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RE: [Telescopes] Re: Klevzov vs SCT?

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  • Scott Horne
    Jesus, I m sorry I asked! Can we knock off the Klevzov arguement now? Scott Horne http://starjourney.net Honolulu, Hawaii ... From:
    Message 1 of 19 , Apr 1, 2001
      Jesus, I'm sorry I asked! Can we knock off the Klevzov arguement now?

      Scott Horne
      http://starjourney.net
      Honolulu, Hawaii


      -----Original Message-----
      From: sbodenheimer@... [mailto:sbodenheimer@...]
      Sent: Saturday, March 31, 2001 7:07 PM
      To: telescopes@yahoogroups.com
      Subject: [Telescopes] Re: Klevzov vs SCT?


      --- In telescopes@y..., idalv@e... wrote:
      > --- In telescopes@y..., Bill Brady <wmbrady@o...> wrote:
      > > From Makscopes Forum:
      > >
      > > > Valery Deryuzhin.
      > >
      > > "Hi,
      > >
      > > I read your post and wondered whether you could identify the laws
      > of
      > > physics to which you refer. In particular you seem to say that
      the
      > > areal value of a spider obstruction is to be treated the same as
      > the
      > > areal value of a central obstruction (c.o.). But this simply
      > > doesn't make sense from a mathematics/physics standpoint. For
      > > example, the areal value of a 200 mm aperture is -- (pi) times
      (100
      > > squared) = (pi)(10,000). If I take away 20 mm from the diameter
      at
      > > the exterior of the aperture, I have an areal value of (pi) times
      > (90
      > > squared) = (pi)(8,100) resulting in a net loss of (pi)(1,900)=
      5966
      > > sq.mm. On the other hand if I remove the same 20 mm from the
      > > diameter at the center of the aperture I have an areal value of
      (pi)
      > > (10,000) minus (pi)(10 squared), resulting in a total loss of (pi)
      > > (100) = 314 sq.mm. Since none of the spider obstrution is
      located
      > in
      > > the center of the aperture (indeed, a substantial portion of the
      > > spider obstruction is located at the periphery of the aperture),
      > how
      > > can one properly equate the area of the spider to the area of a
      > > central aperature? If your analysis were correct, then removing
      > 100
      > > mm from a 200 mm diameter aperture at the center of the aperture
      > > would be the same as removing 27.5 mm from the diameter at its
      > > exterior (i.e., a 200 mm aperture with 50% central obstruction
      > would
      > > be the same as a 172.5 mm aperture with no obstruction). Did I
      > miss
      > > something?
      > >
      > > Also I own a TAL200K and measured the central obstruction -- It
      is
      > > 35% (linear) not 40%. And the images are very bright as
      previously
      > > reported.
      > >
      > > Thanks.
      > >
      > > Steve Bodenheimer"
      > >
      >
      > Have to admit that can't fallow this logic. The idea seems to be
      > simply that what matters is a total of the area blocking the light
      > path.
      >
      > So, if we have a 35% (70mm) central obstruction in a 200mm aperture
      > scope, with three spider vanes 5mm wide and with some 70mm of the
      > length in the light path (being curved), it comes to a total of
      > 3848sq. mm area of the c. obstruction + 1050sq.mm area of three
      > spider vanes = 4898sq.mm area in the light path. This would
      > correspond to a 39.5% (79mm) c. obstruction without spider vanes
      > (say, optical window mounted).
      >
      > It doesn't seem to agree with what Suiter says, since in the
      example
      > in his book (20% obstruction+ 2mm four-vane spider) this aritmetic
      > would result in a total diffraction effect nearly equal to that of
      a
      > 25% (spiderless) obstruction. In effect, it is roughly 21%.
      According
      > to Suiter, % of drop in contrast due to spader vanes (straight or
      > curved, same thing) is approx. equal to the % of aperture's area
      they
      > make. In this case it would be somewhat over 3%, which would
      > correspond to approx. 37% spiderless obstruction. This is still the
      > size of c. obstruction in f/10 Meade SCT's, and that itself
      > doesn't make them third-grade instruments.
      >
      > Vladimir

      Hi Vladimir,

      I'll be darned. Actually you do seem to agree with my logic.
      Contrast is a function of light gathering capacity -- i.e., a
      function of aperture area. Thus percent drop in contrast is simply
      the same thing as losing a portion of the aperture. Bill's post here
      corresponds to my measurements of 4mm thickness and your 70 mm length
      is the same as I measured (using thin solder wire I bent to match the
      curved spiders). Total aperture area loss is 810 sq.mm. Now that
      corresponds to a reduction in aperture size from 200 mm to 191.7 mm.
      Thus this system apparently is the same as a no spider system of
      191.7 mm with 70 mm aperture, or 36.5% linear c.o.

