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From: Pete on 8 Jun 2010 16:09 On 2010-06-08 17:52:25 +0100, David J Taylor said: > "Pete" <available.on.request(a)aserver.invalid> wrote in message > news:2010060815232998946-availableonrequest(a)aserverinvalid... > [] >> Thanks very much indeed, David. >> >> I will try your WXtrack. >> >> As for holidays, my current project is to get myself fit enough to >> travel more than the 30 or 40 miles I can do at the moment. You have >> helped me to "Think Big." > > The great thing about many ships today is that they are well equipped > for disabled folk - wheelchair access, lifts etc. etc. Not saying you > have that problem, but just for others to know. It's a matter of personal pride. While I limit myself to what I can do, I can remain in denial about what I can't do. I thought photography was hard. Admitting the truth and accepting the services available it a hundred times harder. I enjoy learning - a big lesson awaits... -- Pete
From: Peter on 8 Jun 2010 19:23 "Pete" <available.on.request(a)aserver.invalid> wrote in message news:2010060815232998946-availableonrequest(a)aserverinvalid... > As for holidays, my current project is to get myself fit enough to travel > more than the 30 or 40 miles I can do at the moment. You have helped me to > "Think Big." We have different goals Mine is to get myself fit enough to travel more than the 3 or 4 miles I can do at the moment. And lose enough weight to take the mule ride in the Grand Canyon, nest year. -- Peter
From: whisky-dave on 9 Jun 2010 08:59 "Martin Brown" <|||newspam|||@nezumi.demon.co.uk> wrote in message news:GloPn.23481$%u7.19312(a)newsfe14.iad... > On 07/06/2010 13:28, whisky-dave wrote: >> "Martin Brown"<|||newspam|||@nezumi.demon.co.uk> wrote in message >> news:HS6On.18516$%u7.7097(a)newsfe14.iad... >>> 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. > >> But they are photons too. > > Doesn't help you when the optics are close to diffraction limited then the > wave nature of light is very important and cannot be ignored. This is > usually the case for most decent camera lenses at f5.6 or slower. >> >>> 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. > >> Then that must be for everything we observe. > > Yes. A star in focus just gives you the idealised point spread function > for the imaging system you have in front of the CCD detector. >> >>> 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. > >> But that is a problem with the viewing rather than the 'ray', 'wavelengh' >> or >> photon viewed. > > No. It is *intrinsic* to the physics of the problem. A finite size of > aperture necessarily gives a point spread function that is the Fourier > transform of its shape. Bigger aperture samples more of the wavefront and > resolves finer angular detail (all other things being equal). > > A disk is as good as it gets and gives the classic resolution equation > > 1.22.lambda/D > > Where D is the diameter of the lens and lambda is the chosen wavelength. > > Short wavelengths give a slightly higher resolution with the same 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. >> >> So, the planets too were once unresolverable. > > And they are a pretty good approximation to point sources where camera > lenses are concerned. My 1000mm will show Venus and Saturn clearly. Interesting although the word clearly can be misleading. here's a photo of Venus I took with my G10. http://www.flickr.com/photos/whiskydave/3466971234/ I can sill see it's a planet rather than a star. >>> They are as good an approximation to a point object at infinity as you >>> are >>> ever likely to get. >> >> True, but that's not the point. > > It is *exactly* the point. They are good enough as point sources for all > terrestrial telescopes below about 2m diameter. But first you must be sure you're looking a point source of light, my G10 can resolve Venus as not a star. The suns gravatational effect is enough to 'bend' light, so any light we observe from earth is affected by gravity, mostly it's not important, but when talking of exactly parallel rays well there's really no such thing as light bends around the universe and is 'bent' by gravity which changes slightly through space and even ion the earth we can detect changes due to local densities. And itsn't the definition of flat in optics to be less that 1/4 wavelengh of the light used to observe the object, which in the case of visable light means the Earth is flat because the Earths surfacwe curves less than the 1/4 wavelengh. IIRC So it';s all about the actual words we use, so yes according to opticians/optical engineers the Earth can be considered flat, and they sell spectacles ;-)
