Insulating an SCT

[JamesF]

Might now have to change the drab dark coat to a shiny technical one.

For night-time use I'm not sure how much difference the foil will make.  It may be that standard bubble wrap would do an equivalent job.  Or it might depend on what the initial conditions are (eg. OTA at ambient temperature, or OTA at several degrees above ambient).  Need to think about that one more.

James

[JamesF]

I keep reading the statement that the OTA can "cool below ambient temperature". I stand to be corrected, but I believe this is simply impossible. If it were possible, you would have invented a perpetual motion machine. Suspend 1000 OTAs inside a giant thermos flask and put a little windmill below the OTAs, driven by the downdraft of all the scopes cooling below ambient. We've solved the energy crisis!

I guess it ought to be relatively easy to test with an SCT using a pair of thermocouples, one slid up the baffle tube and the other a few feet away from the OTA.  If only the weather would co-operate

I have no idea whether you're right or wrong Agnes and I think it's worth knowing for sure.  Does it help to consider the opposite situation?  Is it possible under suitable circumstances to get the inside of the OTA hotter than ambient?  If so, why should the opposite not be possible?  Is it not just reversing the direction of the energy gradient?

It's at times like this when I realise how little I know about things I think I understand

James

[JamesF]

Apologies, btw, if I'm coming across as wilfully difficult or stupid here.  I'm just trying to avoid making assumptions about things I think I know or are "apparently" obvious, when in fact they may not be, at least to me

James

[Stu]

I keep reading the statement that the OTA can "cool below ambient temperature". I stand to be corrected, but I believe this is simply impossible. If it were possible, you would have invented a perpetual motion machine. Suspend 1000 OTAs inside a giant thermos flask and put a little windmill below the OTAs, driven by the downdraft of all the scopes cooling below ambient. We've solved the energy crisis!

My understanding is that cooling below the ambient temperature is a genuine fact, and is the reason for dew and frost forming. It is a result of radiation into space so the top side of the scope will cool more than the bottom which is facing the earth. I have often wondered about putting some form of shield over my scope to see if it prevents dew and frost forming. In theory it doesn't need to be in contact with the scope, just present a surface which is warmer than the clear night sky.

I found some relevant commentary at this link.... albeit about tents, but still vaguely informative hopefully.

http://www.windowoutdoors.com/WindowOutdoors/Dew%20Frost%20Condensation%20and%20Radiation.htm

Cheers,

Stu

Sent from my iPhone using Tapatalk

[Joves]

My thoughts are that I think I'll just continue taking the scope out a couple of hours prior to high power viewing

[Zakalwe]

I keep reading the statement that the OTA can "cool below ambient temperature". I stand to be corrected, but I believe this is simply impossible. If it were possible, you would have invented a perpetual motion machine. Suspend 1000 OTAs inside a giant thermos flask and put a little windmill below the OTAs, driven by the downdraft of all the scopes cooling below ambient. We've solved the energy crisis!

You are not factoring in the effect of radiative cooling. If you were correct, then a front corrector plate would never dew-up. The corrector plate dews up because it drops below the dewpoint as it radiates it's heat away to the night sky, thus dropping below the ambient air temperature and the dew-point.

http://www.asterism.org/tutorials/tut41DewingFrostingOfOptics.pdf

[perfrej]

I wrote a long piece on insulation (camping mat) and it works wonders for black tubes. Currently in Moscow airport abd don't have time to search, but do ityourself!

/per

[Zakalwe]

The other thing to consider is that the top portion of the OTA is facing the sky and the lower portion is facing the ground. The top portion of the OTA will cool rapidly (I've had ice on the top of the OTA), whereas the lower portion will stay warm as it is facing the ground. So the air inside the OTA will be moving around as the warmer air will rise, cool and then drop down.
I don't know how strong these air currents would be, but if high power imaging was being carried out then it'd surely be better to minimise air currents as much as possible?

My thoughts are that I think I'll just continue taking the scope out a couple of hours prior to high power viewing

It's probably more relevant to imaging than observing to be fair.

[cgarry]

I guess the larger the scope gets the bigger this problem is likely to become.  I'd never have expected there could be such a difference between the top and bottom of the OTA.  How did you measure it, out of interest?  Sounds like it would be worth attempting to duplicate with my C9.25.

James

I figured out that the issues I was having with my C14 were down to tube currents rather than seeing conditions with a simple star test.  To track down the source of the current I used a contactless infrared thermometer to measure the temperature at various points on the OTA surface.  Though sometimes it was quite obvious when there was ice on the upper part of the tube and water dripping off the lower part.  I don't think I had these sorts of issues with my C9.25 but I never did a start test so I really can't be sure.

