Insulating an SCT

[JamesF]

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???

That is indeed a problem.  We (that is, all of us) might perhaps reach an agreement though that tube currents are bad, and that they will be minimised if the temperature differential inside the OTA is as small as possible.  That is at least something we might stand a chance of measuring at an amateur level.

I love the idea of having a pair of C11's to experiment with.  Sadly I don't think my wife would buy the "I found this old dustbin and made it into a telescope" explanation

James

[cgarry]

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???

A star test will tell you if it is seeing or tube currents in a few seconds.

[Davey-T]

Not wanting to divert to another subject but does fitting a fan /fans in the bottom end of SCTs help, there are a few examples of this around You Tube etc.

Dave

[JamesF]

Not wanting to divert to another subject but does fitting a fan /fans in the bottom end of SCTs help, there are a few examples of this around You Tube etc.

For reducing cool-down time I'd have thought it would certainly help.  For maintaining the temperature inside the OTA afterwards I'm unconvinced that it's a good idea.  It seems to me that fans must surely create tube currents when they're running.

James

[Steve Ward]

When trying to minimise the air moving in the tube I suspect that fans would be unhelpful myself.

[cgarry]

Not wanting to divert to another subject but does fitting a fan /fans in the bottom end of SCTs help, there are a few examples of this around You Tube etc.

Dave

With big SCTs there can be an issue if the ambient temperature keeps falling throughout the imaging session and the primary mirror cannot lose heat fast enough to follow it.  When this happens a boundary layer can form on the mirror and degrade the view.

The fans in the bottom of SCTs are designed to solve this by allowing the primary mirror to lose this excess heat and stay close to ambient.  Whether you would run the fans whilst actually capturing data is another question.  I am going to add fans like this to my C14 when they finally arrive.

Chris

[Steve Ward]

Having just watched some videos regarding similar things , I'm convinced that eliminating external sources of heat will be beneficial.

The effect of a warm hand near the mirror on these is pretty dramatic ...

 

[Zakalwe]

With big SCTs there can be an issue if the ambient temperature keeps falling throughout the imaging session and the primary mirror cannot lose heat fast enough to follow it.  When this happens a boundary layer can form on the mirror and degrade the view.

The fans in the bottom of SCTs are designed to solve this by allowing the primary mirror to lose this excess heat and stay close to ambient.  Whether you would run the fans whilst actually capturing data is another question.  I am going to add fans like this to my C14 when they finally arrive.

Chris

Paul Haese cools the rear cell of his C14 with Peltiers:

Details here:

http://paulhaese.net/peltierstoSCT.html

The quality of his images are outstanding, which is probably the ideal combination of good equipment (a C14 is a superb planetary scope), skill and an excellent location. He is very clear that his management of the thermal conditions inside his SCT has made a massive improvement to his imaging  "It was and remains the best modification that I could have conducted on any piece of astro equipment I have owned or currently own"

Now would those mods make my images as could as his? A bloke with much more limited ability, with a C11 and living in a rubbish location (urban location surrounded by houses and under the jetstream)? no it wouldn't. But it may help, and its a very cheap mod to experiment with.

[JamesF]

With big SCTs there can be an issue if the ambient temperature keeps falling throughout the imaging session and the primary mirror cannot lose heat fast enough to follow it.  When this happens a boundary layer can form on the mirror and degrade the view.

The fans in the bottom of SCTs are designed to solve this by allowing the primary mirror to lose this excess heat and stay close to ambient.  Whether you would run the fans whilst actually capturing data is another question.  I am going to add fans like this to my C14 when they finally arrive.

Chris

If what I think we're saying regarding tube insulation is true then I'm not sure that the failure of the OTA to follow falling ambient temperatures during a session are that much of an issue, because the stability of the temperature inside the OTA is more important than its temperature.  In fact, it may well be that the OTA is ahead of falling ambient temperatures in any case.

I can see that by getting to ambient temperature or thereabouts before using the OTA however there may be other benefits, not the least of which is that the focusing of the OTA may remain more accurate because during the course of the session the OTA has remained in a narrow range of temperatures.

James

[jambouk]

I'd have thought the ideal situation was to keep all of the tube as near ambient as possible; insulating the tube would hider heat loss as ambient dropped but tube temperature didn't match it.

On still nights, there is apparently a pocket of much warmer air near the ground (maybe up to 4-5 feet above it) which could bathe the bottom of the tube in warmer air than the top, so insulating the top in this situation may give a more even temperature tube but it would still be well above the ambient at 6 feet above the ground say.

