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what can't a newtonian do well?

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  • mljbw2
    Hi all, this may be opening a can of worms that should not be opened but after nearly a year of reading and hopefully learning I have a general question.
    Message 1 of 19 , May 1, 2008
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      Hi all, this may be opening a can of worms that should not be opened
      but after nearly a year of reading and hopefully learning I have a
      general question. Newtonians seem to be the marque that other
      telescopes are measured against. Is this just a beginners impression
      or is it an accurate beginners impression?

      Now I am not suggesting that a mass produced Newtonian could compete
      with a Takahashi or any other superbly designed limited edition
      magnificently assembled telescope. But I am curious if the Newtonian
      design has been bettered over the years.

      As I said, this may be a can of worms best left unopened but I am curious.
      Brian
    • Jack & Lynn Kramer
      From: mljbw2 ... Brian, I can answer as an old timer (50+ years as an amateur astronomer). As is often said, there is no such thing as a
      Message 2 of 19 , May 1, 2008
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        From: "mljbw2" <mljbw2@...>
        > Newtonians seem to be the marque that other
        > telescopes are measured against. Is this just a beginners impression
        > or is it an accurate beginners impression?

        Brian, I can answer as an old timer (50+ years as an amateur astronomer). As
        is often said, there is no such thing as a "perfect" telescope, which means
        that each design has certain advantages, depending on how it will be used.
        The basic Newtonian has a long history as offering the most telescope for
        the least amount of money. With mirrors of high quality they can equal or
        best any other design in terms of the images they present. Even a Newtonian
        of average to good quality is a good investment, perhaps the best choice in
        many cases. Therefore, anyone interested in acquiring a new telescope needs
        to ask themselves what they will gain by getting some design other than a
        Newtonian.

        Jack Kramer
        Lily Lake, Illinois
      • Doug Bailey
        only real advantages are more aperture per dollar, and no color aberrations. Doug ... From: mljbw2 To: telescopes@yahoogroups.com Sent:
        Message 3 of 19 , May 1, 2008
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          only real advantages are more aperture per dollar, and no color aberrations.
          Doug


          ----- Original Message ----
          From: mljbw2 <mljbw2@...>
          To: telescopes@yahoogroups.com
          Sent: Thursday, May 1, 2008 2:13:48 AM
          Subject: [Telescopes] what can't a newtonian do well?


          Hi all, this may be opening a can of worms that should not be opened
          but after nearly a year of reading and hopefully learning I have a
          general question. Newtonians seem to be the marque that other
          telescopes are measured against. Is this just a beginners impression
          or is it an accurate beginners impression?

          Now I am not suggesting that a mass produced Newtonian could compete
          with a Takahashi or any other superbly designed limited edition
          magnificently assembled telescope. But I am curious if the Newtonian
          design has been bettered over the years.

          As I said, this may be a can of worms best left unopened but I am curious.
          Brian




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        • Geoff Gaherty
          ... Actually, I would say that, for most of us, the apochromatic refractor is the design against which all other telescope designs are measured. There is no
          Message 4 of 19 , May 1, 2008
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            mljbw2 wrote:
            > Hi all, this may be opening a can of worms that should not be opened
            > but after nearly a year of reading and hopefully learning I have a
            > general question. Newtonians seem to be the marque that other
            > telescopes are measured against. Is this just a beginners impression
            > or is it an accurate beginners impression?
            >
            > Now I am not suggesting that a mass produced Newtonian could compete
            > with a Takahashi or any other superbly designed limited edition
            > magnificently assembled telescope. But I am curious if the Newtonian
            > design has been bettered over the years.

            Actually, I would say that, for most of us, the apochromatic refractor
            is the design against which all other telescope designs are measured.
            There is no other design, given top quality in every respect, which can
            equal an apo. The problem is that this optical perfection comes with
            literally a very high price, and that price increases exponentially as
            the aperture increases. When you start comparing telescopes at this
            level of quality, the design differences make a fairly small contribution.

