**ACTION:**- Apply atmospheric-dispersion adjustments to refraction coefficients.
**CALL:**`CALL sla_ATMDSP (TDK, PMB, RH, WL1, A1, B1, WL2, A2, B2)`

**GIVEN:**-

*TDK***D**ambient temperature at the observer (degrees K) *PMB***D**pressure at the observer (mB) *RH***D**relative humidity at the observer (range 0-1) *WL1***D**base wavelength () *A1***D**refraction coefficient A for wavelength WL1 (radians) *B1***D**refraction coefficient B for wavelength WL1 (radians) *WL2***D**wavelength for which adjusted A,B required ()

**RETURNED:**-

*A2***D**refraction coefficient A for wavelength WL2 (radians) *B2***D**refraction coefficient B for wavelength WL2 (radians)

**NOTES:**- 1.
- To use this routine, first call sla_REFCO specifying WL1 as the wavelength. This yields refraction coefficients A1, B1, correct for that wavelength. Subsequently, calls to sla_ATMDSP specifying different wavelengths will produce new, slightly adjusted refraction coefficients A2, B2, which apply to the specified wavelength.
- 2.
- Most of the atmospheric dispersion happens between and the UV atmospheric cutoff, and the effect increases strongly towards the UV end. For this reason a blue reference wavelength is recommended, for example .
- 3.
- The accuracy, for this set of conditions:

height above sea level 2000m latitude pressure 793mB temperature K humidity 0.5 (50%) lapse rate reference wavelength star elevation

is about 2.5mas RMS between 0.3 and , and stays within 4mas for the whole range longward of (compared with a total dispersion from 0.3 to of about ). These errors are typical for ordinary conditions; in extreme conditions values a few times this size may occur. - 4.
- If either wavelength exceeds , the radio case is assumed and the returned refraction coefficients are the same as the given ones.
- 5.
- The algorithm consists of calculation of the refractivity of the
air at the observer for the two wavelengths, using the methods
of the sla_REFRO routine, and then scaling of the two refraction
coefficients according to classical refraction theory. This
amounts to scaling the A coefficient in proportion to and
the B coefficient almost in the same ratio (see R.M.Green,
*Spherical Astronomy,*Cambridge University Press, 1985).

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk