The absence of pixels containing reliable background fluxes in the Lyman
region of the SWP camera image often results in the failure of
BCKGRD's first pathology test for the first swath and sometimes inaccurate
Pass 1 solutions for the first three swaths. These particular extractions may
result in a constant or even monotonically increasing background solution
towards short wavelengths (spatial direction). In contrast,
swaths taken through a more central part of the image and which are not
intercepted by the target ring, show that the background fluxes should decrease
at the short-wavelength (spatial) end of the swath because the detector
response is low below 1200.
Figure 5a shows the example of a final (Pass 2) background solution
for the order containing Lyman for the image SWP04262. Star symbols
represent Pass 1 solutions either at, or constant-flux extensions to,
the line position of this order. Notice that the leftmost Pass 1 value
attains a comparatively high flux level. This is because no pixels
uncontaminated by interorder flux can be sampled at
the low-wavelength end of their Pass 1 swaths. BCKGRD attempts to
correct for this problem (and only for the short-wavelength corner of SWP
images) by interpolating to fluxes sampled by the fourth swath, which does
reach uncontaminated pixels). Experience shows that this action tends to
undercorrect the ``flux turnup" problem in this region of the camera for
spectra of hot stars. We note that the BCKGRD assumes explicitly that maximum
fluxes (within a tolerance criterion set by experimentation) should not be
reached for the first few swaths in the Pass 1 operation. This expectation
is valid for images at most epochs, but as we demonstrate below that it may
not have been accurate for early-epoch images.
(Panel b): A comparison of the final background solutions with the
Pass 1 for SWP35620, a spectrum taken of Sco midway in the IUE mission.
Diamond points in this and the next panel fall off scale.
(Panel c): A comparison of the final background solutions with the
Pass 1 for SWP55997, a spectrum taken of Sco late in the IUE mission.
(Panel a): A comparison of the final background
flux solution for order m = 113 in SWP04262, a spectrum of Sco
taken early in the IUE mission lifetime. Star symbols represent the solutions
from Pass 1 at the intersection of this order, while diamond figures are
those intersections from the first few Pass 1 swath solutions which falsely
rise near the short wavelength corner of the image. The Pass 1 solution is
ordinarily not saved in NEWSIPS processings and was recovered for this
figure from a special processing.
Next: Analysis of Scorpii Background Up: Overview of NEWSIPS Background-Sampling Previous: Final Background Solutions (Pass
(Panel b): A comparison of the final background solutions with the Pass 1 for SWP35620, a spectrum taken of Sco midway in the IUE mission. Diamond points in this and the next panel fall off scale.
(Panel c): A comparison of the final background solutions with the Pass 1 for SWP55997, a spectrum taken of Sco late in the IUE mission.