work progresses on the 13"

[bird]

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]]

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

[asimov]

Sounds good Bird.

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

[bird]

hmm, it works for me :-)

Try it again?

[asimov]

Still nothing here

[[1ponders]]

Nope, still not working.

[iceman]

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

Weird.

[bird]

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

Third time lucky?

Bird

[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?

[[1ponders]]

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. :cool2:

[bird]

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]]

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

[bird]

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]]

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 :cold:

[bird]

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]

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]

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]

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]

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]

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.

[videoguy]

Bird, That's a brilliant image of Tycho on your site!

[bird]

Thanks Steve - was a complete fluke, as most of these things are. It was taken at about 5.30am, only 20 mins before sunrise - I'd been imaging Jupiter and had just finished and decided to pack up but the moon was nearby so I moved over to it and grabbed a few runs including that one.

The scope was properly cooled, the moon was at the zenith and the pre-dawn air was very steady. Turned out to be a good combination :-)

regards, Anthony

[seeker372011]

Bird;
it was great to check out your site. Love your lunar composite project

my personal favourite is your Clavius shot

who needs an Orbiter eh?

[mch62]

Hi bird , interesting idea , but have you used a conical yet as you may find that you may not need to go to all the trouble with the peltiers.

I live out side of Ipswich QLD , which has a reputation of 20-24deg swings from min to max not out of the ordinary , summer or winter and have not had much of a problem with cool down or keeping up with temp drop with my conical.
I do have a large piece of aluminum for the mounting stud and plate like in your diagram and find very little heat turbulence from the mirror strait from the shed and within 1/2 hr the mirror has reached equilibrium.

I also have a very open truss tube .

When I was concidering a mirror I also looked into the use of peltiers and fans but Rob Royce recommended trying with out first .
I never worried about a cooling fan as it has not been necessary.

I keep in touch with a couple of R.Royce's other customers who found a similar result and 2 of them live in the north USA.

May be try with out the peltiers first and do a comparison after wards??

just a thought

Mark

[bird]

Hi Mark, I tried the other cooling options first, and went to the peltiers only after they failed :-(

The conical will be a lot better than a "normal" mirror, it only has about 70% as much glass as a full thickness 13.1" mirror would have, but that's still more glass than my current 10" mirror so I can't see the problem getting any better... I just hope I can stop it from getting worse!

I know Ipswich, I grew up in Glen Innes just south of there...

The problem I'm having in Canberra is that the temp will keep dropping by about 2 degrees per hour from sunset through to 2 or 3am. There's no way my mirror will track that without aggressive cooling - maybe your conditions are better up there, if the temp drops fast and then stabilises you'll be ok.

regards, Bird