IC 166, Open Star Cluster
IC 166 is a very faint but star-rich open cluster in the well-known nothern constellation Cassopeia. It was discovered in 1890 by the British astronomer
William Denning.
Its distance to Earth is given as 15,500 light-years - with an uncertainty of +-1,600 light-years. The apparent diameter of the cluster is 8 arcminutes,
that of the central region about 5 arcminutes. This corresponds to a diameter of 23 light-years - so IC 166 is quite large. The brightest stars of IC 166
have a magnitude of 14mag, but the majority is 16mag and below. There are about 200 stars in the cluster, whose age is about one billion years.
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Even in my 20 inch f/3 Dobson with its large aperture IC 166 is not one of the easiest objects to observe. On the one hand this is because it has a quite
large apparent extent, on the other hand it is quite faint with a visual magnitude of 11.6mag. As a consequence the surface brightness of IC 166 is very
low, so it needs a dark sky to see the cluster at all. With a brightened sky background it would become invisible.
At 120x a faint brightening is visible with 4 stars in front of it. Increasing the magnification to 270x reveals more details. So the softly glowing background
appears grainy and in moments with calm air very faint single stars flash out from time to time, of which about 15-20 can be held with indirect vision.
Overall IC 166 appears quite faint with a roundish shape.
