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A bit about CCD imaging

A bit about CCD imaging. Paul McGale. Signal-to-noise ratio. SNR = C star T / √ (C star T + C sky T+ C dark T + R 2 ) where: T is the total integration time for the image (secs) C star is the count rate of a star in the image (ADU/sec/pixel)

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A bit about CCD imaging

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  1. A bit about CCD imaging Paul McGale

  2. Signal-to-noise ratio SNR = CstarT / √(CstarT + CskyT+ CdarkT + R2) where: T is the total integration time for the image (secs) Cstar is the count rate of a star in the image (ADU/sec/pixel) Csky is the count rate from the sky (ADU/sec/pixel) Cdark is the dark count rate from the CCD (ADU/sec/pixel) R is the readout noise from the CCD (ADU/pixel)

  3. Sub-exposure stacking efficiency (1)e.g. 1 long exposure vs. average of 10 short ones E = √[(x + y) / (x + 10y)] where x is CstarT+ CskyT+ CdarkT y is R2

  4. Sub-exposure stacking efficiency (2)** Dark sky **

  5. Sub-exposure stacking efficiency (3)** Light-polluted sky **

  6. Number of sub-exposures (1) For a bright object SNR ≈√(CstarT) ≡√(N, the number of sub exposures) How does SNR change with increasing N? Rate of change in SNR with N is 1/√(4N) • stack 4, rate = 1/4, SNR decreasing quickly • stack 25, rate = 1/10, SNR decreasing slowly

  7. Number of sub-exposures (2) Faintest part of object visible has SNR=3 i.e. CstarT/ √(CstarT+ CskyT) = 3 Solve for Cstar(snr=3): Cstar(snr=3) = 9 + √(81 + 36CskyT)/(2T) or for N sub-exposures, length t Cstar(snr=3) = 9 + √(81 + 36CskyNt)/(2Nt)

  8. Number of sub-exposures (3)

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