I have a feeling that there are a lot of people with large SCTs who have the problems with tube currents but never bother investigating them, putting it down to bad seeing instead.  I have talked to a few people with large SCTs who complain that the seeing is always bad but refuse to even consider tube currents because their scope is kept in an observatory or has been left to cool down for several hours.  Getting across the point that keeping a scope in an observatory is not enough to rule out thermal problems seems to be impossible with some people!

Cheers,

Chris

[Zakalwe]

While beneficial during telescope cooldown, it now works against us by cooling the air inside the telescope to below the temperature of the corrector plate. So the corrector plate now loses heat to the air inside the telescope as well as to the night sky. If the corrector plate temperature drops below the dew point then dew rapidly forms.

 

 

I've had dew forming once or twice on the primary mirror but not on the corrector. I use a heatband around the corrector which prevented it dropping below the dewpoint. The primary cooled enough to cause the moisture in the air insode the OTA to condense onto the primary. Needles to say, that ruined the nights imaging and the only option was to bring the OTA inside to let the primary warm up.

The only way that could happen is if the primary is losing heat and falling below the dewpoint.

[sharkmelley]

This is very interesting - especially for planetary imaging.  Tube currents are a known problem for an SCT and the usual advice is to let it cool to ambient temperature before imaging.  I've never previously thought about the potential internal currents caused by the top of the OTA being cooler than bottom because of the top radiating heat. 

In my opinion it still makes sense to let the scope cool to ambient and then put a foam camping mat around it.  Since I have two C11 scopes I am minded to try a side by side experiment - one insulated and one not.  Needs a clear night though

Mark

[Zakalwe]

Two C11s??? :eek:

Never mind insulating them....I want to hear that you've made a monster set of 11" binos... :grin:

[cgarry]

This is very interesting - especially for planetary imaging.  Tube currents are a known problem for an SCT and the usual advice is to let it cool to ambient temperature before imaging.  I've never previously thought about the potential internal currents caused by the top of the OTA being cooler than bottom because of the top radiating heat. 

In my opinion it still makes sense to let the scope cool to ambient and then put a foam camping mat around it.  Since I have two C11 scopes I am minded to try a side by side experiment - one insulated and one not.  Needs a clear night though

Mark

The initial cooling down to ambient temperature is important as this allows excess heat held in the scope's glass components to dissipate and should not be compromised by insulating the tube.  I use forced air cooling with my C14 so I do not need to bother removing the scope's jacket for this cool down stage.

Another thing to consider is the dew shield.  On my C14 I found there were also currents in my Astrozap dew shield on some nights - camping mat dew shields really are better than the expensive commercial solutions!

Cheers,

Chris

[Peter Drew]

I have 2 16" SCTs, one in a 30 foot dome and one in a 8 foot dome. The former never dews whilst the latter requires a dew band. Both give excellent views almost immediately provided that the seeing is good, I have noticed however that the image sometimes deteriorates over time. I've always put this down to changes in the seeing but in the light of this discussion it might be worth trying some insulation. 

[ollypenrice]

Two C11s??? :eek:

Never mind insulating them....I want to hear that you've made a monster set of 11" binos... :grin:

Ralf Ottow has such a beast - using self made correctors and secondaries to provide Edge spec flatness and a longer focal length. Although the necessary diagonals will come to focus the result is no longer diff limited so Ralf extended the FL to correct this. The view is rather good! But he's building the Binosaur from scratch. A 40 cm SCT binocular...

It seems that insulating SCTs is very much in favour in the Netherlands at the moment. If I have it right the idea is not to worry about cooldown, which of itself is deemed less important than reducing tube currents. A slow, progressive cooldown produces fewer tube currents than a fast one. I have no opinion on the matter, though, since I know nowt about it!

Olly

[Steve Ward]

Maybe the smaller dome 'collects' more heat from the observers over time ... ?

I also wonder what effect a 'warm' observer can have on a tube , it doesn't take much of a temperature gradient to start convection currents , if the difference between sky-facing and ground-facing sides can create airflow then it must follow that a cold clear sky at one end and a body at the other would do the same ... ?

[Zakalwe]

[thread hijack]

I wonder if this monster ever was finished?

http://www.astronomyshed.co.uk/forum/viewtopic.php?f=35&t=9162

http://stargazerslounge.com/topic/142945-hexy-635mm-bino-update-tubes-skywards/

A 630mm set of binos. :eek:

[/thread hijack]

[Zakalwe]

Maybe the smaller dome 'collects' more heat from the observers over time ... ?

I also wonder what effect a 'warm' observer can have on a tube , it doesn't take much of a temperature gradient to start convection currents , if the difference between sky-facing and ground-facing sides can create airflow then it must follow that a cold clear sky at one end and a body at the other would do the same ... ?

Quite a big effect. I can see the difference if I stand directly under the dewshield when I'm imaging.  I would think that in a RORO obsy the biggest cause of local seeing problems could be the human. How much heat do we pump out? 100 Watts per hour comes to mind?

