def A_pixel(info_dict):
    """
    Compute the projected area of one pixel on the sky

    Parameter
    ---------
    info_dict: dictionnary

    Returns
    --------
    A_pixel: float
             projected area of one pixel on the sky (arcsec2/px) 
    """
    pix_sky_area = info_dict['pixelScale_X']*info_dict['pixelScale_Y']
    info_dict['A_pixel']=pix_sky_area
    return info_dict