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Double Star Question
I have a question regarding the magnitude differences between stars in
a double and how it affects the ability to split them. One of my main reference books is an older one, "How to use an Astronomical Telescope," by James Muirden. In the chapter on double/multiple stars, he gives the basic information: magnitudes, seperations, etc., and a brief discussion about each. I understand that the seperations are mainly for 1985, but that's not important for my question. Under 38 Lyncis (AB 3.9, 6.6; 2".8), he writes, "This close pair can just be divided with an aperture of 100mm, but it is more distinct with larger instruments." Under v Ursae Majoris (AB 3.7, 10.1; 7".2), "The primary is a lovely full yellow, but the companion may be found difficult to see with an aperture of less than 150mm or so." The second pair is more difficult to split, even though it's relatively much wider than the first, because of the difference in magnitudes between the doubles in each pair. I understand the basics of the resolving power of differing apertures, that's not the question. What I'd like to know is, is there any "rule of thumb" that one can use to determine the difficulty or even the possibility of splitting two stars that are "x" magnitude at "y" distance apart in any given aperture telescope (in my case, a 114mm refractor)? BTW, does anyone know of a reference source (book, website) that can tell me the current (or close enough) distances between double stars? Thanks! |
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Double Star Question
"Lisa" wrote in message om... I have a question regarding the magnitude differences between stars in a double and how it affects the ability to split them. No..theoretical angular resolution has nothing to do with magnitude. Only aperture. The minimum resolvable angular separation= 206265 (wavelength of light/D) |
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Double Star Question
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Double Star Question
"Mick" wrote in message
... "Lisa" wrote in message om... I have a question regarding the magnitude differences between stars in a double and how it affects the ability to split them. No..theoretical angular resolution has nothing to do with magnitude. Only aperture. No.. she asked about real stars. Unequal magnitudes can make a big difference. See Sirius for example. The minimum resolvable angular separation= 206265 (wavelength of light/D) Formulas make little sense without specifying the units. Generally, few people use wavelength and would be more familiar with mm or inches. That is (without getting too technical about airy disks etc), 4.5 divided by the aperture (in inches) gives resolvable separation in arc seconds. Of course, this is assuming decent optics and fair observing conditions. Top optics and very stable air will do a little better. Of course, once you move beyond 4-6" seeing will usually limit you before aperture does. You might also try the doubles group at: http://groups.yahoo.com/group/binary-stars-uncensored Clear Skies Chuck Taylor Do you observe the moon? Try the Lunar Observing Group http://groups.yahoo.com/group/lunar-observing/ |
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Double Star Question
Lisa,
magnitude differences will have a significant influence on the difficulty of splitting doubles. A few general rules: The closer they are to each other in magnitude the easier they will be. Even if they are similar in magnitude, the closer they are to the limiting magnitude of your scope the more difficult they will be. One post above refers to the wavelength of light formula. That formula boils down to the Rayleigh criterion. 5.45/D inches (138/Dmm). Without the technical gargon, It is a measure that gives a good indication of what you could see "split". it measures the distance between the centers of two stars Airy disks separated by the radius of the Airy disk. Since the central bright spot is slightly smaller than the Airy disk, this usually shows a fine split. The other numerical reference made above, actually 4.56/D (116/Dmm) is Dawes Limit. It gives a measure of the limit you would be able to see two stars as double if you separate the centers of two stars by the less than the radius of the Airy disk, greater than 50% overlap. This may of may not show a fine split. Neither Rayleigh or Dawes provides a true measurement of a split between two stars. Only if the light in the central bright spot of the Airy disk is small enough will you be able to SEE a split. The answers these two formula give as results is in arcseconds of angular measure. A great reference book for double stars and other deep sky objects is Sky Catalogue 2000.0 Volume 2, Double Stars, Variable Stars and Non Stellar Objects. edz |
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Double Star Question
edz wrote in message . .. Lisa asked in message . .. BTW, does anyone know of a reference source (book, website) that can tell me the current (or close enough) distances between double stars? edz replied - Sky Catalogue 2000.0 Volume 2, Double Stars, Variable Stars and Non Stellar Objects. W Hamblin replied - Look for the Washington Double Star Catalog. Several computer programs include WDS data. Cartes du Ciel (free download!) is one. The WDS itself is on line. Lisa, in addition to these sources, I found the following useful: Mullaney, Jack. 1998-2002. Celestial Harvest. Dover Books. Mullaney culled 300 celestial objects from a number of catalogues, but he has a "thing" for doubles and multiple systems. Half his catalogue are binaries and multiples. His binaries were chosen for asthetic value - e.g. color contrasting pairs like eta Cas and itoa Cas or the classic beta Cygnus (Albireo). Visually striking binaries are typically those with wide spectral classes like O (white) and B (blue white) contrasted with G (yellow) or M (red). You may have to order this book from a used book dealer. |
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Double Star Question
Hello there Lisa,
In Jan 2002 issue of S&T, there are two articles about double star by two experts, Luis Arguelles and Sissy Haas. I think they'll answer your question very nicely. Unequal doubles can be quite challenging: Antares is one that comes to mind. Ron B[ee] -------------- "Lisa" wrote in message om... I have a question regarding the magnitude differences between stars in a double and how it affects the ability to split them. |
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Double Star Question
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