Reducing tube currents in a C11

[brianb]

My CPC1100 has a thin metal tube painted charcoal grey. Tube currents tend to persist even after the scope is properly cooled; I suspected that the top of the tube - open to the sky - was overcooling by radiation from the dark surface.

Yesterday, having reasonable seeing and a nice stable temperature, I did an experiment - lagging the top of the tube by draping a (dry) towel over the top of it whilst observing Jupiter. This did seem to reduce tube currents quite noticeably.

[Ant]

Thats a good tip Brian cheers - I'm just about to get a C925 so may have the same/similar issue.

Taking your idea one step further - I know a lot of people that use the foam camping mats as a dew shield - could a longer version be made that would cover the majority of the length of the tube as well as dew shield part?

Ant

[Mr Spock]

Thats a good tip Brian cheers - I'm just about to get a C925 so may have the same/similar issue.

Taking your idea one step further - I know a lot of people that use the foam camping mats as a dew shield - could a longer version be made that would cover the majority of the length of the tube as well as dew shield part?

Ant

I'm already thinking that for my C9.25. Getting a mat and some velcro to create an extended dew shield.

[brantuk]

"top of the tube - open to the sky"

I thought the cpc had a corrector plate and was closed off to the sky? My 800 does.

*puzzled*

[brianb]

I thought the cpc had a corrector plate and was closed off to the sky?

The tube is closed but the dark grey metal (conductive) tube is thin and the outside of the tube can radiate heat away to the sky ... the top will cool below the temperature of the bottom allowing tube currents to form.

It's useful to have a thin conductive tube so that the scope cools down reasonably quickly but once it's cool it's a nuisance.

[brantuk]

Thanks Brian - I've never seen one of those - sounds very interesting, any chance of a pic please?

[sixela]

Yes, metal tubes will often cool to under ambient temperatures and cause tube currents; that's why some people use cork to insulate the tube wall from the inside of the scope (even a layer of flocking paper helps, according to some people who flocked the interior of their scope).

Of course, wrapping the outside of the tube with something that radiates badly to the night sky will also help (doesn't have to be thick).

[Ant]

Gelert Luxury 9mm Foam Camping Mat

One of these might do the job?

[Mike Hawtin]

Have a look at the Protostar website

http://www.fpi-protostar.com/tubeliner.htm

I ordered one of these for my C9.25 and it seems to do a good job, it's not necessary to have the exact size as it's simple to slit the liner and tape it back up the the correct diameter for a snug fit.

[ahlberto]

Hi.Ive done a mod witch covers all the ota till the back of the scope with rubber foam wich i used too to make the dew shield and the ota if fully flocked too.It prevents the tube currents and the dew over the ota but i think the scope takes more time to cool down but is more stable after it cools:D

[Ags]

Probably a dumb question... How does the metal tube cool to below ambient? I thought that everything would tend to thermal equilibrium... If some substances can tend to 'below ambient' and thus cause currents, then haven't you built a perpetual motion machine?

[ahlberto]

No its not a dumb question.This is a old photto and the scope take ages to cool down so i have removed the rubber insulation.The cool down issue is by far the unique bad thing SCTs have and im getting the courage to drill some holes into the OTA.Ive tried everithing but nothing works and i dont like fans,-they allways suck dust into the OTA no matter what we do.A passive cool down maybe the answer

[philj]

I made a somewhat primitive way of sorting dew and tube currents out when I had a C11. It was a black plastic Homebase swing top bin, lid and base cut off. Built up so it fit snugly onto the tube. Then I made an integral dew heater out of resistance wire and micro bore central heating tube which sat just on the tube end / corrector locating ring.

Worked a treat but you still cant beat leaving the scope out for about 1 1/2 hours to cool properly.

Cheers

Philj

[brianb]

Probably a dumb question... How does the metal tube cool to below ambient?

By radiation to the "cold of space" ... the ambient temperature is that of the surrounding air, but a surface which has nothing except the atmosphere between it and the sky will continue to fall in temperature until it's below that of the air. In fact if the air wasn't there it would cool to 3K (-270C), where the outgoing heat loss by radiation would be balanced by the cosmic background i.e. the residual radiation from the Big Bang.

No perpetual motion, because the cooling below ambient results in the surrounding air being cooled too. You can extract small amounts of energy (e.g. by thermocouples, or by vanes driven by the currents of cooling and warming air) but, given long enough, everything would equalize out and the energy extraction would cease.

[Gary Parkerson]

Hi.Ive done a mod witch covers all the ota till the back of the scope with rubber foam wich i used too to make the dew shield and the ota if fully flocked too.It prevents the tube currents and the dew over the ota but i think the scope takes more time to cool down but is more stable after it cools:D

Interesting Ahlberto. You might consider the contraption the Lymax (Lymax SCT Cooler) markets for cooling SCTs. I've used one and it really does make a difference. It may allow you to leave the insulation on your OTA and still achieve cool-down in a reasonable amount of time. I haven't checked to see if any dealers carry it on your side of the Atlantic though.

Hope this helps.

Gary