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Re: [tmboptical] Re: Guide scope for TMB 130/780 F6

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  • Neil Fleming
    You re most welcome. Yes...definitely try it out and see how it goes. Although many folks feel comfortable at high differences between the focal lengths of
    Message 1 of 8 , Mar 15, 2008
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      You're most welcome. Yes...definitely try it out and
      see how it goes. Although many folks feel comfortable
      at high differences between the focal lengths of the
      two scopes, I like to stay as close as practical.

      ...Neil

      --- Tom <tabbotr@...> wrote:

      > Thanks very much Mike, Neil and Dave, for replying
      > to my post.
      > According to my calculations I have a ratio of about
      > 4:1 arcsec/pixel between the main
      > scope and the guide. I will see how it goes and
      > attempt to measure the guiding error.
      > I now have a better idea of the relationship between
      > the guide and the main scope and
      > pixel size.
      > Mikes page on the pitfalls and solutions of guiding
      > are very instructive.
      > I hope to let you know how it works out.
      > Tom
      > --- In tmboptical@yahoogroups.com, Neil Fleming
      > <neilfleming@...> wrote:
      > >
      > > You're right, Mike. I failed to state that
      > > differences in pixel sizes need to be considered
      > in
      > > the same terms.
      > >
      > > If both scopes had the same focal length, yet the
      > > pixels on the guide scope were twice as big, that
      > > would be equivalent to using a guide scope with
      > half
      > > the focal length of the main scope.
      > >
      > > ...Neil
      > >
      > > --- Mike Dodd <mike@...> wrote:
      > >
      > > > Neil Fleming wrote:
      > > > > Sources like Ron Wodaski state that you can
      > guide
      > > > with
      > > > > a scope that has as little as 1/10th the
      > focal
      > > > length
      > > > > as that of your main scope. Personally, I
      > take a
      > > > more
      > > > > conservative approach and try to stay within
      > > > 1/3rd.
      > > >
      > > >
      > > > > For your TMB with the 780mm focal length, a
      > guide
      > > > > scope of 500mm or higher would be a
      > no-brainer.
      > > >
      > > > I agree in general terms, but of course a lot
      > > > depends on how well your
      > > > mount operates. I use a 300mm guide scope with
      > an
      > > > ST-402 camera
      > > >
      > >
      >
      <http://astronomy.mdodd.com/astronomy-obs.html#GuideScope>
      > > > for an image
      > > > scale of about 6 arcsec/pixel. The image scale
      > for
      > > > the main scope and
      > > > camera is about 2 arcsec/pixel, so I too have a
      > > > ratio of 3:1. I'm using
      > > > a Losmandy G-11 mount. Generally, my guide
      > errors
      > > > are about 0.2 pixels,
      > > > which works out to 0.6 pixels on the imaging
      > camera.
      > > >
      > > > Mike
      > > >
      > > >
      > > >
      > > > --
      > > >
      > > > Mike Dodd
      > > > Montpelier, VA USA
      > > > http://astronomy.mdodd.com
      > > >
      > > >
      > > >
      > > > Yahoo! Groups Links
      > > >
      > > >
      > > >
      > > >
      > > >
      > >
      > >
      > > www.flemingastrophotography.com
      > > Direct from Boston - brilliant diamonds in pea
      > soup
      > > Also check out the astro_narrowband Yahoo group!
      > >
      >
      >
      >
      >
      > ------------------------------------
      >
      > Yahoo! Groups Links
      >
      >
      >
      >
      >


      www.flemingastrophotography.com
      Direct from Boston - brilliant diamonds in pea soup
      Also check out the astro_narrowband Yahoo group!
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