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--- RON WHITE <ron_white@...> wrote:

Hello Albert
Glad to have you join us
How large is the glass from the cat. tubes and how
thick?
Because the light dosen't go thru the type of glass
is less important than how stable it is. It's been a
long time since I've broken a TV screen but I think
the glass is too thin to grind into a good mirror.
let us know we have some real expert mirror makers
here (I'm not one of the experts :-)
Good luck
Ron

Hello Ron,
I am glad of your answer.

When you talk of stability of the glass, it is about
temperature or other factors?
Respect the thickness of the glass, in the front, it
is about 12 milimeters (1/2 inch).
The size of the front of the screen, as you know, the
usual crt(cathode ray tube) are from 14" to 29"
diagonal.
Also is important to consider that already has a
concave shape in the inside.
I am glad to know there are real experts mirror makers
in the group.

I appreciate your interest in the subjet.



=====
Alberto Gasparini
Perito Moreno 1032
Godoy Cruz (5501)
Mendoza
ARGENTINA
telefono 0261-422-0923

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"AGUSTIN GASPARINI" <teveotron@...> wrote:

Respect the thickness of the glass, in the front, it
is about 12 milimeters (1/2 inch).
The size of the front of the screen, as you know, the
usual crt(cathode ray tube) are from 14" to 29"
diagonal.
Also is important to consider that already has a
concave shape in the inside.

12 millimeters (1/2 inch) is pretty thin for any sizable mirror. You might
get away with it for a small mirror if you are careful and the glass is not
tempered (which would surprise me from the way that TV screens shatter).

Any-old concave shape is not necessarily a plus. You want the inside to be
a spherical section of a relatively large sphere before you are done with
grinding.

Radius of curvature is twice the focal length, so an 8in (203mm) f/6 mirror
would have an 8*6=48in (1.224m) focal length and a 96in (2.448m) radius of
curvature. The sagitta (depth of curve at center of mirror) is
approximately S=D/(16*fratio) so for said 8in f/6 mirror, we are looking for
1/12inch (2.12mm) of depth at the center. A 6in f/6 would have a 1/16in
saggita as would a 4in f/4).

One tough part may be trepanning out a small "disk" to work on. Then you
will need to figure out how to work with a blank that has a convex bottom.
(Perhaps the lens makers out there can jump in?)

It sounds like it might be a fun experiment if you have the time and are not
heavily invested in it working out.

Jeff

12 millimeters (1/2 inch) is pretty thin for any
sizable mirror. You might
get away with it for a small mirror if you are
careful and the glass is not
tempered (which would surprise me from the way that
TV screens shatter).

Any-old concave shape is not necessarily a plus.
You want the inside to be
a spherical section of a relatively large sphere
before you are done with
grinding.

Radius of curvature is twice the focal length, so an
8in (203mm) f/6 mirror
would have an 8*6=48in (1.224m) focal length and a
96in (2.448m) radius of
curvature. The sagitta (depth of curve at center of
mirror) is
approximately S=D/(16*fratio) so for said 8in f/6
mirror, we are looking for
1/12inch (2.12mm) of depth at the center. A 6in f/6
would have a 1/16in
saggita as would a 4in f/4).

One tough part may be trepanning out a small "disk"
to work on. Then you
will need to figure out how to work with a blank
that has a convex bottom.
(Perhaps the lens makers out there can jump in?)

It sounds like it might be a fun experiment if you
have the time and are not
heavily invested in it working out.

Hi Jeff:

As far as I know, the glass of crt(catode ray tubes)
is not tempered.
As I told to Ron, I wish I could make the test with
palarized light.
Is there a way to make a home made polariscope?
The radius of curvature of a 21" crt is pproximatelly
120mm wich make a too small radius.

In wich way is affected the cuality
of the telescope, when the radius is that small?

The convex bottom could be filled up wit some epoxy
or similar compound?

