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SFC and Solar Radius

Image name: SFC_and_Solar_Radius.png (click image to enlarge)
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Date submitted: 20-Feb-1998

I have studied the newly generated leak images (SFC) that are stored on the
/isass0data4/yohkoh/ydb/sfc_orb_correct directory at ISAS.  As the name
of the directory indicates, these images were made taking into account the
orbital dependence of the dark images that were subtracted.

I confine my attention to the third stage of the entrance filter opening,
i.e., between August 1996 and January 1998.  Because of the pinholes, it 
seemed to me more important to study the leak images in thin Al filter than
in other filters.   

In January 1998, the SXT-U PI Professor Acton proposed to add the seasonal
variation of apparent solar radius as a criterion for selecting the leak 
images in processing X-ray images.  Previously, a leak image had been 
selected solely on the basis of how close its pointing was with respect
to the X-ray image.

Here, I compare the signals of the leak images in thin Al filter with the 
variation of solar radius.  There are a total of 157 thin Al images, but
only those taken at normal pointing are selected (146 of them).  Three 
signals are extracted: total counts from the entire images, counts from the 
eastern pinhole, and counts from the western pinhole (see the marked areas 
in the image shown at the bottom left corner).  The total counts from the 
entire images generally trace the variation of solar radius, but the signals 
from the pinholes may depend also on the pointing.  The disk center 
positions of these leak images in terms of the CCD coordinate system are 
given in the plot at the bottom right.

Nariaki Nitta, 20-Feb-98

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