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Re: [Telescopes] just a little confused.

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  • Geoff Gaherty
    ... I m not sure about this particular image, but often images of large objects like M42 are mosaics of multiple images made over a period of time. Making
    Message 1 of 15 , Jan 1, 2009
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      Brian W wrote:
      > Hi all, just need a little clarifying. I have been under the
      > impression that as one gets into larger aperture with a reflector, one
      > gets more and more detail but at the expense of less and less area.
      >
      > The Crossley is a 36" reflector and yet it takes an incredibly
      > detailed picture of arguably all of M 42 and also captures M 43 and
      > NGC 1977.
      >
      > So just to clarify... is the Crossley an oddity or will any reasonable
      > quality large fast reflector perform in the same ballpark?

      I'm not sure about this particular image, but often images of large
      objects like M42 are mosaics of multiple images made over a period of
      time. Making such mosaics used to be a laborious process, but RegiStar
      software developed by Rajiv Gupta has made it really simple:

      http://www.aurigaimaging.com/

      RegiStar will take multiple images made at different times and even with
      different telescopes with different image scales by different imagers,
      and combine them into a single image. You ca take high resolution
      Hydrogen Alpha monochrome images and combine them with low resolution
      colour images to produce high resol;ution colour images. Absolutely
      amazing stuff! The Observer's Calendar produced each year by the Royal
      Astronomical Society of Canada usually contains a number of images made
      this way:

      http://www.rasc.ca/calendar/

      Geoff

      --
      Geoff Gaherty
      Foxmead Observatory
      Coldwater, Ontario, Canada
      http://www.gaherty.ca
    • Wayne G
      ... WG: Hi Brian, to try to clear this and a few other things up a bit for anyone confused by all of this or just getting into the hobby and trying to sort
      Message 2 of 15 , Jan 1, 2009
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        >
        > just a little confused.
        > Posted by: "Brian W" mljbw2@... mljbw2
        > Date: Wed Dec 31, 2008 7:22 pm ((PST))
        >
        > Hi all, just need a little clarifying. I have been under the
        > impression that as one gets into larger aperture with a reflector, one
        > gets more and more detail but at the expense of less and less area.
        >

        WG: Hi Brian, to try to clear this and a few other things up a bit for
        anyone confused by all of this or just getting into the hobby and trying
        to sort things out, every telescope is a combination of several factors,
        such as aperture (to which light gathering and resolution are largely
        tied), and focal ratio (f/5 means the focal length or focal plane is
        equal to five times the diameter of the primary objective; 100mm f/5
        would have 500mm focal length.)--- and as such, magnification, scale,
        true FOV and exit pupil are a function of focal ratio AND aperture
        (directly or indirectly).

        A 10" f/5 will give the same magnification, scale and TFOV as a 5" f/10,
        with the same eyepiece or camera because focal length is the same.

        A 10" f/5 and a 5" f/5 will give the same exit pupil (intrinsic
        brightness to the eye), but, not to be get confused, the latter being at
        half the scale (it will have half the magnification--- with the same
        eyepiece). Double the focal length of the eyepiece, you now have the
        same magnification in the second scope as the first, but at the
        sacrifice of being much dimmer now than the 10"!

        If I stop the 10" above down to a smaller aperture (say 8"), I change
        the exit pupil because I've changed the focal ratio, but focal length is
        unaffected so magnification remains the same. Light gathering and
        resolution go down, but freedom from abberations may likely /improve/.

        Light gathering can be influenced by other factors such as central
        obstruction, coatings and system throughput or efficiency.

        Resolution can be influenced as well by telescope design and quality,
        obstruction and seeing, even scatter in the optics, not just aperture
        alone, even though that is the prime factor.

        To the eye, the views in an f/5 scope are no brighter than the views in
        an f/10 scope, if magnification is held constant by using the
        appropriate eyepieces in each.

        Generally, "fast" scopes give wider views (potentially or more easily),
        but also tend to introduce more distortions or difficulties in retaining
        image quality.

        "Slower" scopes have greater depth of field and are thus less touchy to
        focus.

        So you are quite right, larger aperture generally /should/ give more
        resolution if quality is maintained, but also generally, because focal
        ratios tend to stay within a certain practical range, bigger scopes tend
        to have longer focal lengths and thus more magnification or narrower
        fields of view.

        But a big scope can be very fast and violate the rule:

        30" f/3.7 = 2819.4mm focal length (big giant dob)
        14" f/11 = 3911.6mm focal length (much smaller, compact C14)

        to actually have wider scale at the prime focus than a smaller scope, or
        they can employ mosaics in imaging, as Geoff pointed out already, to
        cover wide fields--- the Hubble does this.

        Or the 14" can be made to operate at f/1.8 for a 640mm focal length! An
        enormous gain in area because the scale at prime focus will now be very
        small!


