Light pollution: how it affects what you see and how to work around it
What you actually lose observing from a city, what filters and target selection can compensate for, and what has no fix short of a darker sky.
If you observe from a city or its outskirts, the question isn’t whether light pollution will affect you, but how much and in what way. The answer varies a lot depending on what you want to look at: the Moon and planets barely suffer, while the Milky Way and much of deep sky can be effectively out of reach, filter or no filter. This guide separates what’s genuinely lost from what can be compensated with planning and the right filters, and what has no solution short of a change of location.
What light pollution actually is
The Instituto de Astrofísica de Canarias defines it as the sum of all adverse effects of artificial light introduced into the environment. For astronomical observing, the main problem isn’t so much seeing a nearby streetlight directly, but sky glow: artificial light reflects and scatters off gases and particles in the atmosphere, and that diffuse glow raises the background brightness of the entire night sky, not just the area around the light source.
Three factors determine how much a given light source worsens this glow: direction (light escaping upward instead of being aimed at the ground), intensity (more illumination than actually needed), and color — blue-spectrum light scatters more strongly in the atmosphere (Rayleigh scattering) than warmer tones, which is why modern cool-white LED street lighting is especially harmful for astronomy compared with warmer light sources.
The Bortle scale: how it’s measured (in broad terms)
The most widely used way to describe a sky’s darkness level is the Bortle scale, created by John E. Bortle and published in Sky & Telescope magazine in February 2001. It has 9 levels, from 1 (the darkest possible sky) to 9 (the center of a major city), organized into three broad bands:
- Levels 1-2: exceptionally dark skies, with zodiacal light visible to the naked eye.
- Levels 3-5: transition zones between rural and suburban environments.
- Levels 6-9: increasingly urban environments, with severe light pollution.
At the two extremes of the scale, the difference in naked-eye limiting magnitude (the faintest star visible without an instrument) is significant: under a class 1 sky, that limiting magnitude sits around 7.6 to 8.0; under class 9 (city center), it drops to roughly 4.0. The dossier used for this guide doesn’t detail the specific visual description of each of the 9 intermediate levels (exactly what tells apart a Bortle 4 from a Bortle 6, for instance) — if you need that level of precision for your particular location, it’s worth checking Bortle’s original documentation or an up-to-date light pollution map directly.
Keep in mind that the Bortle scale is a classification based on subjective visual observation, not an instrumental measurement. Anyone wanting a quantitative figure can turn to a Sky Quality Meter (SQM), which measures sky brightness in magnitudes per square arcsecond (mag/arcsec²): the darkest places on the planet — Starlight reserves, certified dark sky sanctuaries — reach a maximum of around 22 mag/arcsec². The dossier doesn’t include a quantitative equivalence table between Bortle levels and SQM values, so it isn’t possible to state, for example, “a Bortle 5 equals so many mag/arcsec²” with precision; these are two related but differently-measured scales (one visual observation, the other instrumental).
Tools like lightpollutionmap.info let you visualize this data interactively, combining satellite measurements with SQM data from ground stations, and are useful for getting a rough idea of your area’s light pollution level before heading out to observe.
What you actually lose observing from a city
Light pollution doesn’t affect everything in the sky equally. The reason has to do with contrast: an object becomes invisible once its brightness falls below the sky’s background brightness, the same way stars — which are still there — aren’t visible during the day because the daytime sky is far brighter than they are. The fainter and more diffuse an object is, the sooner it disappears under that elevated background.
According to the Fundación Descubre, the Milky Way is “the first victim” of light pollution: its diffuse structure and relatively low brightness make it one of the first things to vanish as you get closer to a city. NASA agrees that scattered light “makes it harder to see fainter stars and the Milky Way,” and points to a direct relationship: the bigger and more developed a city is, the more light pollution it tends to produce. Under a pristine sky you can distinguish on the order of 1,500 stars with the naked eye; in an urban area, that’s often reduced to little more than a dozen.
Faint galaxies and nebulae follow the same logic: as sky glow increases, their contrast against the background shrinks until it falls below what the eye can distinguish, and they become unobservable — not because they’ve stopped being there, but because they’re masked by atmospheric light scattering. The dossier consulted doesn’t provide specific surface brightness figures at which an object stops being visible (that kind of threshold depends on too many variables to give a single reliable number), so it’s best understood in qualitative terms: the fainter and more extended the object, the sooner you lose it as you get closer to the city.
What’s still viable from a city
Not everything is lost. Urban observing remains possible with some planning, though the target changes:
- The Moon: observable without any issues, with the same level of crater detail as anywhere else, since its brightness is far greater than the sky’s glow.
- Jupiter and Saturn: excellent urban targets — cloud bands, rings, and moons remain visible because, like the Moon, they’re bright and either point-like or small-disc objects, little affected by an elevated sky background.
