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: http://www.stsci.edu/documents/dhb/webvol2/c12_ghrserrors.fm3.html
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G140L is an exception to this rule because it produces spectra nearly 300 Å long. Spectra from grating G140L may have modest shape effects that have their origin in the sensitivity function. These can be especially pernicious near Lyman- because the breadth of that feature in the standard stars we observed prevented a good determination of the shape of the underlying sensitivity function at those wavelengths.
A simple calculation can give us some idea of how large this effect can be. For example, the G140L spectrum of a point source may fall off the diode array due to drift as the temperature changes. Ignoring the width of the spectrum (something on the order of the size of the SSA or about 8 deflection units) the ends of the spectrum will differ by about 45 defection units and will be within about 9 deflection units of the edge of the diode array. If we assume a worst case drift of about 25 deflection units (seen over 10 hours), we find that at the end of this time about 25% of the spectrum will have fallen off the edge of the diode array!
In the case of an extended object uniformly filling the LSA, the effect is much more pronounced. In this case the width of the spectrum cannot be ignored. The width is equal to the size of the aperture or about 64 deflection units. For the case of a G140L observation of an extended object in the LSA, we start out with a loss of light. The spectrum is already falling off the array with the ends experiencing about 30% light loss. In the time it takes to drift 25 deflection units, some part or all of the spectrum may fall off the array, resulting in a significant reduction in -signal.