        All this means is that a telescope is a combination of many factors and
        as such, no one scope does everything best. Every scope has an area of
        strength and an area of weakness, so when shopping, one must decide to
        chose from three courses of direction:

        1). A general purpose scope which does most things well but few things
        superbly (eg: An 8" f/6 Newt).

        2). A specialized scope which excels at one or a few things at the
        expense of others (eg: An f/15 Maksutov or a 6" f/5 RFT).

        3). Buy several different scopes to cover all the bases! :^)

        Consider light gathering and resolution, portability, ease of use and
        maintenance, and budget for each area of use, be it general star gazing,
        deep sky, lunar, planetary and double stars.

        Isn't it a great hobby?

        Hope this helped a bit.

        Regards,

        WayneG
      • John Bambury
        ... Wayne your comment which I have quoted above, from a technical sense is actually incorrect. I understand what you are are alluding too and possibly trying
        Message 3 of 15 , Jan 1, 2009
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          --- In telescopes@yahoogroups.com, Wayne G <fomalhaut@...> wrote:
          >
          > >
          > > just a little confused.
          > > Posted by: "Brian W" mljbw2@... mljbw2
          > > Date: Wed Dec 31, 2008 7:22 pm ((PST))
          > >
          > > Hi all, just need a little clarifying. I have been under
          > > the impression that as one gets into larger aperture with
          > > areflector, one gets more and more detail but at the expense
          > > of less and less area.
          > >
          >
          > WG: Hi Brian, to try to clear this and a few other things up
          > a bit for anyone confused by all of this or just getting into
          > the hobby and trying to sort things out, every telescope is a
          > combination of several factors ......................

          > and as such, magnification, scale, true FOV and exit pupil are
          > a function of focal ratio AND aperture (directly or indirectly).

          Wayne your comment which I have quoted above, from a technical sense
          is actually incorrect. I understand what you are are alluding too and
          possibly trying to explain, in simple terms for the benefit of
          newcomers. However I think it confuses people new to the hobby who
          will read your post and then think everything revolves around "focal
          ratio", which is certainly not the case. Clearly, focal length is the
          determining factor and not focal ratio. Focal ratio is merely a
          function of focal length and aperture.

          Your comments below explain perfectly why this is so.

          > A 10" f/5 will give the same magnification, scale and TFOV as a 5"
          > f/10, with the same eyepiece or camera because focal length is the
          > same.


          Cheers,
          John B
        • Al Germaine
          The detail or resolution of any telescope, reflector or refractor is a function of the aperture and light gathering is a functiion of the area. The coverage is
          Message 4 of 15 , Jan 1, 2009
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            The detail or resolution of any telescope, reflector or refractor is a
            function of the aperture and light gathering is a functiion of the
            area. The coverage is a function of the focal ratio.

            Al

            --- In telescopes@yahoogroups.com, "Brian W" <mljbw2@...> wrote:
            >
            > Hi all, just need a little clarifying. I have been under the
            > impression that as one gets into larger aperture with a reflector, one
            > gets more and more detail but at the expense of less and less area.
            >

            > Brian
            >
          • John Bambury
            ... Correct ... This is incorrect. The Coverage or more properly The True Field of View (TFOV) is a function of three things; and three things only. 1) The
            Message 5 of 15 , Jan 2, 2009
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              --- In telescopes@yahoogroups.com, "Al Germaine" <trulyyours05@...>
              wrote:
              >
              > The detail or resolution of any telescope, reflector or refractor
              > is a function of the aperture and light gathering is a functiion
              > of the area.

              Correct

              > The coverage is a function of the focal ratio.

              This is incorrect.

              The Coverage or more properly "The True Field of View" (TFOV) is a
              function of three things; and three things only.

              1) The focal length of the telescope.
              2) The focal length of the eyepiece.
              3) The Apparent Field of View (AFOV) of the eyepiece

              Aperture or the Focal Ratio of the Telescope has nothing to do with
              it.

              Let's take a standard 10"/F5 telescope. Assume a 250mm diameter
              primary and a 1250mmm focal length, for ease of the maths. Now we use
              a standard 10mm focal length plossl eyepiece, which has a 50 deg
              Apparent Field of View (AFOV).

              You first need to work out the magnification of the system, which is
              the focal length of the telescope divided by the focal length of the
              eyepiece. In this case it is 1250mm / 10mm = 125X.

              The True Field of View is given by dividing the AFOV of the eyepiece
              (50 deg) by the magnification of the system. In this case it is 50
              deg /125X for a TFOV = 0.4 degrees or 24'.

              Take the second example of a 5"/F10 telescope. Assume a 125mm
              diameter primary and a 1250mmm focal length, for ease of the maths.
              Now we use a standard 10mm focal length plossl eyepiece, which has a
              50 deg Apparent Field of View (AFOV).

              The magnification of the system is the same as for the 10" scope. It
              is given by dividing the focal length of the telescope by the focal
              length of the eyepiece. In this case it is 1250mm / 10mm = 125X.

              Similarly, the Frue Field of View is exactly the same as for the 10"
              scope because the magnification and the AFOV of the eyepiece haven't
              changed.

              As you can see from these two examples a 5"/F10 telescope has the
              same True Field of view (and magnification) as a 10"/F5 telescope
              when the same eyepiece is used. The "focal ratio" is halved yet the
              TFOV remains the same. Clearly, the focal ratio has nothing to do
              with it.


              In answer to the question Brian was alluding to. "Why does the
              Coverage or TFOV reduce as the aperture increases?". Simple, because
              as the aperture of the telescope increases, the focal length of the
              telescope gets longer, although the focal length invariably doesn't
              increase proportionately.

              Cheers,
              John B


              PS: If someone else posts that "focal ratio" has something to do
              with this, I am gonna scream and bury my head in the sand.
            • Brian W
              ... John it is only 2 days into the new year, screaming and burying head has to wait till at least day 5. I accept your math and the truth that the aperture
              Message 6 of 15 , Jan 2, 2009
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                --- In telescopes@yahoogroups.com, "John Bambury" <jbambury@...> wrote:
                >
                > --- In telescopes@yahoogroups.com, "Al Germaine" <trulyyours05@>
                > wrote:
                > >
                > > The detail or resolution of any telescope, reflector or refractor
                > > is a function of the aperture and light gathering is a functiion
                > > of the area.
                >
                > Correct
                >
                > > The coverage is a function of the focal ratio.
                >
                > This is incorrect.
                >

                > PS: If someone else posts that "focal ratio" has something to do
                > with this, I am gonna scream and bury my head in the sand.
                >
                John it is only 2 days into the new year, screaming and burying head
                has to wait till at least day 5.

                I accept your math and the truth that the aperture tends to increase
                quicker than focal length(paraphrasing big time here) so the area you
                see tends to shrink but that if things were kept in balance the fov
                would remain the same.
                Brian
              • Brian W
                ... Hi John and Wayne last night lying in bed listening to the typhoon howl through the trees I had an eureka moment... the problem is that as a new comer to
                Message 7 of 15 , Jan 2, 2009
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                  --- In telescopes@yahoogroups.com, "John Bambury" <jbambury@...> wrote:
                  >
                  > --- In telescopes@yahoogroups.com, "Al Germaine" <trulyyours05@>
                  > wrote:
                  > >
                  > > The detail or resolution of any telescope, reflector or refractor
                  > > is a function of the aperture and light gathering is a functiion
                  > > of the area.
                  >
                  > Correct
                  >
                  > > The coverage is a function of the focal ratio.
                  >
                  > This is incorrect.
                  >
                  > The Coverage or more properly "The True Field of View" (TFOV) is a
                  > function of three things; and three things only.
                  >
                  > 1) The focal length of the telescope.
                  > 2) The focal length of the eyepiece.
                  > 3) The Apparent Field of View (AFOV) of the eyepiece

                  Hi John and Wayne last night lying in bed listening to the typhoon
                  howl through the trees I had an eureka moment... the problem is that
                  as a new comer to the hobby I paid attention to advertising rather
                  than math.

                  In advertising fast focal ratio = wide tfov. So as a newbie I came to
                  believe that it not only equaled it but caused it.

                  Now that you two (and others) have taken the time to put things in
                  their proper relationship for me I realize that the F/R is a 'result
                  / indicator' not a cause.

                  Thanks
                  Brian
                • Wayne G
                  ... WG: Hi John, sorry to all for the longish post here but I will try to explain further; you seem like such a nice, polite person, I feel compelled to
                  Message 8 of 15 , Jan 2, 2009
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                    >
                    > Re: just a little confused.
                    > Posted by: "John Bambury" jbambury@... ausastronomer
                    > Date: Thu Jan 1, 2009 5:05 pm ((PST))
                    >
                    > --- In telescopes@yahoogroups.com, Wayne G <fomalhaut@...> wrote:
                    >
                    >> > WG: Hi Brian, to try to clear this and a few other things up
                    >> > a bit for anyone confused by all of this or just getting into
                    >> > the hobby and trying to sort things out, every telescope is a
                    >> > combination of several factors ...................
                    > > and as such, magnification, scale, true FOV and exit pupil are
                    > > a function of focal ratio AND aperture (directly or indirectly).
                    >
                    >
                    > Wayne your comment which I have quoted above, from a technical sense
                    > is actually incorrect.

                    WG: Hi John, sorry to all for the longish post here but I will try to
                    explain further; you seem like such a nice, polite person, I feel
                    compelled to respond in kind, but sorry, no, there was nothing incorrect
                    in my statement, technical or otherwise. You may not have read it as I
                    truly wrote it.


