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Re: Refraction
From: Kent Nordstr�m
Date: 2008 Jul 4, 16:37 +0200
From: Kent Nordstr�m
Date: 2008 Jul 4, 16:37 +0200
Frank asked: Aha. Very nice! So that does appear to explain it. Kent, did you believe > that your refraction number was for 0 deg C or 10 deg C? Firstly I can confirm that the refraction formula given with the factor 60,35 was for the frequency approx. 5500 Angstroem (same as 520 lamda nanometer). The formula is absed on Po=1013,25x10exp2 Pa and 273,15 degr. K, which should be 0 degr. C. Kent N ----- Original Message ----- From:To: Sent: Friday, July 04, 2008 10:57 AM Subject: [NavList 5668] Re: Refraction > > Dave Walden, you wrote: > "They are functions of pressure, temperature and wavelength. Two sets of > conditions can be calculated for easy comparisons. Those included are for > the Nautical Almanac conditions and a more conventional set to conditions: > 760mb, 0 deg C, lamda .54 microns. The K =60.4 sec, should look > familiar. > The Naut Alm STP yields 58.2, also familiar." > > Aha. Very nice! So that does appear to explain it. Kent, did you believe > that your refraction number was for 0 deg C or 10 deg C? > > And you wrote: > "The formulas can be found in Spherical Astronomy by Green. This is the > "successor" to Smart's work. Green was coauthor with Smart on later > editions and finally wrote his own. (To my way of thinking, it's > better.)" > > Damn. Another book to buy. :-) What's the full title of Green's book? > > And: > "Also attached is a plot of errors of Bennett and simpler formulas > (minutes > refrac vs degrees alt). There is indeed, nothing magic about Bennett. It > get 34 min at 0 deg in case you've forgotten, but the added complication > to > do so only makes things worse at higher altitudes." > > A simple way to keep the best of Bennett (it is indeed more convenient > than > typing in the whole table) is to retain it for altitudes below 15 degrees > (or some other cut-off) and use some simple function of tan(z.d.) for > higher > altitudes. > > Anyone have any thoughts about the best color of light to use for the > refraction formulae/tables? At very low altitudes, the star images really > are smeared out substantially. What's the standard for astronomy? Is it > appropriate for sextant users? > > -FER > > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---