Lecture 4

Question 1

Active pixel sensors

Answer 1

is a detector in which individual pixels contain photosensitive material and an amplifier

each pixel can be read out individually

no moving charge about like in a CCD

Question 2

CCD or CMOS - Electronic Noise

Answer 2

CMOS preferred with low electronic noise, dynamic range is also improved

Question 3

CCD or CMOS - Quantum Efficiency

Answer 3

CCD preferred for low operation at low light levels, but CMOS catching up fast

Question 4

CCD or CMOS - Readout rate

Answer 4

CMOS has a faster readout rate but it is usually unimportant

Question 5

CCD or CMOS - Blooming

Answer 5

CMOS preferred but ‘anti-blooming’ techniques help in CCDs

Question 6

CCD or CMOS - Flat Field

Answer 6

CCD is preferred can be made very uniform. Single substrate, single readout process

Question 7

CCD or CMOS - Dark Current

Answer 7

CCD currently marginally better

Question 8

CCD or CMOS - Spectral Coverage

Answer 8

CCD better outside the optical range. But CMOS can count individual high energy photons

Question 9

CCD or CMOS - Flexibility

Answer 9

CCD readout needs circuits and CMOS readout needs software & computing power

Question 10

CCD or CMOS - Power

Answer 10

Low power means CMOS preferred for space. But few space qualified CMOS detectors

Question 11

Converting DN into physical units

Answer 11

1. Convert DN to photo-electrons per pixel
2. Convert photo-electrons per pixel to photons per pixel and CCD illumination
3. Convert illumination to flux arriving at telescope

Question 12

Converting DN to photo-electrons per pixel (formula)

Answer 12

N = n/g ± σ

N = DN read out

n = number of photo-electrons

g = gain

σ = readout noise

Question 13

the photon-counting noise is given by (formula)

Answer 13

σn = √Ng

Question 14

gain calibration (formula)

Answer 14

σ^2(N) = σ^2 + N/g

Question 15

from photo-electrons to CCD illumination (formula)

Answer 15

nij = Nij g

Question 16

The number of photons incident at that pixel is (formula)

Answer 16

nph = nij/Q

Question 17

the mean wavelength of the photons detected is (formula)

Answer 17

λmean = ∫ P(λ)s(λ)λdλ /
∫P(λ)s(λ)dλ

Question 18

the pixel illumination (formula)

Answer 18

J = nph h vmean/Apix Δt

Question 19

from CCD illumination to flux

Answer 19

J = F/θ^2 (D/f)^2

Question 20

so a pixel of dimension y subtends

Answer 20

θ = y/f