  The absolute brightness temperature of the Sun at millimeter wavelengths is
an important diagnostic of the solar chromosphere. Because the Sun is so
bright, measurement of this property usually involves the operation of
telescopes under extreme conditions and requires a rigorous performance
assessment of the telescope. In this study, we establish solar observation and
calibration techniques at 2.6-mm wavelength for the Nobeyama 45-m telescope and
derive the absolute solar brightness temperature accurately. We tune the
superconductor-insulator-superconductor (SIS) receiver by inducing different
bias voltages onto the SIS mixer to prevent saturation. Then, we examine the
linearity of the receiver system by comparing outputs derived from different
tuning conditions. Further, we measure the lunar filled beam efficiency of the
telescope using the New Moon, and then derive the absolute brightness
temperature of the Sun. The derived solar brightness temperature is 7700+-310 K
at 115 GHz. The telescope beam pattern is modeled as a summation of three
Gaussian functions and derived using the solar limb. The real shape of the Sun
is determined via deconvolution of the beam pattern from the observed map. Such
well-calibrated single-dish observations are important for high-resolution
chromospheric studies because they provide the absolute temperature scale
missing from interferometer observations.
