Insulating an SCT

[JamesF]

I've seen a few people doing this now but not really seen much in terms of the reasons why, so I thought I'd start a thread for discussing it.

We might start with the requirement for OTAs to be at ambient temperature for ideal observing conditions.  I don't know if there are other reasons for this but the one I've always understood is because once the air inside the OTA is all at the same temperature tube currents and thus distortion of the image inside the tube are reduced.

Obviously with a bit SCT or Mak this is a particular problem because there's a lot of air inside and not much opportunity for exchanging it with the external environment, hence some of the coolers on the market.  It's still a problem however as air temperature changes through a session, particularly if it changes fast.

Perhaps however the association of "being at ambient temperature" and "minimised tube currents" is not completely correct.  Perhaps, particularly with SCTs and Maks, we should be focusing (ha! more on stabilising the tube temperature so it is close to the same everywhere.  That being the case, perhaps tube currents should not exist.

(Assume I'm just considering SCTs and Maks from this point on, unless you want to chip in with something relevant to other designs.)

Heat can presumably escape from the OTA by conduction through the tube or radiation.  If that can be stopped or significantly reduced, perhaps the internal tube temperature will stabilise.  So perhaps this is some people are now wrapping their scopes in foil-backed bubble-wrap.

My understanding from the building world where this bubble wrap is installed by charlatans in all sorts of useless places is that this sort of insulating material is ok-ish at reducing conduction, but really needs an air gap to reduce radiation.  So wrapping the OTA directly perhaps doesn't seem likely to help a huge amount unless the significant part of the heat loss is through conduction.

So what are peoples' thoughts?  And of those who already do insulate their OTAs, where did you get the idea?  Is there any empirical evidence to support the practice?  What happens with that big bit at the front that you can't insulate -- the corrector?

I'd like to be persuaded that this works.  I'm not completely convinced yet...

James

[JamesF]

Perhaps it might also be worth discussing if, once insulated, you continue to use a dew shield and/or dew heaters.  And do you go completely mad and insulate the back of the secondary?

James

[Zakalwe]

I'm experiementing with it, but have yet to have the clear weather to try it out.

Firstly, the bubble wrap insulation contains air pockets.  That provides a thermal barrier.

An SCT OTA is normally made of thin steel., which, in my case, is painted black. It's exposed to the sky, so it will quickly radiate away any heat, thus falling below the ambient air temperature. This explains why the front corrector plate is such a dew magnet....it is pointed at the sky and rapidly drops below the ambient air temperature, thus forming a surface below the dewpoint. If the OTA body drops rapidly in temperature then it's conceivable that currents will form in the tube.

By insulating the OTA body you are preventing the OTA outer skin from radiating any heat away.

Will it make a noticeable difference? I don't yet know. If it does, then well and good. If it doesn't I can unwrap it and throw £8 of foild backed insulation into the bin.

I still intend to use the dew heater. In my experience, the corrector will fog in no time without a heatband.

[JamesF]

By insulating the OTA body you are preventing the OTA outer skin from radiating any heat away.

Hmmm.  One of the things this stuff is apparently not supposed to be used for is insulating pipework, at least not where it comes into direct contact with the pipe, precisely because it isn't good at preventing radiative heat loss unless there's a gap.

I wonder if the temperature differential is relevant somehow -- perhaps it's good enough when the differential is, say, 10C, but not close if the differential is 50C?

I'm pleased you're up for experiment with it though.  Are you planning on just making a subjective judgement on whether it improves performance, or do you have some temperature sensors rigged up or anything like that?

James

[Steve Ward]

I've got a conveniently sized off-cut of Actis Triso-Super 10 and am currently doing the Mak180 , it's far from cheap unfortunately so a friendly local builder might be in order.

Mainly doing it to help with being exposed to the heat of a sunny days solar work midsummer but will be interested to see how it does in the dead of night too.

It's a far superior product to the 'mickey mouse' foil bubble wrap which has little or no insulating properties . ( I use it in preference to Kingspan type foams every time ) 

[Steve Ward]

The biggest problem I've encountered with the Mak is the plume of 'warm' air coming off the baffle tube , it gives a very telling spike on a defocused diffraction pattern .