            I'd love to have a 6" or larger apo myself, but I simply cannot afford
            it. I settled on an 11" Newtonian with Zambuto optics instead, and it
            consistently outperforms any 6" apo on the planet. It probably would be
            beaten by an 11" apo, if anyone made such a thing, but the question is
            academic. Every telescope is a compromise. At present my most used
            scope is an 11" Celestron SCT, optically nowhere near as good as my 11"
            Zambuto, but because of convenience it is a better fit for my primary
            observing program, variable stars. It's the best compromise for the
            observing I want to do at this phase in my life.

            The person who probably has more experience testing top level telecopes
            than anyone else is Ed Ting:

            http://www.scopereviews.com/

            Spend a few days reading the material on his web site, and you'll have a
            good feel for what the finest telescopes in the world are like.

            Geoff

            --
            Geoff Gaherty
            Foxmead Observatory
            Coldwater, Ontario, Canada
            http://www.gaherty.ca
          • Al Germaine
            Geoff, I would hardly ever argue against anything you say, but... I think our perspective is as amateur astronomers considering small telescopes. I think the
            Message 5 of 19 , May 1, 2008
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              Geoff,

              I would hardly ever argue against anything you say, but... I think our
              perspective is as amateur astronomers considering small telescopes. I
              think the OP is correct that _reflecting_ telescopes, derivative of
              the Newtonian, are the standard tool of astronomy.

              Al

              --- In telescopes@yahoogroups.com, Geoff Gaherty <geoff@...> wrote:
              >
              > mljbw2 wrote:
              > > Hi all, this may be opening a can of worms that should not be opened
              > > but after nearly a year of reading and hopefully learning I have a
              > > general question. Newtonians seem to be the marque that other
              > > telescopes are measured against. Is this just a beginners impression
              > > or is it an accurate beginners impression?
              > >
              > > Now I am not suggesting that a mass produced Newtonian could compete
              > > with a Takahashi or any other superbly designed limited edition
              > > magnificently assembled telescope. But I am curious if the Newtonian
              > > design has been bettered over the years.
              >
              > Actually, I would say that, for most of us, the apochromatic refractor
              > is the design against which all other telescope designs are measured.
              > There is no other design, given top quality in every respect, which can
              > equal an apo. The problem is that this optical perfection comes with
              > literally a very high price, and that price increases exponentially as
              > the aperture increases. When you start comparing telescopes at this
              > level of quality, the design differences make a fairly small
              contribution.
              >
              > I'd love to have a 6" or larger apo myself, but I simply cannot afford
              > it. I settled on an 11" Newtonian with Zambuto optics instead, and it
              > consistently outperforms any 6" apo on the planet. It probably
              would be
              > beaten by an 11" apo, if anyone made such a thing, but the question is
              > academic. Every telescope is a compromise. At present my most used
              > scope is an 11" Celestron SCT, optically nowhere near as good as my 11"
              > Zambuto, but because of convenience it is a better fit for my primary
              > observing program, variable stars. It's the best compromise for the
              > observing I want to do at this phase in my life.
              >
              > The person who probably has more experience testing top level telecopes
              > than anyone else is Ed Ting:
              >
              > http://www.scopereviews.com/
              >
              > Spend a few days reading the material on his web site, and you'll
              have a
              > good feel for what the finest telescopes in the world are like.
              >
              > Geoff
              >
              > --
              > Geoff Gaherty
              > Foxmead Observatory
              > Coldwater, Ontario, Canada
              > http://www.gaherty.ca
              >
            • Jack & Lynn Kramer
              From: Geoff Gaherty ... Actually, I d have to modify my original reply and agree with Geoff to a point. I have a 6 apochromat and have
              Message 6 of 19 , May 1, 2008
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                From: "Geoff Gaherty" <geoff@...>
                > Actually, I would say that, for most of us, the apochromatic refractor
                > is the design against which all other telescope designs are measured.

                Actually, I'd have to modify my original reply and agree with Geoff to a
                point. I have a 6" apochromat and have found no other telescope of that size
                that can equal it for image quality. But I stand by my earlier statement
                that the Newtonian is the standard of comparison ... taking ALL factors into
                consideration. One of those factors is price per inch of aperture, and cost
                is always a consideration. So my question to anyone who has X dollars to
                spend is "why would you choose anything other than a Newtonian?" There may
                be very valid reasons for not choosing a Newtonian, but it's a question
                worth asking.

