at the eyepiece is a close second. 

I installed insulation around my OTA this summer (material similar to Reflectix; just standard radiator wall insulation) and the improvement has been noticeable. Star images are stable from the start of my observing sessions even if I have not managed to open the roof of my observatory in advance.
This effect is similar to the very beginning of some observing sessions when, not having cooled the telescope, star images would be very good for a short 5-10 mins before thermals appeared inside the OTA. A pre-thermal shock nirvana kind of scenario.
An added bonus of the insulation, which has been mentioned in other posts above, is that my corrector doesn’t dew. I don’t use, and have never used, a dew shield and typically rely on a good dewheater band just behind the front glass.  Humidity is high in England (average >60% and in some nights >90%).  With the insulation material wrapped in a double layer around the OTA, the dew heater band is not very effective, yet my corrector is free of dew. I again, suspect this being down to the much slower cooling rate of the whole OTA. Note the insulation material doesn’t extend more than 1 inch beyond the corrector; ie no Reflectix dew shield.

 

To all considering this solution, I say, go for it!

Roberto

Hi Roberto, etal,

 

I'm wondering (and suspecting) that the radiating ambient and residual heat within your roll-off observatory, and the fact that you're observing within a walled structure (despite it being open to the sky), is helping mitigate or forestall the conditions leading to the formation of dew. I'm curious if your "RoR" is surrounded by grasses or low-laying vegetation and, if so, have you've ever noticed dew (or frost) forming outside the observatory? Also, I'm curious if your RoR is located on a rise or hill or elevated slope vs being located in a slump or valley or other geographic depression?

 

The reason I ask is that regardless of whether I'm observing with my C9.25 or my thick-meniscus SW180 Mak, my (club's) observing location from a paved parking lot edge observing across an expanse of meadow directly to the south and west which is a flat half-mile from a local reservoir, pretty much dictates that I will be contending with either dew or frost with either scope if conditions permit. Invariably when the temperature dips near the dew/frost point the cold, sinking, moisture-laden air pours across the small valley (sometimes in the form of a visible rolling fog) and despite any best counter-measures (dew shields and wraps and heaters) it's pretty much "game over" for the glass-fronted scopes. Meanwhile on the surrounding hills the dew point comes later in the night (or not at all) and its effects are less severe.

 

I bring all of this up to this discussion as there are geographic ground-level climate factors to consider in trying to solve the, um, "dew problem" beyond merely wrapping and/or venting and/or heating one's scope. A moist, dew-prone valley will defeat any best counter-measures.

 

smile.gif

Jimmy G


Edited by Jimmy462, 30 November 2018 - 09:04 AM.


#110 R Botero

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Posted 30 November 2018 - 10:27 AM

Jimmy

 

My RoR is in my back garden. There is lawn leading to it but it sits atop a concrete base and next to it there's a section paved with Indian sandstone which has thermal conductivity similar to marble (https://www.naturals...lletins/rvalue/).  Dew regularly forms on the plants and grass around the observatory but also on surfaces inside it when humitidy is high. I keep a tarpaulin on this scope when I'm not observing and it sometimes drips when humidity is high.  

As for the enclosure, it certainly protects somewhat against the elements - wind being the main one - but I would say that light breeze sometimes helps delay dew formation.

 

Roberto



#111 Sarkikos

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Posted 30 November 2018 - 10:54 AM

Agreed ... this is a very complex hobby with lot's of things that demand precision, attention to detail and an in depth knowledge of how things work and why, and that only gets worse as you progress in the hobby.  Anyone can point a camera at the sky and click the shutter, but as you add CCD's and mounts and tracking and guiding and stacking and postprocessing and automation that becomes more true ... any less and you're just wasting your time and your money. 

 

Those that fail to realize that will suffer from frustration, wasted evenings and eventually put their stuff on AstroMart or CN classified ... I choose not to suffer that fate, and welcome as many "walls of text" by people who know more than I do as they can churn out ...

 

So, ... teach on, "Professor John"! smile.gif

I save more time and money by only doing visual astronomy.  No imaging for me!  

 

I think it's similar to what Mark Twain said about golf:  "Golf is a good walk ruined."  

 

I say:  "Imaging is a good observation ruined."  But maybe that's just me.  YMMV

 

On the other hand, I'm the Topic Starter, so I get to say it.

