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17/08/26 |
oldscope |
very difficult |
separated |
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So, at 9:30, I turned my attention to another, more difficult double star that has always eluded me due to its tight separation and dissimilar magnitudes. Also easy to find, it is Zeta Herculis, “a modestly bright third magnitude (2.90) star orbited by a sixth magnitude (5.53) companion only a second of arc or so away.” It’s the ‘bottom right’ corner of the Hercules keystone, below M-13. How many times have you looked at M-13 with some big dob and ignored this great double? The current separation is 1.28 “, not much greater than the 1.01” theoretical limit for a 4-1/2 inch lens but not too hard if you have a lot more aperture and high quality optics. The faintness of the companion, which is easily lost in the diffraction ring of the primary, requires very steady seeing, and the star needs to be viewed while high up in the sky (less atmosphere). Very high power is required and if the Airy disk and diffraction ring are bobbing around, forget it. You won’t pick out the companion. But, finally, we were in luck! At about 47 deg. altitude, the moon setting, and the air very steady (9 out of 10 seeing), I noticed the faint little companion immediately at 343x and it was distinctly separated at 429 x. Just to be sure, I asked Nancy to confirm its position and without hesitation, she identified the exact location that I saw. This is a tough object for a five inch refractor, so I was very pleased with the performance of the 4-1/2 inch Brashear lens on this difficult double star tests. Not long after, I went to look at it again, but the seeing started to deteriorate and it became all but impossible to resolve. It was as if the double was thumbing its nose at me, saying “That’s all you get for now!” |
