BLO Catalog

When looking at modern digital astronomy, the BLO Data (often referred to as the BLO Catalog) represents one of the most specialized "digital-first" databases for observers. While it doesn't have the 18th-century pedigree of the Herschel or Lacaille lists, it is a cornerstone for 21st-century amateurs who use computerized telescopes.

"BLO" stands for Best of the Largest Objects, and it was specifically designed to bridge the gap between "standard" catalogs and the reality of what modern digital sensors and wide-field telescopes can capture.

History & Who Made It

The BLO Data was compiled by Jim Cornmell, a prominent figure in the digital observing community, around 2005–2006.

At the time, many "Best Of" lists were frustrating for observers with modern equipment. Old lists focused on small, bright points (like planetary nebulae), but new wide-field telescopes and sensitive cameras were capable of seeing massive, faint structures that older catalogs ignored. Cornmell spent years refining a list of targets that were "large" in terms of their physical footprint in the sky, specifically for the Deep Sky Planner software and the AstroPlanner community.

The Purpose: The "Wide-Field" Census

The primary purpose of the BLO list is to identify extended objects.

• The "Scale" Filter: Most catalogs don't care about size—they care about brightness. The BLO list focuses on objects that are at least 20 arcminutes in diameter (about 2/3 the size of the full moon).

• Digital Calibration: It was designed to help imagers choose targets that would actually fill their camera’s field of view, preventing them from accidentally zooming in too far on a massive nebula.

• The "Invisible" Giants: Many BLO objects are so large that a standard telescope at high power "looks right through them." This database highlights them so you know to use your lowest-power eyepiece or a wide-field camera lens.

What Objects Can Be Found In It?

The BLO database is a "Best Of" selection drawn from much larger catalogs like the NGC, IC, and Sharpless lists. It contains exactly 265 objects in its standard version.

BLO Database Breakdown (Flat Text)

Catalog Name: BLO Data (Best of the Largest Objects) Creator: Jim Cornmell Year Established: ~2005 Total Objects: 265 Primary Focus: Objects with large angular diameter (20'+)

Object Category: Approximate Count

• Bright/Diffuse Nebulae: 112

• Open Clusters: 88

• Galaxies: 34

• Planetary Nebulae: 12

• Supernova Remnants: 10

• Dark Nebulae: 9

Key "BLO" Highlights:

• BLO 104 (The North American Nebula): A classic BLO target; so large (120') it requires a wide-field lens.

• BLO 142 (The Andromeda Galaxy): The largest galaxy in the list, stretching nearly 3 degrees across the sky.

• BLO 212 (The Rosette Nebula): A massive star-forming region that is often too big for traditional "small-field" catalogs.

• BLO 55 (The Beehive Cluster): A huge open cluster that fills even a wide-field binocular view.

Why the BLO Data is Different

Unlike the Herschel 400 (which is a "skill" list) or the Messier (which is a "historic" list), the BLO is a "Technical" list. It is designed for the era of the Digital SLR and the Short-tube Refractor.

If you look at a BLO object through a standard telescope, you might see nothing but a few stars because the object is "bigger than the view." But when you look at the BLO list, you're looking at the true giants of our galaxy and beyond—the "landscape photography" of the heavens.

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