 

grin.gif

Mike


Edited by Sarkikos, 30 November 2018 - 10:56 AM.

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#112 Sarkikos

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Posted 30 November 2018 - 10:59 AM

I am enjoying the technical discussion of conductive/convective and radiative heat transfer as it pertains to my SCT and getting better views.   Let's continue the technical discussion by all means!  I just bought a roll of reflectix and am looking forward to some fun in experimenting with it.

 

John, if -30C wasn't the temperature of the sky, did you find a number for what it actually is?

 

Thanks,

John

Wouldn't the sky be warmer than -30C (-22F) due to the intervening atmosphere near the Earth's surface?  

 

Mike


Edited by Sarkikos, 30 November 2018 - 11:02 AM.


#113 Sarkikos

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Posted 30 November 2018 - 11:05 AM

For the EdgeHD 8", as long as I keep the AstroZap black dew shield on the scope and the warming strip inside the dew strip is still receiving power, I can observe all night without condensation forming on the corrector.  I also run the vent fans all night.  This is without Reflectix.  My observing site is very dewy.  

 

So if I were to adopt a Reflectix cover for the EdgeHD 8", it would be to resolve thermal problems, not dew problems.

 

Mike


Edited by Sarkikos, 30 November 2018 - 11:08 AM.


#114 jhayes_tucson

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Posted 30 November 2018 - 11:21 AM

I am enjoying the technical discussion of conductive/convective and radiative heat transfer as it pertains to my SCT and getting better views.   Let's continue the technical discussion by all means!  I just bought a roll of reflectix and am looking forward to some fun in experimenting with it.

 

John, if -30C wasn't the temperature of the sky, did you find a number for what it actually is?

 

Thanks,

John

 

John,

 

Thanks.  

 

It turns out that the temperature of the night sky depends strongly on the humidity, clouds and their altitude, the zenith angle, and (probably) on the total airmass (related to the altitude of the site.)  I started out trying to find the temperature drop on the front surface of a piece of glass exposed to the night sky and I couldn't find any data on this specific question (I'm sure that it's out there, but I couldn't find it.)  So, I got interested in building a heat transfer model that takes all this stuff into account to predict dew & frost with any specified dew shield (both heated and unheated.)  I've got a model that I'm testing and I've found that under most real conditions, the radiometric temperature of the sky varies between about -20C and only a few degrees below the local ambient temperature.  One of the interesting things that the model shows is that it's the difference in emissivity of the sky and the front surface of the telescope that has a strong effect on the heat transfer between the two.  I'm in the process of gathering some data to experimentally show that the model is producing reasonable results--and so far it looks pretty good.  Once I get that done, I'll write up a paper on it.  It's moderately complicated so the challenge will be to present this stuff in a comprehensible way--without writing a book!   It will probably take another month or two to get it done so don't hold your breath.

 

John


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#115 choward94002

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Posted 30 November 2018 - 11:28 AM

John,

 

Thanks.  

 

It turns out that the temperature of the night sky depends strongly on the humidity, clouds and their altitude, the zenith angle, and (probably) on the total airmass (related to the altitude of the site.)  I started out trying to find the temperature drop on the front surface of a piece of glass exposed to the night sky and I couldn't find any data on this specific question (I'm sure that it's out there, but I couldn't find it.)  So, I got interested in building a heat transfer model that takes all this stuff into account to predict dew & frost with any specified dew shield (both heated and unheated.)  I've got a model that I'm testing and I've found that under most real conditions, the radiometric temperature of the sky varies between about -20C and only a few degrees below the local ambient temperature.  One of the interesting things that the model shows is that it's the difference in emissivity of the sky and the front surface of the telescope that has a strong effect on the heat transfer between the two.  I'm in the process of gathering some data to experimentally show that the model is producing reasonable results--and so far it looks pretty good.  Once I get that done, I'll write up a paper on it.  It's moderately complicated so the challenge will be to present this stuff in a comprehensible way--without writing a book!   It will probably take another month or two to get it done so don't hold your breath.

 

John

Write a book, write a book!  :)   Seriously, once you get your model done if you could put that into some kind of equation then folks could code that into the various heater control packages and do the community some real service ... currently most algo's use the air temp, air humidity, SQM (to get cloud coverage) and wind speed to determine if it's "dew or no-dew" and that's empirical at best ... a computational model would be much better ...



