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From: Martin Brown on 4 Jun 2010 08:55 On 03/06/2010 09:33, DanP wrote: > On 2 June, 16:44, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com> > wrote: > >>>> Lenses are imperfect, there's diffraction (and air for us down >>>> here) and stars are definitively larger than 0, just very far away. >>>> Stop waffling and try it. >>> So there is no source of light with perfect parallel rays. > > Your previous quote: "stars are definitively larger than 0, just very > far away." is incompatible with parallel rays. They are close enough to parallel as makes no difference (apart from our sun which subtends about half a degree). The largest stars are a few tens of milliarcseconds across as viewed from the Earth. It is right on the limits of our best optical telescopes to resolve them. Most stars are considerably smaller in angular size. Betelgeuse is just about resolved by the HST it is 20,000 smaller in diameter than the width of the sun as seen from Earth. http://www.spaceimages.com/atofbet.html To all intents and purposes at all ordinary apertures available to photgraphers and amateur astronomers they are ideal point sources with a bit of annoying phase screen in front from the atmosphere. > >> Both are f/4 at maximum aperture. Does that change your claim? >> Or does "the amount of light let in depends only of exposure >> time and f number" hold? > > No, both set at f/4 will let in the same amount of light. That is clearly an insane position to take. What you say is only sort of correct for an image formed by extended objects that are resolved. The key point about stars is that they are unresolved. A star remains unresolved even in the best lenses or telescopes and so all the light that hits the aperture ends up in the image. The larger lens captures more light and gives a smaller image Airey disk. The rough rule of thumb breaks down a bit when the scope is much bigger than 10" aperture as seeing starts to be the limiting factor and the star images stay roughly constant angular size after that point. Astronomers always want bigger apertures to capture more light. http://www.scopecity.net/LightGrasp.php It doesn't help on extended objects the laws of physics see to that but for a point source like a star the bigger aperture always wins. Regards, Martin Brown
From: Martin Brown on 4 Jun 2010 09:05 On 04/06/2010 13:46, whisky-dave wrote: > "Wolfgang Weisselberg"<ozcvgtt02(a)sneakemail.com> wrote in message > news:q2lld7-h5u.ln1(a)ID-52418.user.berlin.de... >> You need Earth's orbit as a baseline to detect differences >> for close stars. > > But the problem is how to you get two exactly parallel 'rays'. There are no such things as light rays they are a useful construction for computing with geometrical optics. The light coming from distant stars arrives as a wavefront and the shape of the image that is formed is determined by the shape of the aperture that it goes through. That is why certain number of leaves of diaphram are preferred. If you want to see some brutal diffraction effects try putting a small square aperture mask in front of your longest telephoto lens. The stars you see at night with the sole exception of the nearby bright planets are all unresolved and unresolvable with any amateur equipment. They are as good an approximation to a point object at infinity as you are ever likely to get. The largest of them subtend angles 1/20000 that of the sun and most are at least another order of magnitude smaller. Regards, Martin Brown
From: James Nagler on 4 Jun 2010 09:11 On Fri, 04 Jun 2010 13:55:52 +0100, Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote: >The rough >rule of thumb breaks down a bit when the scope is much bigger than 10" >aperture as seeing starts to be the limiting factor and the star images >stay roughly constant angular size after that point. It is suggested that for more than 99% of the time a 16" dia. optic will not resolve details any better than the 200" dia. telescope at Mt. Palomar due to atmospheric turbulence ("seeing"), unless that telescope is used for observing from high altitudes. It helps to put things into perspective for those who suffer from aperture-fever when it comes to resolving power.
