Atmospheric Effects

Refraction

• The dominant refraction effect is removed simply by repointing the telescope.

Dispersion

• The degree of refraction depends on wavelength, so the light from a point source is dispersed as a function of Zenith Distance (Z) (or elevation).
• The amount of dispersion is more directly coupled to airmass than to Z.
• The dispersion can be largely compensated by the current ADC design, up to an airmass set by the travel of the prisms.

Differential Refraction across LRIS Field

• Since Z can change across a large field of view, differential rafraction exists, "compressing" the field in elevation.
• Due to changing orientation of the field with respect to elevation, the effects of differential refraction can act in a variety of manners, for example, as a rotation or changing skew (page 22 of Phase A report).
• A good pointing/guiding model for the Keck telescopes should account for the removal of these effects as much as it can, regardless of whether or not images are corrected with an ADC.