                    > I understand what you are are alluding too and
                    > possibly trying to explain, in simple terms for the benefit of
                    > newcomers.

                    WG: Perhaps not. Maybe my meaning and intent was not simple enough. I
                    was trying to convey something not entirely obvious about optics to any
                    novice readers here looking for a deeper understanding. I am sure that
                    somewhere on this group someone read my post and it connected and
                    resonated with them in a more or less profound way--- that is the
                    person(s) I was hoping to connect with.

                    FWIW, I skipped science class in 8th grade because I knew more about
                    astronomy than the science teacher; I jumped to the 12th grade
                    astronomy class in public high school when first entering the 9th grade
                    (there was no 9th grade astronomy); and I taught astronomy for extra
                    credit later in private high school as a preamble to college. I was
                    only in "high school" for 2 years.

                    You see, I have not made a fundamental error concerning something so
                    basic as focal length, focal ratio, et al.: these things are just about
                    as basic as simple arithmetic! :^) One problem is that when you try
                    to convey something like this in a way to draw the reader into thinking
                    deeper about a subject, too often on the Internet if they are not
                    understood overtly, rather than the reader seeing himself as the one
                    failing to grasp, all too often it is just assumed that it is the
                    /writer/ who is wrong or confused!

                    The fundamental problem to most teaching (on the Internet or otherwise)
                    is that too often one teaches the reader /what/ to think, rather than
                    HOW to think. While I did provide some simple "If A+B, then is equal to
                    C" in my other post, in other cases, I am providing two "ends" in my
                    thought but leaving something out in the middle requiring the reader to
                    THINK and put the pieces together themself so that they truly comprehend
                    the matter, to be INVOLVED in the learning process, just as I was taught.

                    The point is not to be just handed stuff to memorize, but to be involved
                    in learning; to /comprehend/. It is not the "whats" that make learning
                    dynamic and interesting, but the "whys!"

                    What I was trying to convey was not that aperture, focal length, etc.,
                    are each separate entities to be "calculated" independent to
                    one-another, but rather to be /connected/, related; I was trying to
                    convey RELATIONSHIPS. Everything in optics and in a telescope in
                    particular is about relationships.

                    Focal ratio is a relationship between the two most important, global
                    factors in a telescope, the aperture and the focal length. From this
                    comes virtually every other common parameter in one way or the other! I
                    was not implying that one uses the f/ number to *calculate*
                    magnification, so please quit banging your head in frustration that we
                    are all "mixed up." I was drawing the reader in to think deeper about
                    what the telescope actually DOES when magnifying, etc., rather to simply
                    memorize formulas in wrote.


                    > However I think it confuses people new to the hobby who
                    > will read your post and then think everything revolves around "focal
                    > ratio", which is certainly not the case.

                    WG: I didn't know I said anything specifically /revolves/ around focal
                    ratio in my original post. Please read my post again. But perhaps you
                    are looking at focal ratio in too narrow a way. Perhaps that is the
                    source of your confusion, for perhaps you missed that "(directly or
                    indirectly)" comment in my original post and what it was implying. It
                    was the "indirectly" part I threw in there hoping to spur a few on to
                    further thought about how all of this fits together. For when you speak
                    of focal ratio, you are encompassing not only the "speed" of the system,
                    but also the aperture and the focal length of the system, since these
                    are the parts which make it up!

                    And if you change aperture in our hypothetical telescope, you change f/
                    ratio, and if you change focal length, you change f/ ratio--- f/ratio
                    is right at the center of it all in this context, and more importantly,
                    it directly ties in with /exit pupil/--- one of the most important
                    factors in observing. While most people always look to choosing
                    telescopes and eyepieces based purely on magnification, they might do
                    better to look at the relationship between their scopes and eyepieces
                    and the exit pupils they will produce.

                    It is the combination of scale and exit pupil that makes for an ideal
                    instrument when choosing to observe a particular class of object. This
                    statement alone is worth 40 years of observing and learning, given here
                    for free.

                    I was not speaking about how to /calculate/ magnification, John, nor
                    meant to imply that the f/ratio was used in the calculation. It is
                    merely an indirect, global /factor/ in influencing how the scope magnifies.

                    You should not assume you know how other people will think by reading
                    what someone else writes just because you do not follow it or it is not
                    expressed in a way that you would choose!

                    Hope this all made sense.

