A check on the Flat field of the Stardust NAVCAM images                       
                                                                              
Jian-Yang Li                                                                  
                                                                              
                                                                              
The flatfield of Stardust images was checked through a simple analysis using  
the images of the Moon obtained during its Earth flyby included in the cruise 
data archive (STARDUST-C/E/L-NC-2-EDR-V1.0).  Three non-saturated pairs of    
images taken through Oxygen, C2, and red filters (centered at 633.6 nm, 513.2 
nm, and 712.9 nm, respective) were used in the analysis.  In each pair, the   
first image that was taken earlier is not compressed, and the second is       
compressed by a square-root lookup table as provided by Stardust team in the  
calibration document.  In these images, the Moon is resolved into more than   
500 pixels in diameter.  For each pair, two images were shifted and rescaled  
to align with each other.  The relative shift of the position of the Moon in  
the field of view changes by about 160 pixel in horizontal direction, and     
from 38 to 85 pixels in vertical direction, in the raw image frame.  The      
change of scale of the Moon is only about 0.5% from one image to another, and 
no relative rotation is found.  The alignment is better than 0.1 pixel.       
Slight rotation of the Moon can be recognized from the two frames taken about 
4 minutes apart, but the shift of features is much less than one pixel.       
Finally the ratio of the two images in each pair was calculated to check the  
flatfield.                                                                    
                                                                              
The result ratio images are shown in moon_ratio_oxygen.gif, moon_ratio_c2.gif,
and moon_ratio_red.gif.  For all three ratio images, there is no variations   
greater than 3% found.  The variation in the ratio disk of the Moon is        
comparable to background noise.  The small-scale features in the disk of the  
Moon that are only several pixels in size are associated with the imperfect   
alignment between two frames due to the rotation of the Moon.  This indicates 
that any non-linear flatfield variation across the center 500 to 700-pixel    
region in the CCD is smaller than 3%.  Further, the average ratios from all   
three filters are all unity, suggesting that any linear flatfield variation   
over the distance of the relative shift of the Moon in the two frames of each 
pair is less than 3%.  Considering the effect of compression/decompression    
noise included in one of the two images in each pair, the ratio images        
suggests that the flatfield of Stardust images is beter than 2%.