This page (revision-25) was last changed on 02-Feb-2017 13:19 by PeterYoung

This page was created on 12-Apr-2007 18:31 by UnknownAuthor

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25 02-Feb-2017 13:19 394 bytes PeterYoung to previous
24 07-Jul-2010 22:42 336 bytes PeterYoung to previous | to last
23 16-Jun-2010 22:15 359 bytes PeterYoung to previous | to last
22 16-Nov-2009 22:14 332 bytes PeterYoung to previous | to last
21 22-Jul-2008 11:12 299 bytes David R Williams to previous | to last

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At line 1 removed one line
About the errors in eis_prep.
At line 3 removed 34 lines
A new version of eis_prep, due to Mike Marsh, has been uploaded in ssw.
It incorparates a corrected expression of the errors.
In the old version, it was assumed that the fractional error of the
intensity in cgs units was equal to the fractional error of the intensity
in DN, rather then in photon units.
That is, given d, p, and I the intensities in DN, photons and erg/cm2/s/sr/A,
it was set in eis_prep
err(I)/I = err(d)/d
instead of
err(I)/I = err(p)/p
Actually, p is almost proportional to d, with a slight dependence on the
wavelength: that is p = f(d) ~ K*d, with K almost constant.
Errors are computed in DN as
err(d) = sqrt(d+2.5^2)
being 2.5 the value of the read out noise. It follows then that
err(p) = sqrt(p+f(2.5)^2) ~ sqrt(K*d+(K*2.5)^2)
being f(2.5) the read out noise in photon units.
Thus, the correct expression of the fractional error
err(I)/I = err(p)/p ~ sqrt(K)/K * err(d)/d
differs from the previous one by the factor 1/sqrt(K).
This generated incorrect error values; the new values can be up to 50%
larger than the previous ones.
This page (revision-25) was last changed on 02-Feb-2017 13:19 by PeterYoungTop