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

[George Anderson <georgea@cam.org>]

Hi Guy

The Celestron Nextstar 114 works the same way, a fast spherical primary
with a barlow in the focuser tube. Works good enough for Joe Average to
get bitten by astronomy and aperture fever.
Makes for a small portable scope.
I won one as a door prize at the ROC convention last spring, it makes a
handy grab and go. When the mirror needs recoating I'll look at
parabolizing it and make it a small rft.

George Anderson
Montreal Canada

Clear skies and good health

Guy Brandenburg wrote:
> 
> 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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