Aperture Orientation and Geometry in the IUE Data                              
                                                                               
The basic layout and geometry of the spectrograph slits is described           
by Schiffer (1980) [SCHIFFER1980].  A brief review is given here, with         
specifics related to the comet data included.                                  
                                                                               
The diagram "spatial_geometry.png" shows the basic geometry of the             
Long Wavelength, Long Aperture (LWLA) slit, with respect to the FES            
reference frame.  The spectrographs and the FES are fixed with respect         
to the spacecraft, so their relative positions do not change.  The +X          
and +Y axes are defined by the rows and columns of the FES camera.  As         
shown in the diagram, the -Z axis is directed along the line of sight,         
which also corresponds to the negative roll axis of the spacecraft.            
The projected sunward direction always lies at an angle of 28.3                
degrees clockwise from the -Y axis, and this direction corresponds to          
the negative pitch axis.  The yaw axis completes the coordinate                
system.                                                                        
                                                                               
Because the spacecraft rolls to keep the sunward direction fixed, the          
North and East directions will change position on the camera as                
different objects are observed.  (Note that there are an odd number of         
reflections in the instrument, so East is measured 90 degrees                  
clockwise from North in the diagram.)  The orientation of the camera           
with respect to North is defined by the Solar Vector Position Angle            
(SVPA) which is the angle between North and the sunward direction, as          
measured clockwise.  The SVPA is listed in the header for each of the          
observations.                                                                  
                                                                               
The axes of both the LWLA and SWLA lie at an angle of 73 degrees               
clockwise from the sunward direction.  The relative layout of the              
four apertures is shown in the diagram "aperture_positions.png", with          
the distances between the apertures listed in arcseconds and the               
relative positions listed in FES pixels.  (The X dimension of a pixel          
is 0.268 arcsec and the Y dimension is 0.262 arcsec.)                          
                                                                               
When spectra are obtained of a comet's coma, there is an offset of the         
slit relative to the optocenter of the comet.  In these cases, the             
images contain the keywords SLIT_OFFSET_ARC_DISTANCE and                       
SLIT_OFFSET_POSITION_ANGLE, which define the offset of the slit in             
distance and postion angle (measured from North through East) from the         
nucleus.                                                                       
                                                                               
Written:  2005-10-11, T.L. Farnham, SBN