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Re: [LX200GPS] Meade F/6.3 on LX200 10?

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  • John Mahony
    Just hold the reducer with a distant light source (the moon or a streetlight) on one side and a piece of paper on the other, and adjust the spacing until the
    Message 1 of 14 , Apr 1 1:10 AM
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      Just hold the reducer with a distant light source (the moon or a streetlight) on one side and a piece of paper on the other, and adjust the spacing until the light forms a focused image on the paper. The "old" Meade reducers will focus at something like 10" (~250mm). The new ones are about half that or slightly more. Note that this spacing is _not_ the same as the proper spacing for imaging with a scope (but it is related to that number).

      -John




      ----- Original Message ----
      > From: bowtie_gps <victor.amdio@...>
      >
      > Thanks for all the replies.
      >
      > I will have to play around to see what work and play with spacing.
      >
      > Is there any way to discern what vintage of Meade FR you have? Markings?
      > Letters? Key physical design changers?
      >
      > Vic
    • wedwards1938
      G day Folks, ... the optimum setting. I find using mine before the electronic focus followed by a T adapter gives me f5.4 and better star images. I got poorer
      Message 2 of 14 , Apr 1 10:28 PM
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        G'day Folks,
        In an earlier input to this topic, Rick wrote the following:
        > Hi,
        >   If you own the Meade FR you have to play with spacing to achieve
        the optimum setting. I find using mine before the electronic focus
        followed by a T adapter gives me f5.4 and better star images. I got
        poorer quality using it at f6.3. Experiment. I guess this applies to
        any brand FR.
        > Rick
        Elsewhere, since recently taking up the hobby of astronomy, I have
        read statements by learned astronomers wherein they speak of 'running
        their telescope at "f something or other"'. Rick speaks of
        positioning the f6.3 FR in a particular way giving him a FR working
        optically at f5.4. In my naivity, I think of a telescope having a
        fixed aperture and focal length, hence a fixed f number.
        How do you alter and then determine the resultant f number of a
        system, be it an entire telescope system, or part of the optical
        train, as in Rick's case?
        Regards,
        Bill.
      • John Mahony
        ... It s just a matter of definition. When you add a reducer or barlow, it changes the image scale, and image scale is a function of focal length, so it s
        Message 3 of 14 , Apr 2 6:42 PM
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          ----- Original Message ----

          > From: wedwards1938 <wedwards1938@...>
          >
          > G'day Folks,
          > In an earlier input to this topic, Rick wrote the following:
          > > Hi,
          > > If you own the Meade FR you have to play with spacing to achieve
          > the optimum setting. I find using mine before the electronic focus
          > followed by a T adapter gives me f5.4 and better star images. I got
          > poorer quality using it at f6.3. Experiment. I guess this applies to
          > any brand FR.
          > > Rick
          > Elsewhere, since recently taking up the hobby of astronomy, I have
          > read statements by learned astronomers wherein they speak of 'running
          > their telescope at "f something or other"'. Rick speaks of
          > positioning the f6.3 FR in a particular way giving him a FR working
          > optically at f5.4. In my naivity, I think of a telescope having a
          > fixed aperture and focal length, hence a fixed f number.
          > How do you alter and then determine the resultant f number of a
          > system, be it an entire telescope system, or part of the optical
          > train, as in Rick's case?

          It's just a matter of definition. When you add a reducer or barlow, it changes the image scale, and image scale is a function of focal length, so it's common to say that the extra lens changes the focal length. Then since f ratio is focal length divided by aperture, the f ratio also changes. Note that exposure time also changes, just as it would if you were comparing two telescopes with the same aperture but with different focal lengths.

          If that bothers you, coinsider that the primary in most SCTs is really only f/2. The convex secondary acts like 5X barlow. But no one refers to an f/10 SCT as really being an f/2 scope.

          Sometimes to keep things clear I use the term "effective" focal length when extra lenses are being used to change the image scale.

          Image scale (in arcsec/pixel) is directly proportional to focal length, so if you know the size of the pixels in your camera, you can measure the effective focal length by taking a picture of two stars that are a known angular distance apart.

          The common 6.3 reducers are really .63 reducers since they "reduce" an f/10 SCT to f6/.3 (Meade used to make some SCTs with a "native" focal ratio of 6.3, and on those the reducer converted them to f/4 (6.3 *.63 ~=4). But the "reduction factor" actually depends on the spacing between the reducer and chip, so it's possible to adjust the final focal length (and focal ratio) somewhat.

          -John
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