Expert: Tom Whiting - 11/28/2004

Question
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Followup To
Question -
hi
In egypt, telescopes are illegal and can't be found. But I am a real fan of astronmy and I want to have a telescope. The question is can I build a home made telescope. I want to build either a refraction or a reflection telescope. I am in the second year of the engineering college and I have no problem dealing with lenses. but how to do it and where to get the lenses or the mirrors.thank you for listening.
Answer -
Hi Hassan,
I've never heard of a country or a civilization that did not
allow their citizens to own and operate a telescope....
I did not know that.  Wonder what the penalty is
for that?  Loss of eyesight.  ;-)

(for what purpose to make them illegal)??
Is that also true for the other optical equipment...microscopes??
How about binoculars??
Eyeglasses allowed??
Can't you apply for a government "waiver", promising
to use it only on the night sky, and not to spy on that
lady sitting on a camel??  ;-)
(Besides, the image in inverted and the field of view is
typically less than  0.5 degree!)

In any case, I would never tell a person to intentionally
break the law...personally I'd move to a different country!

But to your question, I recall John Dobson, president of
the San Francisco Sidewalk Astronomers Club, he would
grind and figure a mirror just out of common ship's port-hole glass from a  large boat. So it can be done.  I've never had to grind a piece of glass,
but I have built several scopes, up to 24 inches (0.6 metre) in diameter, but I always purchased the optics for it.  Then there was the Coulter Corporation telescope who made their mirrors
from regular plate glass of only 1 inch (2.54 cm) thick!
I even owned two of them...an 8 inch and a 13 inch reflector....surprisingly, after the mirror
reached outside air temperature, they gave pretty good
performance.  Not perfect, but still pretty darn good for the low cost of Coulter telescopes.  
So evidently, a telescope mirror-glass doesn't have to be Pyrex glass or this new ceramic- glass (very low expansion
coefficients)...but a pretty good mirror can be ground and
figured just with regular window plate glass.   However,
inexpensive high expansion glass will only hold a 1/4 or 1/8 wavelength of light, at best....I wouldn't even attempt a 1/10 or better wavelength because plate glass just won't hold that high tolerance.

In your position, I would be more inclined to do a mirror
(and a Newtonian Reflector telescope)
as opposed to a refractor with a lens up front....a single
lens imparts, as you know, chromatic aberration; the better
refractor telescopes on the market use multiple lenses glued
together to almost eliminate chromatic aberrations....so if
one does a lens, they'd have to do 2 or 3 of them of the same size, and properly glue them together like the big manufacturers do.  
To me, its much easier and less time consuming, just to grind a glass to spherical or parabolic shape from just a piece
of large glass, silver it (or get it aluminized on the top surface)
and go from there.

 Where to get a chunk of glass?  If you live near a harbor
with big ships coming into harbor, (and surely there are
ship-repair places in that harbor),
that use (and replace) their old porthole glass??
Especially after it gets all scratched up.  (Which you will
polish out in the process of figuring your glass porthole).

Or another idea for glass;
If it were me, and surely you have access to a furnace, say in the metallurgical engineering part of your college (or even
a ceramics kiln department in the pottery section)
I would suggest to gather up old Coke-Cola bottles
(and other soft drink bottles like glass water bottles, or
fruit juice jars....or even common Mason Jars which the ladies
use to perserve jellies, jams, etc.)
and melt down and make my own mirror blank out of that
glass in a furnace.  Of course, it might take several tries
to make your own mirror blank glass.....you can't have any
remaining air bubbles on the surface you will be grinding,
and cooling is a big factor too....has to undergo very slow
cooling so it doesn't crack under stress.   Of course, if it
does fracture, you can always re-melt it and try again.

Another problem I see...how are you going to get eyepieces
for telescope?  You'll have to order them I think, as I've never
heard of anyone making their own eyepieces....although I
guess someone out there must manufacture them.

Well, hope all this helps...never had a question like this
before....and didn't know telescopes were illegal to own
in different countries....what a dumb law.  What is the
Egypt Government afraid of???

Amazing, considering Egypt's contribution and influence
on astronomy, over 2000 years ago.

Clear Skies,
Tom Whiting
Erie, Pennsylvania
USA
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thank you very much for your concern and help. It is a very helpful notes and I well try to do it. the easier solution is to grind the glass myself. I can purchase the glass from the stores but can you tell me what kind is better and how to grind it. by the way, it is illegal here because they think it can be used in spy and eye picking. I know it is stupid that is why I am trying to build that telescope on my own. About that eye piece. what did you mean about that. If you meant the vertical hole with the lens where the light is made visible again. This I can easily do with some calculations and an ordered lens, or I can just adjust the distance verticaly to suit any convex lens. If this is not what you mean then please tell me how. by the way I have only seen a lady on a camel only twice in my life near the pyramids :)  

Answer
Yes, the eyepiece at "the other end of the telescope"
The piece of glass you look into with your eye...
You need to have one.....in fact, I have about 10 of them
so I can change powers at will....they basically are
1.25 inches and 2 inches....stay away from the Japanese
model eyepieces that are 0.965 inch in diameter, as they are
very inferior to the American ones.
Popular names are Plossls, Naglers (very expensive)
and Meade Wide field of view eyepieces.  And they are
labeled by their focal length.....40 mm, 20 mm, 15 mm
9 mm, down to about 3 mm.
Then you can calculate your magnification, which is
focal length of the telescope divided by focal length of
the eyepiece.
Hope all this helps,
Clear Skies,
Tom Whiting