Is this a parabola?

[amdx]

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.
> https://youtu.be/8CLRTa_ocmo

Full length.

https://www.youtube.com/watch?v=8CLRTa_ocmo

Mikek

 

On Thu, 14 Nov 2019 10:26:28 -0600, amdx <nojunk@knology.net> wrote:

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.
https://youtu.be/8CLRTa_ocmo

Full length.
https://www.youtube.com/watch?v=8CLRTa_ocmo


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

 

[Martin Brown]

On 14/11/2019 16:45, jlarkin@highlandsniptechnology.com wrote:

On Thu, 14 Nov 2019 10:26:28 -0600, amdx <nojunk@knology.net> wrote:

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?

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.

Truncated version.
https://youtu.be/8CLRTa_ocmo

Full length.
https://www.youtube.com/watch?v=8CLRTa_ocmo


Mikek

I won't view a Youtube that has a non-cancellable 12 second ad, or
multiple ads.

It was cancellable after about 4 seconds.

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.

I just wait for the skip advert popup.

--
Regards,
Martin Brown

 

[Rick C]

On Thursday, November 14, 2019 at 12:21:49 PM UTC-5, Martin Brown wrote:

On 14/11/2019 16:45, jlarkin@highlandsniptechnology.com wrote:
On Thu, 14 Nov 2019 10:26:28 -0600, amdx <nojunk@knology.net> wrote:

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?

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.

Truncated version.
https://youtu.be/8CLRTa_ocmo

Full length.
https://www.youtube.com/watch?v=8CLRTa_ocmo


Mikek

I won't view a Youtube that has a non-cancellable 12 second ad, or
multiple ads.

It was cancellable after about 4 seconds.

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.

I just wait for the skip advert popup.

Some ads they make you sit for the whole ad. I won't do that either and bail. 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 and 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

 

[Rick C]

On Thursday, November 14, 2019 at 11:26:35 AM UTC-5, amdx wrote:

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.
https://youtu.be/8CLRTa_ocmo

Full length.
https://www.youtube.com/watch?v=8CLRTa_ocmo


Mikek

The proof of the pudding is in the eating. The energy gathered is a function of the area of the mirror, not it's shape. The shape determines how small an area the energy is focused into. Looks to me like the guy got it down to about two inches or so. Not at all bad. Parabolas are not easy to construct. I seem to recall they can be formed by spinning a liquid in a cylindrical 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

 

[amdx]

On 11/14/2019 10:45 AM, jlarkin@highlandsniptechnology.com wrote:

On Thu, 14 Nov 2019 10:26:28 -0600, amdx <nojunk@knology.net> wrote:

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.
https://youtu.be/8CLRTa_ocmo

Full length.
https://www.youtube.com/watch?v=8CLRTa_ocmo


Mikek

I won't view a Youtube that has a non-cancellable 12 second ad, or
multiple ads.

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.

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.

He did no test other than look at reflected light through smoke, which
worked well.
I do bail most of the time.

> I'd pay a modest fee for ad-free Youtube. It's currently $12 a month.

I don't want to encourage bad behavior.

Mikek

 

[Johann Klammer]

On 11/14/2019 05:26 PM, amdx wrote:

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.
https://youtu.be/8CLRTa_ocmo

Full length.
https://www.youtube.com/watch?v=8CLRTa_ocmo


Mikek
It doesn't play. what's in the vid?

 

[Tom Del Rosso]

jlarkin@highlandsniptechnology.com wrote:

I'd pay a modest fee for ad-free Youtube. It's currently $12 a month.

You never see an ad with adblock plus for Firefox. Can't get it for
Chrome because google won't block their own ads.

 

[Rick C]

On Thursday, November 14, 2019 at 2:17:12 PM UTC-5, Johann Klammer wrote:

On 11/14/2019 05:26 PM, amdx wrote:
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.
https://youtu.be/8CLRTa_ocmo

Full length.
https://www.youtube.com/watch?v=8CLRTa_ocmo


Mikek
It doesn't play. what's in the vid?

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 space and the curve in the mylar is like a parabola. His focus seems to be about 2 inches, so it's not bad, but might actually just be a sphere rather than a paraboloid. Don't know. Untill the shape gets a lot deeper there really 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 well.

--

Rick C.

-- Get 1,000 miles of free Supercharging
-- Tesla referral code - https://ts.la/richard11209

 

[Mike Coon]

In article <f4dcca87-ea45-4507-b2a8-e032e7c4705c@googlegroups.com>,
gnuarm.deletethisbit@gmail.com says...

I seem to recall they (paraboloids; 3D parabolas) can be formed by
spinning a liquid in a cylindrical container. Didn't they do that
with mercury for some purpose?

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.

 

[Jasen Betts]

On 2019-11-14, Martin Brown <'''newspam'''@nezumi.demon.co.uk> wrote:

On 14/11/2019 16:45, jlarkin@highlandsniptechnology.com wrote:
On Thu, 14 Nov 2019 10:26:28 -0600, amdx <nojunk@knology.net> wrote:

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?

