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" 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

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.

"Jason" wrote in message ...


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.


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

Surprisingly chromium (~70%) isn't as reflective as polished
aluminium(~80%+). Aluminium mirror coatings are a bit higher (~85%).
Silver (on glass) much higher (~98%). All figures are very aproximate
and assume a fresh polish or coating.

Chris.B

If this is true, who don't they make telescope mirrors with silver coatings
instead of aluminum. Since we're talking about coatings that are only about
3 molecules thick, it doesn't seem that cost would be a relevant factor.

"Chris.B" wrote in message
om...

Surprisingly chromium (~70%) isn't as reflective as polished
aluminium(~80%+). Aluminium mirror coatings are a bit higher (~85%).
Silver (on glass) much higher (~98%). All figures are very aproximate
and assume a fresh polish or coating.

Chris.B

"Kilolani" wrote in message
ink.net...
If this is true, who don't they make telescope mirrors with silver
coatings
instead of aluminum. Since we're talking about coatings that are only
about
3 molecules thick, it doesn't seem that cost would be a relevant factor.

Silver tarnishes. They used to use silver. Aluminum stays bright
essentially forever.

It might be time to revisit this, though, and see if a coating can be put on
over the silver.

Isn't aluminum is cheaper than silver? And any process to put a thin coat
of plastic over the silver is not going to prevent re-silvering the mirror,
since I don't know if any clear cheap plastic that will not color and dull
with time. Then the plastic will have to be replaced, and when you do that
you'll have to resilver the mirror, since the plastic needs to be bonded to
the silver if it's going to prevent tarnishing. And the plastic is going to
have to be hard, to avoid scratching if dust has to be removed.

Seems more trouble than it's worth, two coatings instead of one for just
11% increase in reflectivity. I don't know of any professional scopes that
use silver with a plastic coating, and I think since they have more money
than we do, but they're still under budget contraints, that if plastic on
silver were better and more affordable, they'd have done it by now.
--
----------------------------------------------------------------------
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.


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


"Michael A. Covington"
wrote in message ...

"Kilolani" wrote in message
ink.net...
If this is true, who don't they make telescope mirrors with silver
coatings instead of aluminum. Since we're talking about coatings that
are only about 3 molecules thick, it doesn't seem that cost would be a
relevant factor.

Silver tarnishes. They used to use silver. Aluminum stays bright
essentially forever.

It might be time to revisit this, though, and see if a coating can be
put on
over the silver.

"Michael A. Covington"
wrote in :


"Kilolani" wrote in message
ink.net...
If this is true, who don't they make telescope mirrors with silver
coatings
instead of aluminum. Since we're talking about coatings that are only
about
3 molecules thick, it doesn't seem that cost would be a relevant
factor.

Silver tarnishes. They used to use silver. Aluminum stays bright
essentially forever.

It might be time to revisit this, though, and see if a coating can be
put on over the silver.



Silver has been used for years by Questar. They protect the silver from
tarnish by some sort of durable layer. I don't know the details but
probably SiO2 - the same as used for protected Al coatings by the likes
of Spectrum and others. I vaguely recall that Denton Vaccuum do a
"Protected Silver".

L.

"Kilolani" wrote in message link.net...
If this is true, who don't they make telescope mirrors with silver coatings
instead of aluminum. Since we're talking about coatings that are only about
3 molecules thick, it doesn't seem that cost would be a relevant factor.

Chemically deposited silver tarnishes rapidly. Particularly near the
sea and air-pulluted areas (sulphur). In a clean inland rural area it
might still work.
It isn't the amount of metal involved. It is the quantity of silver
bearing chemicals required in the wet chemical process to deposit the
silver. This makes the process quite expensive. Though I have no idea
how it compares with commercial aluminising. It is also rather fraught
with danger as there is an explosive situation when the chemicals are
allowed to dry. (Which should be avoided) Even the big observatories
used to have occasional accidents with explosions. Though they lost
some light gathering power when they went over to aluminising. (from
silver) They had much less 'down time' for recoating their mirrors.

Chris.B

Chris.B wrote:

Chemically deposited silver tarnishes rapidly. Particularly near the
sea and air-pulluted areas (sulphur). In a clean inland rural area it
might still work.
It isn't the amount of metal involved. It is the quantity of silver
bearing chemicals required in the wet chemical process to deposit the
silver. This makes the process quite expensive. Though I have no idea
how it compares with commercial aluminising.

The newest big mirrors have a sputtered silver deposition with a
nickel-chromium passivating layer I believe

Steve