Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 07/04/2016 - 20:43
Sorry if this has been answered before, but I didn't see it in the Variable Star Plotter (VSP) help:
Do we know the definition of the mag uncertainties listed in parentheses within VSP's CCD photometry tables? Are they 1-sigma, or 2-sigma, or some other meaning? Do we know that the listed mag uncertainties are defined uniformly across data sources?
Recent experiments here show some benefit to using listed uncertainties to relative-weight comp stars in ensemble photometry--but before I go much farther I should know (or make assumptions) about the uncertainties' meaning.
Any guidance helpful. Thank you!
Hi Eric,
Most of the uncertainties (those from APASS, or any of the AAVSOnet telescopes) are one-sigma uncertainties that include both the random error from night to night as well as the systematic error from the calibration onto the standard system. They are single-measure standard uncertainties rather than mean errors, and so should be a bit larger than others might report.
When using multiple APASS sequence stars in a single field, the errors shown in VSP are quite a bit bigger than you might measure during an ensemble solution, since the systematic error of placing the stars on the standard system will be the same for all of those APASS stars. Typically, the differential uncertainty between two well-exposed APASS stars will be under 0.01mag, even though they might be listed with 0.08mag uncertainties individually in the VSP table.
I hope that helps!
Arne
Thanks--your explanation was concise. Happily, given the calculation basis, I can also use the uncertainties as given--that is, as independent from my own measurement errors. (Now that I think about it, that was probably their intent.)