The Hickson catalog is a collection of 100 galaxy groups identified by Dr. Paul Hickson in his 1982 paper, “Systematic Properties of Compact Groups of Galaxies”.  Hickson compact galaxy groups are small and distant relative to most other deep space objects.  

A typical Hickson group has 4 galaxies, but can have up to 8.  The distance of these groups from us range from 40 million to almost 2 billion light years away.

The detailed criteria for a Hickson Galaxy Group are defined in the paper.  Without showing the equations here, a Hickson group is defined by 3 basic traits: 
     1. The group must have at least 4 galaxies, all with decent brightness.
     2. The group must be isolated and not just part of a large cluster. 
     3. The group has to be compact, with spacing between them tight (about a galaxy width).

Note that distance is not part of the criteria above.  Often, Hickson galaxies of the same group will be at different distances and not be part of a true cluster, instead simply a line-of-sight alignment cluster.  Conversely, galaxies which are truly part of the cluster but are either too dim or too far away visually (at the edge of the image) will not meet the criteria to be part of the Hickson group.  This doesn't quite seem right, but that is how the Hickson groups were defined.

The galaxies of these groups are generally small, distant and a challenge to image. The apparent diameter of a circle which surrounds each entire group ranges from 1 arc-minute to 16 arc-minutes.  In comparison, the apparent diameter of the single galaxy M31 is 200 arc-minutes. 

The 100 Hickson galaxies are show here in 4 posters in Revisions H through K.  The 4 posters each show 25 Hickson groups.  On each group image, the individual group galaxies are identified by letter, starting with 'a' for the brightest galaxy in each group (which is usually about magnitude 13).

For each image, the following 7 group parameters are shown on the line below the Hickson Group number:
1. Number of galaxies in the group
2. Object declination in degrees
3. Group diameter in arc-minutes
4. Group magnitude
5. Distance of galaxy ‘a’, in millions of light years
6. Diameter of galaxy ‘a’, in thousands of light years
7. Is this a true cluster? (Are all galaxies at the same distance?)

Below the data, the white dashed line is scaled to be 1 arc-minute long as a visual reference.         

The 5th poster (Revision M) shows my 10 favorite Hicksons at a larger scale.  Here are the 10 Hicksons included and the reason for each:

• Hickson 27 & 55:  Two of the best Hickson galaxy chains.
• Hickson 40:  Beautiful arrangement of 5 galaxies, 2 of which are spirals.
• Hickson 44:  The largest group in areal extent (16.4 arc-minutes), with a lovely yellow edge-on spiral and a blue distorted spiral.
• Hickson 57:  The largest number of galaxies (8), called Copeland's Septet.
• Hickson 61:  The beautiful Box formation, including the largest Hickson galaxy in apparent size ('b' is 4.5 arc-minutes long)
• Hickson 68: A large, bright galaxy grouping with some of the best color of any Hickson group
• Hickson 79: The amazing Seyfert's Sextet, a triangular arrangement of 5 galaxies in a space of just over 1 arc-minute.
• Hickson 92: The best Hickson group, Stephan's Quintet, with gracing curving star streams and a merger in progress.
• Hickson 95: Four interesting galaxy structures, highlighted by a merger with crazy linear star streams.

A table of Hickson group data is shown in Revisions N and O.  I have included highlights from this data as annotations on the Hickson posters, including the following:  

• Brightest Hickson group (magnitude): Hickson 90 (mag 9.1)
• Dimmest Hickson group (magnitude): Hickson 29 (mag 14.8)
• Brightest Hickson group (surface brightness): Hickson 79 (20.5)
• Dimmest Hickson group (surface brightness): Hickson 58 (25.9)
• Nearest Hickson group: Hickson 92 (40 million ly)
• Furthest Hickson group: Hickson 50 (1930 billion ly)
• Largest Hickson group (areal extent): Hickson 44 (16.4 arc-minutes)
• Smallest Hickson group (areal extent): Hickson 54 (0.7 arc-minutes)
• Largest ‘a’ Hickson galaxy (actual diameter): Hickson 45 (290,000 ly)
• Smallest ‘a’ Hickson galaxy (actual diameter): Hickson 54 (25,000 ly)
• Most Hickson group galaxies: Hickson 57 (8 galaxies)
• Furthest north Hickson group: Hickson 84 (+78 deg dec)
• Furthest south Hickson group: Hickson 63 (-33 deg dec)

In the table, I have rated each of the Hickson galaxies on an imaging scale from 0 to 5.  This is the same scale that I have used in the upcoming fall release of my Deep Sky Compendium (the Compendium will show an image, description, and rating for each of 3200 DSOs, as well as technical information such as distance, diameter, brightness and inclination for each object).  The rating scale is as follows:

• 5 (Showcase - Top 2% of all sky objects)
• 4 (Excellent - Top 10% of all sky objects)
• 3 (Good - Top 25% of all sky objects)
• 2 (Majority of objects)
• 1 (Only for hardcore imagers)
• 0 (Not worth imaging)

Each of the 100 Hickson galaxy group images has been posted individually to my Astrobin Hickson Collection, where you can find all of my equipment data for each image as well as a description of the object.

I hope that seeing these galaxies in this format will inspire appreciation for these small, distant, yet incredible objects.   Imaging them is challenge, usually requiring focal lengths of 1000mm and above, but the results can be satisfying especially when considering the incredible scale involved.