#116 Sarkikos

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Posted 30 November 2018 - 11:34 AM

We need a manual on control of thermals and dew prevention for amateur astronomy!  I don't want formulae. I'm an amateur astronomer, not an engineer.

 

I want printed tables or software where the ordinary layman observer can look up the information they need.   

 

In lieu of that, general concepts and rules of thumb based on the science and experience in the field using available materials would be nice. 

 

These are the sorts of guidelines most amateurs will go by.  Probably most are only interested in thermal engineering to the extent that it will help them get better images - visual and photographic - through their telescopes.  

 

Mike


Edited by Sarkikos, 30 November 2018 - 11:46 AM.


#117 jhayes_tucson

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Posted 30 November 2018 - 12:45 PM

We need a manual on control of thermals and dew prevention for amateur astronomy!  I don't want formulae. I'm an amateur astronomer, not an engineer.

 

I want printed tables or software where the ordinary layman observer can look up the information they need.   

 

In lieu of that, general concepts and rules of thumb based on the science and experience in the field using available materials would be nice. 

 

These are the sorts of guidelines most amateurs will go by.  Probably most are only interested in thermal engineering to the extent that it will help them get better images - visual and photographic - through their telescopes.  

 

Mike

 

Mike,

I plan to show how it works in some gory detail but I hope to distill it down to what you are looking for.  I'd like to provide a spreadsheet but right now it involves "solves" and that's not as trivial as I'd like.

 

John


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#118 555aaa

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Posted 30 November 2018 - 02:25 PM

I also thought about modeling this in Matlab Simscape thermal. It has radiative transfer as well as conductive and convective. I agree that the sky radiative boundary is an interesting and difficult question but with the model you can play with those parameters. Simscape is a bulk modelbut the PDE tool can do heat transfer geometrically by FEM or method of moments. The bulk model should be OK to start with.

#119 jhayes_tucson

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Posted 30 November 2018 - 10:33 PM

I’d be delighted to compare results if you build a model.  If you dig into it, you’ll find that there a LOT of conditions that can make a big difference in the results.  My model takes into account humidity, cloud cover (which isn’t very important), zenith angle, radiative heat exchange, dew shield dimensions, dew shield temperature, and all the various emissivities.  It computes the temperature drop at the front surface along with the difference from the dew point.  It would be interesting to see if we could get some agreement..but, I’ll be stunned if we do.  It’s not a trivial calculation and that’s why I think that it’s so hard to find simple numbers on the web.  That’s also why I’m trying to gather at least a little real data to verify that the results are in the right ballpark.

 

John


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#120 Tom Glenn

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Posted 30 November 2018 - 11:23 PM

John, sorry if you already answered this somewhere earlier in the post, but does an IR temp gauge give accurate representations of the temperature of the sky (from a radiative cooling perspective)?  When I have a clear night, and I point my gauge at the sky, I consistently get readings of -40 to -50C, and often get a "Error Lo" display because my unit maxes out at -50C and it apparently reads below that frequently.  During the daytime if I do the same thing with the IR gauge I get readings of about -20 to -30 C.  Obviously if there is cloud cover at low altitude, then taking an IR reading of the sky gives me numbers within 20 degrees of ambient usually.  


Edited by Tom Glenn, 30 November 2018 - 11:26 PM.


#121 jhayes_tucson

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Posted 01 December 2018 - 12:06 AM

John, sorry if you already answered this somewhere earlier in the post, but does an IR temp gauge give accurate representations of the temperature of the sky (from a radiative cooling perspective)?  When I have a clear night, and I point my gauge at the sky, I consistently get readings of -40 to -50C, and often get a "Error Lo" display because my unit maxes out at -50C and it apparently reads below that frequently.  During the daytime if I do the same thing with the IR gauge I get readings of about -20 to -30 C.  Obviously if there is cloud cover at low altitude, then taking an IR reading of the sky gives me numbers within 20 degrees of ambient usually.  

