XSM FLIGTH CALIBRATION PROCEDURE                                              
                                                                              
XSM PIN detector operation is calibrated onboard by using a Fe55 radioactive  
source attached on the inner face of the shutter. When the the shutter is in  
closed position and the acquisition is on, XSM measures the photons coming    
from the Fe55 source.                                                         
                                                                              
The source is also coverd by a thin Ti foil of 5 microns. Hence, the          
calibration spectrum is composed of four distinct lines, which are Mn K alpha 
(5.9 keV), Mn K beta (6.4 keV), Ti K alpha (4.5 keV) and Ti K beta (4.9 keV). 
                                                                              
The gain and energy resolution can be determined by using the 4.5 keV and 5.9 
keV lines. (Gain defines channel number and energy ralation. FWHM defines     
energy resolution as a function af energy.)                                   
                                                                              
Each of the XSM observation is started and stopped with taking at least four  
calibration spectra, which equals the calibration mode (shutter closed and    
acquisition on).                                                              
                                                                              
During the final data reduction the data must be gain corrected. This means,  
that the energy and channel scales are equal in ARF, RMF and data. FWHM is    
inserted into the RMF! (See the SOFTINFO.TXT)                                 
                                                                              
It is assumed that the gain and offset remains constant during each           
observation. In the case those values start to shift, one has to apply        
interpolative methods to insert correct values into respective ARF's and      
RMF's.