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Re: How do I measure (quantify) seeing?

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  • tincanandstring
    Jeff, may I ask about one statement you made? I often start an observing session by observing familiar double stars so I can compare the cleanness of the
    Message 1 of 15 , Apr 1, 2011
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      Jeff, may I ask about one statement you made?

      "I often start an observing session by observing familiar double stars so I can
      compare the cleanness of the split to an estimate of the seeing in arcseconds."

      I would like to do this sort of thing. Do you do this from measurements you personally have made, or is this sort of information available somewhere? Can "cleanness of the split" be described in any way other than graphically?

      Frank



      --- In telescopes@yahoogroups.com, "Jeff (also aka: StarGeezer)" <chipdatajeffb@...> wrote:
      >
      > Some good replies so far.
      >
      > As to specifically what effects sseing has on splitting double stars, I'll offer these (perhaps dangerous) generalizations from a few decades of doing just that:
      >
      > 1) Better seeing always allows you to do better work, especially if you're making measurements instead of just visually trying to discern a split. If a split is difficult in good seeing, it will be more difficult in poorer seeing. The worst case, I think, is when the split is tight and one star is very much dimmer than the other.
      >
      > 2) Sometimes you can do acceptable work even when the seeing is not great. This depends a lot on both the amount of separation and the relative magnitudes of the stars involved.
      >
      > 3) For doubles which show a "snowman split" or "figure-8 split" due to seeing, better seeing may allow a cleaner split. But you can still work the separation by interpolating the centroids of the stars involved ... as long as the seeing isn't highly variable (star images moving all over the place).
      >
      > 4) Magnification can help when the seeing is average or below, as long as the seeing is stable. When the image is blurred but not bouncing around, you can still infer the centroid. By increasing the magnification "just enough" you can make more accurate interpolations ... but if you go too far it becomes less accurate. How much is "just enough" depends on the seeing as well as the specific stars involved.
      >
      > I still do most of my double star measurements with micrometers and astrometric eyepieces. But I do also use a CCD camera. With a camera, you're integrating the incoming photons over a discrete pixel array and this amounts to averaging.
      >
      > If you are observing visual stars just for fun, then often the challenge is more in being able to tell there actually are two stars than in how well you can split them: any split can be "enough". But if you're doing it to contribute measurements, then it's important to note the seeing conditions at the time.
      >
      > I often start an observing session by observing familiar double stars so I can compare the cleanness of the split to an estimate of the seeing in arcseconds.
      >
      > Again, these are generalizations and your mileage may vary for specific cases.
      >
      > By the way, most of my double-star measurements over the years have been with 6" apertures, though I currently am mostly using a C-11.
      >
      > Hope this helps,
      >
      > Jeff B.
      >
      > --- In telescopes@yahoogroups.com, "tincanandstring" <frank@> wrote:
      > >
      > >
      > > I'm a little confused at the relationship between the ability to split double
      > > stars and the effect of differing seeing conditions.
      > >
      > > If I can split Castor cleanly with a 4" f/8 doublet, even if I need 200x to do
      > > so, can I draw any inference about the seeing in numerical terms from that? Does
      > > that mean that the seeing is at least "x" arcseconds?
      > >
      > > If the 4" refractor will not do it but the 8" Newt will, what does that mean
      > > about the seeing, numerically, if anything?
      > >
      > > Thanks for any illumination.
      > >
      > > Frank
      > >
      >
    • tincanandstring
      I ve noticed that, John, it is nice to have multiple scopes set up in order to benefit from the different characteristics of each size and type. Frank
      Message 2 of 15 , Apr 1, 2011
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        I've noticed that, John, it is nice to have multiple scopes set up in order to benefit from the different characteristics of each size and type.

        Frank

        --- In telescopes@yahoogroups.com, "ausastronomer" <jbambury@...> wrote:
        >
        > --- In telescopes@yahoogroups.com, "tincanandstring" <frank@> wrote:
        > >
        > >
        > > If the 4" refractor will not do it but the 8" Newt will,
        > > what does that mean about the seeing, numerically, if anything?
        > >
        > > Thanks for any illumination.
        > >
        > > Frank
        > >
        >
        > Hi Frank,
        >
        > Jeff Barton has summed this up very well. I just wanted to add something in regard to your specific question above.
        >
        > You can't really draw any conclusions from this as the ability to split a double star is dependent on so many factors. What you often find however is that notwithstanding that seeing is poor, a large aperture telescope can split a double star whereas a small aperture scope will not. This is solely due to the fact the larger scope has a much greater resolving power and light grasp. I can split a lot of double stars in the 18" Obsession at 67X under poor seeing which smaller scopes cannot split at higher power. At 67X with a 31mm Nagler the scope is operating at less than 4X per inch of aperture and at these low powers seeing does not affect the size of the airy disk too much. Under the same seeing conditions a 4" scope may be able to push to 120X, but due to the smaller aperture this may not be enough power to split the same double. On the same night you may not get the larger scope to 120X, but you don't need to.
        >
        > Cheers,
        > John B
        >
      • Jack & Lynn Kramer
        From: tincanandstring ... As John said, scope size is important. Type is an issue in view of the sometimes differing levels of optical
        Message 3 of 15 , Apr 1, 2011
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          From: "tincanandstring" <frank@...>
          > it is nice to have multiple scopes set up in order to benefit from the
          > different characteristics of each size and type.

          As John said, scope size is important. Type is an issue in view of the
          sometimes differing levels of optical quality. This was clearly brought home
          a couple of years ago when some of us were looking at Epsilon Lyrae (the
          "double double"). I was able to cleanly split it in my six-inch refractor
          but in a ten-inch Schmidt-Newtonian the flaring around each star made a
          clean split impossible. But a larger Newtonian with good optics will always
          best my refractor.

          Jack Kramer
          Lily Lake, Illinois
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