Skyguide - Selected objects for every season
The Skyguide is primarily intended to give suggestions for your own observations and will briefly describe selected objects for each season of the year. Both easy and difficult objects will be selected. How difficult an object is depends of course on several factors, especially sky quality, telescope aperture and experience of the observer.
For each object the most important information is given in short form. These are supplemented by photos or sketches. Furthermore, a map, created with the free software Cartes du Ciel (Skychart), is available for a rough orientation. In general, I recommend, to create your own finder charts. The visual description of the object is based mainly on own observations and should serve only as a starting point.
Skyguide 2024-3 (Autumn)
Although small and rather inconspicuous, the constellation Triangulum is located in prominent surroundings and is therefore easy to find between Andromeda and Aries. The brightest star is just 3m.0. The brightest deep-sky object is Messier 33 (Triangulum Nebula) and can also be seen with the naked eye under a dark, transparent sky. But today we want to focus on smaller objects.
We start with the physical double star Iota Trianguli (6 Tri), whose components (mv = 5m.3 / mv = 6m.7) are separated by an angular distance of 4 arc seconds, wide enough to be separated even with the smallest telescope. The colors of the components are usually described as yellowish-orange and white. If the magnification is not too high, the double star STF 232 to the east can be observed together with Iota Trianguli at a separation of about 30 arcminutes. Its almost equally bright components (mv = 7m.8 / mv = 7m.9) are somewhat further separated by 6.7 arc seconds. Both double stars together provide a really rewarding sight.
Next we come to the asterism Collinder 21. Although many people certainly don't like things by halves, this asterism is definitely convincing. About 8 stars with magnitudes of 8m.2 to 11m.6 form a beautiful, striking semicircle. It begins in the south with the brightest star and runs over the west to the north. A conspicuous pair of stars is located just above half of the semicircle. Due to the brightness, owners of smaller telescopes should also be able to observe this asterism with ease. There are hardly any brighter stars in the immediate vicinity. The galaxy IC 1731 (mv = 13m.4), which is located to the north of Collinder 21, can also be reached under dark skies with a sufficiently large telescope. However, for most people it will only remain a faint, oval nebula. Nevertheless, it is worth a look.
Those who like nebulous objects will certainly enjoy Arp 166, an interacting pair of quite compact galaxies. The northern NGC 750 (mv = 11m.9) is somewhat larger and brighter than the southern NGC 751 (mv = 12m.5). The patient observer will be able to see these two galaxies under dark skies with an aperture of just 4 inches, even if it is only a small, faint nebula. With an aperture of about 8 inches, a separation of the two galaxies is conceivable. At least Arp 166 is then visible as an oval nebula, with the northern part appearing brighter. The separation succeeds at the latest with an aperture of 12 inches and high magnification, as the sketch by René Merting shows. A faint extension to the northwest can also be seen on long-exposure images, which is why Halton Arp included this pair of galaxies in the group of galaxies with diffuse filaments. This extension is a visual challenge and requires excellent conditions and a very large telescope. Uwe Glahn was able to successfully observe this detail with a 27-inch Dobsonian.