                Jack Kramer
                Lily Lake, Illinois
              • Geoff Gaherty
                ... I think we re talking about two different things. The apochromatic refractor is the standard of _optical perfection_, because of its absence of a central
                Message 7 of 19 , May 1, 2008
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                  Al Germaine wrote:
                  > Geoff,
                  >
                  > I would hardly ever argue against anything you say, but... I think our
                  > perspective is as amateur astronomers considering small telescopes. I
                  > think the OP is correct that _reflecting_ telescopes, derivative of
                  > the Newtonian, are the standard tool of astronomy.

                  I think we're talking about two different things. The apochromatic
                  refractor is the standard of _optical perfection_, because of its
                  absence of a central obstruction, and the diffraction which that causes.
                  The commonest telescope type in _actual use_ is the reflector, to a
                  large extent among amateur astronomers, and virtually exclusively among
                  professionals.

                  Geoff

                  --
                  Geoff Gaherty
                  Foxmead Observatory
                  Coldwater, Ontario, Canada
                  http://www.gaherty.ca
                • Taylor J. Barton
                  ... I m in the same boat as Jack (48 yrs in the hobby). Things have changed a lot over that period, especially the availability of large- aperture telescopes
                  Message 8 of 19 , May 1, 2008
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                    --- In telescopes@yahoogroups.com, "mljbw2" <mljbw2@...> wrote:
                    >
                    > Newtonians seem to be the marque that other
                    > telescopes are measured against. Is this just a beginners impression
                    > or is it an accurate beginners impression?
                    >
                    I'm in the same boat as Jack (48 yrs in the hobby). Things have
                    changed a lot over that period, especially the availability of large-
                    aperture telescopes of decent quality at affordable prices.

                    When I was starting out, not many of us had access to apertures
                    greater than 6 inches for reflectors and 3 inches for refractors.
                    Hard times.

                    Now I would say that has doubled, given the very wide availability
                    of "decent" 10" to 12" newtonians on dobsonian mounts at about the
                    same price as your average EQ-mounted Synta 6" achromat.

                    So, in terms of the mass market scopes of that quality, I'd say the
                    only thing either design has against them is portability: at least as
                    far as, let's say, being "airline portable" or "grab and go", whereas
                    a compact SCT or Maksutov design of 130mm can be airline portable.

                    But in terms of absolute quality, I'd say there's little visual
                    difference the average observer will notice between a 6" apo and an
                    8" newtonian if both are premium scopes optically. Can some of us
                    detect the difference visually? Probably. But is it easily, at-a-
                    glance obvious? Probably not. Please notice I did specify premium
                    optical quality for both scopes!

                    In the case of "what does a Newtonian not do well?" I'd suggest that
                    the answer is "not very much". Newtonians do just about everything
                    I'd want to do, and do it well. Except maybe for travel, if the scope
                    is 12" aperture or larger. Then again, a 6" apo is not something you
                    just throw in the backseat. Mine fills the back end of my pickup, if
                    you include the mount and accessory cases. The larger (and pricier!)
                    the scope, the larger (and pricier!) the cases. All that adds up in
                    weight pretty quickly, too.

                    If you decide that size and weight are issues, then that limits your
                    aperture. And once you get down around 4" or less in aperture, things
                    begin to change. A 102mm to 127mm apo is still rather longish and
                    pricey compared to the same aperture in a Mak or SCT. If you want to
                    view fine planet detail, or to observe planetary nebulae, you need to
                    strike a balance between aperture (resolving power and light) and
                    focal length (magnification), so that tends to make Newts and
                    refractors physically longer. And that, in turn, puts a premium on
                    the mount.

                    So, it's not an easy question to answer except in generalities.
                    Newtonians tend to be Jacks of all trades ... as long as physical
                    bulk is not a problem (and we are talking about premium optics).

                    Personally, my choice comes down to the target type.

                    If I want to chase galaxies, I want all the aperture I can afford. If
                    I want to tackle fine planetary detail, then I want all the focal
                    length I can afford and acceptably illuminate. And if I want
                    widefield observing then I want all the quality I can afford in an
                    easy-to-handle package.