I guess the heat escapes from the tube through the thin walls readily enough but the baffle then radiates for longer , I always wait until the spike disappears before imaging with it at night , a little trickier to determine stability in daytime however .

[JamesF]

I'd not thought about it from the point of view of solar imaging, but yes, I can see it may well be a good plan.  Should be very interesting to see how that goes.

I've also wondered whether performance might be different by combining two types of insulating material.  Some sort of dense foam, say, with the foil-backed bubble wrap or similar around that.

I'm just not quite ready yet to dismantle my C9.25 to fit a load of temperature sensors inside it

James

[Steve Ward]

Put the front cap on , stand it face down in a bucket , and fill with squirty foam , what could possibly go wrong ... 

[JamesF]

We call that stuff "squirty brains", 'cos that's how it looks sometimes.  Wonderful stuff, though.  Has all sorts of uses.  Some of which the gods of DIY certainly aren't going to let me into heaven for

James

[JamesF]

Actually, it would be a bit tedious, but with a hot wire and a bit of patience it might well be possible to make an expanded polystyrene jacket.  A bit like those buoyancy jackets small children have for swimming pools.  Given the recent weather having a scope that floated might be quite handy, too

James

[Steve Ward]

Always got a couple of tins to hand , saved the day more than once.

The old stuff was a 'mare , it just kept expanding for hours , the new stuff is much more forgiving ... 

Don't bother with the nasty cheap one-shot canisters , get a proper gun for it , controllable application and no waste ... 

[Steve Ward]

Expanded polystyrene is actually a pretty poor insulator , and not the most stable product .

And as for the mess ... !

[m.tweedy]

There was an interesting discussion here

http://stargazerslounge.com/topic/139174-insulating-a-telescope/

on this subject. DewNot and the Sky at Night magazine covered this recently also.

[Peter Drew]

I'm familiar with the practice of wrapping an OTA with reflective material to protect one left out during the day from solar heating. I'm not sure about the benefit during nightime observation, specially in the UK. As well as the air contained in the OTA, the optics themselves have to reach thermal equilibrium as both focal plane and optical correction vary during the cool down period. I don't know, but I would have thought that insulation at night would prolong the process. Always good that someone gives it a try. 

[JamesF]

There was an interesting discussion here

http://stargazerslounge.com/topic/139174-insulating-a-telescope/

on this subject. DewNot and the Sky at Night magazine covered this recently also.

There do seem to be a lot of people saying "X does Y" without offering hard evidence though.  I agree that some positions are scientifically plausible, but I do like evidence.  I'd have thought that if it did work, or even if it doesn't and people still say that it does, there'd be at least a few people who have collected hard data and demonstrated that it either is, or isn't, effective.

James

[m.tweedy]

I can only go off my subjective evidence and think the camping mat makes mine a bit better but have no hard evidence. However it does protect it from the knocks which i do give it frequently.

[JamesF]

In the light of what we've got so far perhaps that's as good a place as we can expect to start, Michael.  Can you explain in what way you perceive your scope to perform better when it is insulated?

James

[JamesF]

Oh, and what scope is it?

James

[Zakalwe]

Insulating an OTA will prolong the cooldown, I would think. If the OTA is stored inside the house then insulating the OTA would make the cool-down a lot longer, and worsen the situation.

However, my OTA is kept in the obsy, so it should be near ambinet when I start. What I am trying to see if the isulation of the OTA prevents it cooling below ambient. The black painted, thin-walled OTA easily radiates any heat away as it is facing the night sky, which is a lot colder than the ambient air at ground level (ever notice the difference in ice on a car windshield when you park it facing a wall compared to facing the car away from a wall? The car facing the wall will not ice it's windshield up as much as it cannot radiate heat away. The wall reflects it back. In comparison, the car facing away will ice up much more as the windshield is facing the night sky).

[cgarry]

Hi James,

I absolutely need to have my C14 insulated with radiator foil or I get very nasty tube currents forming, I spent a lot of time diagnosing this issue.

