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In contrast to the original IUESIPS PHOTOM, the raw DN values are
converted to linearized (i.e., photometrically corrected) FN values
directly from the effective exposure time associated with each level of
the ITF. For a given pixel the FN associated with level i of the ITF
is:
FNi = T(i)
where T(i) is the effective exposure time in seconds for level i.
There are four basic methods for the determination of the FN of a raw
science image pixel from a single ITF pixel (for the case where the
displacement coordinates of the science image are 0.125 pixel
from the ITF pixel coordinate). In the following discussion, let
DN(ITF1) represent the DN level of the relevant pixel in the first
ITF level; let DN(ITF12) represent the DN level of the relevant
pixel in the twelfth ITF level; let DNraw represent the DN level of
the science image pixel; and let DNsat represent the DN level of
saturation for the relevant pixel. Then the four alternatives are:
- Fully calibrated data: When , the FN is determined with a linear interpolation
algorithm. The interpolation is performed between the two ITF levels
that bound the input DN in intensity.
-
Saturated data: When and , the pixel is considered to be saturated and the FN is
set to a constant (the FN value for the top level of the ITF; see
Table 6.1). DNsat has been determined on a pixel-by-pixel
basis through analysis of the individual ITF curves and is defined to be
the DN at which the slope of the ITF curve (DN versus exposure level)
approaches zero. Since ITF curves vary considerably from pixel to pixel,
the DN level corresponding to saturation varies from pixel to pixel. The
flagging of saturated data is broken down into two separate cases; in
either case, the FN for the pixel in question is set to the FN of the
top level of the ITF:
- Case 1:
- DNraw = DN(ITF12). These pixels are flagged with
the saturation flag of -1024.
- Case 2:
- DNraw > DN(ITF12). These pixels are flagged with
both the saturation flag of -1024 and a positive extrapolation
flag of -256. In this instance, the assignment of an
extrapolation flag serves only to identify the fact that the DN
exceeds that of the top level of the ITF; no extrapolation of the
FN data is performed.
-
Positively extrapolated data: When DNraw > DN(ITF12) and
DNraw < DNsat, the FN is computed by a two-point extrapolation
from the top two levels of the ITF. These pixels are flagged with the
positive extrapolation flag of -256.
-
Negatively extrapolated data: When DNraw < DN(ITF1), the FN
is computed with a two-point extrapolation from the bottom two levels of
the ITF. However, the pixel is only flagged with an indication of
negative extrapolation if the extrapolation is considered excessive. For
this purpose, a ``negative extrapolation reference image'' has been
created by determining the 50% intensity level of the ITF null image,
and smoothing these data with a 5-point boxcar in two-dimensions.
Consequently, negative extrapolations are only considered excessive and
flagged if DNraw < DNneg. ext. ref. image. Excessive negative
extrapolations are flagged with a value of -128.
When there is significant misalignment (>0.125 pixel) between the raw
science image and the ITF, a
4 × 4 matrix of ITF pixels
surrounding the location in question is used to compute the relevant FN.
Using the raw science image DN, an FN value is computed at each of the
4 × 4 locations in the matrix, using the above described
single-pixel scenarios. The median FN in the matrix is computed, and
then deviant values are eliminated from the FN matrix and replaced with
the median FN. FN values are considered to be deviant if they are
different from the median by more than 100 FN. Once the FN matrix has
been defined and deviant values replaced, the matrix is fit with a
spline surface which is evaluated at the final coordinate location
desired for the final FN value.
The FNs determined during the PHOTOM procedure have values which
range between 1024. Any FN values originally exceeding these
limits are clipped and set to the respective limit value.
Next: 6.4 Associated Flags and
Up: 6.3 Description of the
Previous: 6.3.1 Determination of the
Karen Levay
12/4/1997