                    Can I get all those in a Newtonian? Yes.
                    Can I get them all in ONE Newtonian? No.
                    Can I get them all in ONE SCT or Mak? Maybe, with a focal reducer.

                    Jeff Barton
                  • stargazer_7@cox.net
                    I appreciate your comments, Geoff (and others) . . . but if one can afford a scope with larger-sized, high-quality Newtonian optics, it can be fitted with an
                    Message 9 of 19 , May 1, 2008
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                      I appreciate your comments, Geoff (and others) . . . but if one can afford a scope with larger-sized, high-quality Newtonian optics, it can be fitted with an off-axis mask that bypasses the spider's diffraction. If the mirror is high-quality (to stress it again), when stopped down it will outperform any Apo of the same aperture.

                      For example, a friend once at a star party observed through someone else's scope with an exceptional 16" f/5 Newtonian mirror (paired with a 3.5" diagonal) that was stopped down to 6 inches at f/13.3. It was blowing away APO scopes similar in aperture to the stopped down 16", such as scopes from TeleVue, Starfire (AstroPhysics) and others, on views of Mars, Jupiter and Saturn, as well as a host of DSOs.

                      To this day, my friend's viewpoint is, "Give me a big sack of money for a scope, and I'll bypass everything else and buy a large Newtonian with superb optics."

                      My feelings are similar . . . I had a 10" f/5.6 which I stopped down to a 4" f/14, and I got fantastic lunar and planetary views, easily split close double stars, and star clusters, and nebulae were breathtaking. All of this with no color aberration and much, much lower cost than an APO.

                      Just my $0.02 . . .

                      - Dave G.
                      Northern Virginia


                      ---- Geoff Gaherty <geoff@...> wrote:
                      > mljbw2 wrote:
                      > > Hi all, this may be opening a can of worms that should not be opened
                      > > but after nearly a year of reading and hopefully learning I have a
                      > > general question. Newtonians seem to be the marque that other
                      > > telescopes are measured against. Is this just a beginners impression
                      > > or is it an accurate beginners impression?
                      > >
                      > > Now I am not suggesting that a mass produced Newtonian could compete
                      > > with a Takahashi or any other superbly designed limited edition
                      > > magnificently assembled telescope. But I am curious if the Newtonian
                      > > design has been bettered over the years.
                      >
                      > Actually, I would say that, for most of us, the apochromatic refractor
                      > is the design against which all other telescope designs are measured.
                      > There is no other design, given top quality in every respect, which can
                      > equal an apo. The problem is that this optical perfection comes with
                      > literally a very high price, and that price increases exponentially as
                      > the aperture increases. When you start comparing telescopes at this
                      > level of quality, the design differences make a fairly small contribution.
                      >
                      > I'd love to have a 6" or larger apo myself, but I simply cannot afford
                      > it. I settled on an 11" Newtonian with Zambuto optics instead, and it
                      > consistently outperforms any 6" apo on the planet. It probably would be
                      > beaten by an 11" apo, if anyone made such a thing, but the question is
                      > academic. Every telescope is a compromise. At present my most used
                      > scope is an 11" Celestron SCT, optically nowhere near as good as my 11"
                      > Zambuto, but because of convenience it is a better fit for my primary
                      > observing program, variable stars. It's the best compromise for the
                      > observing I want to do at this phase in my life.
                    • Geoff Gaherty
                      ... While this is true, the view with the Newtonian at full aperture will always beat the view with it stopped down. This has been tried experimentally and
                      Message 10 of 19 , May 1, 2008
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                        stargazer_7@... wrote:
                        > I appreciate your comments, Geoff (and others) . . . but if one can
                        > afford a scope with larger-sized, high-quality Newtonian optics, it
                        > can be fitted with an off-axis mask that bypasses the spider's
                        > diffraction. If the mirror is high-quality (to stress it again),
                        > when stopped down it will outperform any Apo of the same aperture.

                        While this is true, the view with the Newtonian at full aperture will
                        always beat the view with it stopped down. This has been tried
                        experimentally and discussed at great length over the years on the
                        Zambuto Yahoo Group. The gain from using the full aperture (assuming
                        top notch optical quality) far exceeds any advantage of an unobstructed
                        aperture. Aperture always wins.

