insulating a telescope

[Nillchill]

i have been trying to think of ways to minimise dew on my scopes during prolonged evenings viewing.

Has anyone ever thought of, or tried, insulating the outside of the tube of a telescope to minimise the chilling effect of it past the dew point.

something akin to a lagging jacket used to insulate hotwater tanks.

The theory is to try and slow down the loss of heat that the tude experiences, so it never cools past the dew point.

[YorkieGraham]

I have cut up one of those insulated camping mats with the foil covering and have used it on a number of occasions when I think it might be worth the bother. It seems to work quite well, although I haven't done any tests to see if the telescope is better with or without. I will post a picture of my scope with its coat on if I can work out how to get pictures onto here. The dew heater strips all look good value until you realise that you also have to buy the controller and power it. So, the DIY solutions seemed to be worth a try.

[Nillchill]

thanks Graham, I look forward to the pics

[YorkieGraham]

I am hoping that the attachment has worked. If it has here are the pics of my scope with its insulating coat on.

[m.tweedy]

yes I have a camping mat round the tube permanantly

[Nillchill]

ok so this sounds a promising concept,

do you find this helps minimise dew build up, or do you use dew bands in conjunction

[m.tweedy]

ok so this sounds a promising concept,

do you find this helps minimise dew build up, or do you use dew bands in conjunction

I use it to stop thermal currents inside the tube as I feel it can fall below ambient temperature on a cold night. The other use is that it protects the tube from the occasional knocks and scrapes A dew band and dew shield is essential on an 11 inch SCT

[Tim]

Hmmm, some interesting theory here

I wonder if the insulation would make much difference to the change in focus of the metal tube scopes as they expand/contract. Even the Takahashi's appear to shift a bit with temperature swings.

My only concern would be the effect on cool down time though?

Cheers

Tim

[Nillchill]

The theory behind it is that it restricts the heat lost from the metallic tube through radiation.

Thus it would probably affect cooling times, but more importantly stop the tube temperature dropping below the dew point temperature.

Combined with dew shield and a very low power dew band, it could prevent dew altogether.

the limiting factor would be how warm to run the tube to optimise the above without affecting the thermal currents within the tube.

[YorkieGraham]

I did consider letting the tube cool down for a while before fitting the insulation. This would be OK if the few obstruction fitted to the tube to overcome. I have had to cut the insulation into 7 pieces which are secured to the tube in various ways.

[hemihaggis]

ANT: Removed post.

[m.tweedy]

I agree "the same" not below

here is a direct quote from Dew buster

What is Radiation Cooling?

The laws of thermodynamics tell us that heat flows naturally from hot to cold objects. 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.

It may take longer to cool down but cool down it will. The effects from "Too Cold" a scope are just as off putting as "Too warm"

Having had both an 8 inch SCT which did not need anything to the 11 inch SCT I found the longer I was observing with this the more currents were appearing.

I consider VERY CAREFULLY my "good ideas" before i post them always bearing in mind newbies. Hence the reason it has taken me 3 years to post 1000 articles

[beamish]

I do it too

Fans to cool down initially then off with Camp-mat dew shield and carbon fibre OTA lagged with silvered camp-mat, & dew band if/when needed

[calibos]

I'm pretty sure you are both talking about differrent things. M.Tweedy talking about dewshields to slow down dew formation on corrector plates and Haggis talking about the idea proposed by the OP which is wrapping the whole Metal OTA. The fomer is a good idea and the latter not so much. I don't think Haggis is necessarily in disagreement with you Michael.

I understand Haggis' frustration though. One can't help but raise ones eyes to heaven a little bit when one reads something that one knows to be wrong written in an authoritive tone that may be taken as gospel by others. I too would research something comprehensively before wasted my time trying something never mind posting about it.

However, I think you could have been a little more tactful Haggis'

In the conventional Newtonian world its a good idea to insulate the inside of a metal OTA. Not to prevent dew on the OTA. Who cares about dew or frost on the outside of the tube. It affects nothing. The affect a super cooled metal OTA has on the scope are tube currents inside the tube. The cold OTA chills the air against the inside wall of the OTA below the ambient temperature of the rest of the air inside the tube and eddies of this colder aire roll off the inside of the tube into the lightpath causing bad 'Scope generated seeing'. With our open ends we don't have to worry about the whole airmass inside the tube going below ambient, just the eddies rolling off the inside walls. Insulation thus works for us without any negatives.

For a sealed SCT its not so simple. The inside of an SCT can't reach ambient quickly with air exchange like an open ended newtonian (Putting aside the issue of primary mirror cooling, primary fans and boundary layer effects). The air inside can only cool to ambient (putting aside the lymax cooler solution) via conduction with the corrector plate and metal OTA (and from the outside of the corrector plate and metal OTA to space via radiative cooling). Insulating the inside or outside of the metal OTA prevents one or the other, leaving only the corrector plate to conduct/radiate, hence a longer cooldown and longer time till scope generated bad seeing is gone.

The carbon fiber OTA's are not about insulation but to minimise expansion/contraction which can affect focus. Not much of a problem for visual observers but a big problem for imagers. Thus you are more likely to find imagers with carbon fiber SCT's. you will also find that these carbon fiber OTA owners are also aware of the insulation/cooldown problem of carbon fiber OTA's and 99 out of 100 of them will own a lymax cooler I'd wager.

You do indeed run into the problem of the OTA and air inside cooling below ambient which can be as much a generator of bad scope seeing as the opposite, but the solution to one is not to invoke the other.

Dew on the outside of the OTA is not a problem. Dew on the corrector plate is. A temp controlled dew controller like a Dewbuster, dew strap and dewshield is the best solution for dew on the corrector to keep it just above ambient.