Does Damian Peach insulate his tubes?

Jd

Sent from my iPhone using Tapatalk

[cgarry]

If what I think we're saying regarding tube insulation is true then I'm not sure that the failure of the OTA to follow falling ambient temperatures during a session are that much of an issue, because the stability of the temperature inside the OTA is more important than its temperature.  In fact, it may well be that the OTA is ahead of falling ambient temperatures in any case.

I can see that by getting to ambient temperature or thereabouts before using the OTA however there may be other benefits, not the least of which is that the focusing of the OTA may remain more accurate because during the course of the session the OTA has remained in a narrow range of temperatures.

James

I disagree here.  If the mirror gets warmer than ambient then a boundary layer will form on the mirror and this will destroy the view - this is a well known and understood problem.  It is a different problem to the temperature gradient across the tube but it is a problem.

Chris

[cgarry]

Just to throw a bit more into the mix, this is the dew shield I made.  This fixed the currents I was getting in the Astrozap shield.

Cheers,

Chris

[JamesF]

I disagree here.  If the mirror gets warmer than ambient then a boundary layer will form on the mirror and this will destroy the view.  It is a different problem to the temperature gradient across the tube but it is a problem.

And this would occur because the temperature of the air in the OTA had fallen with ambient, but the mirror temperature hadn't?  I'm just trying to work out the mechanics of how we'd arrive at such a situation.

James

[JamesF]

Just to throw a bit more into the mix, this is the dew shield I made.  This fixed the currents I was getting in the Astrozap shield.

That's very space-age

James

[cgarry]

And this would occur because the temperature of the air in the OTA had fallen with ambient, but the mirror temperature hadn't?  I'm just trying to work out the mechanics of how we'd arrive at such a situation.

James

Yes, the mirror's temperature would lag behind the ambient ambient temperature because of its thermal mass.  The fans on the bottom of the SCT help the mirror track the ambient temperature and reduce the problem.

I would speculate that insulating the SCT tube would possibly reduce the problem because the air inside the tube is not able to follow the ambient temperature so easily, but I don't think it would remove the problem.

Chris

[JamesF]

I'd have thought the ideal situation was to keep all of the tube as near ambient as possible; insulating the tube would hider heat loss as ambient dropped but tube temperature didn't match it.

I'm not entirely ready to write ambient temperature off as irrelevant myself, but I think in response one might ask "Does ambient temperature matter at all if the temperature inside the OTA is stable, and if so, how?"

If it were the case that the temperature needed to be as near ambient as possible and given that the OTA temperature may actually drop below ambient, should we also have tube heaters to prevent the latter situation occurring?

James

[Davey-T]

Nice dew shield Chris but it won't fit in my obs'y. 

Perhaps if I fit fans in the obs'y base sucking air out it might help suck down cooler air from above.

As everyone is agreeing to disagree it needs someone to do proper experiments, bit difficult with our weather though, same old problem of needing two identical setups running.

Dave

[JamesF]

I would speculate that insulating the SCT tube would possibly reduce the problem because the air inside the tube is not able to follow the ambient temperature so easily, but I don't think it would remove the problem.

I'd agree that it may well not be possible to remove the problem.  Or at least, not without some sort of active temperature management system that is probably well beyond the ability of most amateur astronomers to assemble.  "Reasonable mitigation" would seem to be the best we can hope for.

James

[Davey-T]

I'd agree that it may well not be possible to remove the problem.  Or at least, not without some sort of active temperature management system that is probably well beyond the ability of most amateur astronomers to assemble.  "Reasonable mitigation" would seem to be the best we can hope for.

James

Lucky Gina hasn't got a big SCT as that sounds like a challenge that would set her off

Dave

[JamesF]

Paul Haese cools the rear cell of his C14 with Peltiers:

"There wasn't room to fit coolers to my C9.25 so I had to buy a C14" has to be one of the weakest excuses I've yet heard

James

[JamesF]

Lucky Gina hasn't got a big SCT as that sounds like a challenge that would set her off

Dave

Hush, before she notices this thread

James

[JamesF]

As everyone is agreeing to disagree it needs someone to do proper experiments, bit difficult with our weather though, same old problem of needing two identical setups running.

