Insulating an SCT

[JamesF]

So, of those people who have or are in the process of insulating their OTAs, what have you used?

Steve Ward has said he intends to use Actis Triso Super 10, which is indeed hideously expensive unless you can find an offcut.

Stephen, I assume you're using something like this? http://www.screwfix.com/p/thermawrap-loft-insulation-400mm-x-5m/76477#

Given that it's radiative heat transfer that's the issue I'd assume something with a reflective layer that doesn't touch the OTA would be preferred.  Any other ideas?

James

[ollypenrice]

In order to tame focus drift during cooldown on the FSQ 106 I always run the dew heaters round the front elements. If they are not needed against the dew I switch them on a little after I start imaging. I might even try heating the rear element. Does it do any good? I don't know (sorry!) but I get the feeling that it does slow down the drift.

Olly

[IanL]

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!

The last remark was slightly fatuous, but actually it is just as valid as the other two approaches.

In all three cases, if the OTA is at a significantly different temperature to the surrounding air, then you will have convection - either rising air if it is warmer, or falling air if it is colder. Bear in mind that the two situations are exactly the same; there is some colder air sinking and there is some warmer air rising so you have convection and air of different densities moving around in front of the OTA.

What you need to do in the third (heating case) is apply just enough heat input to counteract the heat loss due to the net radiation to space plus convection to the surrounding air.  You'd probably be best doing that either by conduction (big dew heater 'nappy' rather than a band) or perhaps by infra-red radiation from some heat lamps or your granny's three-bar electric heater perhaps.  I don't think it would actually be any harder than trying to reach thermal equilibrium by cooling - we just imaging cooling is easy/effective since it is passive, but actually we have even less control than if we were heating, where at least we can turn the wick up and down in response to measurements (in theory at least).

Hence insulation seems like a good strategy as it should be the simplest way to get close to thermal equilibrium.

I cannot see any particular reason why you couldn't make a truss-tube SCT or Mak - the closed tube design is probably due to the extra weight at the front due to the heavy corrector plate (somewhat mitigated by not having a the focuser and eyepiece/camera there though).  Rolling a steel tube is probably just a cheaper solution than making a truss tube which is sufficiently rigid.

[cgarry]

Given that it's radiative heat transfer that's the issue I'd assume something with a reflective layer that doesn't touch the OTA would be preferred.  Any other ideas?

James

Radiator foil, it works a treat. 

Chris

[m.tweedy]

I am loving the discussion on this thread and must say I am now better informed than I was at the beginning of it. Well done for such a healthy and constructive debate.

[IanL]

Radiator foil, it works a treat. 

Chris

How about a sandwich of foam (e.g. camping mat) covered with a layer or radiator foil, another layer of foam, etc.  Closed cell-foam of some sort for preference?

[JamesF]

How about a sandwich of foam (e.g. camping mat) covered with a layer or radiator foil, another layer of foam, etc.  Closed cell-foam of some sort for preference?

You're thinking to reduce loss by conduction as well?

James

[Cath]

Doesn't the metal tube of the scope vary in physical size as it varies in temeperature? .. which would mean the focus would vary with temperature .. which seems to be what happens?

[IanL]

You're thinking to reduce loss by conduction as well?

James

Yes,

Variable gains/losses that need to be minimised:

- Net radiative gain or loss from / to the ground (depending on OTA and ground temperature).

- Net gain or (usually) loss to the air by conduction (depending on air temperature, net wind speed at the OTA, plus any gain/loss due to dew formation).

Fixed losses that can be minimised to a large extent:

- Net radiative loss to the sky (all exterior parts except the corrector)

Fixed losses that can be minimised only in part:

- Net radiative loss to the sky (corrector, after accounting for dew shield and dew heater)

Most high quality insulation products seem to combine a sandwich of materials - often layers of material that trap air (foam, fibreglass, other natural/artificial fibres) sandwiched in layers with other materials including metal foils.  I'm not a builder or engineer, but I don't think a layer of foil alone works well since it is in contact with the metal OTA so conduction still has a route to move heat from inside to outside or vice versa, whereas a sandwich of foam and foil should minimise both radiation and conduction.

[IanL]

Doesn't the metal tube of the scope vary in physical size as it varies in temeperature? .. which would mean the focus would vary with temperature .. which seems to be what happens?

Yes, it is a problem for most scopes, hence the popularity of higher end scopes with carbon fibre tubes as they are more thermally stable.

[Davey-T]

Radiator foil, it works a treat. 

Chris

So radiator foil it is as I've got some of that.

Whats the best way to observe tube currents high mag planetary imaging with video camera ?

How do you distinguish between currents and seeing ?

Dave

[IanL]

So radiator foil it is as I've got some of that.

Whats the best way to observe tube currents high mag planetary imaging with video camera ?

How do you distinguish between currents and seeing ?

Dave

For an SCT or Mak, best way is to focus on a star, reasonably high magnification, and defocus slightly so you get the fresnel pattern. (I.e. the bulls-eye of secondary shadow and concentric dark/light rings around it).

Seeing will usually manifest itself as disturbances all around the pattern.  The most obvious tube current is the one that rises directly off the baffle tube in the middle of the mirror, so you end up with a 'plume' of disturbance running through the fresnel pattern in the up/down direction.  It is harder to tell other tube currents from bad seeing unless you have something to compare with (e.g. a small refractor is usually a good way to tell if seeing is good or bad, and then you can check your SCT to see if it is better / worse, etc.)

[Davey-T]

For an SCT or Mak, best way is to focus on a star, reasonably high magnification, and defocus slightly so you get the fresnel pattern. (I.e. the bulls-eye of secondary shadow and concentric dark/light rings around it).

