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37.3 Spectrum Shape
The shape of the spectrum and its overall level are closely tied, which is why both sensitivity and vignetting were discussed in the previous section. Here we go into vignetting and related effects a little further.
Shape Effects from the Sensitivity Function: G140L
For most GHRS observations, the observed bandpass samples only a small portion of the spectrum produced by a grating. Therefore, undulations in the sensitivity function, which have length scales of many Ångstroms, at most have a small linear effect across a given spectrum.
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.
Light Falling Off the Diode Array
Thermal drifts caused a spectrum to move on the photocathode, so the GHRS routinely performed a SPYBAL (spectrum Y-balance) to properly center the spectrum on the diode array. A SPYBAL was performed every time a new spectrum element was used (i.e., the first use of a different grating) and approximately every two orbits thereafter. This centering can be important because a given spectrum is tilted across the diode array, and lack of proper centering could result in the ends of the spectrum falling off the array. Again, this is routinely corrected for and only becomes a problem if SPYBALs were suppressed for long exposures (i.e., several orbits).
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.
G270M Vignetting Errors
The pre-COSTAR, in-flight sensitivity calibration for this grating is wrong shortward of 2300Å for the period November 11, 1991 through April 1, 1994. The pre-COSTAR vignetting correction for grating G270M is inadequate shortward of 2300 Å, and the post-COSTAR vignetting correction shortward of about 2150 Å is not handled properly by calhrs because the Y-deflection has not been measured below 2300 Å. The program which was to correct this failed. Please see GHRS ISR 077 if you wish to analyze data taken during this time period with this -vignetting.
The Echelle Blaze Function
The echelle blaze function relates relative fluxes to those observed at the center of a given spectral order. This function was determined by observing the standard star µ Col at a number of different wavelengths in different orders, and relating those observations to others made with first-order gratings. It was impossible to cover fully a free spectra range or to sample every echelle order, so the blaze function is meant to be a reasonable approximation to the true function. An underlying, but unstated, assumption, is that observers used the echelles to measure the strengths or positions of weak spectrum features and were therefore not primarily concerned about the absolute flux level in the final, reduced spectrum.
Inappropriate Background Subtraction
In some cases the shape of a spectrum can be distorted if the background is improperly calculated and subtracted. This was seen in the first couple of years of GHRS operation because the background subtraction software fitted a polynomial to the background before subtracting it from the source spectrum. This was done to preserve shape in the background spectrum, but often the background had very few counts so that the fit was spurious. Modification of the procedure to fit a flat line removed the problem.
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