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Starting to realise that thermal management is a big deal...

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#1 johngwheeler

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Posted 04 May 2018 - 07:23 PM

I've been doing some comparisons of my scopes on the planets and Moon now that we can see Jupiter, Saturn & Mars on the same night.

 

I've set up the following over the last few nights:

 

Orion 110mm ED f/7 doublet

SkyWatcher 6" Mak-Cass (f/12)

Celestron C8 (f/10)

GSO 10" Dob (f/4.9)

 

I've aimed to set the scopes up at least an hour before darkness, and vented the C8 & Mak by taking the diagonals out and inverting them as far as the mount would allow.

 

Nightime temperature change is quite slow where I live (Sydney, Australia) - probably only 1-1.5 degrees C per hour. I don't think this is particualry rapid compared to other parts of the world. 

 

I was surprised how long the scopes took to reach thermal equilibrium, even the refractor, which seemed to take at least 30 minutes to become usable at 150x, and took about an hour to really settle down.

 

Both the 6" Mak and C8 took a really long time to show their best (over 3 hours), but the image was never as stable as the refractor. The Mak had marginally sharper images than the SCT, but this could be the optics rather than thermally related (and in any case the 6" mirror should reach equilibrium sooner than the 8" one).

 

The 10" dob (with a small cooling fan) was somewhere in between the refractor and Mak/Cass - after a couple of hours it was pretty good and showed the most detail on Jupiter, Saturn & the Moon - but this is also to be expected given its better theoretical resolution.

 

I'm curious about how I can improve matters - 2-3 hours is a long time to wait for a scope to be "ready" for planetary observation. The biggest problem is the Mak and SCT. I don't really want to be drilling holes is these (& I don't have the tools) - so what can be done to reduce their cool-down times? The scopes are kept in an unheated garage by the way - but this is still probably several degrees warmer than the outside, even in winter.

 

Thanks!

 

John


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#2 DLuders

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Posted 04 May 2018 - 07:28 PM

You could consider INSULATING your Mak -- see this previous Cloudy Nights Forum topic about it:   https://www.cloudyni...jacket-for-mak/


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#3 Eddgie

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Posted 04 May 2018 - 09:06 PM

Here is the really excellent news.  You can make a Newtonain work well on planets by flipping a switch and waiting about 30 seconds.

 

 

What affects the image in a reflector is the boundary layer of turbulent air that clings to the front surface of the primary.  Now what makes this problematic is that the light has to traverse this turbulent layer twice: once on the way to the mirror, and once on the way to the secondary.  Since the light traverses the layer from opposite directions, the refraction is in opposite directions, and this is what does such severe damage to the image.

 

The solution to this is to use a boundary layer scrubber fan.   It is simple as that. 

 

Here is an article that explains everything:

 

http://www.skyandtel...NewtThermal.pdf

 

Again, this works almost instantly and there is no need to run the cooling fan. Is all you need is the boundary layer fan.


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#4 gnowellsct

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Posted 04 May 2018 - 10:03 PM

OK here me now and understand me later.  When I go out after sunset and get temperature readings (using infrared thermometer) the asphalt is about ten to fifteen degrees warmer than the grass lawn next to it.  Makes sense, non?  The grass is emitting moisture which helps keep it cool.  

 

One of the reasons some of us don't have many complaints about cool down is because we routinely observe over grass.  The biggest complainers about cool down tend to come from hard rock desert states where the ground bakes in the day and radiates heat all night.  

 

You might want to repeat your cool down experiments over grass.  

 

I wrapped my c14 in insulation for years but it didn't seem to make any performance difference.  One might think it would, because the ground side temperature of the tube (according to some of my measurements) is often five to six degrees warmer than the sky side.  So insulation should reduce the temperature delta between the two sides and therefore reduce tube currents.  
But there's many other things going on, including cooling of the rear casting and the mirror in the tube (which would be slowed down by insulation on the OTA).

 

The consistent performance issue that I have is not that the C14 or c8 is too hot but that it gets too cold, leading to dew formation.  If it was so hot inside that closed tube the circulating currents of warm air should keep the corrector plate clear.  But in practice it will dew up in 20 minutes or so.  (Not always, but often).  

