DOUBLES.TXT Double stars that are close enough to be suitable for the testing of small telescopes tend to show orbital motion from year to year. For that reason, it is important to have separations and position angles which are calculated for the current year. This file contains predictions for a number of double stars appropriate for the testing of telescopes in the 3" to 30" range. This information is taken, with permission, from the Handbook of the British Astronomical Association for the years 1986 through 1998, and from John Isles, who performed the calculations. Abbreviations: OE = Otto Struve Catalog B = Aitken Catalog Hu = Hussey Catalog I = Innes Catalog h = John Herschel Catalog 2000.0 1998.0 1999.0 RA DEC MAGS PA SEP(") PA SEP(") 85 Peg 0:02.2 +27:05 5.8 8.9 164.0 0.76 173.9 0.75 (1) OE4 0:16.7 +36:30 8.2 8.9 153.0 0.43 151.3 0.42 B395 0:37.3 -24:46 6.3 6.4 271.5 0.28 282.8 0.42 (2) 36 And 0:55.0 +23:38 6.0 6.4 309.7 0.90 311.5 0.92 10 Ari 2:03.7 +25:56 5.9 7.3 344.0 1.11 344.8 1.12 7 Tau 3:34.4 +24:28 6.6 6.7 0.5 0.74 2.0 0.75 (3) OE65 3:30.3 +25:35 5.8 6.2 10.5 0.08 20.3 0.13 (4) Hu445 5:01.7 +20:50 8.6 8.9 296.8 0.45 298.3 0.45 14Lyn 6:53.1 +59:27 5.6 6.8 268.4 0.45 269.4 0.45 9Pup 7:51.8 -13:54 5.6 6.2 316.8 0.36 324.3 0.30 (5) Zeta CncAB 8:12.2 +17:39 5.6 6.0 99.9 0.77 92.8 0.80 B208 8:39.1 -22:40 5.3 6.7 29.6 1.57 30.0 1.60 I 314 8:39.4 -36:36 6.5 7.6 252.4 0.27 250.5 0.32 (6) OE235 11:32.3 +61:05 5.8 7.1 330.9 0.62 336.1 0.63 I83 12:56.7 -47:41 7.4 7.6 226.9 0.85 227.2 0.86 A1609 AB 13:25.8 +44:29 9.0 9.1 0.7 0.46 3.9 0.47 Zeta Boo 14:41.1 +13:44 4.5 4.6 300.1 0.84 299.8 0.82 17 Lib 14:46.2 -21:11 7.1 7.3 107.9 0.24 118.5 0.27 h4707 14:52.2 -66:25 7.6 7.9 293.3 0.84 291.7 0.86 Eta CrB 15:23.2 +30:17 5.6 5.9 52.9 0.89 57.6 0.83 Gam Lup 15:35.1 -41:10 3.5 3.6 274.0 0.68 273.8 0.67 Pi2 UMi 15:39.6 79:59 7.4 8.2 24.9 0.75 24.8 0.75 Xi ScoAB 16:04.4 -11:22 4.9 4.9 255.1 0.23 288.4 0.30 I253 19:19.0 -33:17 7.6 7.7 318.3 0.12 316.4 0.15 Lam Cyg 20:47.4 +36:29 4.9 6.1 6.8 0.89 6.4 0.89 4 Aqr 20:51.4 - 5:38 6.4 7.2 20.4 0.83 21.1 0.82 Eps Equ AB 20:59.1 + 4:18 5.8 6.1 284.5 0.87 284.4 0.85 Tau Cyg 21:14.8 +38:03 3.8 6.4 318.9 0.77 312.6 0.77 A 632 22:52.0 +57:43 8.6 9.1 145.9 0.51 143.7 0.49 B80 23:18.9 + 5:24 8.5 9.1 158.3 0.20 181.6 0.27 72 Peg 23:34.0 +31:20 5.7 5.8 95.7 0.53 96.5 0.53 (1) 85 Peg pased periastron in 1989, when it was a test for a 12" scope. Now, it should be an easy object in a 6". See the file 85PEG.GIF at ftp.mindspring.com/users/labbey. (2) B395 is closing very rapidly. As it does so, it is sweeping through the range of magnitudes suitable for testing small instruments: Year Test for 1991 6" 1992 7" 1993 8" 1994 10" 1998 18" (3) 7 Tau is now an almost perfect test for a 6". It is slightly below Dawes' Limit (see discussion below). (4) OE65 is now widening rapidly. In 1998 it is a test for a 60". In 1999 a 30" should show it. In the next few years it will become a test for a small instrument. It will be interesting to see who can detect it first. (5) 9 Pupis is past maximum separation, and is closing noticeably. When this file was first published, ten years ago, this star was a test for a 24". It widened until it was visible in a 6", and should now be a good exercise for an excellent 12.5". See the file 9PUP.GIF at ftp.mindspring.com/users/labbey for a diagram of this system. (5) In the early issues of this file, I314 was an excellent test for instruments in the 10" range. As the companion reached apastron the pair was effectively out of range for all but giant telescopes. Between 1991.0 and 1994.0 the companion traversed almost 205 degrees of it's orbit! Who said that the stars don't move? Now that the pair is widening again, it will be a good test for a 12.5" in 1999. See the file I314.GIF at ftp.mindspring.com/users/labbey for a diagram of this system. * * * * * * * * * * * A NOTE ON RESOLVING POWER The accepted empirical resolution limit of visual telescopes (Dawes' Limit) is as follows for visual instruments: Closest Star Aperture Resolved 50mm 2.31" 60mm 1.92 80mm 1.44 4" 1.14 4.25 1.07 5 0.91 6 0.76 8 0.57 10 0.45 12.5 0.36 14 0.32 16 0.29 17 0.27 20 0.23 24 0.19 29 0.16 Dawes' Limit actually applies to the case of two yellow stars, both of magnitude 6.0, seen against a perfectly black background. However, the limit reasonably predicts the performance of excellent optics under other conditions, so it is generally accepted as the "resolving power" of a telescope. For reference, my 10" f/8 Newtonian resolves to about 0.42 sec. I define resolution as that degree of separation which will show a dimple in the side of the "figure 8" appearance of the two overlapping Airy disks. Leonard Abbey Atlanta Astronomy Club Labbey@mindspring.com July 21, 1998