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In Fig. 1 we show an example of large format (512
1024 zoomed)
internal flatfield obtained with the F/96 relay of the FOC. The image has had
an unsharp masking technique applied in order to make the various defects and
blemishes more obvious. As a first step in this discussion, I will try to
identify some of the more important features beginning with those which are
there for a purpose.
- The first deliberate feature is the grid of reseau marks. This
grid comprises 17 rows and 17 columns with a nominal spacing (prior to detector
distortion) of 60 pixels. The reseau marks are etched onto the first of the
bialkali photocathodes in the intensifier tube from where they cast shadows onto
the TV target. This grid is used to determine the geometric distortion of the
detector. Two things should be noted, firstly the grid is rotated by
about 7
with respect to the frame axes, and secondly the grid is off
center by about 70 pixels in the 
direction, resulting in the loss of one
complete row of the grid, and an absence of reseaux across the top of the image.
The significance of this is discussed in the section on geometric distortion.
- The second deliberate feature visible in F/96 external images (but not
internal flat fields) are the occulting fingers. The occulting fingers,
which cast very clear shadows on the full format science data, project into the
images from the top right and bottom left edges of the frame.
Several obvious artifacts and blemishes are also visible, most notably:
- Vignetting by a square baffle located just upstream of the detectors
causes
the large wedge-shaped shadow at the top left and the smaller vertical wedge at
the bottom left.
- The central 512512 format causes the square ``burn in'' feature
seen at the center of the image.
- Vertical striping can be seen at the right side of the image. This is
referred to as ``scan distortion'' (see § 4).
- The horizontal ``ripple'' seen at the bottom left, is thought to be due to
a ripple instability of the coil drivers when the frame scan begins.
Other, less obvious features include a narrow line (generally referred to as the
``flyback''), which runs from the bottom left to the upper right corners due to
the read beam not being completely blanked when it returns to the beginning at
the end of each frame scan. This is more noticeable in the smaller formats.
The narrow horizontal features at the right edge, (
= 256, 512, and 768) are
due to noise glitches in the scan coil driver, caused by changes in the most
significant bits of the line counter. Finally there are several patterns
present; these are discussed in § 7.
For any given wavelength, a single full format (10241024) flatfield is
available and smaller formats use the appropriate subset of this image. Since
the full format flatfields are themselves heavily smoothed (using a Gaussian
filter with 
15 pixels), the majority of these blemishes are not
removed by the flatfielding process. Also, some of the features such as the
scan distortion and the rippling at the bottom left, involve changes in the
geometric distortion, not just sensitivity.
It has proven impossible to incorporate the blemish information into a data
quality file primarily because they originate from several different locations,
i.e., the intensifier photocathodes, the TV tube, the data store, etc., but also
because many of the marks are not stationary. As an example, the reseau marks
(or any other blemish associated with the photocathode), can move by 2-3 pixels
(around the edge of the frame) due to small changes in the detector distortion.
None of the items discussed in this section will in any way be affected by the
arrival of COSTAR.
Next: Scan Distortion
Up: Restoration of FOC Imaging
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