Observing
A city sky does not dim the Moon or the planets at all — they are bright enough to ignore it. What it takes is everything faint and extended, and no filter gives all of that back. This guide is about working with what is left.
Extended faint objects — galaxies, most nebulae, the fainter clusters. They are dim against a bright background, and contrast, not brightness, is what has been lost.
The Moon, the planets, double stars, the brighter clusters, and the Sun with the appropriate equipment and the maker’s instructions. All of these are contrast-independent in a way that a galaxy is not.
A narrowband filter passes the wavelengths where emission nebulae emit and blocks the rest. On those specific objects it works well. On a galaxy — which emits across the spectrum, like the sky glow — it removes as much object as background.
Resolution scales with aperture regardless of the sky, so planetary detail improves in a city exactly as it would elsewhere. This is why a folded catadioptric on a small mount is a good city instrument: long focal length, small footprint, and the targets that survive are the ones it is best at.
127 mm Maksutov · 1500 mm focal length
A 127/1500 Maksutov-Cassegrain on the AZ-GTi Wi-Fi GoTo mount, controlled from a phone rather than a handset.
Best for: Lunar, planetary and double-star observing from a small or awkward site
≈ £649 / €784
90 % at 486, 496, 500, 656 and 672 nm · 0.1 % off-band
A CLS broadband filter publishing 90 per cent transmission at the hydrogen, oxygen and sulphur lines and 0.1 per cent at the sodium and mercury lines of artificial lighting.
Best for: Imaging or observing emission nebulae from a town, with a colour camera or by eye
≈ $32 / €46
UHC · passes 486, 496, 501 and 656 nm · 1.25 in
A 1.25 in UHC filter that publishes the emission lines it passes: hydrogen at 486 and 656 nm and oxygen at 496 and 501 nm, blocking everything else.
Best for: An observer who wants to know what a filter passes before buying it
≈ £58 / €58
Variable 1–40 % transmission · 1.25 in · stackable
A two-element polarising filter for 1.25 in eyepieces with transmission adjustable from 1 to 40 per cent by rotating its lower ring.
Best for: The Moon across its whole cycle, where a fixed filter is right at only one phase
≈ $43 / £42 / €38
Six 1.25 in colour filters · #21 and #25 named
A six-filter 1.25 in colour set for planetary observing, with the maker naming #21 orange and #25 red and describing what each brings out.
Best for: A planetary observer with a telescope large enough to spend the light
≈ £57 / €63
It removes a narrow part of the spectrum where certain lamps emit, so the background falls more than the object does. Against broadband white LED lighting there is far less to remove.
They are far brighter than the sky glow. A city sky is a problem of contrast, and objects that are already thousands of times brighter than the background lose nothing.
Not for planets, where it buys resolution. For faint extended objects it collects more sky glow along with more object, so the gain is smaller than the aperture suggests.
Any figure written into the body of this page was true on the day it was written. The amount shown next to each instrument is requested from the retailer as the page loads, and it is that amount, not the text, that stands.
Telescopes and mounts are discounted over and over through the year — hardest of all in the weeks before Christmas, which is exactly when most of them are bought — and a struck-through figure on its own says nothing whatever about what the same model was charged at four weeks ago. UniverTrack keeps a week-by-week record for each retailer, so a discount is measured against a figure somebody genuinely paid instead of one invented for the sale — and it says when the moment to buy has genuinely arrived.