 

This is my way of saying that these systems are actually pretty efficient at cooling in the night air and that the main issue is the heat source, observing on asphalt and desert type heated sand and rocks.   My ten inch Newtonian has a relatively thin mirror--I forget the exact, I think it is something like 1.25 inches, and it too cools very rapidly and never felt the need for fans or other cooling assists.

 

So bear in mind when you introduce all these technologies that you're trying to adjust the temperature of the *scope* when in reality it is the ground underneath you that is the single largest source of heat in the optical system.  GN


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#5 gnowellsct

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Posted 04 May 2018 - 10:05 PM

I have a funny story about fans.  My buddy Pete and I went into the Adirondacks some years ago and the scopes got hot in the car.  We were observing on grass.  But he turned the fans on (Obsession 15) because of that big mirror.

 

So we observed for a while and then he complained he couldn't see anything.  He thought we had clouded out.  It was totally clear.  He had frosted his mirror!


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#6 Jaimo!

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Posted 04 May 2018 - 10:33 PM

Funny thing is...  I have never seen a professional observatory insulating their scopes.

 

Jaimo!


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#7 gnowellsct

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Posted 04 May 2018 - 11:14 PM

Funny thing is...  I have never seen a professional observatory insulating their scopes.

 

Jaimo!

It would be interesting to know more how they handle thermal management.  Maybe they try to keep the dome cooled to approximate night time temps so there is no thermal cool down needed.  I doubt they let the instruments heat up to daytime ambient.


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#8 jhayes_tucson

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Posted 04 May 2018 - 11:38 PM

Funny thing is...  I have never seen a professional observatory insulating their scopes.

 

Jaimo!

They are "insulated" but in different ways.  First, the telescopes are in an observatory that is temperature controlled to be at the same temperature as the surrounding night-time air. Furthermore, the observatory acts as a dew shield exposing the telescope only to a relatively small sliver of sky.  The optics are also typically mounted in open air truss structures that are not subject to tube current issues.  Finally, the larger scopes also have active optics that compensate for seeing effects in real time.

 

John


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#9 garret

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Posted 05 May 2018 - 12:18 AM

 

First, the telescopes are in an observatory that is temperature controlled to be at the same temperature as the surrounding night-time air. Furthermore, the observatory acts as a dew shield exposing the telescope only to a relatively small sliver of sky.

The best we can do is to mount our telescope in a isolated dome (with slit) air conditioned to the outside temperature.

Dome, isolation and a powerful air conditioner are available everywhere, except the cash! grin.gif

 

Garrett


Edited by garret, 05 May 2018 - 12:22 AM.


#10 Asbytec

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Posted 05 May 2018 - 01:21 AM

You could consider INSULATING your Mak -- see this previous Cloudy Nights Forum topic about it:   https://www.cloudyni...jacket-for-mak/

Insulation is getting a lot of positive press these days and may well be worth looking into for MCT and SCTs.

 

I have not seen any thermal problems in years. My secret is a frozen medical gel pack laid across the OTA during dinner. By observing time in our modest tropical climate (with hot days), the scope is cool to the touch as if it had been sitting out all night. Of course, climate matters. I doubt you could take this process all the way to freezing ambient temps. Insulation may be better in most or all cases, but a gel pack works on most spring to autumn evenings. 

 

So, yes, thermal management is important. Whether it's passive or active. Whatever the case, do something that works so you don't have to wait for hours. My MCT 'actively' cools as fast as the refractor during dinner and is ready for 70x per inch and textbook stars right out the door. 


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#11 gnowellsct

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Posted 05 May 2018 - 06:56 AM

They are "insulated" but in different ways. First, the telescopes are in an observatory that is temperature controlled to be at the same temperature as the surrounding night-time air. Furthermore, the observatory acts as a dew shield exposing the telescope only to a relatively small sliver of sky. The optics are also typically mounted in open air truss structures that are not subject to tube current issues. Finally, the larger scopes also have active optics that compensate for seeing effects in real time.

John

Tending those high altitude scopes must be like working in a freezer! With open truss don't they worry about kids tossing candy wrappers on the primary? ( I think your "adaptive optics" got autocorrected.)

Edited by gnowellsct, 05 May 2018 - 07:00 AM.

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#12 jhayes_tucson

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Posted 05 May 2018 - 08:31 AM

Active optical components are those with actuators and they are generally considered to be the same thing as adaptive optics.

