... Meade Instruments Corporation . ... Meade Pictor XT-Series CCD Autoguider/Imagers . ... Take new sets of calibration frames if the temperature changes significantly.(or change the set you are using or the scaling if you are reusing calibration frames). ... You can also use scaleable dark frames called thermal frames. ... However, if you wish to reuse dark frames with images of different exposure times, you must turn the dark frame into a thermal frame by subtracting the bias information. ...