      Clear, calm and dark skies. Thanks.

      Steve Bodenheimer


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    • Michael Hosea
      Don t worry about it, Scott. It s a perfectly legitimate comparison and one that the market needs. Aloha, Mike ... now?
      Message 2 of 19 , Apr 1, 2001
        Don't worry about it, Scott. It's a perfectly legitimate comparison
        and one that the market needs.

        Aloha,

        Mike

        --- In telescopes@y..., "Scott Horne" <gsm2horn@c...> wrote:
        > Jesus, I'm sorry I asked! Can we knock off the Klevzov arguement
        now?
        >
        > Scott Horne
        > http://starjourney.net
        > Honolulu, Hawaii
      • aries57us@yahoo.com
        ... aperture ... the ... here ... length ... the ... Hi Vlad and Steve, In the case with straight thin spiders only a small portion of energy is pulled out
        Message 3 of 19 , Apr 1, 2001
          --- In telescopes@y..., sbodenheimer@c... wrote:

          > >
          > > Have to admit that can't fallow this logic. The idea seems to be
          > > simply that what matters is a total of the area blocking the light
          > > path.
          > >
          > > So, if we have a 35% (70mm) central obstruction in a 200mm
          aperture
          > > scope, with three spider vanes 5mm wide and with some 70mm of the
          > > length in the light path (being curved), it comes to a total of
          > > 3848sq. mm area of the c. obstruction + 1050sq.mm area of three
          > > spider vanes = 4898sq.mm area in the light path. This would
          > > correspond to a 39.5% (79mm) c. obstruction without spider vanes
          > > (say, optical window mounted).
          > >
          > > It doesn't seem to agree with what Suiter says, since in the
          > example
          > > in his book (20% obstruction+ 2mm four-vane spider) this aritmetic
          > > would result in a total diffraction effect nearly equal to that of
          > a
          > > 25% (spiderless) obstruction. In effect, it is roughly 21%.
          > According
          > > to Suiter, % of drop in contrast due to spader vanes (straight or
          > > curved, same thing) is approx. equal to the % of aperture's area
          > they
          > > make. In this case it would be somewhat over 3%, which would
          > > correspond to approx. 37% spiderless obstruction. This is still
          the
          > > size of c. obstruction in f/10 Meade SCT's, and that itself
          > > doesn't make them third-grade instruments.
          > >
          > > Vladimir
          >
          > Hi Vladimir,
          >
          > I'll be darned. Actually you do seem to agree with my logic.
          > Contrast is a function of light gathering capacity -- i.e., a
          > function of aperture area. Thus percent drop in contrast is simply
          > the same thing as losing a portion of the aperture. Bill's post
          here
          > corresponds to my measurements of 4mm thickness and your 70 mm
          length
          > is the same as I measured (using thin solder wire I bent to match
          the
          > curved spiders). Total aperture area loss is 810 sq.mm. Now that
          > corresponds to a reduction in aperture size from 200 mm to 191.7 mm.

          > Thus this system apparently is the same as a no spider system of
          > 191.7 mm with 70 mm aperture, or 36.5% linear c.o.
          >
          > Clear, calm and dark skies. Thanks.
          >
          > Steve Bodenheimer



          Hi Vlad and Steve,

          In the case with straight thin spiders only a small portion of
          energy is pulled out from Airy disk and it spreads only in four
          narrow direction. All rest space around Airy disk is undamaged
          and equal for simple obstructed aperture. In the case with curved
          siders - an energy spreads very uniformly around Airy disk and
          approximation you mentioned is almost 100% valid.

