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4 years ago in Astronomy , Photometry By Meghna R
In photometry, is it better to calculate exposure time or determine it experimentally?
I'm preparing for a photometric observing run on a 1-meter telescope to measure exoplanet transits. I've calculated exposure times using the standard S/N formula, factoring in read noise, sky background, and target magnitude. However, experienced observers tell me to always test exposures on the first night. What's the wiser strategy: trusting the calculation or prioritizing an on-sky test?
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By Sumitra R Answered 1 year ago
You must do both, in sequence. I always start with the calculation it's your essential baseline that prevents wildly over or under-exposing on the first night. However, I have seen perfect calculations rendered useless by unmodeled factors: thin cirrus, worse-than-average seeing, or subtle telescope focus issues. Therefore, I would recommend this protocol: use your calculation to choose a starting exposure on night one. Then, take a short series of test images at that setting and measure the achieved S/N in your actual reduction pipeline. This lets you empirically adjust the time to hit your target S/N, factoring in the real, not theoretical, observing conditions.
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