                        Geoff

                        --
                        Geoff Gaherty
                        Foxmead Observatory
                        Coldwater, Ontario, Canada
                        http://www.gaherty.ca
                      • Jack & Lynn Kramer
                        From: ... My experience as well, Dave. I too had a 10 f/5.6 that I stopped down to 4 , and the contrast was great because of bypassing
                        Message 11 of 19 , May 1, 2008
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                          From: <stargazer_7@...>
                          > I had a 10" f/5.6 which I stopped down to a 4" f/14, and I got fantastic
                          > lunar and planetary views, easily split close double stars, and star
                          > clusters, and nebulae were breathtaking. All of this with no color
                          > aberration and much, much lower cost than an APO.

                          My experience as well, Dave. I too had a 10" f/5.6 that I stopped down to
                          4", and the contrast was great because of bypassing the secondary mirror.
                          But I quit using it after awhile because ... guess what ... upon careful
                          observation I found I was actually seeing LESS detail in things like the
                          moon and planets. I had sabotaged the resolution of the 10" in favor of
                          greater contrast. I think a lot of people confuse higher contrast with
                          better resolution. They're not the same. Larger optics provide higher
                          resolution, which allows you to see finer detail. I'll take resolution any
                          day. If I can get good contrast too, that's a bonus.

                          Jack Kramer
                          Lily Lake, Illinois

                          Jack Kramer
                          Lily Lake, Illinois
                        • stargazer_7@cox.net
                          Geoff, For the content of what you said, you re right on the money. *However* :-) what *I* was doing, was comparing the performance of an APO of the *same
                          Message 12 of 19 , May 1, 2008
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                            Geoff,

                            For the content of what you said, you're right on the money.

                            *However* :-) what *I* was doing, was comparing the performance of an APO of the *same aperture* as the stopped-down Newt (in the example) . . . i.e. I was comparing what is essentially an off-axis Newtonian with an APO of similar aperture.

                            As far as a Newtonian of whatever aperture -- the full aperture will of course out-perform the views with the stopped-down aperture, on the same scope . . . or even another scope of same aperture and optical quality.

                            cheers,
                            Dave G.

                            ---- Geoff Gaherty <geoff@...> wrote:
                            > stargazer_7@... wrote:
                            > > I appreciate your comments, Geoff (and others) . . . but if one can
                            > > afford a scope with larger-sized, high-quality Newtonian optics, it
                            > > can be fitted with an off-axis mask that bypasses the spider's
                            > > diffraction. If the mirror is high-quality (to stress it again),
                            > > when stopped down it will outperform any Apo of the same aperture.
                            >
                            > While this is true, the view with the Newtonian at full aperture will
                            > always beat the view with it stopped down. This has been tried
                            > experimentally and discussed at great length over the years on the
                            > Zambuto Yahoo Group. The gain from using the full aperture (assuming
                            > top notch optical quality) far exceeds any advantage of an unobstructed
                            > aperture. Aperture always wins.
                            >
                            > Geoff
                            >
                            > --
                            > Geoff Gaherty
                            > Foxmead Observatory
                            > Coldwater, Ontario, Canada
                            > http://www.gaherty.ca
                          • stargazer_7@cox.net
                            ... Jack, I agree with all that you said. However, to clarify myself (as I did with Geoff) . . . Yes -- aperture wins, as long as the optical quality is good.
                            Message 13 of 19 , May 1, 2008
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                              ---- Jack & Lynn Kramer <llranch339@...> wrote:
                              > From: <stargazer_7@...>
                              > > I had a 10" f/5.6 which I stopped down to a 4" f/14, and I got fantastic
                              > > lunar and planetary views, easily split close double stars, and star
                              > > clusters, and nebulae were breathtaking. All of this with no color
                              > > aberration and much, much lower cost than an APO.
                              >
                              > My experience as well, Dave. I too had a 10" f/5.6 that I stopped down to
                              > 4", and the contrast was great because of bypassing the secondary mirror.
                              > But I quit using it after awhile because ... guess what ... upon careful
                              > observation I found I was actually seeing LESS detail in things like the
                              > moon and planets. I had sabotaged the resolution of the 10" in favor of
                              > greater contrast. I think a lot of people confuse higher contrast with
                              > better resolution. They're not the same. Larger optics provide higher
                              > resolution, which allows you to see finer detail. I'll take resolution any
                              > day. If I can get good contrast too, that's a bonus.


