The Bortle Scale, Target by Target: What Disappears at Each Sky Class
The Bortle scale runs 1 to 9 and maps to a measurable sky brightness in magnitudes per square arcsecond — roughly 21.8 at class 1, under 18.0 at class 8. Galaxies and faint nebulae lose ground fast as the number climbs, while planets, the Moon and double stars are effectively unaffected. Aperture buys back around one Bortle class; driving to a darker site buys back two or three.
Your sky class caps what any telescope can show you, and it caps the faint stuff hard. At Bortle 8, a 10-inch reflector delivers roughly what a 6-inch delivers at Bortle 4 on galaxies and faint nebulae. On planets, the Moon and double stars, the two skies are indistinguishable.
So the useful question before buying is not which telescope. It is which targets your sky permits, and therefore which telescope is worth the money.
The scale itself, devised by John Bortle in 2001, runs from 1 (a genuinely dark site where the Milky Way casts shadows) to 9 (inner city, where the sky is bright enough to read by). It correlates well with a sky quality meter reading in magnitudes per square arcsecond, which is the number to actually measure. The table below pairs the two, and the second table gives the verdict per target per aperture.
The scale, class by class
Bortle class SQM (mag/arcsec²) Naked-eye limiting magnitude Milky Way appearance Zodiacal light Realistic deep-sky reach in a 6-inch Filter benefit 1 — Excellent dark site 21.7-22.0 7.6-8.0 Casts diffuse shadows; structure obvious from horizon to horizon Bright, colourful, reaches across the sky; gegenschein visible Around mag 14 — hundreds of NGC galaxies in reach None needed; a filter costs more transmission than it buys 2 — Typical truly dark site 21.5-21.7 7.1-7.5 Highly structured, summer Milky Way is a striking object Clearly visible, extends well up from the horizon Around mag 13.5 None needed 3 — Rural sky 21.3-21.5 6.6-7.0 Shows structure overhead; light domes visible low down Visible in spring and autumn Around mag 13 Marginal; OIII still helps on faint planetaries 4 — Rural/suburban transition 20.4-21.3 6.1-6.5 Well defined overhead, washing out near the horizon Faint, only when conditions are good Around mag 12.5 UHC and OIII start earning their keep on emission nebulae 5 — Suburban sky 19.1-20.4 5.6-6.0 Weak overhead, absent near the horizon Very weak or invisible Around mag 12 UHC clearly worthwhile; nothing helps galaxies 6 — Bright suburban 18.4-19.1 5.1-5.5 Only detectable near the zenith, and barely Invisible Around mag 11.5 OIII essential on nebulae; galaxies are a losing fight 7 — Suburban/urban transition 18.0-18.4 5.0 Invisible Invisible Around mag 11 OIII on nebulae only; broadband filters do almost nothing against LED lighting 8 — City sky Under 18.0 4.1-4.5 Invisible; sky is grey or orange to the zenith Invisible Around mag 10 — brighter Messiers as smudges OIII salvages a handful of nebulae; galaxies effectively gone 9 — Inner city Under 17.8 4.0 or worse Invisible; brighter stars of the constellations are hard to trace Invisible Around mag 9.5 — M31 core, M42, brightest clusters OIII for a couple of nebulae; otherwise plan around planets and the Moon Find your class from an SQM reading or a light-pollution map, then use the reach column to judge whether the object list you want is realistic before you buy aperture for it. The reach column is a rough integrated magnitude and should be treated as such. What actually governs whether a faint object is detectable is surface brightness — how the object's light spreads across its apparent area. A mag 9.5 galaxy packed into two arcminutes is a far easier target than a mag 8.5 one spread over a degree, because sky glow competes with the second one across a much larger patch of sky.
That is why M33, at magnitude 5.7, is invisible in a telescope from a suburb while much fainter compact galaxies remain findable. Integrated magnitude flatters diffuse objects badly.