                    Best regards,

                    WayneG
                  • Wayne G
                    WG: Hi Brian, Just a few additional thoughts to you and others following this thread--- ... WG: As Al pointed out, that is of course quite correct. But
                    Message 9 of 15 , Jan 3, 2009
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                      WG: Hi Brian,

                      Just a few additional thoughts to you and others following this thread---

                      > Re: just a little confused.
                      > Posted by: "Brian W" mljbw2@... mljbw2
                      > Date: Fri Jan 2, 2009 4:26 pm ((PST))
                      >
                      > > > The detail or resolution of any telescope, reflector or refractor
                      > > > is a function of the aperture and light gathering is a functiion
                      > > > of the area.
                      >

                      WG: As Al pointed out, that is of course quite correct. But optics are
                      a very strange (and fascinating) matter, you can have three different
                      scopes all of the same aperture which yield somewhat different levels of
                      resolution. Never mind why, but assuming no egregious defects, it often
                      comes down to the differences between theoretical physics (optics in
                      this case), and /applied/, or practical physics. Under certain
                      circumstances, a smaller scope can sometimes show better detail than a
                      larger one. But that is a protracted topic that seems to get bandied
                      and argued about ad nauseum within the groups. Point is that it is
                      often best to take many basic formulas and calculations from them as
                      close approximations or points of reference.

                      Even resolution is something of a hairy topic because if you want to get
                      more specific, there are actually TWO different kinds of resolution!
                      Angular and linear. The former is a function of aperture (how far apart
                      the opposite sides of the entrance aperture are), and the latter
                      actually a function of the f/ratio.

                      Now yes! If you just thought about that, that implies that a 4" f/5
                      system has greater linear resolution than a 16" f/8! What has happened
                      to aperture now? (It's a trick question: I'm sure many people know the
                      answer to this and there is no need to "explain" it, it is really a
                      rhetorical question--- if anyone is curious, I would invite you to
                      investigate further on your own with Google, or a good book).

                      Most calculations do not take into account ALL the variables which might
                      enter into the result, and in amateur astronomy, /experience/ is an
                      important factor in also predicting how a telescope will perform,
                      especially when making a buying decision.

                      Back in the Sixties when I started out, there were no groups like this
                      to ask advice; you were "winging" it on your own. You took your
                      chances and learned to predict matters accordingly (often from your own
                      mistakes). Nor was there the vast selection one has now!


                      >>> > > The coverage is a function of the focal ratio.
                      >>>

                      WG: Of course, communication is always hard over the Internet and it is
                      all too easy for people to gather a different meaning from what you
                      intended, if you even put into writing what you yourself were trying to
                      convey! :^) And there is often more than one right answer and more
                      than one right (or wrong) person. It is often a matter of semantics.

                      Coverage is a term I don't see used much in observational astronomy, but
                      to me, implies more the photographic field taken at the prime focus of a
                      camera. The above statement can be made correct if you assume all other
                      factors are held constant (there is that slippery slope of
                      relationships). For instance, if you reduce focal ratio (as the
                      variable) while holding aperture and everything else the same, then you
                      have actually reduced the /focal length/ of the system and will indeed
                      increase the coverage or true FOV.

                      It is all in how you look at it, or the CONTEXT in which you were
                      speaking, which is why I don't like to argue too much that anything must
                      be all this way or that, or that one person must be wrong for another to
                      be right; often that CONTEXT is not very apparent when we write things
                      and I have seen many people argue vociferously about something in heated
                      disagreement that they were actually in agreement upon, but merely
                      saying the same thing in two different ways!


                      >> Hi John and Wayne last night lying in bed listening to the typhoon
                      >> howl through the trees I had an eureka moment... the problem is that
                      >> as a new comer to the hobby I paid attention to advertising rather
                      >> than math.
                      >>

                      WG: Forget the advertising claims! :^) It is mainly there to get you
                      to buy something.


                      >> In advertising fast focal ratio = wide tfov.

                      WG: This is how, OTOH, optics is an exact science, but on the other,
                      still sometimes very relative or relational: such a statement or claim
                      can be and is true depending on your POINT OF REFERENCE that you use---
                      if you are implying that the aperture and the eyepiece do not change,
                      then, faster focal ratio will reduce the f.l. of the system and give
                      lower magnification and wider TFOV. But you can have an f/4 system in
                      one scope that has longer f.l. than another f/6 system; it depends on
                      the number of variables you allow.


                      >> So as a newbie I came to
                      >> believe that it not only equaled it but caused it.
                      >>
                      >> Now that you two (and others) have taken the time to put things in
                      >> their proper relationship for me I realize that the F/R is a 'result
                      >> / indicator' not a cause.
                      >>

                      WG: It is all interrelated. I steer clear Brian of saying that one
                      necessarily causes the other. The f/ratio describes several things,
                      most importantly the relationship between the aperture and focal
                      length. But you can have a hundred different sized scopes all with
                      different f.l.'s that will have the same f/ratio. And they will all
                      give different magnifications and different TFOV's. It can be very
                      messy and confusing if you are not clear on the context with which you
                      are speaking. This is why John was probably clenching his teeth over my
                      use of the term in this instance. But you can also learn a great deal
                      about optics if you see things or describe them in the context of being
                      a function of the f/ ratio.

