In 2004, this binary was easily separated — even in the smallest of telescopes. Any 5cm telescope can probably resolve it 50% of the time, though combination of the brightness of the pair and poor seeing, sometimes make this difficult. I have seen it resolved with 7×50 binoculars, but it had to be firmly fixed to a tripod. A minimum aperture for resolution was in 1999) between 4cm and 4.5cm. On good nights, my small 5cm refractor has little trouble with powers greater than about 25× magnification. In the city some six stars occupy a quarter of a degree field and this increasing to about ten in darker country skies. I find that both stars can easily be found during daylight and separated cleanly in apertures above about 7.5cm or more. In daylight the colours lose their yellowness, appearing like brilliant white diamonds against the blue sky. Due to the overwhelmingly bright sky, I had some trouble separating the pair with the 5cm refractor. This is reverse to my understanding of optical and double star theory as this should become easier as the images cannot “blur” together so easily. Even three attempts by me have been made and none were successful. (Maybe I was just unlucky!) To find Alpha Centauri in daylight hours, some equatorial telescope is distinctly advantageous, mainly as the position can be simply dialled up using the setting circles.
In the decades to come the decreasing separation will make resolution far more difficult. Observers may tend to make calculations on telescopic resolution using Dawes Limit where visual separation is stated as 11.58″/ Aperture (cm) or 4.54″/ A (inch) Frankly applying this is useless, as the brightness of the two stars just overwhelms the true narrow separation. Problems with this will apply to α Centauri just after 2010 AD, and between the years 2023 and 2031 AD. The minimum aperture to separate the pair when 10 arcsec apart will be 11.5cm telescope.
As apparent separations reduce below 5 arcsec, this will become even more difficult, due to the general problems with the atmospheric seeing and so-called proximity. A good comparative test of this particular problem is the other “pointer” of Hadar / Agena (β Centauri). (See below) Its value in the Dawes” equation this instance should be set to about 13.5″ / A(cm). Between 2013 to 2017, and the years 2035 and 2039, the minimum size telescope required will be at least 20cm but 25cm will have no difficulties. A neutral density filter may have to be used to cleanly separate the two stars during periods of poor seeing.
Closest approach in the orbit occurs (9th) November 2037, where the separation decreases to its meagre 1.71 arcsec on the eastern or preceding side. (PA of 112°) Changes in the position angle for five or six months reaches about 5° per month. For a few years, depending on the seeing conditions, the pair will be very hard to resolve. It is best using hexagonal diaphragm, a hexagonal shaped cover placed over the mirror or lens, or by using neutral density filters. Estimating value in the constant used in the Dawes equation will likely be between 14″/A (cm). and 15″/A (cm).
At minimum separation, H.C. Russell did measured the pair on 18th February 1878; was 1.66 arcsec, but he was using a sizeable refractor! A 25cm may be able to glimpse the duplicity, and 30cm and above will have no real problems! It is fortunate for the visual observer that within two years this difficult period of observation will soon pass.
Southern Astronomical Delights © (2012)