Long IUE exposures characteristically contain ``bright spots'', i.e., pixels with unusually high data number (DN) values which are comprised of discrete impulse noise often reaching the saturation level. Such bright spots are thought to be caused either by permanent blemishes in the target surface, by extraordinarily sensitive (``hot'') pixels which result in recurrent bright spots at fixed locations, or by radiation-induced events within the UV converter which result in randomly placed, nonrecurrent bright spots (Ponz, 1980).
Ponz (1980) has described an algorithm for detecting in raw images bright spots of either kind on the basis of their limited spatial extent and unusual brightness values, primarily through a median filtering technique. The NEWSIPS bright-spot detection algorithm is based on the procedure used in standard IUESIPS processing, which incorporates this method to flag bright spots as described below.
Let DN (i,j) be the DN value of the pixel at line i, sample j. Further, let AVE and MED represent operators which return the weighted average and median values of their argument, respectively. Then the pixel at (i,j) is detected as a bright spot if:
is a DN threshold value, and (k,l) are positional
elements of a 7-pixel spatial window centered on the pixel at (i,j)
and oriented on a diagonal (i.e., nearly along the dispersion direction).
The condition in the first equation is included to reduce the number of
times the median operation in the second equation is performed.
In practice, the spatial windows are weighted according to the weights
(0, 0, 1, 0, 1, 0, 0), and a threshold value of = 90 DN is
employed. The area of the image searched for bright spots corresponds to
the entire camera faceplate regardless of dispersion. This differs from
the IUESIPS approach, which only examined the regions containing
spectral information. Pixel locations detected as bright spots are
written to an output flag file
(Chapter 3) subsequently read by the
spectral extraction routines (
Chapters 9
and
10) so that extracted fluxes
derived from bright spot pixels may be flagged appropriately.
Several reports have been written that list the permanent or recurrent
bright spots for the three cameras. These bright spots are not
automatically flagged like the permanent ITF artifacts (see
Chapter 6.4.3.1
), but rather only if they trigger the bright-spot
detection algorithm. Ponz (1980) has published partial listings of
recurrent bright spots in the LWR and SWP cameras which are listed in
Tables 4.1 and 4.2. The table entries include the line and
sample positions in the raw frame of reference and the approximate
corresponding wavelengths for the various dispersion modes and
apertures. The ``B'' notation means the background spectrum is affected.
This work has been supplemented by Imhoff (1984a), who provided
positions of additional permanent blemishes in the LWP and LWR cameras,
given in Table 4.3.
| Source Type | ||
| Camera | Point | Extended |
| LWP | None | None |
| LWR | 2256 | 2256 |
| 3087 | 3087 | |
| SWP | 1279 | 1279 |
| 1288 | 1288 | |
| 1491 | ||
| 1535 | ||
| 1663 | ||
| 1750 |
2482Å in order 93