 

John


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#13 George N

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Posted 05 May 2018 - 09:21 AM

.......  The biggest complainers about cool down tend to come from hard rock desert states where the ground bakes in the day and radiates heat all night.  

 

.  GN

 

Is that not one of the reasons why "Mr. Pluto" Clyde Tombaugh put his 16-inch up on an open platform (https://www.youtube....h?v=Q_EKg-YA8yo) and was still climbing up there at age 87? He claims in the vid that the planetary detail from the scope is outstanding.

 

About 15 years ago I remember a long article in the 'NY Times' about the luckiest 16 year old kid in the world. His parents built him an observatory with a Meade 16 SCT at their 'weekend home' in the Catskills (they lived in NYC). At first they were very disappointed with the performance of the 16 SCT on planets - plus their location (forest and mountains) blocked a lot of the sky. So Mom, a professional building designer, designed an open tower with a clam-shell observatory at treetop level - (open floor grating - isolated pier up the center - spiral staircase). The design used a 'Japanese Garden' esthetic, so it looked good too. While they loved the new complete access to the sky - and their new grand panoramic view of the Catskill Mountains from above the treetops - the article said the thing that most amazed them was the great improvement in the telescope's optical performance - because it was now well above the ground heating and in steady air currents.

 

Want better seeing? Get your scope 6 to 10 feet off the ground...... and don't have your observatory close to your house or driveway or other heat radiators.

 


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#14 George N

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Posted 05 May 2018 - 09:31 AM

Clyde T's smaller 9" scope - Quote "....has parts from a cream separator, 1910 Buick, and a V2 rocket." See vid: https://www.youtube....h?v=LXkFe2atgkU

 

'nother vid of Clyde's 16 "platform scope" that he says is used visually for planets and moon - https://www.youtube....h?v=NmGZwwYcBuo


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#15 gnowellsct

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Posted 05 May 2018 - 10:15 AM

Active optical components are those with actuators and they are generally considered to be the same thing as adaptive optics.

John


I was thinking you'd come up with something like that. So now I know.

#16 gnowellsct

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Posted 05 May 2018 - 10:19 AM

Is that not one of the reasons why "Mr. Pluto" Clyde Tombaugh put his 16-inch up on an open platform (https://www.youtube....h?v=Q_EKg-YA8yo)

Want better seeing? Get your scope 6 to 10 feet off the ground...... and don't have your observatory close to your house or driveway or other heat radiators.


Life in the 1%! Ordinary mortals can set up on lawns. Wonder what that kid is doing now.
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#17 Special Ed

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Posted 05 May 2018 - 11:11 AM

With the warm weather finally getting here, I too, have been considering this cool down issue.  I have a C14 permanently pier mounted in a ROR observatory.  I got the scope eight years ago so it does not have any vents to place cooling fans like the newer ones.  The observatory is surrounded by grass and trees, so I have that advantage that Greg mentioned.  Here's an old picture and a newer one.

 

sized_TS&Obs_10_April_2012 003.JPG

 

Obs_resize.v2.jpg

 

The obs has a window (which I keep open), several louvered vents, and an insulated white roof so on hot days it can be as much as 10 degrees F cooler inside than out.  But in the mountains, the temperature can drop rapidly when the Sun goes down.  As soon as the obs is in the shade of the ridge west of it, I prop the door open, take the insulated cover off the scope, and roll back the roof, all to facilitate scope and building reaching equilibrium with the surroundings.

 

I'm just wondering if this is enough.  My seeing is often pretty rough after the brief calm of evening twilight and I wonder how much tube currents might be contributing to this, if at all.  Do the effect of tube currents look any different than poor seeing?

 

I have an oscillating fan that I use after a session to blow dry everything when there is a heavy dew.  I'm thinking about turning it on to stir up the air and add to the cool down measures I described above.  Any thoughts?


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#18 Cpk133

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Posted 05 May 2018 - 12:47 PM

If I'm planning a planet observing session and the seeing forecast is good, I check the temperature forecast and try to match that as close as possible with the temperature in my house.  I can get a bit of flexibility by varying location from the basement to some rooms upstairs.  If it's too cold or hot, I'll find a place to get the scope sitting hours before observing.  There are only a few month out of the year where the scope can go straight outside and be perfectly acclimated.


