A wacky telescope making idea...

I had this idea of putting a 4.5-inch mirror into my CD player to see if it
plays. I'll probably need to drill a hole first...

Edwin.

In message , jagbot
writes
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?

No. You would have a simple solar flux concentrator. Maybe if you do it
right one good enough to boil water at the focus with a bit of care.
Wear sunglasses when playing with it the glare is pretty awful.

But it would be much easier to cover it with kitchen foil shiny side out
- at least that way you might retain some of the parabolic figure.
Either way it is never going to give you a telescope the tolerances are
wrong by orders of magnitude. Even using a shaving mirror would be a
better bet!

Bad time of year to be making solar concentrators though - the sun is
feeble and hardly gets above the horizon in the UK during winter.

Regards,
--
Martin Brown

In message , jagbot
writes
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?

No. You would have a simple solar flux concentrator. Maybe if you do it
right one good enough to boil water at the focus with a bit of care.
Wear sunglasses when playing with it the glare is pretty awful.

But it would be much easier to cover it with kitchen foil shiny side out
- at least that way you might retain some of the parabolic figure.
Either way it is never going to give you a telescope the tolerances are
wrong by orders of magnitude. Even using a shaving mirror would be a
better bet!

Bad time of year to be making solar concentrators though - the sun is
feeble and hardly gets above the horizon in the UK during winter.

Regards,
--
Martin Brown

In message , Jason
writes

"jagbot" wrote in message
. com...
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

May I suggest the following:-
1. The surface you would obtain by either proposal would be quite
useless for this purpose since very much greater accuracy is needed.
2. A 'secondary mirror' at the focal point would be in the wrong place,
irrespective of anything else.
3. Most, if not all, TV dishes are 'off-axis' sections of a paraboloid
and so designed to keep the LNB and its arm out off the way..Try a quick
sketch.

I do not mean to dampen your interest in mirror making, but I feel that
you need to do some more reading on optics before you proceed any
further.
There is a great deal of satisfaction to be gained from making your own
telescope mirrors but I'm afraid there just aren't any shortcuts of the
type you suggest.

However, may I point out that if you ever get interested in radio
astronomy then a TV dish is apparently useful as a ready-made starter
aerial.
--
Best wishes,
John.

In message , Jason
writes

"jagbot" wrote in message
. com...
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

May I suggest the following:-
1. The surface you would obtain by either proposal would be quite
useless for this purpose since very much greater accuracy is needed.
2. A 'secondary mirror' at the focal point would be in the wrong place,
irrespective of anything else.
3. Most, if not all, TV dishes are 'off-axis' sections of a paraboloid
and so designed to keep the LNB and its arm out off the way..Try a quick
sketch.

I do not mean to dampen your interest in mirror making, but I feel that
you need to do some more reading on optics before you proceed any
further.
There is a great deal of satisfaction to be gained from making your own
telescope mirrors but I'm afraid there just aren't any shortcuts of the
type you suggest.

However, may I point out that if you ever get interested in radio
astronomy then a TV dish is apparently useful as a ready-made starter
aerial.
--
Best wishes,
John.

On Fri, 21 Nov 2003 12:33:43 +0000, in uk.sci.astronomy , Edwin
Spector wrote:

I had this idea of putting a 4.5-inch mirror into my CD player to see if it
plays. I'll probably need to drill a hole first...

Sell your newt and buy a cassegranian. Then you have the hole already.
:-)

--
Mark McIntyre
CLC FAQ http://www.eskimo.com/~scs/C-faq/top.html
CLC readme: http://www.angelfire.com/ms3/bchambless0/welcome_to_clc.html

On Fri, 21 Nov 2003 12:33:43 +0000, in uk.sci.astronomy , Edwin
Spector wrote:

I had this idea of putting a 4.5-inch mirror into my CD player to see if it
plays. I'll probably need to drill a hole first...

Sell your newt and buy a cassegranian. Then you have the hole already.
:-)

--
Mark McIntyre
CLC FAQ http://www.eskimo.com/~scs/C-faq/top.html
CLC readme: http://www.angelfire.com/ms3/bchambless0/welcome_to_clc.html

jagbot wrote:
Have I got a telescope?

No, but you may have a decent-sized shaving mirror. The maximum surface
error for a reflective telescope optic needs to be around one eighth of
the wavelength of the light. Your sky dish is probably accurate to about
one eighth the wave length of the radio signal it receives (several
orders of magnitude greater than the light) and your CD chips make it
worse.

Use the CDs to shingle your house and be satisfied that you then have a
better view from your front window than anyone else in your street.

Best,
Stephen

Remove footfrommouth to reply

--
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+ Stephen Tonkin | ATM Resources; Astro-Tutorials; Astro Books +
+ (N51.162 E0.995) | http://www.astunit.com +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

jagbot wrote:
Have I got a telescope?

No, but you may have a decent-sized shaving mirror. The maximum surface
error for a reflective telescope optic needs to be around one eighth of
the wavelength of the light. Your sky dish is probably accurate to about
one eighth the wave length of the radio signal it receives (several
orders of magnitude greater than the light) and your CD chips make it
worse.

Use the CDs to shingle your house and be satisfied that you then have a
better view from your front window than anyone else in your street.

Best,
Stephen

Remove footfrommouth to reply

--
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ Stephen Tonkin | ATM Resources; Astro-Tutorials; Astro Books +
+ (N51.162 E0.995) | http://www.astunit.com +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

(jagbot) wrote in message . com...
Hello all:


Have I got a telescope?

Thank you.

No, I don't think you have a telescope, but I bet you could cook hot dogs with it.

Steve O.