The Bortle scale for smart telescope owners
The Bortle scale grades the night sky from 1, a pristine dark site, to 9, an inner-city sky where only the brightest stars survive. For a smart telescope it is the number that matters most, because skyglow lands on every pixel and sets the noise floor that your target has to climb out of.
The scale, and what it costs you
| Class | Sky | Brightness (mag/arcsec²) | Andromeda, S50, solid | Veil, S50, dual-band, solid |
|---|---|---|---|---|
| Bortle 1 | Excellent dark site | 22.0 | 29 min | 1 h 02 min |
| Bortle 2 | Typical dark site | 21.8 | 31 min | 1 h 03 min |
| Bortle 3 | Rural | 21.5 | 34 min | 1 h 04 min |
| Bortle 4 | Rural / suburban transition | 21.0 | 42 min | 1 h 09 min |
| Bortle 5 | Suburban | 20.5 | 55 min | 1 h 16 min |
| Bortle 6 | Bright suburban | 19.8 | 1 h 27 min | 1 h 33 min |
| Bortle 7 | Suburban / urban transition | 19.1 | 2 h 29 min | 2 h 05 min |
| Bortle 8 | City | 18.5 | 4 h 04 min | 2 h 56 min |
| Bortle 9 | Inner city | 17.8 | 7 h 27 min | 4 h 43 min |
Each magnitude is a factor of 2.5 in sky brightness. From Bortle 8 to Bortle 4 the sky drops by roughly 2.5 magnitudes, so the background is about ten times fainter, and a faint broadband target like Andromeda takes about a tenth of the time. No telescope upgrade in this price range comes close to that.
Finding your class
- Press "Use my location" in the calculator. It reads satellite night-lights data for your position and estimates your zenith sky brightness; expect it to land within one class of a sky-meter reading.
- Check a light-pollution map (lightpollutionmap.info shows the same VIIRS data and, where available, sky-meter measurements).
- Look up. Milky Way plainly visible overhead: Bortle 4 or better. Milky Way only hinted at: 5. Only the brightest stars, sky grey rather than black: 7 to 9.
Moving down the scale without moving house
- Drive. Half an hour out of most towns buys two classes, which is worth more than any accessory.
- Use the dual-band filter on emission nebulae. From Bortle 8 the Veil takes 6 h 59 min without the filter and 2 h 56 min with it. That is the filter behaving like a multi-class darker sky, for targets that emit in the two bands it passes. It does nothing for galaxies and clusters, and hurts them. See when dual-band filters help.
- Avoid the Moon. A first-quarter Moon adds about a class; a full Moon two or three. Broadband targets wait for the dark fortnight.
- Aim high. Skyglow is worst near the horizon, and so is atmospheric extinction. The same target an hour later, higher in the sky, is a better target.
- Shoot late. Many towns dim or switch off lighting after midnight; a sky that measures Bortle 6 at 10 pm may be Bortle 5 at 2 am.
- Shield the scope. A dew shield keeps nearby streetlights and security lights out of the tube. It does not change the sky, but it removes gradients that make a bright sky worse.
Every target page has a Bortle 1 to 9 table for each scope, for example Andromeda Galaxy with the Seestar S50.
More guides
- Which smart telescope should you buy?
- How long should my subs be?
- Integration time explained
- EQ mode and field rotation on smart telescopes
- Dual-band filters on smart telescopes: when they help and when they hurt
- Filters explained: VIS, Astro, UV/IR-cut, dual-band, CLS and solar
Updated 2026-08-27. Corrections: about page.