From: Martin Brown on 9 Jun 2010 10:19 On 09/06/2010 13:59, whisky-dave wrote: > "Martin Brown"<|||newspam|||@nezumi.demon.co.uk> wrote in message > news:GloPn.23481$%u7.19312(a)newsfe14.iad... >> On 07/06/2010 13:28, whisky-dave wrote: >>> "Martin Brown"<|||newspam|||@nezumi.demon.co.uk> wrote in message >>>> The stars you see at night with the sole exception of the nearby bright >>>> planets are all unresolved and unresolvable with any amateur equipment. >>> >>> So, the planets too were once unresolverable. >> >> And they are a pretty good approximation to point sources where camera >> lenses are concerned. My 1000mm will show Venus and Saturn clearly. > > Interesting although the word clearly can be misleading. > here's a photo of Venus I took with my G10. > http://www.flickr.com/photos/whiskydave/3466971234/ > I can sill see it's a planet rather than a star. Actually you can't. Venus is so very much brighter than the illuminated moon surface that it is massively over exposed and spread into several adjacent pixels. Think brilliant white cloud for Venus and dark basalt or tarmac for the moons surface - both in direct sunlight but Venus somewhat closer to the sun and so more brightly illuminated. You need a roughly 3" aperture and 15" focal length to get Venus clearly resolved. See for example the images of the moon and venus conjunction of 19 May 2007 complete with varying exposures. <http://www.trivalleystargazers.org/gert/moon_venus_20070519/moon_venus_20070519.html> Actually done with a 4" aperture which is a bit overkill. > >>>> They are as good an approximation to a point object at infinity as you >>>> are >>>> ever likely to get. >>> >>> True, but that's not the point. >> >> It is *exactly* the point. They are good enough as point sources for all >> terrestrial telescopes below about 2m diameter. > > But first you must be sure you're looking a point source of light, > my G10 can resolve Venus as not a star. When overexposed it gives a bigger blob for a point object. That is not resolved. To get an idea of how much detail would be visible on the moon if you had resolved Venus take a look at the URL above. There is an outside chance to photograph Venus with a really high quality 2" working aperture, a suitably long focal length and a very steady tripod when it is at its largest apparent size - when it will look crescent shaped. But that is not what you have recorded. It is easier with binoculars. A good pair of 10x50s will just show Venus at thin crescent phase and Jupiters satellites > The suns gravatational effect is enough to 'bend' light, so any light we > observe from earth is affected by gravity, mostly it's not important, but > when talking of exactly parallel rays well there's really no such thing as > light bends around the universe > and is 'bent' by gravity which changes slightly through space and even ion > the earth we can detect changes due to local densities. The extent to which the sun bends light rays is barely measurable even with the best professional optical gear available. Modern measurements rely on VLBI radio astronomy. > > And itsn't the definition of flat in optics to be less that 1/4 wavelengh of > the light > used to observe the object, which in the case of visable light means the > Earth is flat because the Earths surfacwe curves less than the 1/4 > wavelengh. IIRC I don't know who sold you that one. Diffraction limited is sometimes considered to set in when optical surfaces are within lambda/4 of the ideal perfect shape. But most serious optics are more like lambda/10. > So it';s all about the actual words we use, so yes according to > opticians/optical engineers the Earth can be considered flat, and they sell > spectacles ;-) Spectacle makers may well think the Earth is flat. They must find new members of the flat Earth society from somewhere. Regards, Martin Brown
From: bugbear on 9 Jun 2010 10:53
whisky-dave wrote: > > Interesting although the word clearly can be misleading. > here's a photo of Venus I took with my G10. > http://www.flickr.com/photos/whiskydave/3466971234/ > I can sill see it's a planet rather than a star. In that image (at 768x1024) the moon is around 39 pixels across, so roughly speaking (the moon is around 31 minutes of arc) we're at a pixel per arc-minute. Venus varies from 10 - 69 arc *seconds*, so Venus should be around a single pixel. Well, on the image, Venus is a bit fuzzy, but even the central fully exposed area is 8 pixels. I don't that image resolves Venus very well. BugBear |