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Southern Cross - September 1999

Constellation of the Month - Microscopium

Albert Brakel

Riding high across our zenith at this time of year is a rarely visited constellation south of Capricornus called Microscopium, the microscope. It was invented by Lacaille in 1752, and is one of several of his creations named after various instruments and tools. The brightest star, Theta-1 Microscopii, is only magnitude 4.8. This unremarkable patch of sky does not get much attention because, apart from some double stars, the only telescopic targets are a scattering of small, faint galaxies.

I found that using an 8" (20 cm) SCT from suburban Downer put me at a disadvantage, because the brightest galaxy, NGC 6925 (RA 20h 34.3m, Dec. -31 59'), was too faint to detect at mag. 11.3. It is described as an edgewise spiral, appearing as a hazy ellipse about 2.5' long. I tried also for IC 5105, just NE of the Theta-1/Theta-2 optical pair, and NGC 7057, just west of the binary Melbourne 6, with similar results, but larger scopes should be more successful.

So on to the double stars. Alpha Mic (20h 50.0, -33 47'), which despite its designation can't manage a total magnitude of better than 4.9, is easily resolved at low power into a mag. 5.0 yellow primary with a mag. 9.5 ashen companion a considerable 21" distant to the south.

Theta-2 Mic, or Burnham 766 (21h 24.4m, -41 00'), could not be divided with any magnification up to 235x. The separation of the stars has varied over the years. In 1962 Hartung reported 0.8", when they could be split with 20 cm aperture, but by 1980 they had closed to 0.3", when 30 cm couldn't resolve them.

Melbourne 6 (21h 27.0m, -42 33'), an unequal mag. 5.7 and 8.3 pair of yellow and white components at 2.8", was just resolved with 117x, but more convincingly with 154x.

Border Raids

With all the accessible objects in Microscopium accounted for, I strayed across the border into neighbouring southeastern Sagittarius, another region rarely looked at, in search of more objects of interest. The galaxies here are no brighter than in Microscopium, so I didn't even attempt them, but larger apertures could try IC 4991, IC 4946, NGC 6890, and NGC 6902, all in the far SE corner of the constellation.

Four double stars are worth a look. h5188 (20h 20.5m, -29 11') lies in a quite a nice field of stars, and consists of a mag. 6.4 white star with a mag. 9.5 companion following it at 4.0". The latter is quite faint, and could only be seen with 117x or higher power. The pair points to another binary system not far away (2.5') to the NE in the same field, that has equal (m10.5 and 10.8) members at 4.7" separation and easy to tell apart with 77x.

In h5173 (20h 11.2m, -36 06') the orange-yellow A star (mag. 5.3) is impressive, but the mag. 12.0 partner could not be seen at all, despite being a generous 8.3" away. Although grossly unequal pairs are often difficult, with a spacing like this a resolution should have been possible, and failure to sight the B star was probably because it was below the limiting magnitude for the night.

The white primary of Burnham 763 (Kappa Sgr) (20h 23.9m, -42 25') could only just be split from its fainter attendant with 235x magnification. The magnitudes are 5.9 and 7.5. They were 1.2" apart in 1879 and about 1" in 1960, and judging by the high power need to separate them now, have since drawn closer.

So this is not a particularly exciting region if you're after targets other than double stars, but next month we'll have something more diverse and interesting.

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