Lecture 2

Question 1

CCDs and CMOS detectors work

Answer 1

via the photoelectric effect in a semiconductor

Question 2

Semiconductors

Answer 2

In an isolated atom, the atomic energy levels are well spaced out

in solids, atomic levels form blended bands. the low energy bands are filled by electrons up to the fermi level.

in a semiconductor the last filled level is the ‘valence’ band

the valance and higher energy conduction bands are separated in energy by the band gap

electrons must be able to move between energy levels. so full bands cannot participate

electronic conduction arises when an electron moves from the valence band into a higher energy state.

Question 3

How can a valence electron gain energy and jump the band gap?

Answer 3

Thermally
or from the photoelectric effect

Question 4

to improve a semiconductors conduction, and help store the charges from the photo-electrons, we use

Answer 4

doped semiconductors

• adding a small amount of a different atom to our semiconductor

Question 5

p-type

Answer 5

fewer valence electrons so extra hole

Question 6

n-type

Answer 6

more valence electrons so extra e-

Question 7

Silicon (MOS) transistor

Answer 7

a thin insulator layer of SiO2 is placed onto the p-type Si and on top of this metal electrode

the positive holes are driven away from the small positive bias voltage near the electrode = depletion region

electrons migrate to near electrode

Question 8

depletion region

Answer 8

is the region where the positive holes are driven away from the small positive bias voltage near the electrode

Question 9

full well capacity

Answer 9

maximum number of electrons that can be held before pixel saturates

Question 10

CCD readout

Answer 10

CCDs are readout by applying sequences of voltages along the columns and down the rows of the CCD, transferring charge from one pixel to the next.

For speed all the columns are shifted at the same time.

Question 11

Analogue to digital converter:

Answer 11

converts voltage to data numbers (DN)

Question 12

3 phase read out scheme

Answer 12

each pixel has 3 electrodes connected in parallel at voltages Φ1, Φ2, Φ3

voltage is varied, allowing charge to migrate but also be kept separate

Question 13

contributions to CCD data numbers (formula)

Answer 13

Xij = Pij + Tij + Eij + Cij

Question 13

Pij =

Answer 13

contributions from pixel charge due to photons from the astronomical source

Question 14

Tij =

Answer 14

contribution from pixel charge due to thermal effects (dark current)

Question 15

Eij =

Answer 15

contribution from readout process

Question 16

Cij =

Answer 16

contribution from pixel charge due to cosmic ray hits

Question 17

Thermal noise Tij or dark current arises from

Answer 17

thermal energy in the CCD material, leading to lattice vibrations, called phonons

energy of these vibrations can create electron-hole pairs in the absence of illumination

Question 18

the electrons give a DN signal, Tij and carry a

Answer 18

statistical fluctuation, ΔTij which varies in time

Question 19

dark frames

Answer 19

are exposures with no illumination falling on the CCD

dark frames are exposed for long enough to capture thermal patterns

Question 20

Electronic noise Eij

Answer 20

each stage of the photo-electrons to DN conversion can contribute noise

Question 21

electron noise can arise in

Answer 21

transfer of charge from pixel to pixel

amplification of readout voltage - need low-noise amplifier

measurement of amplified voltage

Question 22

quantisation noise

Answer 22

conversion of the analogue voltage into a digital signal (DN) in the ADC introduces an additional error term

Question 23

Bias frames

Answer 23

are exposures of zero duration, without light falling on the CCD, which capture the various sources of Eij (not thermal noise)

Question 24

bias frames are needed to

Answer 24

quantify the effect of the ADC offset

Question 25

bias frames provide

Answer 25

pixel-to-pixel structure in the electron noise

Question 26

a series of bias frames is acquired and averaged to

Answer 26

reduce the SNR on the bias frame values

Question 27

output of a CCD is an

Answer 27

analogue signal, time-varying voltage

Question 28

CCD voltage is typically composed

Answer 28

of bias voltage and small fluctuations due to the electrons in the potential wells

Question 29

pedestal voltage

ADC offset voltage

Answer 29

a small offset voltage is added to the CCD output voltage when the output is less than the reference voltage