LearnInstruments & observingSome maths
Focal ratio and image scale
Focal ratio is focal length divided by aperture; image scale is how many arcseconds of sky land on one pixel.
Why it matters
Together they decide framing, exposure length and whether a rig is sampling sensibly. They are the numbers you check before buying anything.
The maths, in layers
1 · Intuition
Focal length sets how large the sky is projected; pixel size sets how finely it is chopped up.
2 · The equation
scale ["/px] = 206.265 × pixel [µm] / focal length [mm]
- 206.265
- arcseconds in a radian, divided by 1000
In practice
Every photograph in this archive has its image scale computed from exactly this, never stored.
Leads to
Nothing here builds on it yet.