If I owned an SCT I would own the above. I would actually insulate the inside of the OTA as contradictory as it sounds with a thin layer of flocked cork(kill two birds with one stone)....but only if I had a Lymax cooler. ie. Be able to cool the inside air quickly to ambient bypassing the negative affects of insulation of an SCT OTA at which point the insulation becomes a benefit rather than a negative. It prevents further radiative/conductive cooling from chilling the inside air below ambient.

[jamespels]

I am reading this with interest and did wonder about the effect of insulating a newt ota on internal air currents. Happy to read any views anyone else has...

Onr thing that did make me chuckle though (not that there is anything wrong with what calibos says - just taking it out of context in light of previously recommended insulation materials)

In the conventional Newtonian world its a good idea to insulate the inside of a metal OTA.

So, I should cover the matt black flocking with some lovely shiny silver insulation, then?

[YorkieGraham]

I do not know what has been said by Haggis, as the post thas been removed, however, reading between the lines, some people are unhappy that statements have been made which have not been tested and conflict with accepted well thought out views. (Whether this refers to myself or another poster, I am unclear). Further, others state that ideas should be very well thought out before they are posted. I agree that if a claim is being made, then this should be tested. But surely what is happening here is that Amateur Astronomers are discussing whether it is beneficial to insulate a telescope tube or not. My opinion is that this is heathy debate, far more beneficial than some of the posts you get on here, - and as a result this type of debate should be encouraged and not slapped down by those who may have a superior knowledge. People having good ideas is a good thing as these good ideas can then be discussed, accepted or rejected. I also feel that insulating the OTA without heating the primary has not been covered and I would be interested as to what others think in this regard. I would love to own a SCT but like many other cannot afford this luxury. So, what do folks think about insulating the tube of a hummble refractor or newtonian? Please be gentle with me! I am a reborn newbie!

[Nillchill]

good grief, when I posted this thread it was to open up a discussion, and raise a question "if" insulating a telescope tube would be beneficial.

I posted what my theory was based upon, after doing some reading, I thought an open and honest discussion would ensue.

Has this caused offence to some people?

[YorkieGraham]

Glad you are still in the debate Nillchill.

[Thirdway]

http://www.protostar.biz/tubeliner.htm link for an insulated and flocked tube liner.

I have no idea if it's a good or bad idea, I do know that my Dob doesn't work as well at -7 degrees. I'm not site if that's down to the mirrors getting misted or the lens? Either way, the view becomes just like it does through fog. Up to an hour or so it cools down and works brilliantly, then it begins to exhibit this mustiness which causes flare around stars.

[Carbon Brush]

My approach to temperature and dew is very simple.

Step 1. Store the scope in a cold place. If not permanently, at least on the day of use. That way the effects of moving from storage temperature to use are minimised.

Step 2. When in use, any dew in unwanted places is taken off by a hair dryer. This evaporates as much by stirring up air as by heating the scope. When used correctly, the residual heat in the metal and glass is minimal.

The metal is a good conductor. It has a large surface area to volume ratio. It therefore returns to ambient temperature rapidly.

Glass is a poor conductor and therefore does not heat all the way through a thick lens/mirror, if the hair dryer is used properly. A demonstration of this comes when heating to remove dew from the inside and outside of glass. Outside dew is gone in a second or two. Inside dew takes much longer.

Using the above, I have enjoyed good viewing. That is until contrast and resolution are lost as dew forms. The view is awful while warm turbulent is around the light path. Good viewing returns in seconds though.

Ready now to take cover while the comments fly...

[Nillchill]

david, we appreciate the input and thanks for responding.

i have used the above method you posted, however, it was pointed out by a neighbour(who leaves their window ajar through the night) that the hairdryer I was using was a tad noisy at 2am.

I have since been investigating on whether to use dew bands, I have been looking into the science behind dew formation, and found that the metal surfaces of the tude radiate heat away faster hence why they tend to become colder than the ambient, hence dew forms.

so that is what made me post the question, as an aside to this, I am postulating that if a dew band is used in conjunction with tube insulation it would make a dew band far more efficient.

[lrpingel]

This is an old thread but interesting.  Hi, I'm a new guy to serious astronomy, my background is software engineer who worked on lots of computerized electro-mechanical devices.  

I am strongly considering the use of Reflectix insulation on outside of tube.  This because telescope will be permanently mounted in a metal roll off shed.  So I'm dealing with heat issues from sun during the day.  Fans and ventilation will help but there is still radiated heat from metal shed.  Inside of shed will also be insulated with Reflectix to try and cut down on that.

Any comments or new experience with tube insulation, inside or out?

[m.tweedy]

Interesting article in the Sky at Night magazine that has just landed on this very subject. here is an extract regarding tackling dew "you can wrap the body of your telescope in shiny, low emissivity aluminium. central-heating radiator foil or an aluminised mylar space blanket is great for this job"

this is just part of several actions to combat the dreaded dew.

dont know if dew is your problem. the reverse i suspect but it is all about minimizing the temperature difference between the scope and the external environment.

[Kropster]

"You do indeed run into the problem of the OTA and air inside cooling below ambient "

How is this possible?

The moment it reaches a fraction below ambient, will it not start to be a net absorber of heat by conduction from the air?
 

If not, couldn't we have cheap to run camping fridges shaped like OTAs? :-)

[m.tweedy]

i am not a physicist but know it exists after seeing it through my C11.  you think great the scope is nice and cool then have terrible currents inside the tube 

Quote from Sly at Night magazine just received.

"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".

also check out the dew not site that explains.

as i said i don't know how or why but know it happens.

hope that helps.