Advanced CCD Workshop Arne A. Henden Arne@aavso.org

Advanced CCD Workshop

Arne A. Henden

Arne@aavso.org

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