Aluminum composite reportedly stronger, lighter than carbon

On Wed, 10 Oct 2007 15:49:48 -0700, Fred the Red Shirt
wrote:

On Oct 9, 11:53 pm, Steve Hix
wrote:
In article .com,
Fred the Red Shirt wrote:


...

Obviously you have not seen how a telescope mirror ages over the
years.

Guess again.

Well then you should have observed that the transparency increases
between cleanings.


It's easy to recoat the mirror, so it's not so much of a problem,
certainly less of one than dealing with silver corrosion.

While it is not a technologically daunting task, it is not cheap for

IF you have the equipment (vacuum chamber, heater for vaporizing
Aluminum) it's relatively simple. One of our Astronomy club members
does up to 10 or 12" for the locals. IIRC his favorite source of
Aluminum is peeling the foil off gum wrappers although regular
Aluminum foil works. It just takes more power.

It doesn't take a super high vacuum either. Just one high enough to
develop a plasma although that's not part of the process.

On local used the chemical process for coating which worked pretty
well, but the coating isn't nearly as even as vacuum deposition.

larger mirrors especially when you consider shipping. I have a 17.5"
mirror that will need realuminizing when I finally get around to
putting
it into a scope. It has been cleaned exactly once, but the coating
is nearly gone entirely after 30 years in storage.

Offhand, do you know anyone who recoats mirrors that size?

Not that size, but there are a number of labs that do the work and I
think I saw a couple of links listed.

Roger (K8RI).

On Oct 15, 4:44 am, "Roger (K8RI)" wrote:
On Wed, 10 Oct 2007 15:49:48 -0700, Fred the Red Shirt



wrote:
On Oct 9, 11:53 pm, Steve Hix
wrote:
In article .com,
Fred the Red Shirt wrote:

...

Obviously you have not seen how a telescope mirror ages over the
years.

Guess again.

Well then you should have observed that the transparency increases
between cleanings.

It's easy to recoat the mirror, so it's not so much of a problem,
certainly less of one than dealing with silver corrosion.

While it is not a technologically daunting task, it is not cheap for

IF you have the equipment (vacuum chamber, heater for vaporizing
Aluminum) it's relatively simple. One of our Astronomy club members
does up to 10 or 12" for the locals. IIRC his favorite source of
Aluminum is peeling the foil off gum wrappers although regular
Aluminum foil works. It just takes more power.

Surface Preparation (cleaning) is extremely important as traces
or organics (from handling( will prevent adhesion of the aluminum.

Texereau recommends final cleaning by electron bombardment
in the vacuum chamber.



It doesn't take a super high vacuum either. Just one high enough to
develop a plasma although that's not part of the process.

On local used the chemical process for coating which worked pretty
well, but the coating isn't nearly as even as vacuum deposition.

Described by Texereau also, assuming you mean the chemical silvering
process.


larger mirrors especially when you consider shipping. I have a 17.5"
mirror that will need realuminizing when I finally get around to
putting
it into a scope. It has been cleaned exactly once, but the coating
is nearly gone entirely after 30 years in storage.

Offhand, do you know anyone who recoats mirrors that size?

Not that size, but there are a number of labs that do the work and I
think I saw a couple of links listed.


The place to ask would be sci.astro.amateur. I asked
OP as it looked like maybe he might know one.

--

FF

On Fri, 26 Oct 2007 09:53:04 -0700, Fred the Red Shirt
wrote:

On Oct 15, 4:44 am, "Roger (K8RI)" wrote:
On Wed, 10 Oct 2007 15:49:48 -0700, Fred the Red Shirt



wrote:
On Oct 9, 11:53 pm, Steve Hix
wrote:
In article .com,
Fred the Red Shirt wrote:

...

Obviously you have not seen how a telescope mirror ages over the
years.

Guess again.

Well then you should have observed that the transparency increases
between cleanings.

It's easy to recoat the mirror, so it's not so much of a problem,
certainly less of one than dealing with silver corrosion.

