The Unhappy Me thread

[kladner]

Good News, Chris!

[kriesel]

Originally Posted by xilman

The engineering problems are simply too great.

That's not something that should be said around a German engineer. If the total number of joules for a cooldown and use session is not too much, maybe an ice bath flask, thermally connected to the peltier hot side, and no plumbing involved to bridge and complicate scope/camera motion.

[xilman]

Originally Posted by kriesel

That's not something that should be said around a German engineer. If the total number of joules for a cooldown and use session is not too much, maybe an ice bath flask, thermally connected to the peltier hot side, and no plumbing involved to bridge and complicate scope/camera motion.

Fairy Nuff.

You design and build something, at your expense, and I'll field test it for you.

To compensate you for your investment, I will release you from any obligation to me other than for you to permit me to continue to use your prototype should it prove successful,

Do we have a deal?

[ewmayer]

Careful what you wish for, Paul - just saw this report about wild weather a little ways NE of you, in Spain proper:

https://sputniknews.com/europe/20190...drid-in-chaos/

And speaking of waterworks, the large marble ball in the foreground of the main picture of the above article appears to be one of these:

https://en.wikipedia.org/wiki/Kugel_fountain

[xilman]

Originally Posted by ewmayer

Careful what you wish for, Paul - just saw this report about wild weather a little ways NE of you, in Spain proper:

https://sputniknews.com/europe/20190...drid-in-chaos/

And speaking of waterworks, the large marble ball in the foreground of the main picture of the above article appears to be one of these:

https://en.wikipedia.org/wiki/Kugel_fountain

Happens in these parts too.

Some 20-ish years ago it rained so hard that the beach just outside the restaurant where we were eating (El Pulpo in Los Cancajos) was completely underwater. Not bad going for well-drained coarse volcanic sand. The restaurant staff had to block the doors with towels to keep the rain out. On returning to our 2nd-floor apartment we discovered that rainwater had come in under the door.

Afterwards, we found that something like 18-months worth of rain had fallen in about three hours. Many cars which had been parked in otherwise dry barrancos ended up somewhere in the Atlantic, as did basalt boulders 2 m or more in diameter. There were several fatalities. At least one corpse was found on the beach at Tazacorte, having been washed several km from the caldera where they had been hiking. SWMBO and I were up at El Roque de los Muchachos the following day from where we could see helicopters a kilometer or more below us which were searching for other hikers MIA.

Tenerife suffered similar problems from the same storm on the following day but without, AFAIK, any fatalities.

About the only people happy with the storm were the farmers. That one event gave them security from drought for well over a year, despite losses from the surface run-off.

[kladner]

Originally Posted by xilman

I'd thought about it but it's not worth it.

Very high air temperatures are almost always a daylight phenomenon and the cooling can reach 45C below ambient. It is very unusual for the night time temperatures to exceed 25C and are usually below 20C.

Actually, it is very unusual for day time maximum to exceed 30C; this summer most exceptional.

A fan just above floor level in the dome pumps air 24/7 to keep the interior at close to ambient even when the slit is closed. It's to minimize thermal gradients which can distort optics and introduce turbulence.

I think I get it. If everything is at the same temp, then convection is minimized and the air is less disturbed. I hope I am thinking in approximately the right direction on this.

Is the fan an intake or exhaust? If intake, is the air filtered?

[xilman]

Originally Posted by kladner

I think I get it. If everything is at the same temp, then convection is minimized and the air is less disturbed. I hope I am thinking in approximately the right direction on this.

Correct, as far as it goes. The other effect is that optical components such as the primary mirror may be at different temperatures within their structure, the most common being a delta-T from front to back. The different parts expand differently and so a surface which has been engineered to within 50nm or so of a particular shape is distorted to another shape which is still very smooth but of the wrong shape.

Originally Posted by kladner

Is the fan an intake or exhaust? If intake, is the air filtered?

TBH, I don't know whether inlet or outlet. There's certainly no filter fine enough to keep out anything smaller than birds. Geckos, insects, dust, etc, can certainly be found in the dome.

[kriesel]

Originally Posted by xilman

The restaurant staff had to block the doors with towels to keep the rain out. On returning to our 2nd-floor apartment we discovered that rainwater had come in under the door.

Reminds me of a trip to Brookhaven National Lab around that time frame. Certain conference sessions had to be relocated because of buildings flooding. Here and there a building entrance was sandbagged to combat flooding. The accelerator tour was canceled and access to the vault was prohibited for a while. Evacuation of the accelerator vault was successful before the flood waters reached the level of the knee-high wall mounted electrical service.

[kriesel]

Originally Posted by xilman

Correct, as far as it goes. The other effect is that optical components such as the primary mirror may be at different temperatures within their structure, the most common being a delta-T from front to back. The different parts expand differently and so a surface which has been engineered to within 50nm or so of a particular shape is distorted to another shape which is still very smooth but of the wrong shape.

In addition to axially symmetric distortions, producing (nearly) symmetric but undesired optical surface changes and spacing changes, lateral temperature gradients are unwelcome, producing nonsymmetric changes in optical surface shape, and optical element tilt relative to the nominal optical axis. Minimizing thermal excursions is a first cut at the problem. Second cut is equalizing temperatures. "Athermal design" uses such strategies as different materials with different expansion coefficients in proper proportion to produce nominal net zero expansion in the operating temperature range, and optical designs that are relatively insensitive to thermally produced changes.That's fine for steady state, but to dynamically track, thermal time constants must also be as similar as practical. Thermal expansion coefficient is a function mainly of composition, and the practical limits of mixing, impurities introduced by crucible dissolving in the melt, etc gave a few percent local variation in thermal coefficent, with the highest impurity level and variation near the outside of a melt or cast billet. Vukobratovich described in a class he gave, making optics centered on the center of a perpendicular slice of a billet to put those irreducible resulting composition & thermal property variations on-axis and symmetric about the optical axis, and cut off the worst variation around the edges. He had dealt with meter-scale Space Defense Initiative optics at that point.

https://www.goodreads.com/author/sho..._Vukobratovich

[kriesel]

Originally Posted by xilman

Fairy Nuff.

You design and build something, at your expense, and I'll field test it for you.

To compensate you for your investment, I will release you from any obligation to me other than for you to permit me to continue to use your prototype should it prove successful,

Do we have a deal?

Need some specs to design by (full thermal and mechanical). Also will need full travel expense advance for installation.

[ewmayer]

Originally Posted by xilman

Happens in these parts too.

Some 20-ish years ago it rained so hard that the beach just outside the restaurant where we were eating (El Pulpo in Los Cancajos) was completely underwater. Not bad going for well-drained coarse volcanic sand. The restaurant staff had to block the doors with towels to keep the rain out. On returning to our 2nd-floor apartment we discovered that rainwater had come in under the door.

Afterwards, we found that something like 18-months worth of rain had fallen in about three hours.

Speaking of raining hard, hurricane Dorian, now category 5 and the second-strongest Atlantic hurricane since 1950, is pounding the Bahamas. Besides 185mph sustained winds, "Over two or three days, the slow-moving hurricane could dump as much as 4 feet (1.2 m) of rain."