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6.3.2 Determination of the Flux Values

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 $\leq$ 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:

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 $\pm$1024. Any FN values originally exceeding these limits are clipped and set to the respective limit value.


next up previous contents
Next: 6.4 Associated Flags and Up: 6.3 Description of the Previous: 6.3.1 Determination of the
Karen Levay
12/4/1997