Deep Sky Objects: What to See and How to Find Them

Deep-sky objects — galaxies, nebulae and clusters — are the reason most people buy a telescope, and the reason most people are initially disappointed. Understanding why closes that gap.

Why photographs mislead

A camera integrates light for minutes or hours and uses sensors that are far more sensitive at low light levels than the human eye. Your eye integrates for a fraction of a second and, below about magnitude 6, uses rod cells that do not signal colour at all.

So visually, the Orion Nebula is a grey-green shape with hints of structure. The Andromeda Galaxy is an elongated smudge. The Ring Nebula is a small grey donut. They are genuinely spectacular — but the spectacle is in the detection, not in the colour.

The catalogues

Messier's catalogue of 110 objects is a list of things that look like comets but are not — compiled in the eighteenth century by a comet hunter who kept being fooled. It happens to contain most of the brightest deep-sky objects visible from northern latitudes, which is why it became the standard beginner list.

The New General Catalogue (NGC) runs to nearly 8,000 objects and goes considerably fainter. Beyond that, the Index Catalogue and various specialised catalogues exist for those who want them.

Star-hopping, properly

Star-hopping is the technique of moving from a star you can identify to a star you can identify, in steps small enough to stay oriented, until you arrive at the target. It works where GoTo does not, needs no power, and teaches you the sky.

A working target list by season

Winter

• M42 Orion Nebula — the finest diffuse nebula in the sky; structure visible from 80 mm upward.
• M45 Pleiades — best at the lowest power you have, ideally in binoculars.
• M1 Crab Nebula — a faint smudge; a genuine test in small apertures.
• M35 and M36/37/38 — open clusters that reward low power.

Spring

• M81 and M82 — a bright spiral and a starburst in the same medium-power field.
• M51 Whirlpool — spiral structure needs 200 mm and a dark sky.
• M104 Sombrero — an unmistakable dust lane across a bright bulge.
• M3 and M13 — globular clusters, the latter the best in the northern sky.

Summer

• M57 Ring Nebula — an OIII filter makes it obvious.
• M27 Dumbbell — large, bright and responsive to filters.
• M8 Lagoon and M16 Eagle — big emission nebulae in Sagittarius.
• NGC 7000 North America Nebula — huge; a UHC or H-beta filter and a very wide field.

Autumn

• M31 Andromeda — visible to the unaided eye; the dust lane needs dark skies.
• M33 Triangulum — large, faint, and entirely a dark-sky object.
• NGC 869/884 Double Cluster — one of the finest low-power sights in the sky.
• M15 and M2 — compact globulars that take high magnification.

Filters, briefly

A UHC filter darkens the sky background while passing the lines emission nebulae emit, raising contrast. An OIII filter is narrower and more aggressive — superb on planetary nebulae, useless on galaxies. Neither helps on clusters or galaxies, where there is no line emission to isolate.