                      Too many groups that allow themselves to delve deeper into the details
                      of optics often end up with just a very few throwing a lot of
                      testosterone around trying to prove one person right or wrong. I've
                      seen rudeness, personal attacks, character assassinations and more, as a
                      result. It isn't worth it.

                      Here there are perhaps more beginners than many groups and nothing is
                      more rewarding than helping or sharing something with someone who may be
                      retracing the same steps you once took.

                      This group is exceptional in the sense that it offers a disclaimer to
                      essentially not believe anything you are told; well, I have my own
                      feelings on that, but then, I have seen some of the most well known and
                      respected people in this industry make absurd claims on occasion! I
                      guess no one is right, accurate or even honest 100% of the time (he was
                      a manufacturer), but you have to believe/ trust some source(s).

                      If there is one thing I can share with anyone interested to understand
                      the terms and mechanics of how telescopes, light and optics basically
                      work, without putting their head in a vise, one of the first really
                      great books I bought will do more to teach someone new to this hobby
                      nearly everything they might ever need or want to know in order to
                      understand, predict, use and select telescopes, eyepieces and the like
                      with total assurance and success; and it is a fun, interesting and easy
                      book to read!

                      I consider it the amateur's bible of reference and strongly recommend
                      it. If it is no longer in print, SEEK IT OUT on one of the many
                      used-book search engines and GET A COPY and read it. It is:

                      *Amateur Astronomer's Handbook*
                      by J. B. Sidgwick
                      Enslow Publishers, Hillside New Jersey

                      Clear Skies,

                      WayneG
                    • Brian W
                      ... Hi, seeing as it was my post that started all this perhaps no one will object if I step in at this point? WG: Hi Brian, to try to clear this and a few
                      Message 10 of 15 , Jan 3, 2009
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                        --- In telescopes@yahoogroups.com, Wayne G <fomalhaut@...> wrote:

                        Hi, seeing as it was my post that started all this perhaps no one will
                        object if I step in at this point?


                        "WG: Hi Brian, to try to clear this and a few other things up a bit
                        for anyone confused by all of this or just getting into the hobby and
                        trying to sort things out, every telescope is a combination of several
                        factors, such as aperture (to which light gathering and resolution are
                        largely tied), and focal ratio (f/5 means the focal length or focal
                        plane is equal to five times the diameter of the primary objective;
                        100mm f/5 would have 500mm focal length.)--- and as such,
                        magnification, scale,true FOV and exit pupil are a function of focal
                        ratio AND aperture
                        (directly or indirectly).

                        Wayne you have stated that ...

                        You may not have read it as I > truly wrote it.

                        Wayne as you wrote it magnification, scale, true FOV and exit pupil
                        are a 'FUNCTION' of focal ratio and aperture (directly or indirectly).
                        That means (at least to this reasonably well educated person) that thy
                        are caused by them. You also questioned whether or not you had made
                        your meaning simple enough.

                        > WG: Perhaps not. Maybe my meaning and intent was not simple enough.

                        In my opinion it was certainly was about as simple as it could have
                        been. Which is not to say that it was correct or incorrect but simply
                        simple.

                        >
                        > You see, I have not made a fundamental error concerning something so
                        > basic as focal length, focal ratio, et al.: these things are just
                        about
                        > as basic as simple arithmetic! :^)

                        Even the Big E. admitted to errors in his math on occasion.

                        >
                        > What I was trying to convey was not that aperture, focal length,
                        etc., > are each separate entities to be "calculated" independent to
                        > one-another, but rather to be /connected/, related; I was trying to
                        > convey RELATIONSHIPS. Everything in optics and in a telescope in >
                        particular is about relationships.

                        Coming from a Buddhist perspective I admire your attempt at showing
                        ultimate interconnectedness.
                        >
                        > Focal ratio is a relationship between the two most important, global
                        factors in a telescope, the aperture and the focal length.

                        Actually it is an expression of the relationship. However earlier (see
                        above) you stated that it was a 'cause'.

                        > I was not implying that one uses the f/ number to *calculate*
                        > magnification, so please quit banging your head in frustration that
                        we > are all "mixed up." I was drawing the reader in to think deeper
                        about > what the telescope actually DOES when magnifying, etc., rather
                        to simply > memorize formulas in wrote.

                        You certainly did state that it was part of the cause of
                        magnification. (see above)
                        >
                        > > However I think it confuses people new to the hobby who
                        > > will read your post and then think everything revolves around
                        "focal ratio", which is certainly not the case.
                        >
                        > WG: I didn't know I said anything specifically /revolves/ around
                        focal ratio in my original post.

                        Again you said that many things were cause by F/R



                        > But perhaps you are looking at focal ratio in too narrow a way.
                        Perhaps that is the > source of your confusion, for perhaps you missed
                        that "(directly or > indirectly)" comment in my original post and what
                        it was implying. It > was the "indirectly" part I threw in there
                        hoping to spur a few on to > further thought about how all of this
                        fits together. For when you speak of focal ratio, you are
                        encompassing not only the "speed" of the system, but also the aperture
                        and the focal length of the system, since these are the parts which
                        make it up!