=====
Alberto Gasparini
Perito Moreno 1032
Godoy Cruz (5501)
Mendoza
ARGENTINA
telefono 0261-422-0923

__________________________________
Do you Yahoo!?
Yahoo! Calendar - Free online calendar with sync to Outlook(TM).

"AGUSTIN GASPARINI" <teveotron@...> wrote:

As far as I know, the glass of crt(catode ray tubes)
is not tempered.

Good. That helps.

As I told to Ron, I wish I could make the test with
palarized light.
Is there a way to make a home made polariscope?

I'm not an expert but two things come to mind. (1) a pair of polarized
sunglasses can be taken apart to make two polarizers, or (2) reflected light
is plane polarized, so a mirror plus a a super-bright led might provide a
polarized light source.

The radius of curvature of a 21" crt is pproximatelly
120mm wich make a too small radius.

In wich way is affected the cuality
of the telescope, when the radius is that small?

Sounds like a bit too much curvature for a primary mirror. Even a small
mirror would have an extremely small f-ratio which would lead to major coma.
However it might be fine to use the convex side for a secondary for a
Cassegrain.

The convex bottom could be filled up wit some epoxy
or similar compound?

From what I hear, you don't want to glue on something which might have a
different stress pattern than the glass, otherwise it makes it hard to
create the proper shape as you remove glass.

Jeff

Hi AGUSTIN,
I have considered using the front screen of a tv before but it didnt break where I wanted it. However I came to the conclusion that the best way to proceed would be to first
1. trepann out the circle size you want.
2. Make a mould of the shape you desire, ie the curve, and include sides.
3. line the sides of the mould with kiln shelf paper to allow a bit of expansion
4. find someone with a temperature controlled kiln who can slump the glass and then anneal it for you.
Find a glass artist as they have plenty of practice in these procedures and discuss exactly what you want to do and use their experience in what mould material to use.
Further grinding and polishing with a curved back surface will require excellent support.
I am experimenting here myself and will be sitting the mirror on a flat surface front face down. Placing a ring around the mirror and flooding with dental stone to form a level flat surface.
Rubbing the dental stone down to make sure the mirror sits dead flat and that there is no wedge.
Hope that this does not stress the mirror.
Mount, grind, polish in the normal manner.
Sand of the dental stone when finished. (I hope)
I will report back when I have some hard data.
I must stress that I haven't done this yet and the screen glass is very thin.
Maybe this will generate some other ideas.
regards
Charles


I just got lost in thought. It was unfamiliar territory.

--- Hi Charles Mitchard:
About annealing the glass i have some experience, I
have an oven for crt(cathode ray tubes) procesisng
which can reaach more than 430 centigrades.

The temperature of 430 centigrades is the annealing
point for this tipe of glass.
About the further grinding and polishing I am
thinking of holding the glass from the center an make
it to rotate.
I am not worry about not stressing the mirror, I
think the important cuestion is to have an even
stress.
Mount, grind, polish in the normal manner could be
changed for faster and more accurrate results.
Did eny one in the group trayed other alternatives?
Maybe this will also generate some other ideas.
I thanks your attention, regards.


=====
Alberto Gasparini
Perito Moreno 1032
Godoy Cruz (5501)
Mendoza
ARGENTINA
telefono 0261-422-0923

__________________________________
Do you Yahoo!?
Yahoo! Calendar - Free online calendar with sync to Outlook(TM).

--- In VacuumX@..., AGUSTIN GASPARINI <teveotron@y...>
wrote:

Maybe this will also generate some other ideas.
I thanks your attention, regards.


=====
Alberto Gasparini

Alberto,

Welcome...

I am the owner and moderator of this YAHOO Group, I am also the
owner/moderator of an Amateur Telescope Making Group on YAHOO.

Check us out:


You will find MANY MORE members there that are more familiar with the
questions you are asking here... But please remain a member here
also... you will eventually need to have an aluminum coating on your
glass and that is where you will need the resources of THIS GROUP!

From past experience, getting a useful mirror out of a CRT is not
likely at least in sizes over 150mm diameter. The glass (if larger)
is too easily stressed while working and difficult to finish into an
acceptable figure but... in smaller sizes it is a cheap source of
glass and easily worked.

Good luck!

Ken Hunter