What each aperture shows, target by target
Seven objects, three apertures, four sky bands. These verdicts assume a dark-adapted observer, no Moon, the object reasonably high, and magnification chosen appropriately — low power for the large diffuse targets, higher power for the globular.
Target Aperture Bortle 1-3 Bortle 4-5 Bortle 6-7 Bortle 8-9 M31 Andromeda 60mm Bright elongated glow, dust lane hinted, M32 and M110 both visible Obvious oval core with extension, no dust lane Small faint core smudge Core only, averted vision, easy to sweep past M31 Andromeda 6-inch Both dust lanes, extended halo well beyond the field Main dust lane visible with averted vision Bright core, halo shrinks to a small oval Core only, no extension M31 Andromeda 10-inch Dust lanes and star cloud NGC 206 with patience Main dust lane clear and direct Dust lane hinted at best Bright core, little else — the extra aperture buys almost nothing here M42 Orion Nebula 60mm Wings obvious, Trapezium splits into four at 40x Clear nebulosity, fish-mouth visible Core glow plus a hint of wings Still visible — a genuine city target M42 Orion Nebula 6-inch Extensive wings, fish-mouth sharp, E component of the Trapezium at 150x Wings clear, strong contrast Core and inner wings, helped a lot by a UHC Core region only; OIII recovers some structure M42 Orion Nebula 10-inch Wings across the field, green-grey colour, E and F both routine Wings clear, Trapezium E straightforward Good core detail with a UHC Bright core, wings suppressed; filter mandatory M13 globular 60mm Fuzzy ball, never resolves — 60mm cannot granulate it Fuzzy ball, smaller Faint fuzzy ball Findable but feeble M13 globular 6-inch Granular at 150x, outer stars resolving Partial resolution at 150-200x Grainy edge only Fuzzy ball again — the background has caught up with it M13 globular 10-inch Resolved almost to the core at 200x, a genuinely spectacular sight Well resolved outer two-thirds Outer stars resolve, core stays mushy Grainy at best, washed background M51 Whirlpool 60mm Two faint blobs with averted vision One blob, sometimes two Nothing, or a suspicion Nothing M51 Whirlpool 6-inch Spiral arms hinted at class 1-2, two bright cores otherwise Two cores, arms only on the best nights Two faint cores Nothing to marginal M51 Whirlpool 10-inch Arms clear, bridge to NGC 5195 visible Arms hinted with averted vision Two cores, no structure One faint core if anything Veil Nebula 60mm NGC 6960 arc detectable at low power with an OIII Nothing, filter or not Nothing Nothing Veil Nebula 6-inch Both main arcs with structure, OIII on Both arcs clearly, OIII essential NGC 6960 detectable with OIII Marginal at absolute best Veil Nebula 10-inch Braided filaments in NGC 6992 — one of the finest sights available Both arcs with internal structure Both arcs visible with OIII NGC 6992 detectable with OIII Double Cluster 60mm Superb, both clusters richly framed Excellent, star count slightly down Still a fine pair Both clusters visible, sparse Double Cluster 6-inch Extremely rich, colour in the brighter members Rich and colourful Good, fainter members lost Perfectly worthwhile — a reliable city target Double Cluster 10-inch Overwhelming star count, field too tight to frame both cleanly Rich, framing is the limit not the sky Very good Good — light pollution barely touches it Jupiter and Saturn 60mm Belts, four moons, ring separation Identical Identical Identical Jupiter and Saturn 6-inch Belt structure, Cassini Division on steady air Identical Identical Identical Jupiter and Saturn 10-inch Festoons, barges, Cassini and occasional Encke minimum Identical Identical Identical Read across your own sky class. Where a row says the aperture jump buys nothing, spend the money on fuel to a darker site instead. Planets do not care
Look at the last three rows again. Jupiter and Saturn are identical from Bortle 1 to Bortle 9, and that is not an approximation — it is the physics. Their surface brightness is enormously higher than the sky background even in a city, so adding sky glow changes nothing about the contrast within the disc.