                        Nope I didn't miss it. Perhaps it is your use of the English language
                        that is the problem? F/R is indeed the 'speed' of the system but you
                        are implying, in the above, that aperture and F/L are merely 2 parts
                        that make up the F/R. Would it not be more accurate to state that the
                        relationship berween aperture and F/L is stated in terms of F/R?


                        > I was not speaking about how to /calculate/ magnification, John, nor
                        > meant to imply that the f/ratio was used in the calculation. It is
                        > merely an indirect, global /factor/ in influencing how the scope
                        magnifies.

                        Again you are stating that the F/R is a cause when it is merely a result.

                        Let me close this by thanking both of you because it was the
                        interconnection between all of our thoughts that have helped to
                        clarify this issue for me.
                        Brian
                      • Gregg
                        http://tinyurl.com/7yg6nr to see the photos by that scope in 1900. There were camera films of larger format than what we are used to using in astrophotos.
                        Message 11 of 15 , Jan 3, 2009
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                          http://tinyurl.com/7yg6nr to see the photos by that scope in 1900.
                          There were camera films of larger format than what we are used to using
                          in astrophotos. There are special field-flattening lenses that makes
                          it possible to expose a 9x9 inch or larger film plate. That let Keeler
                          put larger areas of the sky in one shot.
                          http://www.scribd.com/doc/6715299/Telescopes-With-Fixed-Position-
                          Eyepieces see figure 11. (And see the figure 28 for the Crossley.)
                          "The Crossley reflector, because of its high light efficiency at f/5.8
                          in the classical photography with large plates at the Cassegrain focus
                          was..." from http://www.springerlink.com/content/t101x848t250274r/
                          "For Curtis had been engaged for nearly a decade on the photography of
                          nebulae with the Crossley reflector, and for much of that time had been
                          an enthusiastic convert to the island universe theory; only that March
                          he had dined with Hale in Washington within a week of lecturing
                          on 'Modern Theories of the Spiral Nebulae'" from
                          http://antwrp.gsfc.nasa.gov/htmltest/gifcity/cs_real.html
                          It seems that one telescope's pictures had quite an impact on the
                          cosmology of the day, and laid the groundwork for Hubble's effort a
                          generation later.

                          --- In telescopes@yahoogroups.com, "Brian W" <mljbw2@...> wrote:
                          >
                          > Hi all, just need a little clarifying. I have been under the
                          > impression that as one gets into larger aperture with a reflector, one
                          > gets more and more detail but at the expense of less and less area.
                          >
                          > The Crossley is a 36" reflector and yet it takes an incredibly
                          > detailed picture of arguably all of M 42 and also captures M 43 and
                          > NGC 1977.
                          >
                          > So just to clarify... is the Crossley an oddity or will any reasonable
                          > quality large fast reflector perform in the same ballpark?
                          >
                          > Brian
                          >
                          > PS my temperature may be rising and it might just be aperture fever.
                          > B
                          >
                        • Al Germaine
                          John, Who said anything about an eyepiece??? Coverage refers to the image at the film plane, AFAIK, and what I said is still incorrect. It is related to focal
                          Message 12 of 15 , Jan 4, 2009
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                            John,
                            Who said anything about an eyepiece??? Coverage refers to the image
                            at the film plane, AFAIK, and what I said is still incorrect. It is
                            related to focal length.

