** Up:** The gain-elevation correction of

The IRAM 30-m telescope (Baars et al. 1987, 1994) is a homologous construction
(von Hoerner 1967a,b) so that for all elevation angles () the
best-fit reflector surface is a paraboloid. The homology does not eliminate
the gravity deformations of the reflector [(i,) for
the surface elements *i* = 1, 2, ..., *N*]; however, the root mean square (rms)
value of the surface deformations of the best-fit parabolic reflector
is

| |
(1) |

for all elevation angles (0 90) and
wavelengths , with the shortest wavelength of observation of the telescope. If at the elevation
angle _{0} the special adjustments
-(i,_{0}) are applied on the reflector
surface, then (von Hoerner & Wong 1975) the reflector is free of gravity
deformations at _{0} (= 43 for the 30-m telescope) and the
rms-value of the residual surface deformations is

| |
(2) |

with (_{0}) = 0, and (0) and (90)
(see below) the gravity induced deformations at horizon
( = 0) and zenith ( = 90). The
elevation-dependent surface deformations () introduce
an elevation-dependent loss of gain for which astronomical observations must
be corrected. The rms-values () are derived from
finite element (FE) calculations of the reflector backstructure or from
radiometric measurements of astronomical sources. We use both methods and
explain, in particular, the gain-elevation dependence as function of the
source diameter (). We provide information how to correct
observations made with the 30-m telescope. (See Baars et al. (1987) for
an earlier emphasis that a gain-elevation must be applied, at least at
short wavelength.)

** Up:** The gain-elevation correction of

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