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Astronomical Data Analysis 2 > Definitions and Explanations > Flashcards

Definitions and Explanations Flashcards

1
Q

CCDs readout the image by

A

moving the stored charge across the device

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2
Q

CMOS readout the image by

A

reading out each pixel individually

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3
Q

CCDs and CMOS detectors work via the

A

photoelectric effect in a semiconductor

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4
Q

full well capacity

A

maximum number of electrons that can be held before pixel saturates

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5
Q

CCD readout method

A

Applying sequences of voltages along the columns and down the rows of the CCD, transferring charge from one pixel to next

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6
Q

Charge transfer efficiency

A

describes the fraction of charge transferred per pixel within the semiconductor

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7
Q

Analogue to digital convertor

A

converts voltage to ‘data numbers’

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8
Q

3 phase readout scheme

A

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

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9
Q

Thermal Noise / Dark Current arises from

A

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

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10
Q

Dark frames

A

are exposures with no illumination falling on the CCD

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11
Q

Electronic Noise

A

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

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12
Q

Electronic noise can arise in

A

transfer of charge from pixel to pixel

amplification of readout voltage

measurement of amplified voltage

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13
Q

quantisation noise

A

conversion of the analogue voltage into a digital signal in the ADC

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14
Q

Bias frames

A

are exposures of zero duration without light falling on the CCD

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15
Q

Bias frames are needed to

A

quantify the effect of the ADC offset

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16
Q

ADC offset voltage

A

the CCD output voltage is compared to a steady reference voltage, and the small difference is amplified.

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17
Q

Flat field

A

represents the response of each pixel to illumination

corrects for non-uniform CCD response,

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18
Q

Taking a flat field

A

exposing the CCD to a uniform light source, then normalising each pixel value by dividing by the average value over all the pixels.

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19
Q

underpreforming

A

pixels producing < 10 DN

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20
Q

Cosmic Ray Spikes

A

if a CCD is exposed for a long time, or a CCD is in space, cosmic rays impact it and cause pixels or groups of pixels to saturate

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21
Q

Correcting for Cosmic Ray Spikes

A

Mean or Median filtering

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22
Q

CCD quantum efficiency peaks

A

in the optical, but the wavelength response can be broadened into the UV by coatings

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23
Q

Anti-reflection coatings improve

A

QE down to about 350nm

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24
Q

Rear-side illumination

A

gives increased sensitivity at λ < 400nm

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25
Q

the limited full well of a CCD pixel limits

A

CCD dynamic range and can lead to blooming

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26
Q

dynamic range

A

is the ratio between the brightest and faintest sources that can be recorded

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27
Q

Blooming

A

photo-electrons overflow from one potential well to the next along conduction paths leading to bright streaks which cannot be corrected.

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28
Q

Active Pixel Sensor

A

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

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29
Q

CCD vs CMOS Electronic Noise

A

CMOS preferred with low electronic noise as each pixel has its own amplifier -> low bandwidth -> low noise

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30
Q

CCD vs CMOS Quantum efficiency

A

CCD preferred for operation at low light levels

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31
Q

CCD vs CMOS Readout rate

A

CCD slower

CMOS faster

usually unimportant

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32
Q

CCD vs CMOS Blooming

A

CMOS preferred but anti-blooming techniques help in CCDs

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33
Q

CCD vs CMOS Flat Field

A

CCD preferred can be made very uniform.

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34
Q

CCD vs CMOS Dark Curent

A

CCD preferred

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35
Q

CCD vs CMOS Spectral Coverage

A

CCD better outside the optical range

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36
Q

CCD vs CMOS Flexibility

A

CCD readout needs circuits

CMOS readout needs software and computing power

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37
Q

CCD vs CMOS Power

A

low power means CMOS preferred for space

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38
Q

In a lab you can take a flat field image using

A

an artificial light source

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39
Q

for a CCD onboard a telescope in space you can take a flat field by

A

using the earth as a flat field source.

You could fix the telescope’s pointing on a distant source, such that the Earth would eclipse the telescope for part of its orbit.

40
Q

Why are the resulting images divided by the flat field

A

if a pixel in a flat field has a value of > 1 means the pixel is ‘over detecting’ compared to the average.

If <1 it is ‘underdetecting’ so to correct for excess in over detecting pixel, and lack in underdetecting pixel divide by flat field.

41
Q

mean-filtering

A

pixel replaced by mean value of neighbours

Mean is not a good representative of this cosmic ray pixel and the neighbouring pixels.

42
Q

median filtering

A

pixel replaced by the median value of neighbour median.

For Cosmic Ray removal, CR pixel value is a lot higher than the neighbouring pixels, a statistical outlier. Hence median value better than mean

43
Q

bright features cause

A

blooming of saturated pixels

44
Q

faint source need

A

highest quantum efficiency

45
Q

wide-field optical imaging needs

A

low readout/dark current

46
Q

Convolution

A

Describes the effect of one signal on another

47
Q

cross-correlation

A

Describes the similarity of two signals

48
Q

Auto-correlation

A

Measures how well a signal matches a time shifted version of itself

49
Q

Point spread function

A

is the distribution of intensity in the image plane when a point source is viewed through a telescope.