IR temp gauges are used by sky monitoring systems to look for clouds because of the clear difference in temperature that you've seen with and without clouds.  It's a pretty sensitive way to detect clouds!  I don't know enough about how these systems are configured and calibrated to say for sure if the numbers that they produce are very good for the purposes of computing heat loss to the sky.  To do that, you need a calibrated radiometric temperature that's measured over a wide wavelength range with good response in the 8-14 micron range and I just don't know if those IR temperature gauges make good measurements in that bandwidth.  I suspect that they are producing a value that's way too low.

 

I've looked through the literature and found an expression for the relationship between the hemispherical irradiance on a surface under the night sky as a function of humidity and cloud cover.  You can use that equation to work backward to find the sky temperature and using the equations that I have, the only way that the sky temp goes below about -20C is when the air temp is -20C and humidity is really low...like 5%.  Under those circumstances it can get to -50C.  I'm still trying to figure out if the equations that I have produce "reasonable and expected" results, which is why I'm not yet willing to post any numbers.  I just can't say with enough confidence that my numbers are really "right."  

 

BTW, this stuff is really important for folks developing radiant cooling systems so it's pretty easy to find a lot of papers on this stuff.  Unfortunately that doesn't mean that everyone agrees on the numbers!  It seems that the agreement between various measurements is only within 30%-50%.  I have to believe that there's a lot of stuff going on in the 6-11 mile deep column of air leading to the Tropopause that the models don't account for very well.  That's what makes this stuff so hard to measure with enough accuracy to generate a good model.

 

John


Edited by jhayes_tucson, 01 December 2018 - 12:15 AM.

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#122 Tom Glenn

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Posted 01 December 2018 - 12:21 AM

Thanks John.  And I assume that this "sky temp" value that you are reporting would be like an average black body temperature for the entire sky column above an observer?  Because at certain altitudes, such as jet aircraft cruising altitudes of 35,000 feet, the local air temperature definitely gets to about -40C or colder, but I have no idea how the various sky temperatures at different altitudes work together to produce a final effect on heat loss at the ground, which is what I gather you are trying to calculate.  My IR temp gauge is certainly not designed for these extreme measurements, and probably only has limited accuracy in general, but it was super useful for quickly diagnosing that my thermostat was stuck open in the cooling system of my car! 



#123 jhayes_tucson

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Posted 01 December 2018 - 01:54 AM

You have to be a bit careful.  You are right that the International Standard Atmosphere (ISA) shows the temperature lapse rate going to zero at 36,000' at a temperature of -56.5C; however, there's a temperature inversion that occurs high in the Stratosphere due to the ozone layer where the temperature goes way up again.  The big issue is that the optical bandpass of water in the lower atmosphere is complicated and it blocks a lot of the long wavelength radiation responsible so the light that reaches the ground may correlate with the temperatures at different altitudes.  Take a look at these curves: https://en.wikipedia...rption_by_water and you'll see the problem.  Humidity is what makes the whole thing so tough to accurately model and why I think that IR cameras may not report a very accurate radiometric temperature.

 

At this point, this stuff is starting to get pretty far afield from the OPs question so we should probably take it to a new thread if you want to discuss it further.

 

John


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#124 Tom Glenn

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Posted 01 December 2018 - 02:17 AM

Got it, thanks for the info.  



#125 Jimmy462

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Posted 01 December 2018 - 09:41 AM

Jimmy

 

My RoR is in my back garden. There is lawn leading to it but it sits atop a concrete base and next to it there's a section paved with Indian sandstone which has thermal conductivity similar to marble (https://www.naturals...lletins/rvalue/).  Dew regularly forms on the plants and grass around the observatory but also on surfaces inside it when humitidy is high. I keep a tarpaulin on this scope when I'm not observing and it sometimes drips when humidity is high.  

As for the enclosure, it certainly protects somewhat against the elements - wind being the main one - but I would say that light breeze sometimes helps delay dew formation.

 

Roberto

Thanks for those insights, Roberto, they're helpful to what I've been trying to work through in regards to the relationships of instrument- and immediate-environment thermal-seeing issues. In my experience, those nights where the temperatures will reach or near the dew point are most problematic for high-magnification planet viewing when viewing at the previously-mentioned field location, ergo my interest in knowing your seeing experiences while observing in the slightly-warmer tub of air in your RoR.

 

smile.gif


Edited by Jimmy462, 01 December 2018 - 09:43 AM.



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