The issue I was seeing is the upwards facing part of the scope was largely facing the sky whilst the bottom facing part of the tube was largely facing the ground. In my situation I found that the top of the tube was up to 5 degrees cooler than the bottom of the tube and this generated horrendous tube currents than never stopped as the cause of the temperature differential was always there.  Once I wrapped the tube in radiator foil the problem went away!

Cheers,

Chris

[m.tweedy]

C11 stored in a shed so nearly at ambient.

Observing with the society at our dark (cold) site I was getting shimmering that was getting worse as the night went on. not replicated in other scopes (smaller aperture) around me. Looked like thermal currents but in reverse ie worse when cold.

Was told by more experienced member about scope getting too cold.

this Quote from Sly at Night magazine seems to tally with my experience.

"The chilling comes from the fact that a clear night sky gives off very little thermal (heat) radiation and its radiative temperature can be many tens of degrees colder than than the local air temperature. Surfaces facing this cold night sky that are radiating their heat away get back little from the sky in return, and as a result can cool down  by several degrees relative to the ambient air temperature. it is only warmth conducted back into them from the surroundings that stops them cooling further".

 

and this from Dew Not

This means that the telescope (warm) gives up its heat to outer space (cold). A dew shield helps because it reduces the area of night sky that is robbing heat from the corrector plate. The telescope tube is also affected by radiation cooling because it has a large surface area exposed to the night sky and metal is very efficient at radiating heat (this is why metal objects dew up quickly). 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.

 

So it now wears an overcoat of camping mat, dew band and dew shield.

Again Subjectively this seems to have solved the problem For me. I have no objective proof though so am unable to back up my ramblings with any scientific proof.

[m.tweedy]

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

[Ags]

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!

[JamesF]

I absolutely need to have my C14 insulated with radiator foil or I get very nasty tube currents forming, I spent a lot of time diagnosing this issue.

The issue I was seeing is the upwards facing part of the scope was largely facing the sky whilst the bottom facing part of the tube was largely facing the ground. In my situation I found that the top of the tube was up to 5 degrees cooler than the bottom of the tube and this generated horrendous tube currents than never stopped as the cause of the temperature differential was always there.  Once I wrapped the tube in radiator foil the problem went away!

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

[JamesF]

C11 stored in a shed so nearly at ambient.

Observing with the society at our dark (cold) site I was getting shimmering that was getting worse as the night went on. not replicated in other scopes (smaller aperture) around me. Looked like thermal currents but in reverse ie worse when cold.

Was told by more experienced member about scope getting too cold.

this Quote from Sly at Night magazine seems to tally with my experience.

"The chilling comes from the fact that a clear night sky gives off very little thermal (heat) radiation and its radiative temperature can be many tens of degrees colder than than the local air temperature. Surfaces facing this cold night sky that are radiating their heat away get back little from the sky in return, and as a result can cool down  by several degrees relative to the ambient air temperature. it is only warmth conducted back into them from the surroundings that stops them cooling further".

 

and this from Dew Not

This means that the telescope (warm) gives up its heat to outer space (cold). A dew shield helps because it reduces the area of night sky that is robbing heat from the corrector plate. The telescope tube is also affected by radiation cooling because it has a large surface area exposed to the night sky and metal is very efficient at radiating heat (this is why metal objects dew up quickly). 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.

 

So it now wears an overcoat of camping mat, dew band and dew shield.

Again Subjectively this seems to have solved the problem For me. I have no objective proof though so am unable to back up my ramblings with any scientific proof.

Hmmm.  I'm not convinced that with tube currents the absolute temperature is the necessarily the problem.  I think it's the temperature differential.  Chris's post seems to me to reinforce that hypothesis.  If all the air in the OTA is at -10C, or 0C, or 10C or even 20C, why would there be any tube currents?  But if the top of the OTA is at 0C and the bottom is at 10C (whether the top be the top half of the tube, or the corrector plate facing upwards), then surely convection must occur and tube currents will happen.  Perhaps your subjective evidence supports that view as well.

To my way of thinking the S@N piece suggests that if you insulate the OTA then it might get even colder than it was before because there's no heat getting into the OTA at all.  The difference perhaps being that the temperature drop may occur more slowly or more evenly, perhaps, when the tube is insulated.  Not sure about that one.

James