50
Q

the point spread function arises

A

because of a variety of effects: e.g. poor focusing, diffraction, scattered light.

51
Q

can correct for effects of the PSF by

A

deconvolving using the convolution theorem

52
Q

PSF can be measured

A

by observing a bright point source near the target object

53
Q

Aperture photometry

A

place apertures of different sizes around the source and measure the total intensity.

plot curve of growth

54
Q

Lines recorded have a profile of

A

intensity versus wavelength that is a combination of the true profile and the instrumental line profile

55
Q

Phase folding

A

is used for irregularly and poorly sampled data.

56
Q

Phase folding procedure

A

For a range of guess periods T(i), the data is reorganised into bins within this trial period and then averaged to find a systematic pattern. If no pattern try a different Ti.

57
Q

where wavelet analysis is more suitable than the auto-correlation approach

A

looking for example where oscillation is changing in time

So one example is gravitational wave signal of merging blackholes.

58
Q

All parts of a galaxy along a line-of-sight

A

Contribute to its observed spectrum , where different parts have different LOSV which effectively broadens a spectral line

59
Q

if the spectral velocity is dominated by a single v(LOS) we can use

A

cross-correlation to find it

60
Q

if v(LOS) does not align

A

the two signals will be small -> CCF is small

61
Q

if v(LOS) does align

A

the two signals will be large -> CCF is large

62
Q

can estimate v(LOS) by calculating

A

the CCF for many trial values of v(LOS) and S(u-v(LOS)) and finding its maximum value

63
Q

if the signal has characteristics of white noise

A

then the power spectrum is flat i.e. PSD = const

one part of the signal is entirely uncorrelated with any other

64
Q

high pass filter

A

cuts off low frequency signals

65
Q

low pass filter

A

cuts off high frequency signals

66
Q

how to obtain a cleaner time series

A

inverse fourier transform

67
Q

example of phase folding

A

extra-solar planet transits

68
Q

cone of influence

A

the region where the boundary effects are important, resulting in unreliable wavelet power

69
Q

wavelength calibration

A

done using absorption lines from the Earth’s atmosphere

or using a reference spectral emission lamp

identify strong lines and fit the dispersion curve

70
Q

Extracting spectrum

A

often CCD is a 2D image of (λ,y)

get spectrum for particular y or spatially integrated Σy

71
Q

Correcting for tilt

A

when the spectrum not aligned to CCD x,y

(i,j)tilted ->(rotated) (i’,j’)

72
Q

Interpolation

A

after tilt correction the pixel values might not be integrs

-> interpolate tilt corrected values into int pixel locations

73
Q

Sky background

A

Background sky sources due to atmospheric scattering.

sky background unlikely constant with λ.

74
Q

Spectral Diagnostics

A

properties of remote source from spectral measurements

75
Q

spectral lines are produced by

A

a hot tenuous gas i.e. gas clouds

76
Q

significance of an observation

A

is defined as the signal expressed as a number of standard deviations

77
Q

Equivalent width

A

is a way to measure the total absorption in a spectral line

78
Q

optical depth

A

Describes the absorption of photons

79
Q

atomic line diagnostics

A

involve measuring the ratio of intensities between two emission lines from the same element

80
Q

term or Grotrian diagrams

A

Illustrate the possible transitions in an element

81
Q

most common spectroscopic diagnostics rely on

A

assuming that the radiating gas/plasma is in local thermodynamic equiliibrium

82
Q

in local thermodynamic equilibrium (LTE)

A

the matter is in thermal equilibrium in some small neighbourhood around a point in space

assumes slow changes

83
Q

LTE also means that the system reaches equilibrium via

A

collisions between particles

84
Q

optically thin approximation

A

the radiation escapes the gas/plasma without interaction

85
Q

a collision

A

a free electron perturbs an orbital electron

86
Q

the population ratio of two levels depends on the

A

temperature

87
Q

detailed balance

A

in a true equilibrium the average state does not change, so for every upward transition i -> j must be accompanied somewhere by a downwards transition j -> i

88
Q

ionisation potential, χ

A

the energy required to remove an electron

89
Q

collisional ionisation

A

collisions between fast electrons and atoms in a hot gas can result in the ejection of a bound electron into a free state

90
Q

the statistical weight of a particle

A

is the number of states that it can occupy

91
Q

metastable level

A

the upper level of a forbidden transition

92
Q

finite lifetime of electrons in the excited states

A

Lead to natural broadening and collisional broadening

93
Q

doppler/motion on micro or macroscale

A

thermal, turbulent and rotational

94
Q

particle distribution f(v)dv for LTE

A

is a Maxwell-Boltzmann distribution

95
Q

Ionisation balance curves

A

Shows T range each ion is most abundant

Astronomical Data Analysis 2 (11 decks)

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