          Steve, if only the matter will be in % brightenss lost.
          You can place a neutral density filter (which absorbs 15% of
          energy) over a true APO and you will not see any contrast lost,
          except highest magnifications.
          But if you will place a c.o. with total 15% energy shadowed
          (15% area lost), you will see a significant contrast lost,
          because not only 15% energy lost, but also significant portion
          of Airy disk energy will be spreaded in first, second and third
          diffraction rings - THIS will decrease a contrast.


          Valery Deryuzhin.
        • andy_vollmer@yahoo.com
          ... now? ... Dear Scott, I for one have enjoyed the very informative , if somewhat overheated , discussion of the TAL-200. I doubted that Bill s assertion that
          Message 4 of 19 , Apr 1, 2001
            --- In telescopes@y..., "Scott Horne" <gsm2horn@c...> wrote:
            > Jesus, I'm sorry I asked! Can we knock off the Klevzov arguement
            now?
            >
            > Scott Horne
            > http://starjourney.net
            > Honolulu, Hawaii
            >
            >
            Dear Scott,
            I for one have enjoyed the very informative , if somewhat
            overheated , discussion of the TAL-200. I doubted that Bill's
            assertion that his Klevzov is "just as bright and twice as sharp" as
            a C 9.5 would go unchallenged . That is quite a bold statment for a
            spherically designed scope of 8" with 4.5 mm spider vanes . My 130mm
            Newt has similarly sized straight vanes and I see large diffraction
            spikes on bright objects . Irregardless of whether the vanes are
            straight or curved they still obstruct the aperature and cause a loss
            of contrast at best , large spikes like I see at worst .
            Through discussions like these I (and other 'newbies' like me) am
            learning the various experiences of these gentlemen . I think that's
            a good thing . That there are differences of opinion being strongly
            debated is also a good thing . But let us try to keep to the facts
            without getting personal as that can get ugly indeed .

            Thanks
            AndyV in Cloudyville
          • Bill Brady
            ... I try to limit my reports to what I see. I really don t know if the TAL200k is a superlative performer or not, I let other people deterimine that if they
            Message 5 of 19 , Apr 1, 2001
              andy_vollmer@... Wrote:

              >I doubted that Bill's
              >assertion that his Klevzov is "just as bright and twice as sharp" as
              >a C 9.5 would go unchallenged . That is quite a bold statment for a
              >spherically designed scope of 8" with 4.5 mm spider vanes

              I try to limit my reports to what I see. I really don't know if the
              TAL200k is a superlative performer or not, I let other people deterimine
              that if they care too.

              I noted in my initial report that the scope was different, that I was
              seeing different patterns and images, but I didn't know it was also
              different in how it would invoke controversy. As Andrea Tasselli said:

              > Indeed Yuri, it was quite a good wine. But not as strong as your brandy. In
              > a couple of weeks I can predict the TAL CO will reach the 100% mark while
              > TEC/Aries/Intes will plunge to the 0% line, only because you can't have
              > negative CO :-))
              >
              > Clear Skies
              >
              > Andrea
              >

              In any case, TAL200k scopes are now arriving at their new owners sites
              and soon someone else can take up the reporting.

              Wm. "Bill" Brady, Harwood MD 38°51'30"N 76°41'00"W - Clouds - Rain -
              Clear then WIND!
            • idalv@earthlink.net
              ... here ... length ... the ... mm. ... Hi Steve, The reason I said can t follow your logic is that what Valery said (spider area should be treated same as
              Message 6 of 19 , Apr 1, 2001
                --- In telescopes@y..., sbodenheimer@c... wrote:

                > Hi Vladimir,
                >
                > I'll be darned. Actually you do seem to agree with my logic.
                > Contrast is a function of light gathering capacity -- i.e., a
                > function of aperture area. Thus percent drop in contrast is simply
                > the same thing as losing a portion of the aperture. Bill's post
                here
                > corresponds to my measurements of 4mm thickness and your 70 mm
                length
                > is the same as I measured (using thin solder wire I bent to match
                the
                > curved spiders). Total aperture area loss is 810 sq.mm. Now that
                > corresponds to a reduction in aperture size from 200 mm to 191.7
                mm.
                > Thus this system apparently is the same as a no spider system of
                > 191.7 mm with 70 mm aperture, or 36.5% linear c.o.




