SENSFUNC, CALIBRATE: Units and definition of the sensitivity function
Frank Valdes wrote on Aug 31, 1999
Q: For the graph type `s' -- what exactly is the y-axis scale
(this is `sensitivity' vs wavelength. I would have guessed
that a total-instrument efficiency would have been a good
candidate, but the numbers don't look right.
A: Below is the relevant section of the help page for SENSFUNC. I admit
that the answer to your question is not completely obvious. What you
want is from equations (1) and (2):
S = 2.5 log (O / (T B F))
It is clear in the help for CALIBRATE, also given below, which is what
applies the sensitivity curve derived by SENSFUNC.
Cheers,
Frank Valdes
>From help for SENSFUNC:
The calibration factor at each point is computed as
(1) C = 2.5 log (O / (T B F)) + A E
where O is the observed counts in a bandpass of an observation, T
is the exposure time of the observation, B is the bandpass width, F
is the flux per Angstrom at the bandpass for the standard star, A
is the airmass of the observation, and E is the extinction at the
bandpass. Thus, C is the ratio of the observed count rate per
Angstrom corrected to some extinction curve to the expected flux
expressed in magnitudes. The goal of the task is to fit the
observations to the relation
(2) C = S(W) + AE(W)
where W is wavelength, S(W) is the sensitivity function, and E(W) is
a residual extinction function relative to the extinction used in
(1). In later discussion we will also refer to the residual
sensitivity which is defined by
(3) R = C - S(W) - AE(W)
The sensitivity function S(W) is output as an one dimensional image
much like the spectra. The sensitivities are in magnitude units to
better judge the variations and because the interpolation is
smoother in the logarithmic space (mags = 2.5 log10[sensitivity]).
>From help for CALIBRATE:
The sensitivity spectrum is in units of:
2.5 * Log10 [counts/sec/Ang / ergs/cm2/sec/Ang].
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Last post on Aug 31, 1999