About the errors in eis_prep.

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.