What the three prefixes designate
M is the Messier catalogue, a list of about a hundred objects compiled precisely so that a comet hunter would stop mistaking them for comets. The first edition, in 1774, held 45 entries; the list grew over the following decades to the hundred-odd it is known by. It is short, it is bright, and it is the list most beginners work through.
NGC is the New General Catalogue, published in 1888, running to several thousand entries and ordered by position in the sky rather than by brightness.
IC is the pair of Index Catalogues that extend it — published in 1895 and 1908 — adding objects found after the NGC was compiled.
The three overlap heavily. An object in Messier’s list is almost always in the NGC too, which is why handsets and charts show, for example, both designations for the same target.
What they do not promise
Visibility. A catalogue entry says an object was recorded, not that it is within reach of any particular instrument under any particular sky. A great many NGC entries are beyond a small telescope from a suburban garden.
Nor do they promise consistency of naming. Popular names — the ones people actually use — are not part of any catalogue, and two charts can disagree about them.
And a catalogue size printed on a box promises nothing at all: the count includes entries no amateur telescope will show.
What they work with
GoTo handsets and smart telescope applications, both of which organise their target lists by these prefixes. Planispheres and charts, which label the brighter entries. And the filters and apertures chosen to reach particular classes of object.
On this site the designations appear wherever a page names a target, so that a reader can find it on their own chart rather than only in ours.
The questions that keep arriving
Why does one object have several numbers?
Because the catalogues were compiled independently and overlap. An object bright enough for Messier’s list is almost always in the NGC as well, so it carries both, and handsets differ in which they show.
Is a bigger catalogue number a fainter object?
Not reliably. The NGC is ordered by position rather than brightness, so a high number says where an object is rather than how hard it is to see.
Do the catalogue sizes on a GoTo box mean anything?
Very little. A handset advertising tens of thousands of objects is counting entries that its telescope cannot show. What matters is whether the useful lists are easy to reach in the dark.
Last reviewed 17 September 2026