[ATM] Strange Chinese Telescope Design f/3.6, spherical mirror

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

This evening, a fellow brought in a no-name, low-cost,
short-tube, 6-inch reflecting telescope that he had
bought in China when he was stationed (or something)
in Japan a while back. He wanted help in collimating
it. We tried, but it wasn't easy, but that's not what
I wanted to talk about here. He asked me to bench-test
the primary mirror, so we did. We found that the scope
had a RADIUS OF CURVATURE of about 43 inches, or a
focal length of about 21.5 inches (about 550 mm), or
an f/ ratio of about 3.6. We also found that the
Ronchi lines, while very nice and smooth, were also
were very close to being straight! (except for a
curved edge and being a bit oblate esp. in the middle)


However, the scope said that the focal length of the
scope was 1400 mm, and it had something like a
permanent barlow at the bottom of the focuser tube; I
take it that the barlow has about about a 2.6
magnification power.

I was not able to do a star test nor to test the
entire optical path of the entire scope. The owner had
expressed concern with the optics, specifically the
collimation. It seems to me that even if the barlow
was really extending the focal length by about 2.6
times, there still ought to be some parabolization on
the mirror at f/9.3.

Oh, and there was also a turned-down edge over about
1/4 inch or so.

Does anybody else have any experience with commercial
scopes like this? Any thoughts?

Guy B

=====
Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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