                              Jack, I agree with all that you said. However, to clarify myself (as I did with Geoff) . . .

                              Yes -- aperture wins, as long as the optical quality is good. And in light of my agreement with you, I wasn't and would not argue the issue of aperture, and resolution vs. contrast.

                              To continue clarifying: What I was doing was comparing the views given by an *unobstructed* mirror versus an APO. An example of what I'm saying, would be to take a 16" Newt stopped down to about 6" unobstructed, and compare it to the views through a 6"-ish APO scope such as from Astro-Physics.

                              In my example, it doesn't matter that the 6" aperture on the Newt is derived from a 16" mirror . . . 6" is 6".

                              Again with the same example -- if I understood you correctly, you're saying a 16" Newt at full aperture will outperform *itself* stopped down to whatever aperture. Unarguably true . . . we were just saying two different things.

                              Unless, of course, I've lapsed into a crazed coma after eating a tainted tuna sandwich for lunch . . . :-)

                              - Dave G.
                            • Jack & Lynn Kramer
                              Dave, I guess we are on the same page then. I would be interested to see a side-by-side comparison between a large Newtonian stopped down to 6 and a 6 apo.
                              Message 14 of 19 , May 1, 2008
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                                Dave, I guess we are on the same page then. I would be interested to see a
                                side-by-side comparison between a large Newtonian stopped down to 6" and a
                                6" apo. (Being a refractor guy, my bet is on the apo :-)

                                Jack Kramer
                                Lily Lake, Illinois
                              • David Stein
                                All telescope designs (at least, all the ones that are actually used) are equally good as designs. They are, however, different engineering solutions, each
                                Message 15 of 19 , May 1, 2008
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                                  All telescope designs (at least, all the ones that are actually used)
                                  are equally good as designs. They are, however, different engineering
                                  solutions, each designed to maximize performance in particular
                                  respects under specific constraints.

                                  Trying to get maximum field of view with minimum size and weight,
                                  regardless of cost? It will be hard to beat an apochromatic
                                  refractor.

                                  Trying to get maximum light gathering power and resolution for a
                                  particular cost (bang for the buck)? You probably want a Newtonian.

                                  Trying to get maximum light gathering power with minimum tube length?
                                  You may want an SCT or MCT, (or perhaps a truss-tube Dob if you're
                                  really only concerned about transportation).

                                  Trying to take very high quality flat-field deep sky astrophotographs,
                                  regardless of cost? A Ritchie Chr├ętien may be what you're looking
                                  for.

                                  And so forth--if a design were "bad", no one would bother to build
                                  scopes using it (or stay in business long doing it). The real problem
                                  is to figure out which design matches your particular requirements of
                                  cost, size, weight and so forth while providing the best performance
                                  for the observing and/or photography that you personally want to do,
                                  and then find a manufacturer who does the best job executing that
                                  design within your price range.

                                  Clear skies,
                                  David
                                • Tim Kamel
                                  Jack I have been following this thread with great interest. I have a mask for a 10 Newtonian but have never used it, did not see much point in doing so.
                                  Message 16 of 19 , May 2, 2008
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                                    Jack

                                    I have been following this thread with great interest. I have a mask for a 10" Newtonian but have never used it, did not see much point in doing so. This thread is encouraging me to at least try it to do some experimenting.

                                    My questions - should this discussion be taking into consideration the focal length? Is it reasonable to compare a stopped down Newtonian at f/13 to a similar sized Apo at, say, f/5? Would it be better to compare to an Apo at the same focal length (I am sure that there is no such animal)? And, as a follow up, how would an achromat at f/13 compare with a stopped down newtonian at the same focal length?

