Tube material vs. cooldown?

Hi all,

I've been perusing this forum and whatever other ATM information sources I can find on the web, and it seems that there is some disagreement about the effect of tube material on cooldown issues.

Some people seem to think that an insulating tube material is best (sonotube, fiberglass, etc.) whereas others favor a high thermal conductivity material such as aluminum. In my case I'm leaning towards aluminum because of weight issues anyway, but it seems to me intuitively that initial cooldown will be faster, and ongoing thermal effects throughout an observing session less disruptive, with a material that conducts heat readily.

Is this an old controversy that has already been settled? Can anyone point me to an article or something that definitively settles it? I'm inclined to think that the more thermal conductivity the merrier, and in fact I'm thinking of using an aluminum tube and leaving the outer surface unpainted altogether. I can't get away from flocking the interior - aluminum is awfully shiny! - but finishing the outside seems unnecessary.

Am I all wet here?

Thanks,

Mike

Mike,

My guess is you will get diverse opinions. Personally, I think an unpainted aluminum tube with a thin insulating layer inside (flocking) is the best choice for a Newt. We have a 6" f/8 done this way and it does not have thermal issues. A friend has done much experimentation with tubes over the years, and he is sold on the same approach. For appearance, you can "jewel" the outside of the tube (overlapping swirls).

Unpainted aluminum has good thermal conductivity, but poor emissivity, so it will cool rapidly to ambient air temperature but will tend not to cool below ambient. Before we removed the paint from the 6" f/8, it had terrible problems with dewing of the optics.

Clear skies, Alan

Some people seem to think that an insulating tube material is best (sonotube, fiberglass, etc.) whereas others favor a high thermal conductivity material such as aluminum. In my case I'm leaning towards aluminum because of weight issues anyway, but it seems to me intuitively that initial cooldown will be faster, and ongoing thermal effects throughout an observing session less disruptive, with a material that conducts heat readily.
----

There does seem to be a fair amount of controversy over tube material. I don't think it is quite a simple as figuring aluminum will cool faster because it also responds faster to outside events. Touching a metal tube, whether steel or aluminum causes tube currents. Another issue to consider is the heat capacity of the OTA, many insulators have a low heat capacity.

As far as weight goes, I have done quite a few calculations on wall thickness between aluminum and sonotube. I don't see a lot of savings by going to aluminum if the wall is sufficiently think the make for a rigid tube. One needs to consider the inside covering and some sort of tube rings at the ends for addition support around the mirror cell and the focuser spider.

Jon

Mike Damato said:

Hi all,

I've been perusing this forum and whatever other ATM information sources I can find on the web, and it seems that there is some disagreement about the effect of tube material on cooldown issues.

Some people seem to think that an insulating tube material is best (sonotube, fiberglass, etc.) whereas others favor a high thermal conductivity material such as aluminum. In my case I'm leaning towards aluminum because of weight issues anyway, but it seems to me intuitively that initial cooldown will be faster, and ongoing thermal effects throughout an observing session less disruptive, with a material that conducts heat readily.

Is this an old controversy that has already been settled? Can anyone point me to an article or something that definitively settles it? I'm inclined to think that the more thermal conductivity the merrier, and in fact I'm thinking of using an aluminum tube and leaving the outer surface unpainted altogether. I can't get away from flocking the interior - aluminum is awfully shiny! - but finishing the outside seems unnecessary.

Am I all wet here?

Thanks,

Mike

Hi

Phenolic tubes used for high power rocketry are supposed to be quite good.

Here's a link to a web site for this kind of tube.



Apparently, an advantage this type of tube has over aluminum is less thermal expansion.

Hi,

Looking back on all the newtonians that I have owned over the past 30 years my opinion is that the tube issue has been blown well out of porportion. Since the majority of telescope tubes are oversized (to eliminate thermal issues because the currents tend to stick to the walls) one should begin to look at the optics as the main source of tube currents.

Aluminum will cool to ambient temperature 99 times faster than glass. Under the night sky aluminum tubes will be close to the ambient temperature. The real cause of tube currents is the primary mirror. A simple out of focus star test will show where the currents are coming from in any newtonian telescope. In my experience it has always been the primary mirror. This is why most of my newtonians were cooled. I have used fiberglass, phenolic, steel, wood and aluminum for tubes. It never made much of a difference what I used. The tube decision was based on the telescope design and what was availiable at the time.

If you are into deep sky-low power views, don't fret yourself over tube materials but if you are into high power lunar planetary, multiple star observing, consider cooling the scope. In my research I found that 1/2 degree temperature difference in 36 inches of air is equal to 1/4 wave error.

Keith

Thanks everyone for letting me pester you with so many questions!! Wow, nice to have a forum like this to consult.

Eventually I'll be much less clueless and start answering other people's questions...

Mike