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Time Accurate time under your control Most computers -> use NTP Necessary for simultaneous observations
(satellite, flare) Exposure time has error (open/close/syst) Exposure length is “error”
Flat Fields Adds noise to every science frame Goal: 10x lower noise contribution from flats
compared to object Typical fwhm area = 10pix, so for 0.01mag error,
want flat pix to have 10000*sqrt(100/10) (or 30000) electrons minimum.
For mmag photometry, need even more, plus lamp color match.
Accurate Photometry Transformations Scintillation Differential airmass Signal/noise
Transformations Necessary to place everyone on same
system (0.01mag level) Due to differences between standard
system and yours (filters) Use set of standard stars and
transformation equations, least squares Only good for non-pathological stars
Scintillation Caused by earth’s atmosphere Important for small aperture and/or short
exposure Reduce effect by working close to zenith
Differential airmass For precise work, need to account for
airmass difference from top to bottom of frame
Avoid by never working at high airmass
Signal/noise considerations For precise work, must consider other
factors besides Poisson noise Different noise factors important in different
regimes
Aperture selection Example: WZ Sge good/bad seeing,
crowded field Curve of growth analysis for maximum
signal/noise Bright stars - use big aperture, spread light
to get maximum dynamic range
Differential photometry (V-C), (K-C) common Accounts for majority of sky variations Ensemble techniques for higher precision Uses “mean comparison”: sum(Cmag)/N Reduces error by sqrt(N) (9 comps, 3x less
noise contribution from comp star)
Stacking images Useful to remove cosmic rays, cosmetic
errors No penalty if sky background limited Median worse than straight average/mean
by about 20percent Other rejection algorithms
Faint stars, bright background Use smallest possible aperture (psf fitting
best) Stacking method (average, rejection)
makes a difference Compare all methods against average on
clean stars
Exoplanets High precision (millimag level) Usually bright stars. Scintillation and finding
comp stars important Use ensemble methods where possible High time resolution not important, but
transformation important if combining datasets
Gamma-Ray Burst Afterglows Early time observations require cookbook
procedure (you can’t be thinking about exposure times)
Rapid fade, so need to get on it fast Filters highly important (Rc,Ic,Z) Watch stacking techniques to avoid
rejecting high/low points
