Occasionally circumstances in the interorder fluxes lead to solutions that are slightly unstable, producing wiggles in an interpolated region that go beyond the flux range of sampled pixels at the two spatial ends of the camera. Such occurrences may be caused by abnormal conditions affecting the image (e.g. target-ring glow, cosmic ray hits, LWR flare, flux down-turns at camera edge). A series of eight "pathology tests" has been added to BASISMOD to protect against blind solutions at the end of Pass 1 that do not agree with simple and often correct interpolations. These checks generally rely on a comparison of fluxes at two or more pixels along the swath or on a ratio of smoothed flux ranges. The rms statistic computed from local raw background fluxes is a convenient unit of measure for flux ranges because it does not rely upon source brightness, exposure time, or an arbitrary flux level. In most cases a failure of a solution in a pathology test causes either the PSF information not to be used in Pass 1, the degree of the polynomial fit to the interorder data to be reduced, or both. Lowering the fitting degree has the effect of removing extra wiggles in the solution; however, the degree of the Chebyshev fit is never reduced below 3. In those cases where superfluous wiggles in the IOR region persist stubbornly after a few trials, a simple linear interpolation is adopted between "good" regions. This may occur for spatial positions toward the short wavelength (spatial) end of the IOR for certain swaths having reliable background samplings at the target edge. These tests are used only for continuum source images in Pass 1. Therefore, the final output background vectors from Pass 2 are still guaranteed to be pure continuous Cheybshev functions.
The data pathology tests keep track of the number of iterations through the swath-fitting routines, as well as a history of previous failure modes. Certain pathologies have been found to oscillate sometimes between two types of failures. If such patterns are detected, a fall-back exit option is adopted. Table 1 summarizes the pathology checks and the strategies for circumventing them in subsequent iterations of the swath-fitting routine.
BASISMOD establishes a hierarchy of severity for the various pathologies it identifies. Tests 1-4 and 8 are the most critical, and the responses to failure of them are therefore more severe. Each test looks for a different and well known pathology. For example, Test 4 and Test 6 search for subtlely different degrees of a similar pathology, so their response strategies are different. Test 2 searches for a solution minimum within the IOR (exceeding a tolerance) compared over the rest of the swath whereas Test 8 searches for a flux minimum in the solution anywhere within the swath. Test 2 permits a milder fix while Test 8 looks for a more severe IOR minimum. If Test 8 detects this condition, a linear interpolation between nc and 50 pixels beyond nd is used, with points in the halation ramp taken from the measured interorder flux values, without consideration of the PSF. If Test 2 repeatedly fails, the IOR-minimum pathology is guaranteed to be addressed by linear interpolation in Test 8.
Experience has shown that Test 7 is one of the most commonly occurring rejections of an initial solution. This test rejects having a local maximum at the high-sample number end of a swath. These rejections are often caused by an abnormal global shift of the spectral image format combined with target ring glow, but they can result also from any systematic rise in flux toward the short wavelength (spatial) end of the camera as well. If the pathology cannot be circumvented by decreasing the degree of the Chebyshev fit or by resorting to ignoring the PSF information in the swath-fitting routine, the solution is admitted. Test 7 is similar to Test 3 but checks for two maxima in the the middle of the short-wavelength (spatial) end of the image. In Test 3 one of the maxima searched for must be within the IOR, but this is not necessarily true for Test 7. Test 3 also checks for flare discharges which can occur in the long-wavelength corner of LWR images and computes two possible solutions: the first is a pure Chebyshev solution with the flare points deweighted and the second is a quadratic extrapolation of the solution without the flared region included. The solution with lower fluxes in the upturned flare region is then adopted, as this solution causes ringing less frequently.
A rejection by Test 5a, which searches for a maximum in the IOR, causes the degree of the fit to be decreased on the first iteration and halves the trial PSF slope factor. It interpolates linearly over this region if additional iterations are necessary. Test 5b searches for an excessive minimum for spatial positions at the short wavelength (spatial) end of the image and the beginning of the IOR. A minimum in the IOR is usually caused either by a bright target ring at the beginning of the swath, or an overestimate of the PSF. Such conditions cause a false overcorrection for background contamination within the IOR and hence a background solution which is too low. Both tests are addressed by decreasing the Chebyshev degree.
Table 1: Recourses to Various Pathological Trial Solutions
TEST NUMBER CONDITION/RECOURSES
1 Solution monotonic between nd & nf:
Short Swaths If poly. degree > 3, decrease degree;
otherwise linearly interpolate
2 Min. in IOR between nc & nd;
IOR Min. decrease degree,
on third attempt, give up
3 Max. at high sample #:
Max. near swath end decrease degree
[LWR Flare] [Lower weights for points in flare]
4 Max. in IOR is max. for swath:
IOR Max. decrease degree
5a Max. at low sample #:
Max. at or near swath start decrease degree & reduce
slope of PSF model
5b Min. at low sample #:
Min. at or near swath start 1st time: go to Test #6;
thereafter: decrease degree or
interpolate linear solution
6 Max. within IOR:
IOR max. decrease degree or
interpolate linearly across IOR
7 Max. at high sample #:
Max. at end decrease degree
8 Absolute min. is in IOR:
IOR min. interpolate linearly in IOR
Absolute!