Estimates the radial elongation of a stellar image caused by wavelength-dependent
refraction across a filter bandpass, using the Edlén (1966) dispersion
formula for standard air with a Filippenko (1982) temperature / pressure
correction.
Inputs
nm
mm
Used only to draw the diffraction-limited resolution reference (Rayleigh criterion,
computed at the short-wavelength edge of the bandpass — the sharpest the instrument
can theoretically resolve within the filter).
°C
hPa
Defaults are standard conditions (15°C, 1013.25 hPa — dry air). Colder / denser
air increases the effect; the correction assumes dry air and does not include
water-vapor humidity terms, which are a small secondary effect.
Result
Elongation, full bandpass
0.00arcsec
= 0 mas
Altitude below ~15–20° — the tan(z) approximation and the underlying refraction
formula lose accuracy near the horizon; treat this result as order-of-magnitude only.