A wacky telescope making idea...

Hello all:

I have no experience making telescopes. Advance apologies if this idea
is stupid and shouldnt have been posted at all. OTOH, I would like
some feedback. Please read on:

I look at a Sky TV dish and it seems parabolic. The radio/tv waves are
reflected off it and brought to focus at the reciever. To convert this
dish into a mirror, I assemble a large number of CDs that reflect
light decently, cut them up to small pieces and stick them along the
dish without leaving any gaps. I now have a parabolic mirror whose
radius is the radius of the dish with focal point at the reciever. I
replace the reciever with a secondary mirror and direct the light to a
focusser where an eyepiece can be fitted. Have I got a telescope?

Thank you.

"jagbot" I have no experience making telescopes. Advance apologies if this
idea
is stupid and shouldnt have been posted at all. OTOH, I would like
some feedback. Please read on:
I look at a Sky TV dish and it seems parabolic. The radio/tv waves are
reflected off it and brought to focus at the reciever. To convert this
dish into a mirror, I assemble a large number of CDs that reflect
light decently, cut them up to small pieces and stick them along the
dish without leaving any gaps. I now have a parabolic mirror whose
radius is the radius of the dish with focal point at the reciever. I
replace the reciever with a secondary mirror and direct the light to a
focusser where an eyepiece can be fitted. Have I got a telescope?


Not a telescope ~ ( image quality would be terrible ) ~ but I guess it could
make a nice little solar furnace on sunny days :-) maybe kindle a few
brushwood fires in the outback ?

Ant

"jagbot" wrote in message
om...
Hello all:

I have no experience making telescopes. Advance apologies if this idea
is stupid and shouldnt have been posted at all. OTOH, I would like
some feedback. Please read on:

I look at a Sky TV dish and it seems parabolic. The radio/tv waves are
reflected off it and brought to focus at the reciever. To convert this
dish into a mirror, I assemble a large number of CDs that reflect
light decently, cut them up to small pieces and stick them along the
dish without leaving any gaps. I now have a parabolic mirror whose
radius is the radius of the dish with focal point at the reciever. I
replace the reciever with a secondary mirror and direct the light to a
focusser where an eyepiece can be fitted. Have I got a telescope?

Thank you.

I'm not a telescope maker, either, but I've been thinking about this, and
have to wonder about the CDs. Why not just coat the dish with a reflective
surfice, like chroming it, etc.? You end up with a much more even
reflective surface, rather than your "fractured" one the CDs would produce.

Other than that, sounds interesting, though I'm not sure if it'd work or
not. You might need to put baffling up around the edges of the dish, and on
the end of the "arm" that held the LNB, where your secondary mirror's gonna
go. Also, you're going to have a big blank spot in your view, caused by
that big "arm".

--Jason (newbie astrophotographer)
http://www.websown.com/~jdonahue/astro/astrophoto.htm

Go figure out how many wavelengths or parts thereof that the surface will be
accurate to. When you do this, remember that the smaller that error, the
better the telescope.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!

"Bob May" wrote in message
...
Go figure out how many wavelengths or parts thereof that the surface will
be
accurate to. When you do this, remember that the smaller that error, the
better the telescope.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink
less.
Works evevery time it is tried!



Bob,

Jagbot's idea has got me thinking. Do you have any links/recommended
reading for determining this? Dishes are relatively cheap (In the US, $49
at Radio Shack with an LNB in place), and this could make an interesting
project to try for kicks.

--Jason (newbie astrophotographer)
http://www.websown.com/~jdonahue/astro/astrophoto.htm

Jason @websown wrote:

"Bob May" wrote in message
...

Go figure out how many wavelengths or parts thereof that the surface will

be

accurate to. When you do this, remember that the smaller that error, the
better the telescope.

CLIP

Jagbot's idea has got me thinking. Do you have any links/recommended
reading for determining this? Dishes are relatively cheap (In the US, $49
at Radio Shack with an LNB in place), and this could make an interesting
project to try for kicks.

Just compare the wavelength of the radio signal the dish is made to
receive to the wavelength of light to see the problem...

"lal_truckee" wrote in message
...

Just compare the wavelength of the radio signal the dish is made to
receive to the wavelength of light to see the problem...


OK, so, with a freq. range of 12.2 - 12.7 GHz for the DirectTV setup, we're
talking wavelengths of around .023 - .024 meters approx. Visible light,
meanwhile, runs from around 400 nm to 700 nm or so, so we're talking a
significant difference in wavelength.

