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Astronomical Seeing & Bortle Class Helper

Combine sky brightness (Bortle class), seeing conditions and telescope aperture for a quick, opinionated guide to which targets are worth chasing tonight.

Step 1

Describe your sky & scope

Pick your Bortle class, judge the seeing, enter your aperture, then let the helper grade different target types.

Bortle class (1 = darkest, 9 = brightest)

If you’re not sure, use an online light-pollution map or a rough guess based on how washed-out the Milky Way looks.

Seeing quality (steadiness of the air)

Seeing mainly affects planets, the Moon and double stars at high magnification.

Telescope aperture (mm)

For binoculars, enter one objective size (e.g. 10×50 → 50 mm).

If in doubt, start with Bortle 5 and “Good” seeing.

Step 2

What tonight favours

Once you’ve entered your conditions, this section will summarise how your sky and scope are likely to behave.

Target types

Each category combines darkness, seeing and aperture into a simple “how hopeful should I be?” label.

Suggested magnifications

You’ll see ballpark magnification ranges here once you’ve entered your aperture.

Guide

FAQ & notes

The Bortle scale is a nine-level description of how bright your sky is from light pollution. Bortle 1 is an extremely dark site where the Milky Way is rich and structured; Bortle 9 is an inner-city sky where only the brightest stars punch through the glow.

You can estimate your class using online light-pollution maps, or by comparing what you actually see (Milky Way visibility, number of stars, sky glow) with published Bortle descriptions.
Seeing is how stable the air is. Good seeing means stars look sharp and planets hold fine detail at high magnification. Poor seeing makes everything wobble or “boil” in the eyepiece.

Transparency is how clear the air is (haze, moisture, thin cloud). It affects how bright faint objects appear. This helper mostly uses Bortle class as a rough stand-in for transparency, plus your seeing rating and aperture.
The grades (Excellent / Good / Workable / Challenging / Very tough) are deliberately rough. They combine three ingredients:
- How dark the sky is (Bortle).
- How steady the air is (seeing).
- How large your aperture is.
Real observing also depends on target altitude, your experience, collimation, optics quality, local obstacles, your eyes and more. Treat this as a planning nudge, not a promise.
For binoculars, enter the size of one objective lens (e.g. 10×50 → 50 mm). If you own several scopes, just run the tool multiple times to get a feel for how each one behaves under the same sky conditions.

Being within 5–10 mm is absolutely fine – this is an approximate helper, not a lab instrument.