Charge Transfer Efficiency
Charge Transfer Efficiency ( CTE ) is a property of a camera with a CCD sensor .
The charge induced in a pixel of the camera is shifted column by column and row by row and then recorded in the readout amplifier. The CTE is the ratio of the charge after moving to the charge before moving from one pixel to the next. The CTE describes the loss of charge when the charges are moved on the way to the readout amplifier.
Significance of the CTE for CCD sensors
The value of the CTE is usually very close to 1 (never higher than 1, otherwise charge would be created when moving). Typical values are between 0.9999 and 0.999995. Over many shifting steps this multiplies to a measurable effect (example: 0.9999 1000 = 0.90; this means approx. 10% loss for 1000 steps with CTE = 0.9999). Determining the CTE is particularly important for calibrating CCD sensors in X-ray astronomy in order to obtain correct measurements. See e.g. B. the EPIC camera of the X-ray satellite XMM-Newton .
Measurement of the CTE
The CTE can be measured in that the surface of the CCD is illuminated exactly evenly and the difference between the measured charge of pixels near the readout amplifier and far away from the readout amplifier is determined. Since a sufficiently uniform illumination is difficult to achieve and the light sensitivity of the individual pixels is not exactly the same, an X-ray method is also used. A radioactive source is used to generate X-ray quanta of defined energy which, when absorbed in the CCD, generate a precisely known amount of charge. The iron isotope 55 Fe is mostly used for this. In the beta decay of 55 Fe to 55 Mn, so-called Mn-Kα x-rays with 5.899 k eV energy are generated, which in silicon generate 1620 electron-hole pairs during absorption. In addition to the CTE, this measurement also determines the amplification of the readout amplifier.
literature
- James R. Janesick, Scientific Charge-coupled Devices , Chapter 5, Charge Transfer , pp. 387ff