I'm very much a fan (hah!) of the empirical approach.  I think there would be a good deal to be learnt by testing out people's ideas of what is happening.  It is sadly disappointing that our weather makes such a venture so difficult.  And even more disappointing that I don't have a pair of C14s to work with

I have also begun to wonder if there's any reason that an SCT should be solid at all, and if it a truss-style arrangement similar to some RCs might improve performance.  I'm really not ready to go there yet though

James

[IanL]

A scope can indeed fall below 'ambient' temperature due to radiation. It's the same mechanism that causes radiation fog, ground frost and dewing/frost on car windows. The OTA radiates heat in to space, and space radiates heat back.  So your scope will be radiating at say +10C (initially), and on a clear night space will be radiating back at approximately -270C, thus there is a net loss of heat to space due to radiation. On a cloudy night however, the clouds radiate back at a similar temperature to the ground so there is much less net heat loss if any at all; hence no frost or dew on your scope (but no stars to observe either).

Of course the scope will also be gaining heat by radiation from the ground; let's be kind and say the ground is a balmy +5C and also that were on a billiard-table flat field with no buildings or trees above the horizon.  So the OTA 'sees' 50% of its view as sky at -270C and 50% as ground at +5C; a difference of 260C. (By view I don't mean the image through the eyepiece, I mean the exposure of the whole OTA to the environment).

Eventually the scope will cool to way below freezing, since the net heat gain from the ground is not sufficient to compensate for the much larger net heat loss to the sky.

Next we can bring in the air at say +15C too; problem is that air is a very good insulator and the net heat gain (by conduction) is usually too small to make up the difference (unless it is a hairdryer applying much hotter air to the corrector!) There is also an even smaller amount of thermal conduction via the mount/tripod to the ground.

You can satisfy yourself of this because dew and frost do not form on windy nights (in temperate zones at least). The layer of air in contact with the cold surface of the tube / corrector / lens is blown away and replaced by warmer air before it can cool by conduction to below the dew point.  Conversely on still nights, the layer of air closest to the scope is in contact for a longer time and can cool below the dew point.

Eventually it will be replaced by new/warmer air as it sinks away from the scope due to convection, the new air cools, dews, sinks, etc.  (This is why cutting a small hole at the lowest point of your foam dew-shield may help with preventing dew formation - the colder air pooling near the corrector will sink through the hole to be replaced by slightly warmer air from the front of the dew-shield - conversely if this convection is too effective, you are creating your own bad seeing!)

So no perpetual motion machine required, move along, nothing to see here!  If there was no or insufficient heat input to the system, eventually the surface of the planet would cool to near -270C to reach thermal equilibrium with the sky.  If you want to see what that looks like, New Horizons will be reaching Pluto in July 2015; I believe the mission was timed to get there before whatever atmosphere it has completely freezes out as the ex-planet moves further away from the Sun on its orbit.

That doesn't happen on the Earth since we have plenty of heat input from the Sun on a daily basis (even if we don't get to see it as much as we'd like on the ground!) Bear in mind that there is a massive temperature difference between the equator and the poles (especially during winter when parts above the Arctic circle receive no direct radiation at all during the course of the day for part of the winter, and due to the way the atmosphere circulates the cold air at the poles tends to stay there). Basically a large component of the temperature difference is due to the difference in the amount of incoming solar radiation (further exacerbated by snow and ice being very effective at reflecting it back out in to space again).

Some other planets are heated by solar radiation, internal heat from their formation and/or radioactive decay and tidal effects due to the gravitational pull of other nearby bodies (e.g. Io).

Now back to the point of this thread:

Our problem is differences in temperature between components of the OTA which causes convection in a fluid (the air in the tube), which affects the quality of the image we see.  If the SCT was a closed system, eventually it would reach thermal equilibrium and there would be no convection and no tube currents or boundary layers to worry about.

But the OTA is not a closed system; it is exposed to the environment which can cause the different components (the tube, the castings, the mirror, the baffle tube, the corrector, etc.) to be at different temperatures. The layer of air closest to a component will be heated or cooled by conduction, and the resulting differences in air temperatures in the different parts of the tube create the unwanted convection currents or boundary layers.

You'll notice above that I put quotes around the term 'ambient' temperature.  I think that's the key to the whole conundrum.  There is no such thing as 'ambient' temperature in a complex system like this; we have already established that the ground, the air and the sky are at markedly different temperatures:

- We're most concerned with net gain or loss of heat by radiation, as there is a huge temperature difference between the sky and the ground.  (Even if the ground is covered in ice and it is -30C out, you have a difference of about 240C between ground and sky).  It's not uniform as others have noted, parts of the OTA 'see the ground' and don't cool as rapidly, or indeed may warm up if the scope started out below the ground temperature (keep it in a fridge?)

