How is photometric error calculated in AstroImageJ?

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How is photometric error calculated in AstroImageJ?

karenacollins
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Re: How is photometric error calculated in AstroImageJ?

karenacollins
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    First, the error for each target and comparison star is individually
    calculated using equation 25 in MERLINE, W. & HOWELL, S.B.,
    1995, EXP. ASTRON., 6, 163. The error value is reported as
    Source_Error_Xnn in the measurements table and accounts for CCD
    gain, read out noise (RON), dark current, quantization noise, and
    poisson statistics in the source and sky background signals. As of
    release 2.1.5, scintillation noise is not included in the error
    calculation.
   
    The error equation is:
   
    SourceError = SquareRoot(IntSourceCnts * Gain + NSourcePixels * (1.0
    + NSourcePixels / NSkyPixels) * (SkyPerPixel * Gain + DarkPerPixel +
    RON * RON +  Gain * Gain * 0.083521)) / Gain;
   
    where:
    IntSourceCnts = net integrated source counts (calculated by AIJ)
    SkyPerPixel = average sky background per pixel (calculated by AIJ)
    NSourcePixels  = number of pixels within the sources aperture
    (calculated by AIJ)
    NSkyPixels = number of pixel within the sky background annulus
    (calculated by AIJ)
    DarkPerPixel = total dark counts per pixel (entered by user on set
    aperture panel as [e-/pix/sec], which is then multiplied internally
    by the exposure time found in the FITS header {or by 1.0 if no
    exposure time header info is found}, or is read from the FITS header
    keyword specified in the set aperture panel, but in this case should
    be in [e-/pix] in the header {i.e. is not multiplied by exposure
    time in this case})
    RON = CCD read out noise [e-] (entered by user on set aperture
    panel)
    Gain = CCD gain (entered by user on set aperture panel)
   
    For relative photometry error calculations, the individual source
    errors are propagated through the summations and divisions using
    standard error propagation techniques assuming independent variables
    (see http://en.wikipedia.org/wiki/Propagation_of_uncertainty ). Thus,
    the error in the comparison star ensemble is calculated by summing
    the individual comparison star errors in quadrature {i.e. in the
    measurements table, tot_C_err = SquareRoot(Source_Error_C1^2 +
    Source_Error_C2^2 + ... + Source_Error_CN^2)}. When forming the
    relative flux terms, error is propagated through the division and
    reported in the measurements table as rel_flux_err_Xnn. For example,
    rel_flux_err_T1 = (Source-Sky_T1 / tot_C_cnts) * SquareRoot(
    Source_Error_T1^2 / Source-Sky_T1^2 + tot_C_err^2 / tot_C_cnts^2).
   
    If data are normalized or converted to magnitude in the plot window,
    error is normalized or converted to magnitude before plotting as
    well. For normalization, the error value is divided by the same
    baseline average value used to normalize the data. For conversion to
    magnitude, the following calculation is used: rel_flux_error_T1(mag)
    = 2.5 * Log_base10 (1.0 + rel_flux_err_T1/rel_flux_T1).
    Normalized error and/or error in magnitude can be saved to the
    measurements table by using the "New Col" button ( ) on the
    left-hand side of the "Multi-plot Y-data" panel corresponding to the
    displayed curve that shows the normalized or magnitude converted
    data.
   
   
   
 



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