The same holds for the Moon, for bright double stars, for the brighter planetary nebulae at high magnification, and largely for open clusters, which are resolved points rather than diffuse glow.
This reframes the buying decision completely for anyone stuck in a city. A 6-inch or 8-inch on a solid mount, used mainly for lunar, planetary and double-star work from a balcony, will deliver its full advertised performance every clear night. The same money spent chasing galaxies from the same balcony delivers close to nothing. Seeing conditions, not sky darkness, is what governs how good those nights are — and a warm urban rooftop is often worse for seeing than a cold rural field, which is the one way city observing genuinely loses on planets.
Filters: what they fix, and what no filter can
Narrowband filters work by passing the specific wavelengths that emission nebulae radiate — principally the doubly ionised oxygen lines near 496 and 501 nm, and hydrogen-beta at 486 nm — while blocking everything else, including most of the sky glow. Against the right target they are transformative.
Against the wrong target they are useless, and the distinction is absolute. Galaxies emit across a broad continuum. Star clusters emit across a broad continuum. Filter out the sky glow and you filter out the object by exactly the same proportion, then lose a further 10-20% to the glass. There is no filter that helps a galaxy.
A word on broadband light-pollution filters. They were designed to notch out the narrow emission lines of low-pressure sodium and mercury street lighting, and against those sources they worked reasonably well. Municipal lighting has moved almost entirely to broad-spectrum white LEDs, which emit right across the visible band. There is nothing narrow left to notch. Broadband LPR filters are close to obsolete for visual use under modern street lighting, and this is the single most out-of-date piece of advice still circulating in the hobby.
- OIII — the strongest performer on the Veil, the Helix, most planetary nebulae. Dark filter, needs aperture and low power
- UHC — a wider band covering OIII plus hydrogen-beta. Better on M42, the Lagoon and the North America Nebula, and easier to use in smaller apertures
- Hydrogen-beta — a specialist filter for a handful of targets, the Horsehead being the famous one. Not a general purchase
- Broadband LPR — largely superseded by LED street lighting; do not expect the results described in older books
- Nothing at all — the correct choice for galaxies, globulars, open clusters, the Moon and the planets
Measure your own sky rather than trusting the map
Light-pollution maps are modelled from satellite data and average across a wide area. Your garden may sit a full class either side of the map value depending on a neighbour's security light, a hedge line, or which direction you observe.
Two methods worth the effort. A sky quality meter gives a direct reading in magnitudes per square arcsecond in about a second, and readings are comparable between sites and across years — the SQM value is the number to record in an observing log, not the Bortle class.
Without a meter, count stars. Pick a well-defined region — the trapezium of Ursa Minor is the classic — and record the faintest star you can hold with direct vision, then cross-reference the naked-eye limiting magnitude column. Do it after twenty minutes of dark adaptation with no phone screen, which is the part most people skip and the part that costs a full magnitude.
Take readings at the zenith and towards each horizon. Most suburban sites are a class or two better overhead than towards the nearest town, which changes which half of the sky is worth observing on a given night.
Aperture buys about one class. Driving buys three
Doubling aperture gains roughly 1.5 magnitudes of stellar limiting magnitude and, on diffuse objects, roughly the equivalent of one Bortle class of improvement. That is a real gain and it costs the price of the next telescope up plus a heavier mount.
Moving from a Bortle 7 suburb to a Bortle 4 site forty minutes away gains three classes, and it costs fuel. On the target list in the second table, that is the difference between two faint cores and visible spiral arms in the same 6-inch.
The honest recommendation follows from that. If you observe mostly from a bright suburb and will not travel, buy the aperture that suits planets, the Moon and clusters — and buy the mount quality to go with it, because those targets punish a shaky mount at high magnification far more than a galaxy does. If you will travel, buy the largest aperture you will genuinely carry to the car in the dark, because a 10-inch left at home is a 0-inch.
The Delivered Light Index treats sky class as an input rather than an afterthought for this reason. Aperture-normalised light grasp means nothing without a stated background against which it is being delivered.