Representative SCUBA sensitivities
SCUBA's sensitivity: NEFDs |
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SCUBA's sensitivity is measured by the Noise Equivalent Flux Density (NEFD), which is equivalent to the flux of an object which can be detected at S/N = 1 in 1 second of integration. At all wavelengths the NEFD is a function of the transmission of the atmosphere, where
transmission = exp(-opacity*airmass).
In this plot the data points show measured values for the current wideband filters at 450 and 850 microns; the red lines show fits to these data, formulae for which are shown on the plot. Note that the NEFD can only reliably be determined from observations of faint sources or blank sky; with obvious sources the shot noise causes an overestimate of the NEFD. Simple instructions for calculating the NEFD of SCUBA at any time are given below.
- From Photometry observations
- Reduce calibrator and science data in standard way, being careful to use the correct opacity value in both.
- Use the SURF command "stats" to determine the rms - look for the "_PEAK" result.
- Calibrate this value using the result of the calibration observation.
- Multiply calibrated rms by sqrt(18s) (since each integration takes 18s) - result is the NEFD, in mJy/sqrt(Hz) (which is the unit in the plot above).
The NEFD derived in this way relates to the time taken to reach a given noise level (sigma) as follows:
t = (NEFD/sigma)^2
e.g. to achieve an rms noise of sigma = 5 mJy at 850 microns assuming a "typical" NEFD of 90 mJy/sqrt(Hz) takes about 5.4 minutes, not including any overheads.
- From Jigglemap observations
- Reduce calibrator and science data in standard way, being careful to use the correct opacity value in both.
- Use the "image regions" option in GAIA or other technique to determine the rms.
- Calibrate this value using the result of the calibration observation.
- Multiply calibrated rms by (sqrt(Number of integrations * 128s))/4 (since each integration takes 128s, but oversamples by a factor of 4) - result is the NEFD, in mJy/sqrt(Hz)
The NEFD derived in this way relates to the time taken to reach a given noise level (sigma) as follows:
t = 16 * (NEFD/sigma)^2
e.g. to achieve an rms noise of sigma = 5 mJy at 850 microns assuming a "typical" NEFD of 90 mJy/sqrt(Hz) takes about 1.44 hours, not including any overheads. This represents about 40 integrations.
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