                Hi Steve,

                The reason I said can't follow your logic is that what Valery
                said (spider area should be treated same as that of c. obstruction -
                the way you put it) doesn't imply that image contrast is a function
                of the remaining clear aperture area in the way that it is
                appropriate to an unobstructed aperture of that area. Those are
                different assumptions.

                As for the vanes thickness, that was an estimate on my side.
                Actually, it was closer to 4mm, but that Markus's photo in my memory
                tricked me into thinking that it has to be thicker. Doesn't make much
                difference, anyway.

                I arrived to my figure using my shortcut for contrast% and Suiter's
                mention of contrast drop caused by spider wanes being aprrox. same
                as % of their area in that of the aperture's. I don't follow your
                figures (again!), but there's little difference in the end result.

                When it comes to figuring out contrast level in an obstructed vs.
                aprropriate contrast-wise unobstructed aperture, there is no simple
                answer. It is different for high-contrast/high-brightness (Moon,
                doubles), low-contrast/high-brightness(planets) and low-contrast/
                low-brightness (nebulae) objects, with all in-betweens. For the
                first group, the obstructed is worse in the lower resolution range,
                and slightly better in the higher one (a 35% obstructed aperture here
                would have greatest contrast loss at about 1/3 of its resolution
                range, being not much better than a 40% smaller unobstructed
                aperture - note that the obstruction takes out only 12% of the area).
                For the other two, always worse, but not necessarily in the same
                degree. What about the in-betweens? It is complex enough without
                factoring in optical quality and optics' alignment, possible effects
                on contrast of either corrector, focusing mechanism, baffling,
                changing factors (personal, weather-wise, atmospheric, as Mike
                mentioned), should I go on?

                As for the spider effect, it is there and it's minor, even for
                one with 4mm thick vanes. Combined c.o./spider effect is not.
                Valery's objections have merit, although my impression is that
                he went too far towards the opposite (downgrading) end. How about
                your impressions? We all would be interested to hear.

                Vladimir
              • sbodenheimer@carolina.rr.com
                ... ... Hi Vladimir, Thank you for your detailed explanation and analysis. As you must have guessed, I am no expert on telescope design or
                Message 7 of 19 , Apr 2, 2001
                  --- In telescopes@y..., idalv@e... wrote:
                  > --- In telescopes@y..., sbodenheimer@c... wrote:

                  <chop> <chop>

                  > Hi Steve,
                  > <snip>
                  > When it comes to figuring out contrast level in an obstructed vs.
                  > aprropriate contrast-wise unobstructed aperture, there is no simple
                  > answer. <snip>
                  > It is complex enough without factoring in optical quality and
                  > optics' alignment, possible effects on contrast of either
                  > corrector, focusing mechanism, baffling, changing factors
                  > (personal, >weather-wise, atmospheric, as Mike mentioned), should I
                  > go on?
                  >
                  > As for the spider effect, it is there and it's minor, even for
                  > one with 4mm thick vanes. Combined c.o./spider effect is not.
                  > <snip>
                  > How about your impressions?
                  >
                  > Vladimir

                  Hi Vladimir,

                  Thank you for your detailed explanation and analysis. As you must
                  have guessed, I am no expert on telescope design or optics; I do have
                  a sufficient physics background and interest to be dangerous. :-)

                  I am sure that I dropped the ball on my quantitative estimate to
                  which you kindly replied. I was basing my analysis on the general
                  optical principle that the total effect of multiple obstructions is
                  simply additive, unless the shape and regularity of arrangement of
                  the obstructions is sufficient to result in interference. A single
                  central obstruction, on the other hand, inherently generates
                  interference. However, you are undoubtedly correct that with so many
                  different real world factors involved, the theoretical is a mere
                  guesstimate -- the best test is the real world.

                  I am sad that my comments may have fueled a fire that hurt
                  Bill Brady. He has such wonderful enthusiasm -- a friend once told
                  me he believed enthusiam to be the most important predictor of
                  success -- that the word itself had origins in the Greek
                  word 'theus' -- and just as 'theus' is the heart of 'enthusiasm', a
                  person with enthusiam has a heart filled with a spirt that positively
                  impacts everyone around them. Perhaps theus will bring Bill back to
                  this group.

                  Again, thanks for your analysis and help. Clear skies.

                  Steve Bodenheimer
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