It looks like a good focus if not long. But does it follow y=ax^2 as I understand it? If not a parabola, does it still focus as much energy as a true parabola. Why would the compressed air automatically make a parabola? Truncated version.
Full length.
Mikek
I won't view a Youtube that has a non-cancellable 12 second ad, or multiple ads.
I'm guessing that they may have a way to find out that I didn't download it all. If enough people bail on long ads, and mid-video ads, maybe they will back off.
I'd pay a modest fee for ad-free Youtube. It's currently $12 a month.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
It is the boundary conditions and uniform deforming force of air pressure that make it form a parabola (of sorts).
I am not convinced it will be an exact parabola but I can't be bothered to do the integrals. It is forming the 2D equivalent of a catenary.
1D analytic solution would be cosh(ax) ~ 1 - (ax)^2/2! + (ax)^4/4! - ...My instinct is that the solution for the deformation of a drumskin will be J0(x) so a bit closer to a parabola than that but not quite perfect
J0(x) ~ 1 - x^2/4 + x^4/64 - ...
It might even be slightly better than that on reflection. You can see in the smoke that it isn't quite free from spherical abberation.
Another way to make a perfect parabola is spin casting on an old record turntable ( the Canadian mercury mirror scope works this way). This generates a perfect parabola when done exactly right - getting the bearings really smooth is key to useful optics though. And you can only look at the zenith with it.
It was cancellable after about 4 seconds.
I just wait for the skip advert popup.
-- Regards, Martin Brown
I didn't get the ad, but when I do, I close it and try again, that often starts the video, occasionally I need to restart twice.
He did no test other than look at reflected light through smoke, which worked well. I do bail most of the time.
I don't want to encourage bad behavior.
Mikek
a.
The proof of the pudding is in the eating. The energy gathered is a functi on of the area of the mirror, not it's shape. The shape determines how sma ll an area the energy is focused into. Looks to me like the guy got it dow n to about two inches or so. Not at all bad. Parabolas are not easy to co nstruct. I seem to recall they can be formed by spinning a liquid in a cyl indrical container. Didn't they do that with mercury for some purpose?
-- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code - https://ts.la/richard11209
bola.
Some ads they make you sit for the whole ad. I won't do that either and b ail. I think YouTube gets that since most ads have the five second exit. Even that is a PITA for some ads, like when they come on with loud noises a nd I can't hit the mute fast enough. Time for some competition to YouTube.
-- Rick C. + Get 1,000 miles of free Supercharging + Tesla referral code - https://ts.la/richard11209
It doesn't play. what's in the vid?
You never see an ad with adblock plus for Firefox. Can't get it for Chrome because google won't block their own ads.
la.
He makes a parabolic mirror by gluing a sheet of metalized mylar to a piece of what amounts to plywood with a ring of glue. He then inflates the spac e and the curve in the mylar is like a parabola. His focus seems to be abo ut 2 inches, so it's not bad, but might actually just be a sphere rather th an a paraboloid. Don't know. Untill the shape gets a lot deeper there rea lly isn't much difference between the two.
Not a bad video and it's not long. He had one false start using spray foam as a backing and pulled it off the base rather than cutting. Didn't end w ell.
-- Rick C. -- Get 1,000 miles of free Supercharging -- Tesla referral code - https://ts.la/richard11209
Yes, IIRC to make telescopes with the major limitations they only look vertically upwards. Also by making large rotating kilns in which to cast glass blanks for telescopes. Of course when the blanks have been polished and silvered they can be mounted in a large gimbal, maybe thousands of miles away from the kiln.
Mike.
Does that blocker work for Youtube ads, inside videos?
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
no it's not. a catenary has unidirectional force per unit length, the baloon has the force normal to the surface.
-- When I tried casting out nines I made a hash of it.
It's going to make a spherical surface.
A circlular arc is a fairly good approximation of the base of a parabola.
It focuses "as much anergy" as a true parabola, but not as tightly.
-- When I tried casting out nines I made a hash of it.
Not the version I have. However, you still may run into problems with Youtube videos, such as garbled sound on some videos. I also hate the ads in the middle of the video that will not allow you to delete them and continue watching. Sometimes there is an ad every 5 minutes, which completely disrupts the message in the video.
You can eliminate all ads, popups and youtube problems by downloading the file and playing it in VLC. If you don'e have VLC, you can download it here:
Get the Youtube link and go to Free YouTube Downloader at
Paste the Youtube link in KeepVid. Do not press the "Download Now" button to the right of the search box.
Instead, scroll down to the Video Quality section and select the appropriate file quality and format.
A window will pop up in yout Firefox browser allowing you to play the video in VLC or download it to your default folder.
When the download is complete, rename the file to some name with the appropriate extension.
If you download to your defaut folder, you can go back to Youtube and select another video while the first one is downloading.
You will end up with a pop-up free, ad-free, ungarbled VLC video you can watch with no interruptions.
torsdag den 14. november 2019 kl. 21.12.25 UTC+1 skrev John Larkin:
yes and it also works fine in Chrome
once you try it you'll never do without it
I never get those, not any other ads on youtube. The only ads I get are if the person making the video included it in their original upload. I have adblock plus and a few other extensions like umatrix.
.
There are a few web sites that can tell you are using it and won't let you view content, like the Washington Post. I thought reader view mode might g et me around that and it seemed to work, but it doesn't actually load the f ull article. Reader view only gets rid of the popup. Reader view mode doe s get rid of ads and such though. And it gets rid of the goofy formatting some sites use, like hackaday. But some of the images might not show up, l ike at hackaday.
-- Rick C. -+ Get 1,000 miles of free Supercharging -+ Tesla referral code - https://ts.la/richard11209
Yeah, lowest order Bessel function would be my first guess.
George H.
Probably not, for two reasons: first, Mylar is rolled out in production, it is NOT a homogeneous material (like a cast plastic is) but has been pre-stretched in one (or more) direction.
Second, the elastic properties as it stretches will make the center of the sheet stiffen (because the disc is, to first order, not stretched at all at the outer radius) in the usual work-hardening way (assuming, of course, inelastic deformation).
If the focal length is long enough, minor deviation from parabolic doesn't much matter.
Spinning a mass of molten glass (or mercury) under gravity DOES make a parabola rather accurately. You could do this with plaster of paris, a turntable, and finish with some aluminum paint.
I'm pretty sure you are right. In which case it has classical spherical aberration which gets worse the shorter the focal ratio. Anything longer than f8 and you can more or less get away without parabolising a mirror.
At f2 which this one is the focal length gets shorter as a function of the angle that a paraxial ray makes with the normal to the surface
f = (2cos(a)-1).r/2
-- Regards, Martin Brown
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