While it is not a technologically daunting task, it is not cheap for

IF you have the equipment (vacuum chamber, heater for vaporizing
Aluminum) it's relatively simple. One of our Astronomy club members
does up to 10 or 12" for the locals. IIRC his favorite source of
Aluminum is peeling the foil off gum wrappers although regular
Aluminum foil works. It just takes more power.

Surface Preparation (cleaning) is extremely important as traces
or organics (from handling( will prevent adhesion of the aluminum.

Texereau recommends final cleaning by electron bombardment
in the vacuum chamber.

Which reminded me, my friend uses that plasma for the final cleaning.
Thats a lot of electronic bombardment. :-))




It doesn't take a super high vacuum either. Just one high enough to
develop a plasma although that's not part of the process.

On local used the chemical process for coating which worked pretty
well, but the coating isn't nearly as even as vacuum deposition.

Described by Texereau also, assuming you mean the chemical silvering
process.

I do and it was.



larger mirrors especially when you consider shipping. I have a 17.5"
mirror that will need realuminizing when I finally get around to
putting
it into a scope. It has been cleaned exactly once, but the coating
is nearly gone entirely after 30 years in storage.

Offhand, do you know anyone who recoats mirrors that size?

Not that size, but there are a number of labs that do the work and I
think I saw a couple of links listed.


The place to ask would be sci.astro.amateur. I asked
OP as it looked like maybe he might know one.

I had forgotten about that group and I used to follow it.

Roger (K8RI)

On Oct 26, 2:04 pm, "Roger (K8RI)" wrote:
On Fri, 26 Oct 2007 09:53:04 -0700, Fred the Red Shirt
...
Surface Preparation (cleaning) is extremely important as traces
or organics (from handling( will prevent adhesion of the aluminum.

Texereau recommends final cleaning by electron bombardment
in the vacuum chamber.

Which reminded me, my friend uses that plasma for the final cleaning.
Thats a lot of electronic bombardment. :-))



How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

--

FF

Fred the Red Shirt wrote:

How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber? Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?

Mark "under pressure" Hickey

"Mark Hickey" wrote in message
...
Fred the Red Shirt wrote:

How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber? Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?


Maybe, maybe not. Round pressure vessels keep their shell walls in tension,
hence the more pressure the better they hold their shape. Vacuum vessels are
just the opposite, and quite often much easier to collapse than one might
naturally assume.

I can say I once built a round vacuum chamber out of rolled 1/4" aluminum.
It was approximately 18" long and 18" in diameter. The bottom was 3/8"
aluminum, and the top was 1" clear plastic. The chamber was successful with
up to an near perfect vacuum, and used many times without failure. At
maximum vacuum, the bottom would dish approximately 1/8 to 3/16" inch, an
the plastic top would dish about 1/2".

I have a chamber I use now for another purpose, but it is only 6" in
diameter. The top for it is just 3/16" tempered glass.

Hope the number might help your estimates.

Max

Maxwell wrote:

"Mark Hickey" wrote in message
...

Fred the Red Shirt wrote:


How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber? Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?



Maybe, maybe not. Round pressure vessels keep their shell walls in tension,
hence the more pressure the better they hold their shape. Vacuum vessels are
just the opposite, and quite often much easier to collapse than one might
naturally assume.

I can say I once built a round vacuum chamber out of rolled 1/4" aluminum.
It was approximately 18" long and 18" in diameter. The bottom was 3/8"
aluminum, and the top was 1" clear plastic. The chamber was successful with
up to an near perfect vacuum, and used many times without failure. At
maximum vacuum, the bottom would dish approximately 1/8 to 3/16" inch, an
the plastic top would dish about 1/2".

I have a chamber I use now for another purpose, but it is only 6" in
diameter. The top for it is just 3/16" tempered glass.

Hope the number might help your estimates.

Max




You know, an ABSOLUTE vacuum is only 15 psi...