                                    Tim Kamel

                                    --- On Thu, 5/1/08, Jack & Lynn Kramer <llranch339@...> wrote:

                                    From: Jack & Lynn Kramer <llranch339@...>
                                    Subject: Re: [Telescopes] Re: what can't a newtonian do well?
                                    To: telescopes@yahoogroups.com
                                    Date: Thursday, May 1, 2008, 5:00 PM






                                    Dave, I guess we are on the same page then. I would be interested to see a
                                    side-by-side comparison between a large Newtonian stopped down to 6" and a
                                    6" apo. (Being a refractor guy, my bet is on the apo :-)

                                    Jack Kramer
                                    Lily Lake, Illinois
















                                    ____________________________________________________________________________________
                                    Be a better friend, newshound, and
                                    know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ
                                  • Jack & Lynn Kramer
                                    From: Tim Kamel ... Tim, I would say that any comparison should be done at the same magnification, not necessarily the same focal
                                    Message 17 of 19 , May 2, 2008
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                                      From: "Tim Kamel" <timonium27@...>
                                      > should this discussion be taking into consideration the focal length? Is
                                      > it reasonable to compare a stopped down Newtonian at f/13 to a similar
                                      > sized Apo at, say, f/5?

                                      Tim, I would say that any comparison should be done at the same
                                      magnification, not necessarily the same focal ratio. Long focal length
                                      achromats don't exhibit nearly the amount of chromatic aberration as faster
                                      refractors, so the image should be similar to that in a stopped-down
                                      reflector. An apo is a different story. But I'll defer to someone who may be
                                      better versed than I in optical principles.

                                      Jack Kramer
                                      Lily Lake, Illinois
                                    • jpcannavo
                                      Dave, John It would be interesting... ... to see a ... 6 and a ... First star test both the apo and the stopped down newt, side by side, to ensure that other
                                      Message 18 of 19 , May 2, 2008
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                                        Dave, John
                                        It would be interesting...
                                        > Dave, I guess we are on the same page then. I would be interested
                                        to see a
                                        > side-by-side comparison between a large Newtonian stopped down to
                                        6" and a
                                        > 6" apo. (Being a refractor guy, my bet is on the apo :-)
                                        >
                                        > Jack Kramer
                                        > Lily Lake, Illinois

                                        First star test both the
                                        apo and the stopped down newt, side by side, to ensure that other
                                        factors -optical quality, cooling, collimation etc - are not
                                        operating.
                                        If then thermal issues, differences in optical quality,
                                        collimation are not operating; there really SHOULD be no difference
                                        in the on-axis image. In fact, since a paraboloid focuses light
                                        absolutely perfectly on-axis, if anything, you might expect a
                                        theoretical advantage for the newt.
                                        But this does raise one remaining question (that was recently
                                        discussed on the Zambuto mirror group) that provides a possible
                                        advantage to the APO. Specifically: Do reflective surfaces - due to
                                        unavoidable residual micro-roughness in the aluminum coating -
                                        scatter more light than lenses? I don't know the answere, but I have
                                        heard this claim made from time to time...an "urban" myth of sorts?
                                        If this is a myth, then I don't see whats left - from a theoretical
                                        standpoint - that could confer an advantage to the APO in the on-axis
                                        image? On the other hand if its true...
                                        Joe
                                      • Robert
                                        Dear David, Very succinct and I think yours are the most helpful and thoughtful comments, but, the best telescope is the one you actually use! I have seen so
                                        Message 19 of 19 , May 3, 2008
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                                          Dear David,

                                          Very succinct and I think yours are the most helpful and thoughtful
                                          comments, but, the "best" telescope is the one you actually use!

                                          I have seen so many folks agonize over this issue, then become
                                          paralyzed in their purchase decisions and never get out under the stars!

                                          Clear Dark Skies

                                          Robert in Hershey
                                          (an owner of a big 18" newt, small 5" mak, 8" sct, 66, 102, and 180mm
                                          achros and an 90mm fluorite triplet apo, yep, I use 'em all; but not
                                          at the same time, LOL)

                                          --- In telescopes@yahoogroups.com, "David Stein" <david.r.stein@...>
                                          wrote:
                                          >
                                          > All telescope designs (at least, all the ones that are actually
                                          > used) are equally good as designs. They are, however, different
                                          > engineering solutions, each designed to maximize performance in
                                          > particular respects under specific constraints...
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