- Venus and Mars: visible with a bit more patience.
- Bright open clusters, such as the Pleiades, and globular clusters like the one in Hercules: more resistant to light pollution than faint galaxies, since they’re relatively concentrated, bright groupings of stars.
- Andromeda and some planetary nebulae like the Ring Nebula: visible from the less affected spots within the city itself.
What’s not realistically viable from a city with significant light pollution is the full Milky Way and most faint galaxies and nebulae, with their fine structure and real contrast. There’s no filter or trick there that substitutes for a darker sky.
Choosing the right target and time, the first compensation
Before spending money on a filter, the most cost-effective strategy is adjusting what you observe and when:
- Prioritize bright targets: the Moon, planets, and bright open and globular clusters, rather than trying to chase faint galaxies from the balcony.
- Observe the target at its highest point (near the zenith), where it passes through less atmosphere and therefore fewer layers of light-scattered air.
- Take advantage of the early morning hours, when lighting in many cities is reduced compared with peak nighttime activity hours.
- Look for areas with vegetation (parks) rather than asphalt surfaces within the city itself, and position yourself away from direct light sources like streetlights — pavement retains more heat during the day and generates more local atmospheric turbulence at night, on top of reflecting more light.
- Prioritize aperture, within what your budget and storage space allow: a larger telescope gathers more light regardless of the pollution level, which helps with both planets and brighter deep-sky objects. If you’re still not sure what minimum aperture makes sense for your case, this guide on what you can see with a beginner telescope goes into it in more depth.
As a rough reference gathered from amateur forums (not a manufacturer specification), for reliable planetary and lunar observing from an urban setting, minimum apertures around 80-90 mm for refractors, 150 mm for reflectors, and 100-125 mm for Maksutov catadioptrics are commonly mentioned.
Filters: what they compensate for and what they don’t
A light pollution filter doesn’t “clean” the sky or add light that isn’t there — it selectively blocks the wavelengths associated with the most common artificial lighting sources (sodium vapor, mercury vapor) while letting through more of the light from nebulae’s own emission lines, which improves the contrast between the object and the sky background. That’s why they work reasonably well on emission and planetary nebulae, but do nothing for a galaxy (which emits across a continuous spectrum, not in specific lines) or for resolving a globular cluster.
A few concrete examples, with the specifications given by each manufacturer or distributor:
- Optolong CLS (City Light Suppression), in 1.25” format: priced around €52 and rated at roughly 95% transmission at the main nebula emission lines (H-alpha 656 nm, OIII 496/500 nm, H-beta 486 nm, SII 672 nm), with under 0.1% transmission outside that passband and specific blocking of sodium (589 nm) and mercury (435/578 nm) wavelengths. Suitable for telescopes between 4” and 11” of aperture.
- Celestron UHC/LPR (model 94126A), also in 1.25”: rated above 97% transmission across its entire passband, with a 60 nm bandwidth and particular efficiency at H-alpha. At the time the source was checked it was out of stock at the retailer, so it’s worth confirming current availability and pricing before relying on any of these figures.
- Svbony, as the more budget-friendly option in the same category (CLS and UHC), is a common choice for those starting out. It’s worth treating some user reviews with caution, as they report poor anti-reflective coating on its UHC and CLS models, with visible reflections of the observer’s own eye and ghost images around stars — an optical quality issue, not a problem with the filter technology itself. If you’re considering this option, it’s worth reading several recent reviews before buying. (The dossier consulted doesn’t provide up-to-date prices in euros for these models.)
As a general functional difference: the CLS has a wider passband and is recommended for light-to-moderate light pollution, while the UHC is more restrictive and performs better under severe pollution and on specific nebulae — but the dossier doesn’t include a line-by-line transmission table that would allow a precise numerical comparison between the two filter types, so this difference should be taken in qualitative terms, not as a closed technical comparison.
These filters are for visual use (threaded onto the eyepiece); if your interest is long-exposure astrophotography, the approach changes considerably and isn’t the focus of this guide — the dossier consulted documents CCD/DSLR camera filters, but only in general terms, without going into the specific impact of light pollution on long-exposure photography versus live visual observing.
If you already know you want a filter and are looking for more detail on the different types depending on your telescope and your sky, the guide telescope filters: which ones you actually need goes through the lunar, planetary, and light pollution variants one by one.
Final recommendation
If you observe from a city, the order of priorities matters more than any filter: first choose targets that tolerate urban skies well (Moon, planets, bright clusters), observe near the zenith during the hours with less lighting, and only then consider a CLS or UHC filter to squeeze out a bit more contrast on specific nebulae. No filter is going to give you back the full Milky Way or a faint galaxy from the center of a major city — for that, the only real compensation is heading out to a darker sky. If you want to go deeper into how to find those objects once you know which ones are viable from where you observe, how to find objects in the sky without a GoTo mount is a good next step.