[Steve Ward]

Sounds to me like the ideal thing is sufficient cooldown for the glass , long dewshield and an insulating jacket to isolate tube from external thermal influences.

Always amazes me that those poor little photons have travelled relatively unscathed for umpteen million miles only to be buffeted and battered in the last two feet of their journey ... 

[Zakalwe]

Sounds to me like the ideal thing is sufficient cooldown for the glass , long dewshield and an insulating jacket to isolate tube from external thermal influences.

Always amazes me that those poor little photons have travelled relatively unscathed for umpteen million miles only to be buffeted and battered in the last two feet of their journey ... 

Or how they manage to make it across hundreds of millions of light years without getting absorbed by dust clouds, gobbled by a black hole or red-shifted into the infra-red. And then get blocked and absorbed in the all-pervading blanket of moisture that seems to exist permanently above my observatory :mad:

[Steve Ward]

Not to mention the initial million years or so that they need to get from their point of creation deep inside a star to actually escaping from the photosphere and 'beginning' the journey.

[JamesF]

I did some reading on radiative temperature transfer last night and decided that I can justify to myself that it is possible to lower the tube temperature below ambient this way.

Radiation (in the form of heat in this instance) happens continuously for any body that isn't at 0K, so an OTA is constantly emitting heat.  In a closed system everything reaches the point where it's all radiating heat at the same rate and it's all then at the same temperature.  With an OTA however, heat can be radiated out into the sky resulting in the cooling of the tube, but there's no requirement that this be matched by the amount of heat it absorbs from other sources.  In the instance that a few posters have documented with the "top" and "bottom" of the OTA being at different temperatures, one might imagine that's because the OTA is radiating heat in all directions, but the half of the scope facing downwards is also absorbing heat radiated by the planet, whereas the half facing the sky is receiving very little heat radiation from other sources by comparison.

The process of radiation itself cares nothing for the ambient temperature, so a mirror, say, at 10C will radiate just as much heat when the ambient temperature is 20C as it will when the ambient temperature is 0C.  What makes the perceived difference is the amount of radiation the mirror is absorbing from the environment.  The OTA will therefore continue radiating heat and the temperature dropping until the amount of heat radiated out is in balance with the amount absorbed.  If an OTA starts at ambient temperature and is radiating heat in all directions and is only absorbing heat through that half facing the (ambient temperature) ground then it appears logical that it might well cool below the ambient temperature.  The rate of radiation apparently varies with the fourth power of the absolute temperature, so I guess it might well be possible to roughly model how much the temperature would have to drop inside the tube for the emission and absorption from a fixed temperature environment to balance each other.

I think it probably also supports the hypothesis that the effect of insulating the OTA is actually to slow down radiation, thus slowing the cooling of air in the OTA and reducing the effect of tube currents.

James

[JamesF]

I keep reading the statement that the OTA can "cool below ambient temperature". I stand to be corrected, but I believe this is simply impossible. If it were possible, you would have invented a perpetual motion machine. Suspend 1000 OTAs inside a giant thermos flask and put a little windmill below the OTAs, driven by the downdraft of all the scopes cooling below ambient. We've solved the energy crisis!

I think I see the flaw in this, Agnes.

Presumably the air forming the downdraft will pool at the bottom of the "flask" because it's cold.  Whilst the downdraft is getting colder and colder the air will cycle.  Eventually however the air will reach the point where radiation to the sky is matched by radiation inwards through the walls of the flask and the system will stop.

That's very similar to what I now believe happens in the OTA.

James

[Zakalwe]

^^That's pretty much how I see it James. Good post.

I have noticed that when imaging Jupiter I have started out with nice sharp details. I then get into a rhythm of running off R, G, B data sets. I do a trial stack and sharpening on the first couple to check focus and so-on. When I come to process the data the next day, the last sets are invariably worse, hence my thoughts on insulating the OTA. I also need to check focus more frequently during the night, but I'm hoping that the tube insulation removes one variable.

The problem with this gig is that so much is subjective and variable. Is the degradation down to worsening seeing (though I'd hope that the seeing would improve as the night moves on), or contraction of the tube altering focus or some other issue???

[Zakalwe]

I think I see the flaw in this, Agnes.

Presumably the air forming the downdraft will pool at the bottom of the "flask" because it's cold.  Whilst the downdraft is getting colder and colder the air will cycle.  Eventually however the air will reach the point where radiation to the sky is matched by radiation inwards through the walls of the flask and the system will stop.

That's very similar to what I now believe happens in the OTA.

James

There's 3 ways of moving heat:

Conduction

Convection

Radiation

A Dewar flask tries to control and minimise all three. Enclosing the OTAs within a Dewar flask makes the system a closed loop system. In comparison, an OTA open to the night sky (which will be at something like -50C) is very much an open-loop system.