Add references for optical calibration using the solar distance

See issue #1808

added 12 commits

• d3b8f3d4...7ee8c6d8 - 11 commits from branch master
• 0a9e6822 - Merge branch 'master' into 1808_solar_variability_as_input

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added 7 commits

• 0a9e6822...9e80c891 - 6 commits from branch master
• 51bdd5b6 - Add reference for optical calibration using the solar variability

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merged

mentioned in commit 539ecaa3