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#19 Peter Besenbruch

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Posted 05 May 2018 - 05:07 PM

I'm just wondering if this is enough.  My seeing is often pretty rough after the brief calm of evening twilight and I wonder how much tube currents might be contributing to this, if at all.  Do the effect of tube currents look any different than poor seeing?

That's a lovely area you live in. To distinguish between lousy seeing and tube currents, try defocusing a star by a fair amount. Tube currents will show up by the star not being round. I'm not talking about commatic images, but things like one side chopped off, or slow moving columns in the defocused star. Turbulence shows up as faster moving waves and ripples in the defocused star. You shouldn't see shape deformations.


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#20 Asbytec

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Posted 05 May 2018 - 05:20 PM

That's a lovely area you live in. To distinguish between lousy seeing and tube currents, try defocusing a star by a fair amount. Tube currents will show up by the star not being round. I'm not talking about commatic images, but things like one side chopped off, or slow moving columns in the defocused star. Turbulence shows up as faster moving waves and ripples in the defocused star. You shouldn't see shape deformations.

I've not tried it, but I believe you can actually focus on seeing conditions by defocusing the scope so you get a defocused pattern at infinity. This defocused image is also a focused pattern of the much closer atmosphere imposed onto the defocused diffraction pattern. It's like focusing on the atmosphere, but a more distant star happens to be in the background. It lets us see what's passing in front of the star allowing us to actually focus on the atmosphere - which sounds impossible to do otherwise. I believe it requires being outside infinity focus. 


Edited by Asbytec, 05 May 2018 - 05:23 PM.

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#21 jeremiah2229

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Posted 05 May 2018 - 05:32 PM

I've not tried it, but I believe you can actually focus on seeing conditions by defocusing the scope so you get a defocused pattern at infinity. This defocused image is also a focused pattern of the much closer atmosphere imposed onto the defocused diffraction pattern. It's like focusing on the atmosphere, but a more distant star happens to be in the background. It lets us see what's passing in front of the star allowing us to actually focus on the atmosphere - which sounds impossible to do otherwise. I believe it requires being outside infinity focus. 

Yes. Like focusing on the storm clouds instead of the moon and getting some super great views. I chase them storm clouds when Luna is out and bringing them to life.

 

 

Peace...


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#22 contrailmaker

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Posted 05 May 2018 - 05:33 PM

I wish someone would come up with adaptive optics for amateur telescopes. Seems doable with today’s technology. How about it John? Want to come out of retirement?

 

CM



#23 johngwheeler

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Posted 05 May 2018 - 07:16 PM

I like the idea of an insulating jacket.

Is this something that you leave off during cooldown, and only put on once the thermal equilibrium is reached to prevent the scope from dropping below it?

I was also wondering whether standing the scope on an insulating mat (event a foam camping mat) could help when you don’t have grass surfaces? My garden is all gravel or paving because I was too lazy to mow the lawn :-)

#24 Asbytec

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Posted 05 May 2018 - 07:28 PM

I like the idea of an insulating jacket.

Is this something that you leave off during cooldown, and only put on once the thermal equilibrium is reached to prevent the scope from dropping below it?

I was also wondering whether standing the scope on an insulating mat (event a foam camping mat) could help when you don’t have grass surfaces? My garden is all gravel or paving because I was too lazy to mow the lawn :-)

No, an insulating jacket stays on all the time. The beauty of the insulation jacket is the scope, itself, does not have to be cooled to ambient. It only has to have a stable temperature to preclude any gradient forming inside the OTA. It's the gradient in the OTA that causes less dense warmer air to rise into the light path. It's the density of the cold and warm air that causes unwanted refraction in much the same way as the atmosphere does.

 

The other option is to cool the scope to ambient to the same effect. The scope can be hot or cold, regardless of ambient temperatures, but it has to have a homogeneous, stable temperature and thus uniform density. Air density acts like a lens, so it's really not a temperature problem. It's a density problem. 

 

An insulating mat may help keep warm air from rising into the open bottom of a Dob, but it'll likely have only minimal effect, if any, a few feet off the ground above the OTA. That's the realm of local seeing. Just a guess, thought about it myself. 


Edited by Asbytec, 05 May 2018 - 07:33 PM.

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#25 Jeff B

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Posted 05 May 2018 - 08:26 PM

All I know is that insulating both my TEC 7 and C11 with Reflectix made a vast difference in the usability of both scopes...for the better.

 

Jeff

 

 




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