- The air is more complex; it may be still or it may be moving. The more the air is moving, the more important it becomes since it will carry more heat away more quickly than still air. (I.e. the 'wind chill' factor, but without the evaporative mechanism that makes it even worse for humans).  

Cooling the scope to the local air temperature temperature in and of itself is meaningless; we've already established that the scope will continue cooling through the night due to net radiative heat loss to the sky, so getting it to the same temperature as the air outside the tube (which is what I assume most writers mean by 'ambient') is no help. What we really want to do is get all the components of the scope to the same temperature and keep them there so that there is no difference in internal air temperature and thus no convection currents. Getting the scope to the outside air temperature may seem helpful, since there is no net heat gain/loss by conduction once you do, but it may be a false dawn as the scope will tend to fall below the air temperature fairly quickly afterwards due to net radiative loss (hence dewing) so you still have a temperature gradient (albeit reversed now).

- Cooling the scope is one method to try to reach thermal equilibrium.  Some parts of the scope are exposed to the sky and cool rapidly, others are not (the mirror and baffle are enclosed and can't radiate directly except by way of the relatively small area of the corrector, plus they are in contact with a nicely convecting bunch of warm air which will provide some additional heat transport from component to component and location to location). Eventually the internal components and the exposed ones may reach a similar temperature through radiation, and presumably by using fans to cool the biggest thermal mass of all (the mirror) we can use conduction to speed up that process.  Equally it might be that the outside continues to cool faster than the inside components through the night and they never reach equilibrium.  I'd guess the external air temperature and wind speed are probably the governing factors here, but I don't know how you'd model it - you need an engineer of some sort, but experience shows that some nights you get lucky with the conditions and the tube currents reduce, but on other nights they continue for the whole night.

- I'd say insulating the scope should be equally valid an approach and perhaps better.  If you completely prevent heat loss by radiation from the exposed elements, then they should reach thermal equilibrium with the enclosed elements eventually.  It would work fine if you put everything in a well insulated box of some sort, but in practice the corrector has to be exposed to the part of the sky you want to image.  Using a dew shield and dew heater should compensate for radiative heat loss via the corrector to some extent. I think insulating ought to be more effective than cooling, since you are removing a lot more of the variability due to the air temperature and wind speed. and for a decent amount of good insulation you should be able to produce a much more predictable behaviour for the scope.  If the insulation of the scope and dew shield was perfect, your only variables for a given scope would be the air temperature on the corrector and its interaction with the dew heater.  I.e. far less variable than the external air temperature and the wind.  Of course the insulation won't be perfect, but it wouldn't be hard to make it very good.

- Finally you could reach thermal equilibrium by heating the whole scope to balance out the net radiative loss to space.  Maybe a couple of infra-red lamps or a patio heater  I don't think it is a serious/viable suggestion but theoretically it is just as valid as the other two options above.

[Zakalwe]

- Finally you could reach thermal equilibrium by heating the whole scope to balance out the net radiative loss to space.  Maybe a couple of infra-red lamps or a patio heater  I don't think it is a serious/viable suggestion but theoretically it is just as valid as the other two options above.

Another great post, but I disagree with the last piece. It's not valid as you would be creating a heat plume around the scope. This is what causes bad seeing...thermal air currents and pockets of air of different densities.

Again, AFAIK, you are aiming to get the mirror and the air within the scope to be as close to each other's temperature as possible. This alone prevents tube currents and boundary layers from forming. You can't insulate the corrector (well you could, but you wouldn't be able to see through it!), so a dew band is used to stop that dropping below the dewpoint. The perfect solution, I assume, would be to have the corrector, mirror and the air inside the OTA all at the same temperature and just above the dewpoint.

With regards to a truss SCT, I guess that there's no other reason other than manufacturing costs for not having them. A truss would cost a lot more than a simple rolled steel tube though. I have heard of people cutting openings in SCT tubes to allow fans to blow air across the mirror. I guess that's what Celestron is attempting to do when they fit air-holes to their Edges-series SCTs. Taking a hole-saw to my C11 is fair few steps too far for me though!. As a modification, reversible it isn't!

[JamesF]

As a modification, reversible it isn't!

Quite.  I'm not ready to go there yet.

James