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.

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.

 

[John Larkin]

On Thu, 14 Nov 2019 14:24:25 -0500, "Tom Del Rosso"
<fizzbintuesday@that-google-mail-domain.com> wrote:

jlarkin@highlandsniptechnology.com wrote:

I'd pay a modest fee for ad-free Youtube. It's currently $12 a month.

You never see an ad with adblock plus for Firefox. Can't get it for
Chrome because google won't block their own ads.

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

 

[Jasen Betts]

On 2019-11-14, amdx <nojunk@knology.net> wrote:

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.

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.

 

[Steve Wilson]

John Larkin <jlarkin@highland_atwork_technology.com> wrote:

On Thu, 14 Nov 2019 14:24:25 -0500, "Tom Del Rosso"
fizzbintuesday@that-google-mail-domain.com> wrote:

jlarkin@highlandsniptechnology.com wrote:

I'd pay a modest fee for ad-free Youtube. It's currently $12 a month.

You never see an ad with adblock plus for Firefox. Can't get it for
Chrome because google won't block their own ads.

Does that blocker work for Youtube ads, inside videos?

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:

https://www.videolan.org/vlc/index.html

Get the Youtube link and go to Free YouTube Downloader at

https://keepvid.pro/

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.

 

[Lasse Langwadt Christense]

torsdag den 14. november 2019 kl. 21.12.25 UTC+1 skrev John Larkin:

On Thu, 14 Nov 2019 14:24:25 -0500, "Tom Del Rosso"
fizzbintuesday@that-google-mail-domain.com> wrote:

jlarkin@highlandsniptechnology.com wrote:

I'd pay a modest fee for ad-free Youtube. It's currently $12 a month.

You never see an ad with adblock plus for Firefox. Can't get it for
Chrome because google won't block their own ads.



Does that blocker work for Youtube ads, inside videos?

yes and it also works fine in Chrome

once you try it you'll never do without it

 

[Rick C]

On Thursday, November 14, 2019 at 4:55:47 PM UTC-5, Lasse Langwadt Christensen wrote:

torsdag den 14. november 2019 kl. 21.12.25 UTC+1 skrev John Larkin:
On Thu, 14 Nov 2019 14:24:25 -0500, "Tom Del Rosso"
fizzbintuesday@that-google-mail-domain.com> wrote:

jlarkin@highlandsniptechnology.com wrote:

I'd pay a modest fee for ad-free Youtube. It's currently $12 a month..

You never see an ad with adblock plus for Firefox. Can't get it for
Chrome because google won't block their own ads.



Does that blocker work for Youtube ads, inside videos?

yes and it also works fine in Chrome

once you try it you'll never do without it

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 get me around that and it seemed to work, but it doesn't actually load the full article. Reader view only gets rid of the popup. Reader view mode does 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, like at hackaday.

--

Rick C.

-+ Get 1,000 miles of free Supercharging
-+ Tesla referral code - https://ts.la/richard11209

 

[George Herold]

On Thursday, November 14, 2019 at 12:21:49 PM UTC-5, Martin Brown wrote:

On 14/11/2019 16:45, jlarkin@highlandsniptechnology.com wrote:
On Thu, 14 Nov 2019 10:26:28 -0600, amdx <nojunk@knology.net> wrote:

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?

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 - ...
Yeah, lowest order Bessel function would be my first guess.

George H.

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.

Truncated version.
https://youtu.be/8CLRTa_ocmo

Full length.
https://www.youtube.com/watch?v=8CLRTa_ocmo


Mikek

I won't view a Youtube that has a non-cancellable 12 second ad, or
multiple ads.

It was cancellable after about 4 seconds.

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.

I just wait for the skip advert popup.

--
Regards,
Martin Brown

 

[Chris Jones]

On 15/11/2019 07:12, John Larkin wrote:

On Thu, 14 Nov 2019 14:24:25 -0500, "Tom Del Rosso"
fizzbintuesday@that-google-mail-domain.com> wrote:

jlarkin@highlandsniptechnology.com wrote:

I'd pay a modest fee for ad-free Youtube. It's currently $12 a month.

You never see an ad with adblock plus for Firefox. Can't get it for
Chrome because google won't block their own ads.



Does that blocker work for Youtube ads, inside videos?

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.

 

[whit3rd]

On Thursday, November 14, 2019 at 8:26:35 AM UTC-8, amdx wrote:

It looks like a good focus if not long.
But does it follow y=ax^2 as I understand it?

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.

 

[Martin Brown]

On 14/11/2019 20:17, Jasen Betts wrote:

On 2019-11-14, Martin Brown <'''newspam'''@nezumi.demon.co.uk> wrote:
On 14/11/2019 16:45, jlarkin@highlandsniptechnology.com wrote:
On Thu, 14 Nov 2019 10:26:28 -0600, amdx <nojunk@knology.net> wrote:

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?

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.

no it's not. a catenary has unidirectional force per unit length,
the baloon has the force normal to the surface.

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