On Nov 1, 12:21 pm, "Maxwell" wrote:
"Mark Hickey" wrote in message

...



Fred the Red Shirt wrote:

How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber? Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?

Maybe, maybe not. Round pressure vessels keep their shell walls in tension,
hence the more pressure the better they hold their shape. Vacuum vessels are
just the opposite, and quite often much easier to collapse than one might
naturally assume.

I can say I once built a round vacuum chamber out of rolled 1/4" aluminum.
It was approximately 18" long and 18" in diameter. The bottom was 3/8"
aluminum, and the top was 1" clear plastic. The chamber was successful with
up to an near perfect vacuum, and used many times without failure. At
maximum vacuum, the bottom would dish approximately 1/8 to 3/16" inch, an
the plastic top would dish about 1/2".

I have a chamber I use now for another purpose, but it is only 6" in
diameter. The top for it is just 3/16" tempered glass.

Hope the number might help your estimates.


Ok so at 18" diameter that clear plastic dish had a surface area
of about 254 square inches, so it saw a force of about 3700 lbs,
less if you were significantly above sea level.

--

FF

On Thu, 1 Nov 2007 12:21:03 -0500, "Maxwell"
wrote:


"Mark Hickey" wrote in message
.. .
Fred the Red Shirt wrote:

How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber? Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?


Maybe, maybe not. Round pressure vessels keep their shell walls in tension,
hence the more pressure the better they hold their shape. Vacuum vessels are
just the opposite, and quite often much easier to collapse than one might
naturally assume.

I can say I once built a round vacuum chamber out of rolled 1/4" aluminum.
It was approximately 18" long and 18" in diameter. The bottom was 3/8"
aluminum, and the top was 1" clear plastic. The chamber was successful with
up to an near perfect vacuum, and used many times without failure. At
maximum vacuum, the bottom would dish approximately 1/8 to 3/16" inch, an
the plastic top would dish about 1/2".

We used to use this on glass plates to grind the corrector for a
schmidt casagrain. Warp the glass with the vacuum, grind to a
parabola, and then release the vacuum. In this case it took a lot
more work than to explain. :-))

Roger (K8RI)


I have a chamber I use now for another purpose, but it is only 6" in
diameter. The top for it is just 3/16" tempered glass.

Hope the number might help your estimates.

Max

On Nov 1, 8:53 am, Mark Hickey wrote:
Fred the Red Shirt wrote:

How big is his vacuum chamber?

What sort of vacuum pump(s) does he use?

One large enough for a 17.5" mirror is rather non-trivial. Assuming
a 20" diameter cylindrical chamber, the top and bottom would need
to support over 3000 pounds each, if the work is done at sea level.

Is it really that hard to build a vacuum chamber?

No, but it is much easier to build a small one than a large one.

Seems to me that
the most pressure it'll ever experience is about 15psi (1 bar), while
it's trivial to build/buy pressure containers that can handle 10-100x
that much (positive) pressure. Certainly if building a 1 bar vessel
20" in diameter is daunting, building a submarine (or worse, a
deep-sea bathyscaphe, which have reached depths of almost 36,000 feet
below sea level, resisting a pressure of about 1,100 bar) would be
unthinkable.

Or am I missing something?

Buckling.

The skin of a pressure vessel is in almost pure tension, so they can
be thin and not buckle. Any bending moment on a flat section bows
it outward reducing the bending moment (essentially converting it to
tension) The sides of vacuum vessel see compression and bending,
and any flat sections will buckle inward which will increase the
bending
moment.

The bathyscape and similar vessels are cylindrical with hemi-spherical
ends so that their skin is in almost pure compression with very little
bending moment.

A 55-gallon drum can be cut down to make the sides of the vacuum
chamber but I'm not clear on how to make the end hemispherical.
An option is to use nested vessels, with partial vacuum between them.

There is a reason why bell jars have a hemispherical top, and it is
not esthetics.

--

FF