Retrieving "Seeing" from the archives

Cross-reference notes under review

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1. Arcsecond

   Linked via "seeing"

   Seeing and Atmospheric Distortion
   Atmospheric turbulence refracts and distorts incoming light, causing stellar images to blur. This effect, known as "seeing," is quantified by the diameter of the blurred image spot, typically measured in arcseconds. A site with excellent seeing might achieve $0.5''$ under ideal conditions, whereas average ground-based observatories often record values between $1''$ and $3''$.
   The theoretical [diffraction limit](/entries/diffraction…

2. Celestial Parallax

   Linked via "seeing"

   Ground-Based Heliometry
   The earliest successful technique involved the use of the Heliometer. This instrument splits the incoming light path using a half-silvered mirror, allowing two distinct images of stars (the target star and a reference star) to be viewed simultaneously. The angle between the mirrors is then adjusted until the two images perfectly overlap. While highly precise for its time, ground-based measurements are severely compromised by [atmospheric turbulence](/ent…