Index Catalogue (IC)

The Index Catalogue (IC) is the direct successor and essential partner to the NGC with over 5,000 objects, often discovered through early photography.. It represents a pivotal moment in history when technology changed the way we look at the universe.

1. The Origin: A Necessary Sequel

The IC was created by the same man who compiled the NGC, John Louis Emil Dreyer. After he published the NGC in 1888, the world of astronomy was moving fast. New telescopes were being built, and more importantly, a new technology was emerging: astro-photography.

Astronomers began finding thousands of objects that were too faint for the human eye to see through a telescope but appeared clearly on long-exposure photographic plates. Dreyer realized the NGC was already becoming outdated, so he published two supplements:

• IC I: Published in 1895 (containing 1,520 objects).

• IC II: Published in 1908 (containing 3,866 objects).

Together, these are known simply as "The IC."

2. The Purpose: The "Catch-All" Supplement

The primary goal of the IC was to expand and correct.

• New Discoveries: It captured all the deep-sky objects found between 1888 and 1907.

• Corrections: Dreyer used the IC to publish a "errata" list for the original NGC, fixing coordinates and descriptions that were found to be wrong.

If you think of the NGC as the "Volume 1" of the deep sky, the IC is "Volume 2." In Stellarium, they are almost always used together as the NGC/IC database.

3. What is in the Database?

The IC contains 5,386 objects. Because many of these were discovered via photography, they tend to be significantly fainter than the objects in the Messier or NGC lists.

The catalogue is dominated by:

• Faint Galaxies: Thousands of distant "island universes."

• Emission and Reflection Nebulae: Vast clouds of gas, some of which are quite famous today.

• Star Clusters: Smaller or more distant groupings of stars.

4. Why it Matters Today

For modern amateur astronomers and astrophotographers, the IC is where the "hidden gems" are located. While a beginner might stick to the bright Messier objects, an experienced observer with a large telescope or a sensitive camera will dive into the IC to find more exotic and challenging targets.

Like the NGC, the IC is organized by Right Ascension, meaning the objects are numbered from west to east across the sky.

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