Why were solar constant measurements before TSIS-1 all about 0.3% high?
From the abstract of Kopp &amp; Lean (2011) &quot;A new, lower value of total solar irradiance: Evidence and climate significance&quot;:

Scattered light is a primary cause of the higher irradiance values measured by the earlier generation of solar radiometers in which the precision aperture defining the measured solar beam is located behind a larger, view‐limiting aperture. In the TIM, the opposite order of these apertures precludes this spurious signal by limiting the light entering the instrument.

(emphasis mine)
For example, in the case of the ACRIM instruments used on the Solar Maximum Mission, the Upper Atmosphere Research Satellite, and ACRIMSat:

Notably, for the ACRIM instrument NIST determined that diffraction from the view‐limiting aperture contributes a 0.13% signal not accounted for in data from the three ACRIM instruments [Butler et al., 2008]. This correction lowers the reported ACRIM values, resolving part of ACRIM's difference with TIM. In ACRIM and all other instruments, the precision aperture used to define the measured solar beam is deep inside the instrument with a larger view‐limiting aperture at the front, which, depending on edge imperfections, in addition to diffraction can directly scatter light into the absorbing cavity. Additionally, this design allows into the instrument interior two to three times the amount of light intended to be measured; if not completely absorbed or scattered back out, this additional light produces erroneously high signals. In contrast, the TIM's optical design places the precision aperture at the front so only light intended to be measured enters (Figure 4a).

The paper goes into a bit more detail about the testing done to determine the reasons for the instrumental differences.