It's mind boggling the accuracy of those things. It seems incredible
that people can grind lenses out by hand. Maybe it's easier than it
sounds, but in my experience hardly anything is easier than it
sounds...

(Parallax) wrote in message . com...
"George Anderson" wrote in message ...
They also have really bad coma in addition to a crappy figure. The mirrors
tend to be in the order of f/0.7 for speed.
When I looked at one (for a laugh) on my Ronchi tester it looked more like a
topo map for upstate New York.


George Anderson
Montreal Canada

Clear skies and good health
"Jon Isaacs" wrote in message
...
Where's the problem with this theory? Are these mirrors ridiculously
expensive or something? Why can't I use a shaving mirror (jp)?

Normally the cost of the primary mirror is more that one half the cost of
the
OTA, in a DOB, the entire scope might cost $400 and the primary mirror
$200.

One can use a shaving mirror to shave with, they work quite nicely for
that.
However the mirror in a telescope needs to be accurate to fractions of a
wave
length of light, or about 1/2000th the thickness of that hair one shaves
off.

Jon

For a truly basic reflector, see the old ATM books where they make one
with just a mirror, a couple of boards, a diagonal holder made from a
truck rear view mirror holder. They musta had an old eyepiece to use.

However, the other posters are correct, the primary mirror gots to be
expensive.

But, dont hesitate to take it (the telescope) apart and figger it out
as long as you dont scratch the mirror. Even if you do scratch it,
you will probably never be able to see the decrease in performance.