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From: Wolfgang Weisselberg on 7 Jun 2010 21:50 George Kerby <ghost_topper(a)hotmail.com> wrote: > On 6/5/10 7:20 AM, in article 62spd7-tk3.ln1(a)ID-52418.user.berlin.de, >> You can't see the stars during the day anyway. > Almost right: Only one. The sun is a star, but is not part of the collective "the stars" for the viewing location Earth. -Wolfgang
From: Pete on 8 Jun 2010 03:46 On 2010-06-08 02:50:00 +0100, Wolfgang Weisselberg said: > George Kerby <ghost_topper(a)hotmail.com> wrote: >> On 6/5/10 7:20 AM, in article 62spd7-tk3.ln1(a)ID-52418.user.berlin.de, > >>> You can't see the stars during the day anyway. > >> Almost right: Only one. > > The sun is a star, but is not part of the collective "the stars" > for the viewing location Earth. It's also the only visible star that's the most difficuly to see during the night. -- Pete
From: Martin Brown on 8 Jun 2010 05:48 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. > >> 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. Regards, Martin Brown
From: David J Taylor on 8 Jun 2010 08:48 "Pete" <available.on.request(a)aserver.invalid> wrote in message news:2010060812300565633-availableonrequest(a)aserverinvalid... [] > If you know of any software that would show me the twilight times, I'd > really appreciate it. My WXtrack includes all five definitions of sunrise/sunset: http://www.satsignal.eu/software/wxtrack.htm Use the View, Sunrise & Sunset menu. >> Ever hear of "The Land of the Midnight Sun"? > > Yes. For me, it would be very difficult to both get there and survive > there, but I'd love to go. If you prefer the Winter nights: http://www.satsignal.eu/Hols/2010/NorthernNorway/index.html Cheers, David
From: Gill Collins on 8 Jun 2010 09:17
On Tue, 8 Jun 2010 12:30:05 +0100, Pete <available.on.request(a)aserver.invalid> wrote: > >If you know of any software that would show me the twilight times, I'd >really appreciate it. You might try "Kalendis". http://individual.utoronto.ca/kalendis/ http://individual.utoronto.ca/kalendis/kalendis.htm I've been using it for many years, and in more recent years to create full-year TXT file lists of sun/moon rise/set, lunar-phase, twilight times to read back on my CHDK cameras using the EVF or LCD display, right from the SD card using CHDK's built-in text reader. (Save a month or year of data as comma or tab delimited file then import into a spreadsheet program to select which required columns to print.) This way I have a list for any latitude I might be at or heading to, right in my hands in my camera. It can be important information if you are hoping to catch that full-moon peeking above the horizon during sunset (or sunrise). Or if you want to have a new(er) moon illuminated by earth-shine with its bright sliver of sunlit portion in part of your frame. There's no sense heading to that summit for a spectacular photograph if the players and stage-hands for lighting aren't in the house that day. For a sample of its HTML output format (how it looks when imported into a spreadsheet as well from its .csv or .tab files): http://individual.utoronto.ca/kalendis/exports/twilight/Greenwich/jun.htm For a good page of information on "twilight" http://individual.utoronto.ca/kalendis/twilight/index.htm |