work progresses on the 13"

bird (Anthony Wesley) · #1 17-10-2005, 09:32 PM

Hi people,

poor weather and other things have conspired to keep me from any new imaging, but in the meantime I have made some progress on the construction of the new 13" scope.

I've finished the mirror cell with integrated cooling, it was quite an interesting job to design and build it so I took some pictures along the way - you can find the details here:

http://www.acquerra.com.au/astro/cooling/version3/

With any luck I'll be back out and imaging this week - if the weather cooperates.

regards, Bird

[1ponders] (Paul) · #2 17-10-2005, 09:34 PM

Hi bird. The link doesn't seem to be working>

asimov (John) · #3 17-10-2005, 09:37 PM

Sounds good Bird.

There's something wrong with your link, could be my end though.

bird (Anthony Wesley) · #4 17-10-2005, 09:37 PM

hmm, it works for me :-)

Try it again?

asimov (John) · #5 17-10-2005, 09:40 PM

Still nothing here

[1ponders] (Paul) · #6 17-10-2005, 09:40 PM

Nope, still not working.

iceman (Mike) · #7 17-10-2005, 09:43 PM

I get a blank page, but view->source shows html code.

Weird.

bird (Anthony Wesley) · #8 17-10-2005, 09:45 PM

aha - I found a typo. It was working for me in Mozilla, but maybe IE didn't like it.

Third time lucky?

Bird

iceman (Mike) · #9 17-10-2005, 09:47 PM

That's better..

Nice page Anthony, a very elaborate system you've developed there! I guess you subscribe to the theory that the mirror must be absolutely at the ambient temperature?

[1ponders] (Paul) · #10 17-10-2005, 09:52 PM

If it works better than your previous design bird I cant wait to see what you're going to turn out. Especially with the new camera.

bird (Anthony Wesley) · #11 17-10-2005, 09:53 PM

Quote:

Originally Posted by iceman

That's better..

Nice page Anthony, a very elaborate system you've developed there! I guess you subscribe to the theory that the mirror must be absolutely at the ambient temperature?

Absolutely!! :-)

I've seen the difference that it makes in practice - on Jupiter earlier this year there were tiny storms and swirls that completely disappear when the mirror is even a teeny bit warmer than the air next to it.

From my own research and stuff I've read from other people it seems that the boundary layer between cold and warm air next to the mirror can distort the apparent paraboloid surface by a couple of wavelengths, making it perform like a very poor mirror indeed.

I think this is the "secret" to why some people get amazing images and other don't. A mirror that's thermally out of balance is going to perform so badly that it really doesn't matter how well you've collimated or focussed...

Smaller mirrors are easier to cool, but once you get to 10" or larger then this can become a big problem - especially here in Canberra.

Bird

[1ponders] (Paul) · #12 17-10-2005, 09:57 PM

Move to Queensland then bird. PLUS the planets are higher up

bird (Anthony Wesley) · #13 17-10-2005, 10:05 PM

That's the Long Term plan...might take me another few years to get there but that's definitely where I'll be in 5 or 10 years from now...

Tropic of Capricorn seems to be about the best place in australia for planetary work, so I might end up in Rockhampton :-)

Bird

[1ponders] (Paul) · #14 17-10-2005, 10:10 PM

Stick near the coastal plain then. My folks managed a caravan park out near Emerald and it get damn near as cold as Canberra out there at night in winter

bird (Anthony Wesley) · #15 17-10-2005, 10:13 PM

Yeah, spent a couple of weeks at a place about 15k north of Rocky in April, and it was a lot of fun. That's pretty close to where I'd like to be - still 20k or so inland so we miss some of the coastal cloud but not elevated so the temps stay very stable.

Bird

asimov (John) · #16 17-10-2005, 10:25 PM

I've always maintained that my full thickness 12.5" Parks primary has trouble reaching ambient. I read Alan Adlers article on thermal management some time ago: using a fan/s blowing across the face of the primary to eliminate the boundary layer. But then I've often thought this would perhaps create another problem: tube currents. Plus the mirror would not actually be at thermal equilibrium.

You've obviously spent considerable time & effort researching this Bird! Well done.

bird (Anthony Wesley) · #17 17-10-2005, 10:29 PM

Well, Canberra has extreme swings between day and night, usually around 15 degrees or more, so thermal issues are exaggerated here. But I tried fans and they didn't make enough difference for it to be worthwhile - as you say it was only stirring up the warm air and not really solving the real problem - the primary was too hot.

If you can cool the primary properly then you can switch off all the fans etc, let the air settle down in the tube so it's all stationary and you'll get fabulous views :-)

Bird

asimov (John) · #18 17-10-2005, 10:51 PM

What I have been doing lately (& it sound lame) is Placing the OTA in the lounge room where my aircon is....2 thermometers, 1 on the primary & one outside. I then wait until I get both temps correlating & rush out & setup.

The trouble with that is the mirror still can't keep up with the dropping ambient temp over time, even with a fan blowing on the back of the mirror. Once the mirror temp is 1 deg above ambient, back comes the boundary layer. I'm not game enough to cut a hole/s in my tube as yet & try the Adler technique lol..

bird (Anthony Wesley) · #19 17-10-2005, 11:04 PM

I can usually get a window of 30 minutes or so before I have to switch the cooling back on - so the cooling runs from sunset when I set up until 2am or whenever, then I switch it off and let it sit for about 30 minutes, after that I get about 30 minutes to an hour of useful time before I see the image starting to break up again as the boundary layer comes back. Then it's time to turn the cooling back on and go inside for a cuppa :-)

I've got a computer that watches these temps and switches the cooling off if the mirror reaches ambient. That way I dont have to babysit it too much, except making sure I don't get ice on the primary in winter ..

Bird

asimov (John) · #20 17-10-2005, 11:29 PM

Me being a welder (namely a mig welder) I know of a similar device somewhat similar to yours. Only it works in reverse:

In mig welding an inert gas is blown over your work as the flux. In most cases these days they use co2 (cheap) When co2 is discharched at a given rate through a regulator, the regulator will freeze up causing damage to the regulator. Enter the heating element inside the regulator to stop this happening. Relatively recently, someone came up with a totally new concept: A special shaped regulator comprising of a block of aluminium with fins on it. This allows the ambient air to 'heat' the co2 as it runs through the regulator eliminating the freezing up. It still runs very cold: only a few deg above freezing, but it's enough to stop this problem. Enter the heaterless gas regulator.

It's all very interesting! (sorry if I've gone off subject/topic etc lol.