The data files

Degree of difficulty: rating of the observational difficulty:

1. 1 object easily visible to the naked eye

2. 2 object difficult to see with the naked eye

3. 3 object easily visible in 8×30 binoculars

4. 4 object easily visible in 10×50 binoculars

5. 5 object difficult to see with 10×50 binoculars

For more information about visual and photographic difficulty, see page 63.

Minimum Aperture: minimum aperture required to see the object under a dark mountain sky, according to the personal experience of the first author. There are four categories:

1. • naked eye

2. • 15mm

3. • 30mm

4. • 50mm

Designation: catalog number in the NGC (New General Catalogue) or the IC (Index Catalogue).

Type: Object type. For a more detailed introduction to the different types, see page 53.

Class: Classification of the object, specific to its type:

1. • Galactic nebulae: distinction between emission nebula and reflection nebula, see page 53

2. • Open clusters: Trümpler classification, see page 55

3. • Globular clusters: concentration class, see page 56

4. • Galaxies: Hubble classification scheme, see page 61

Distance: Distance from Earth in light-years. As far as possible, uniform sources have been used, i.e.:

1. • galactic nebulae and open clusters: K2005 (Kharchenko, N.V., et al.: “Astrophysical parameters of galactic open clusters,” Astronomy and Astrophysics 438, 1163 (2005)

2. • globular clusters: R2005 (Recio-Blanco, A., et al.: “Distance of 72 galactic globular clusters,” Astronomy and Astrophysics 432, 851 (2005)

3. • galaxies: H2000 (multiple authors: “The Hubble Space Telescope Key Project on the Extragalactic Distance Scale,” Astrophysical Journal 529, 698, 745, 786 (2000)

4. • Virgo cluster galaxies: V2004 (Sanchis, T., et al.: “The origin of HI-deficiency in galaxies on the outskirts of the Virgo cluster. II. Companions and uncertainties in distances and deficiencies,” Astronomy and Astrophysics 418, 393 (2004)

5. • Virgo cluster galaxies: V2002 (Solanes, J.M., et al.: “The Three-dimensional Structure of the Virgo Cluster Region from Tully-Fisher and HI data,” Astronomical Journal 124, 2440 (2002)

6. • extragalactic HII regions: HK83 (Hodge, P.W., Kennicutt, R.C., Jr.: “An atlas of HII regions in 125 galaxies,” Astronomical Journal 88, pp.