The SXT makes use of a front-illuminated, virtual-phase CCD to detect both X-ray and visible photon fluxes. While it is possible to operate CCDs in a photon counting mode, the SXT CCD is operated in a camera-integrating mode. This means that the shutter is opened for a short interval during which photon events are summed into the CCD's pixels. The amount of charge read out of the CCD by the camera represents the energy in the photon flux which has been integrated in each pixel, not the actual number of photons. If the incident beam is mono-energetic, then the amount of charge is directly proportional to the photon flux. However, since the Sun's X-ray flux is far from mono-energetic, we must model the response of the CCD for a given solar spectrum.
When a photon is absorbed in the CCD it produces electron-hole pairs in
the silicon. The number of photons, 
that produced the
charge or number of electrons, 
, which is read out of the
CCD is given by
where
is the incident photon wavelength (Å) and 3.65 eV/e
is the energy required to produce one electron-hole pair in silicon. Note that this relation applies only to X-ray energies. The SXT CCD pixels have a full-well capacity of about 250,000 e
The CCD camera electronics contain analogue-to-digital converters. In this Guide the digital camera output is referred to as DN or Data Number. These values are read from the camera with 12-bit accuracy, but in the case of X-ray images, are compressed to 8-bit values using a pseudo-square root compression algorithm prior to being telemetered. Thus, when looking at raw SXT images, one should remember that the intensity values are typically compressed. The relation between the DN values and the integrated charge or number of electrons is given by
(e
/DN) is the so-called gain constant and 13 is a digital offset in the CCD camera for full resolution images (see Section 3.2.9 for more information). The value of
was adjusted to be about 100 e
/DN prior to launch. This value is a compromise to enable adequate performance at both X-ray and visible light energies. The routine SXT_DN2PH will convert SXT DN values into photon counts for a user-supplied wavelength. The digital offset is removed automatically during dark frame subtraction (SXT_LEAKSUB, SXT_DARKSUB), since the dark frame images also have the same digital offset.