Deep Field

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Deep Field
29 September 2026 2518 stories tracked

Glossary · Telescopes & Instruments

Seeing

How much the atmosphere is blurring the stars tonight, measured in arcseconds.

Turbulence in the air smears a point of light into a disc. Seeing of one arcsecond is respectable, half an arcsecond is superb, and it is why observatories sit on isolated peaks with smooth airflow rather than simply high ones. No telescope on the ground resolves better than the seeing allows unless it corrects for it — which is what adaptive optics does, and why the same 8-metre mirror can be either a modest instrument or an extraordinary one depending on the night and the hardware.

Related terms

Lagrange point Five places where two bodies' gravity lets a third sit still relative to both. Adaptive optics Bending a mirror hundreds of times a second to cancel out the atmosphere. Interferometry Combining separate telescopes so they resolve detail like one enormous dish. First light The night a new telescope is pointed at the sky and actually works.

Questions

What is Seeing?

How much the atmosphere is blurring the stars tonight, measured in arcseconds.

Why does it matter?

Seeing of one arcsecond is respectable, half an arcsecond is superb, and it is why observatories sit on isolated peaks with smooth airflow rather than simply high ones. No telescope on the ground resolves better than the seeing allows unless it corrects for it — which is what adaptive optics does, and why the same 8-metre mirror can be either a modest instrument or an extraordinary one depending on the night and the hardware.

Written and computed by . Corrections and errors to hello@cantinalabs.dev.