From: J. Clarke on 4 Jun 2010 09:36 On 6/4/2010 8:55 AM, Martin Brown wrote: > On 03/06/2010 09:33, DanP wrote: >> On 2 June, 16:44, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com> >> wrote: >> >>>>> Lenses are imperfect, there's diffraction (and air for us down >>>>> here) and stars are definitively larger than 0, just very far away. >>>>> Stop waffling and try it. >>>> So there is no source of light with perfect parallel rays. >> >> Your previous quote: "stars are definitively larger than 0, just very >> far away." is incompatible with parallel rays. > > They are close enough to parallel as makes no difference (apart from our > sun which subtends about half a degree). The largest stars are a few > tens of milliarcseconds across as viewed from the Earth. It is right on > the limits of our best optical telescopes to resolve them. Most stars > are considerably smaller in angular size. > > Betelgeuse is just about resolved by the HST it is 20,000 smaller in > diameter than the width of the sun as seen from Earth. > > http://www.spaceimages.com/atofbet.html > > To all intents and purposes at all ordinary apertures available to > photgraphers and amateur astronomers they are ideal point sources with a > bit of annoying phase screen in front from the atmosphere. >> >>> Both are f/4 at maximum aperture. Does that change your claim? >>> Or does "the amount of light let in depends only of exposure >>> time and f number" hold? >> >> No, both set at f/4 will let in the same amount of light. > > That is clearly an insane position to take. What you say is only sort of > correct for an image formed by extended objects that are resolved. > > The key point about stars is that they are unresolved. > > A star remains unresolved even in the best lenses or telescopes and so > all the light that hits the aperture ends up in the image. The larger > lens captures more light and gives a smaller image Airey disk. The rough > rule of thumb breaks down a bit when the scope is much bigger than 10" > aperture as seeing starts to be the limiting factor and the star images > stay roughly constant angular size after that point. > > Astronomers always want bigger apertures to capture more light. > http://www.scopecity.net/LightGrasp.php > > It doesn't help on extended objects the laws of physics see to that but > for a point source like a star the bigger aperture always wins. I think that you need to investigate the assumptions being made with those limiting magnitude calculations. Comparing telescopes to photographic lenses is comparing apples to oranges. Photographic lenses are used with the primary lens projecting directly on the sensor, astronomical telescopes almost always use secondary lenses.
From: James Nagler on 4 Jun 2010 10:19
On Fri, 04 Jun 2010 09:36:19 -0400, "J. Clarke" <jclarke.usenet(a)cox.net> wrote: >On 6/4/2010 8:55 AM, Martin Brown wrote: >> On 03/06/2010 09:33, DanP wrote: >>> On 2 June, 16:44, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com> >>> wrote: >>> >>>>>> Lenses are imperfect, there's diffraction (and air for us down >>>>>> here) and stars are definitively larger than 0, just very far away. >>>>>> Stop waffling and try it. >>>>> So there is no source of light with perfect parallel rays. >>> >>> Your previous quote: "stars are definitively larger than 0, just very >>> far away." is incompatible with parallel rays. >> >> They are close enough to parallel as makes no difference (apart from our >> sun which subtends about half a degree). The largest stars are a few >> tens of milliarcseconds across as viewed from the Earth. It is right on >> the limits of our best optical telescopes to resolve them. Most stars >> are considerably smaller in angular size. >> >> Betelgeuse is just about resolved by the HST it is 20,000 smaller in >> diameter than the width of the sun as seen from Earth. >> >> http://www.spaceimages.com/atofbet.html >> >> To all intents and purposes at all ordinary apertures available to >> photgraphers and amateur astronomers they are ideal point sources with a >> bit of annoying phase screen in front from the atmosphere. >>> >>>> Both are f/4 at maximum aperture. Does that change your claim? >>>> Or does "the amount of light let in depends only of exposure >>>> time and f number" hold? >>> >>> No, both set at f/4 will let in the same amount of light. >> >> That is clearly an insane position to take. What you say is only sort of >> correct for an image formed by extended objects that are resolved. >> >> The key point about stars is that they are unresolved. >> >> A star remains unresolved even in the best lenses or telescopes and so >> all the light that hits the aperture ends up in the image. The larger >> lens captures more light and gives a smaller image Airey disk. The rough >> rule of thumb breaks down a bit when the scope is much bigger than 10" >> aperture as seeing starts to be the limiting factor and the star images >> stay roughly constant angular size after that point. >> >> Astronomers always want bigger apertures to capture more light. >> http://www.scopecity.net/LightGrasp.php >> >> It doesn't help on extended objects the laws of physics see to that but >> for a point source like a star the bigger aperture always wins. > >I think that you need to investigate the assumptions being made with >those limiting magnitude calculations. Comparing telescopes to >photographic lenses is comparing apples to oranges. Photographic lenses >are used with the primary lens projecting directly on the sensor, >astronomical telescopes almost always use secondary lenses. How many elements in how many groups do you think are behind that main objective lens in an average camera-lens design? |