Bearing in mind that my physics is more than a little rusty, it looks like
the dish isn't really the right diameter for this, so you wouldn't really
get a clean image with it, rather than being an issue of the "point" of the
"light cone" the dish would generate not being at the position of the LNB -
be it a RF wave or a visible light wave, they should reflect the same way,
correct?

--Jason (newbie astrophotographer)
http://www.websown.com/~jdonahue/astro/astrophoto.htm

On Thu, 20 Nov 2003 12:06:36 -0800, "Jason"
wrote:

OK, so, with a freq. range of 12.2 - 12.7 GHz for the DirectTV setup, we're
talking wavelengths of around .023 - .024 meters approx. Visible light,
meanwhile, runs from around 400 nm to 700 nm or so, so we're talking a
significant difference in wavelength.

Bearing in mind that my physics is more than a little rusty, it looks like
the dish isn't really the right diameter for this, so you wouldn't really
get a clean image with it, rather than being an issue of the "point" of the
"light cone" the dish would generate not being at the position of the LNB -
be it a RF wave or a visible light wave, they should reflect the same way,
correct?

The problem is that the dish isn't very parabolic. At long radio wavelengths, it
only needs to be accurate to a millimeter or so. A parabolic optical reflector
needs to be accurate to nanometers. You'll get better optical performance with a
shaving mirror than with a microwave antenna.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"jagbot" wrote in message
I have no experience making telescopes. Advance apologies if this idea
is stupid and shouldnt have been posted at all. OTOH, I would like
some feedback. Please read on:

I look at a Sky TV dish and it seems parabolic. The radio/tv waves are
reflected off it and brought to focus at the reciever. To convert this
dish into a mirror, I assemble a large number of CDs that reflect
light decently, cut them up to small pieces and stick them along the
dish without leaving any gaps. I now have a parabolic mirror whose
radius is the radius of the dish with focal point at the reciever. I
replace the reciever with a secondary mirror and direct the light to a
focusser where an eyepiece can be fitted. Have I got a telescope?

Sure, for lambda ~0.1m, if you use small pieces of "mirror".

What kind of "light" did you wanna look at?

You'd do better at smaller wavelengths to glue aluminium foil to the
dish; then you might be ok at lambda above a centimetre or two.

But then, at these long wavelengths, you might as well just use the
dish "unadorned", because the reflectivity in visible light isn't going
to be much of an indicator of its reflectivity at these much longer
wavelengths. So there's no point gluing stuff to the dish, I reckon.

The key thing here, assuming the underlying dish's "figure" is a good
parabola, is the "roughness" (RMS deviation from true parabola) compared to
lambda. The lower the value of (RMS "roughness" / lambda), the better,
assuming equal reflectivity at lambda in all cases.

By the way, I'm bull****ting, but I think I'm pretty close here.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

Aside from the fact that the requirements for focusing radio waves is less
than the requirements to focus light waves . . .

A group of flat surfaces does not focus light to a point, which is needed
if you're going to get anything close to a clear view. The segmented
mirrors used in the Keck and other scopes are themselves parabolic, so each
can focus the light without the need of the other mirrors. CDs, even if
reflective enough, are not curved in parabolas, hence none of them will
bring the light to a focus, and hence the entire setup won't come to any
focus, hence no images. Hence no telescope.
--
----------------------------------------------------------------------
There is a fifth dimension beyond that which is known to Man.
It is a dimension as vast as space and as timeless as infinity.
It is the middle ground between light and shadow,
Between Science and superstition
And it lies between the pit of Man's fears
and the Sunlight of his knowledge.
It is the dimension of imagination.
It is an area that might be called. . . The Twilight Zone.


----------------------------------------------------------------------

"jagbot" wrote in message
om...
Hello all:

I have no experience making telescopes. Advance apologies if this idea
is stupid and shouldnt have been posted at all. OTOH, I would like
some feedback. Please read on:

I look at a Sky TV dish and it seems parabolic. The radio/tv waves are
reflected off it and brought to focus at the reciever. To convert this
dish into a mirror, I assemble a large number of CDs that reflect
light decently, cut them up to small pieces and stick them along the
dish without leaving any gaps. I now have a parabolic mirror whose
radius is the radius of the dish with focal point at the reciever. I
replace the reciever with a secondary mirror and direct the light to a
focusser where an eyepiece can be fitted. Have I got a telescope?

Thank you.