I bet no one knows enough to answer THIS question... !
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From: jgh on 19 Jul 2010 06:19 On Thu, 15 Jul 2010 20:48:43 -0700, Paul Furman wrote: > In the case of an enormous lens > or let's say telescope mirror... there might be significant savings but > I never heard of a telescope built that way... maybe it's been done? Not square, but hexagonal - so you can tile segments, when it's too big to build in one piece. And hex isn't as far away from circular as square or rectangle. - Jeremy
From: Martin Brown on 19 Jul 2010 06:44 On 19/07/2010 11:19, jgh wrote: > On Thu, 15 Jul 2010 20:48:43 -0700, Paul Furman wrote: >> In the case of an enormous lens >> or let's say telescope mirror... there might be significant savings but >> I never heard of a telescope built that way... maybe it's been done? Some early radio telescopes were built that way in the days before we had massive computer power available for processing results. Most notably the phased array that discovered pulsars at Cambridge. Basically the diffraction effects from a square or rectangular aperture hurt too much. You get best value for money with a circular aperture or as close to that ideal as you can reasonably get. That is why some designs of camera lens iris work better than others. You can try out a square aperture with a cardboard mask in front of your camera lens with some loss of field of view as well. The effect is not as brutal as a 4-star cross filter but is in the same vein: http://www.hoyafilter.com/products/hoya/pro1d-08.html > > Not square, but hexagonal - so you can tile segments, when it's > too big to build in one piece. And hex isn't as far away from > circular as square or rectangle. Plenty of radio telescopes are made of rectangles or close to rectangles of curved sheet metal or mesh. Regards, Martin Brown
From: Paul Furman on 19 Jul 2010 11:23 Martin Brown wrote: > On 19/07/2010 11:19, jgh wrote: >> On Thu, 15 Jul 2010 20:48:43 -0700, Paul Furman wrote: >>> In the case of an enormous lens >>> or let's say telescope mirror... there might be significant savings but >>> I never heard of a telescope built that way... maybe it's been done? > > Some early radio telescopes were built that way in the days before we > had massive computer power available for processing results. Most > notably the phased array that discovered pulsars at Cambridge. > > Basically the diffraction effects from a square or rectangular aperture > hurt too much. You get best value for money with a circular aperture or > as close to that ideal as you can reasonably get. That is why some > designs of camera lens iris work better than others. > > You can try out a square aperture with a cardboard mask in front of your > camera lens with some loss of field of view as well. The effect is not > as brutal as a 4-star cross filter but is in the same vein: > > http://www.hoyafilter.com/products/hoya/pro1d-08.html >> >> Not square, but hexagonal - so you can tile segments, when it's >> too big to build in one piece. And hex isn't as far away from >> circular as square or rectangle. > > Plenty of radio telescopes are made of rectangles or close to rectangles > of curved sheet metal or mesh. Oh, here's the square flare from my old super-8: http://edgehill.net/Southwest/1995-paria-canyon/pg2pc11
From: Scotius on 28 Jul 2010 17:03
On Wed, 14 Jul 2010 22:36:14 -0400, krishnananda <krishna(a)divine-life.in.invalid> wrote: >In article <i1loqb$36i$2(a)news.eternal-september.org>, > John McWilliams <jpmcw(a)comcast.net> wrote: > >> Scotius wrote: >> > Why is it that camera lenses are round, but pictures are >> > square? Hmmm? Tell me that ya' smarteys! >> >> It's a mystery that'll never, ever be explained. > >The Nikon 8mm 180-degree fisheye produces a circular image on >rectangular film, as do other non-full-frame fisheyes. > >But a much more important question is why are hot dogs sold in packs of >10 but hot dog buns are sold in packs of 8? > >In philosophy this is known as the "Hot Dog Dilemma". It's a very irksome dilemna, whatever you call it. |