detector temperature is derived from the bias level using a standard procedure for NICMOS [@Pirzkal2009b]. In addition, @Burke2010 showed that the detector electronics vary between 7 states, resulting in photometric variations approximately at the millimagnitude amplitude, peak to valley (Figure \[fig:get7phases\]). The detector state in a given image is inferred by comparing the bias level in two different quadrants. These 7 states are clearly identified in the XO-2 data.

-   Filter wheel position.\
    The filter wheel has a constant position during an orbit, but experiences small shifts between orbits due to the blank filter element insertion during Earth occultation of the target. These variations are believed to affect the position of the spectral trace, in particular its angle [@Gilliland2003]. The filter wheel as 2 positions.

-   HST phase.\
    The HST orbital phase is known to influence NICMOS data. However, this parameter is responsible for variations of other parameters that are more directly linked to the images, such as the FWHM and the detector temperature [@Wiklind2009].

![image](fig4.png){width="16cm"}

![Variations of the bias level of quadrant 1 (lower left quadrant) versus quadrant 2 (lower right quadrant) for the 3[$^{\textrm{\scriptsize rd}}$ ]{}orbit of the 1[$^{\textrm{\scriptsize st}}$ ]{}visit, relatively to the bias level of the 