Seeing will usually manifest itself as disturbances all around the pattern.  The most obvious tube current is the one that rises directly off the baffle tube in the middle of the mirror, so you end up with a 'plume' of disturbance running through the fresnel pattern in the up/down direction.  It is harder to tell other tube currents from bad seeing unless you have something to compare with (e.g. a small refractor is usually a good way to tell if seeing is good or bad, and then you can check your SCT to see if it is better / worse, etc.)

I've got a 100mm  refractor permanently mounted on top of the SCT so will give it a try, when the clouds go away.

Dave

[Peter Drew]

A major operational problem with large SCTs is the change of focal plane during cooling of the optics. During this process the focal length of the primary changes, if this changes by x then the focal plane changes by 25x in the common form of SCT that has a secondary amplification of 5x. A SCT focused correctly at the start of the cooling requires significant refocusing subsequently, not so much a problem for lunar and planetary imaging for which this design excels but problematic for long exposures. 

[ChrisLX200]

I used a fan on my SCT only to speed up cooling until it reached near ambient, then it was switched off for the night. I reckoned it reduced cool-down time from about an hour to around 20mins. After doing that job it was of no further use. There were no extra ventilation slots in my OTA so I had to leave it with no camera/eyepiece in, the air was drawn in via a port on the rear cell onto and around the rear of the primary, then exits out through the visual back. I kept the OTA horizontal whilst this was going on.

ChrisH

[Zakalwe]

So, of those people who have or are in the process of insulating their OTAs, what have you used?

Steve Ward has said he intends to use Actis Triso Super 10, which is indeed hideously expensive unless you can find an offcut.

Stephen, I assume you're using something like this? http://www.screwfix.com/p/thermawrap-loft-insulation-400mm-x-5m/76477#

Given that it's radiative heat transfer that's the issue I'd assume something with a reflective layer that doesn't touch the OTA would be preferred.  Any other ideas?

James

Yes, thats the stuff.

Its nothing more than shiny bubble wrap- pockets of air encapsulated between thin sheets of plastic. Air is an excellent thermal insulator (ask any sheep ). Plastic is also an excellent insulator, so a substantial portion of the heat is blanketed and held in the OTA.

Its important to note that my OTA is stored in the obsy. If you store the OTA inside a central heated house then insulating the OTA is exactly what you don't want to do. The OTA will be a heck of a lot warmer than the outside air and you want to dump that heat as quickly as possible.

[Davey-T]

So is the general consensus that it doesn't matter what the temp inside the tube is as long as you can stabilise it ?

Dave

[perfrej]

I still vote for camping mat. Now that I have made it home from Moscow I have had time to find the post... Here it is: http://stargazerslounge.com/topic/119924-do-reflectors-need-dew-shields/#post1982389

/per

[Zakalwe]

I still vote for camping mat. Now that I have made it home from Moscow I have had time to find the post... Here it is: http://stargazerslounge.com/topic/119924-do-reflectors-need-dew-shields/#post1982389

/per

Same idea really. Foam is air bubbles in a substrate. Air is a good insulator.

I did laugh at this though:

If the neighbours see you hauling that out of the house in the dead of night they'll think that you are trying to dispose of a body wrapped in carpet

[ollypenrice]

So is the general consensus that it doesn't matter what the temp inside the tube is as long as you can stabilise it ?

Dave

I think this might well be correct.

Olly

BTW, I used to cool my SCT by having it 'corrector down' and the visual back removed since hot air rises. You can take precautions against the ingress of bumble bees and scorpions, of course...

[JamesF]

From what I was reading last night I'm not at all sure colour is relevant when considering radiation, so I don't think it matters what colour the tube is.  Actually, I'm not sure colour matters for absorption either, unless a significant proportion of the incident radiation is in the visible spectrum.  If an object is white, or black or green in the visible spectrum, I can't see that tells you a whole lot about it's reflectivity or emissiveness in, say, the IR part of the spectrum.

James

[perfrej]

I beg to differ

You can google "black body" and find tons of info. Basically, a black body absorbs all radiation that hits it, and thud also radiates against the lower temperatures in all wavelengths. One simple explanation is here: http://voyager.egglescliffe.org.uk/physics/astronomy/blackbody/bbody.html

/per

[cgarry]

How about a sandwich of foam (e.g. camping mat) covered with a layer or radiator foil, another layer of foam, etc.  Closed cell-foam of some sort for preference?

As far as I am concerned there is no issue with conduction as that happens equally on all sides of the tube, stopping heat radiating away is all I am interested in.  Having said that the radiator foil is not really a foil as such as it has a few mm of material with low thermal conductivity too.

Cheers,

Chris

[Zakalwe]

I think this might well be correct.

Olly

BTW, I used to cool my SCT by having it 'corrector down' and the visual back removed since hot air rises. You can take precautions against the ingress of bumble bees and scorpions, of course...

That'd be pointless as the baffle tube extends into the OTA body.

It'd be better to remove the Fastar secondary (assuming it had one) and point the OTA upwards.

[JamesF]

I beg to differ

You can google "black body" and find tons of info. Basically, a black body absorbs all radiation that hits it, and thud also radiates against the lower temperatures in all wavelengths. One simple explanation is here: http://voyager.egglescliffe.org.uk/physics/astronomy/blackbody/bbody.html

/per

But even if a "black body" appears black, that does not mean that all things that appear black in the visible spectrum are "black bodies".  A "black body" is just a name for a (theoretical) perfect absorber of radiation.  Somewhat counter-intuitively, stars are a reasonable approximation of a "black body".

I am not convinced that the properties of an object that make it a good absorber of visible EM radiation (and hence appears black) tell you anything definite about its properties as an absorber or emitter of near- or far-IR radiation.

James