DIY Parabolic

[JohnSadlerAstro]

Hi,

A really interesting video popped up in my recommended list, a video about making parabolic mirrors using mylar sheeting. It's quite an interesting method, I'm wondering how effective mirrors made in this way would be for astronomy? In theory it would be possible to make a large crude light bucket with a mirror like this? The one made in the video had an almost impossible short focal length, probably about f/1.5! Surely with a large enough mirror, enough light would be correctly focused onto a secondary to get it to work?

You can see the light path in the smoke at the start of the vid, and it actually looks quite distortion-free.

John

[Merlin66]

The Mylar mirror has been a subject of investigation for the last twenty years!!

many suggestions have been trialed, without success.

not a quality optical solution......... sorry.

 

[michael.h.f.wilkinson]

Good enough for a solar furnace, perhaps, but not good enough to form a decent image

[Gina]

What a pity...  Maybe some day someone will discover a reflective material that will be good enough.

[JohnSadlerAstro]

Just now, Gina said:

What a pity...  Maybe some day someone will discover a reflective material that will be good enough.

It would be wonderful to be able to make a large, lightweight mirror like that.... I guess grinding for 12 hours or so is the only way!

John 

[Merlin66]

More success has been achieved using thin optical sheets of glass vacuum distorted....limitation seems to be around f8 parabolic curves.

 

[michael.h.f.wilkinson]

It's not just a matter of getting a good reflective material with the right optical figure, it is very much a matter of keeping it in that figure. Using a very thin material will almost immediately lead to flexure in the optics, and require active compensation of some kind. Even the Hale telescope's honeycomb mirror has mechanical actuators to compensate for flexure when the scope isn't pointing at zenith

[vlaiv]

Does anyone know if spin casting has been used to make mirrors?

Gravity and centrifugal force can be used while glass is still melted to get to roughly parabolic shape - then it would be just matter of figuring. I have no idea about internal stress of the glass in such case - it might pose a problem.

[JohnSadlerAstro]

You might end up with the glass at "high tension". I have heard of sugar bowls and things like that splintering because of stresses induced in this way. 

Jonh

[Merlin66]

Angel and his team is the US have been spin casting 4m telescope Mirrors for the past twenty  years.

Don’t know about the potential for amateur sized optics.....

[Gina]

Yes, I've heard of it too.  I wouldn't mind a 4m mirror ?  But it would need to have a very short focal length and would take up pretty much all of my grass area.  Have to be covered in daytime or the focused sun could instantly fry a passing bird!!

[Astrobits]

Commercial Plastic film is simply not of optical surface quality. It varies in thickness, and hence surface flatness, sufficiently give a very poor wavefront for imaging.

However, I like Gina's idea of BBQing passing birds. I might get rid of some of the seagulls around here?

Nigel

[Stub Mandrel]

Mercury mirrors.

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

Mounting is bit of a challenge...

[Gina]

But that only works pointing straight up.

[skybadger]

Which is what the naval observatory in Washington use as a transit scope.

Been to that one.

M

[Demonperformer]

21 hours ago, Merlin66 said:

Don’t know about the potential for amateur sized optics.....

 Read somewhere that the glass has to be cooled at about 1/2 a degree per day, so it takes weeks, and has to be kept spinning through the entire time. For small mirrors where they can set up grinding facilities to make hundreds, the cost would be prohibitive. But for a one-off 4m mirror it works out a lot cheaper.

[Demonperformer]

21 hours ago, Gina said:

I wouldn't mind a 4m mirror ?  But it would need to have a very short focal length and would take up pretty much all of my grass area.

Do you think my NEQ6 could cope?

[JohnSadlerAstro]

14 hours ago, Stub Mandrel said:

Mercury mirrors.

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

Mounting is bit of a challenge...

Imagine the noise of a centrifuge going off in your ear. "lets make it transportable and power it with a 2 stroke engine! "

I imagine vibration would be a bit of an issue too. It would have to be kept perfectly balanced.

John

[Demonperformer]

And mercury is not particularly nice stuff to have around in any sort of quantity, let alone this much.

[JohnSadlerAstro]

Especially if its spinning around in a shallow mirror cell ? 

Collimation, too would be a bit nasty. Perhaps glass mirrors are the only way.  

John