                            Al

                            --- In telescopes@yahoogroups.com, "John Bambury" <jbambury@...> wrote:
                            >
                            > --- In telescopes@yahoogroups.com, "Al Germaine" <trulyyours05@>
                            > wrote:
                            > >
                            > > The detail or resolution of any telescope, reflector or refractor
                            > > is a function of the aperture and light gathering is a functiion
                            > > of the area.
                            >
                            > Correct
                            >
                            > > The coverage is a function of the focal ratio.
                            >
                            > This is incorrect.
                            >
                            > The Coverage or more properly "The True Field of View" (TFOV) is a
                            > function of three things; and three things only.
                            >
                            > 1) The focal length of the telescope.
                            > 2) The focal length of the eyepiece.
                            > 3) The Apparent Field of View (AFOV) of the eyepiece
                            >
                            > Aperture or the Focal Ratio of the Telescope has nothing to do with
                            > it.
                            >
                            > Let's take a standard 10"/F5 telescope. Assume a 250mm diameter
                            > primary and a 1250mmm focal length, for ease of the maths. Now we use
                            > a standard 10mm focal length plossl eyepiece, which has a 50 deg
                            > Apparent Field of View (AFOV).
                            >
                            > You first need to work out the magnification of the system, which is
                            > the focal length of the telescope divided by the focal length of the
                            > eyepiece. In this case it is 1250mm / 10mm = 125X.
                            >
                            > The True Field of View is given by dividing the AFOV of the eyepiece
                            > (50 deg) by the magnification of the system. In this case it is 50
                            > deg /125X for a TFOV = 0.4 degrees or 24'.
                            >
                            > Take the second example of a 5"/F10 telescope. Assume a 125mm
                            > diameter primary and a 1250mmm focal length, for ease of the maths.
                            > Now we use a standard 10mm focal length plossl eyepiece, which has a
                            > 50 deg Apparent Field of View (AFOV).
                            >
                            > The magnification of the system is the same as for the 10" scope. It
                            > is given by dividing the focal length of the telescope by the focal
                            > length of the eyepiece. In this case it is 1250mm / 10mm = 125X.
                            >
                            > Similarly, the Frue Field of View is exactly the same as for the 10"
                            > scope because the magnification and the AFOV of the eyepiece haven't
                            > changed.
                            >
                            > As you can see from these two examples a 5"/F10 telescope has the
                            > same True Field of view (and magnification) as a 10"/F5 telescope
                            > when the same eyepiece is used. The "focal ratio" is halved yet the
                            > TFOV remains the same. Clearly, the focal ratio has nothing to do
                            > with it.
                            >
                            >
                            > In answer to the question Brian was alluding to. "Why does the
                            > Coverage or TFOV reduce as the aperture increases?". Simple, because
                            > as the aperture of the telescope increases, the focal length of the
                            > telescope gets longer, although the focal length invariably doesn't
                            > increase proportionately.
                            >
                            > Cheers,
                            > John B
                            >
                            >
                            > PS: If someone else posts that "focal ratio" has something to do
                            > with this, I am gonna scream and bury my head in the sand.
                            >
                          • Brian W
                            Point is that it is often best to take many basic formulas and calculations from them as close approximations or points of reference. as my old engineer friend
                            Message 13 of 15 , Jan 4, 2009
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                              Point is that it is often best to take many basic formulas and
                              calculations from them as close approximations or points of reference.

                              as my old engineer friend used to put it... the theory she's great,
                              the practical not so good!


                              >
                              > Most calculations do not take into account ALL the variables which
                              might
                              > enter into the result, and in amateur astronomy, /experience/ is an
                              > important factor in also predicting how a telescope will perform,
                              > especially when making a buying decision.

                              astronomy is much like flying ( I used to do it for a living) most of
                              it is a 'science' but the really fun parts are an art.
                              >

                              >
                              >
                              > >>> > > The coverage is a function of the focal ratio.
                              > >>>
                              >
                              > WG: Of course, communication is always hard over the Internet and
                              it is > all too easy for people to gather a different meaning from
                              what you > intended,

                              now there's a truth that pastors soon learn.


                              > and I have seen many people argue vociferously about something in
                              heated
                              > disagreement that they were actually in agreement upon, but merely
                              > saying the same thing in two different ways!

                              you have been listening to my wife and myself discussing things.
                              >
                              >
                              >
                              >
                              >
                              > >> So as a newbie I came to
                              > >> believe that it not only equaled it but caused it.
                              > >>
                              > >> Now that you two (and others) have taken the time to put things in
                              > >> their proper relationship for me I realize that the F/R is a 'result
                              > >> / indicator' not a cause.
                              > >>
                              >
                              > WG: It is all interrelated. I steer clear Brian of saying that one
                              > necessarily causes the other. The f/ratio describes several things,
                              > most importantly the relationship between the aperture and focal
                              > length.

                              absolutely it does 'describe' but does it cause?


                              > Too many groups that allow themselves to delve deeper into the details
                              > of optics often end up with just a very few throwing a lot of
                              > testosterone around trying to prove one person right or wrong. I've
                              > seen rudeness, personal attacks, character assassinations and more,
                              as a
                              > result. It isn't worth it.

                              very true

                              >
                              > This group is exceptional in the sense that it offers a disclaimer
                              to > essentially not believe anything you are told; well, I have my
                              own > feelings on that, but then, I have seen some of the most well
                              known and > respected people in this industry make absurd claims on
                              occasion!

                              undoubtedly that includes me.

                              but you have to believe/ trust some source(s).

                              only the ones with good reputations and which can be verified


                              >
                              > I consider it the amateur's bible of reference and strongly recommend
                              > it. If it is no longer in print, SEEK IT OUT on one of the many
                              > used-book search engines and GET A COPY and read it. It is:
                              >
                              > *Amateur Astronomer's Handbook*
                              > by J. B. Sidgwick
                              > Enslow Publishers, Hillside New Jersey
                              >
                              >
                              next e-mail out will be the one asking my friend the used bookstore
                              fanatic to keep his eye out for it.

                              Brian
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