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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
the limited full well of a CCD pixel limits
CCD dynamic range and can lead to blooming
26
dynamic range
is the ratio between the brightest and faintest sources that can be recorded
27
Blooming
photo-electrons overflow from one potential well to the next along conduction paths leading to bright streaks which cannot be corrected.
28
Active Pixel Sensor
an APS detector is a detector in which individual pixels contain the photosensitive material and an amplifier
29
CCD vs CMOS Electronic Noise
CMOS preferred with low electronic noise as each pixel has its own amplifier -> low bandwidth -> low noise
30
CCD vs CMOS Quantum efficiency
CCD preferred for operation at low light levels
31
CCD vs CMOS Readout rate
CCD slower CMOS faster usually unimportant
32
CCD vs CMOS Blooming
CMOS preferred but anti-blooming techniques help in CCDs
33
CCD vs CMOS Flat Field
CCD preferred can be made very uniform.
34
CCD vs CMOS Dark Curent
CCD preferred
35
CCD vs CMOS Spectral Coverage
CCD better outside the optical range
36
CCD vs CMOS Flexibility
CCD readout needs circuits CMOS readout needs software and computing power
37
CCD vs CMOS Power
low power means CMOS preferred for space
38
In a lab you can take a flat field image using
an artificial light source
39
for a CCD onboard a telescope in space you can take a flat field by
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
Why are the resulting images divided by the flat field
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
mean-filtering
pixel replaced by mean value of neighbours Mean is not a good representative of this cosmic ray pixel and the neighbouring pixels.
42
median filtering
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
bright features cause
blooming of saturated pixels
44
faint source need
highest quantum efficiency
45
wide-field optical imaging needs
low readout/dark current
46
Convolution
Describes the effect of one signal on another
47
cross-correlation
Describes the similarity of two signals
48
Auto-correlation
Measures how well a signal matches a time shifted version of itself
49
Point spread function
is the distribution of intensity in the image plane when a point source is viewed through a telescope.
50
the point spread function arises
because of a variety of effects: e.g. poor focusing, diffraction, scattered light.
51
can correct for effects of the PSF by
deconvolving using the convolution theorem
52
PSF can be measured
by observing a bright point source near the target object
53
Aperture photometry
place apertures of different sizes around the source and measure the total intensity. plot curve of growth
54
Lines recorded have a profile of
intensity versus wavelength that is a combination of the true profile and the instrumental line profile
55
Phase folding
is used for irregularly and poorly sampled data.
56
Phase folding procedure
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
where wavelet analysis is more suitable than the auto-correlation approach
looking for example where oscillation is changing in time So one example is gravitational wave signal of merging blackholes.
58
All parts of a galaxy along a line-of-sight
Contribute to its observed spectrum , where different parts have different LOSV which effectively broadens a spectral line
59
if the spectral velocity is dominated by a single v(LOS) we can use
cross-correlation to find it
60
if v(LOS) does not align
the two signals will be small -> CCF is small
61
if v(LOS) does align
the two signals will be large -> CCF is large
62
can estimate v(LOS) by calculating
the CCF for many trial values of v(LOS) and S(u-v(LOS)) and finding its maximum value
63
if the signal has characteristics of white noise
then the power spectrum is flat i.e. PSD = const one part of the signal is entirely uncorrelated with any other
64
high pass filter
cuts off low frequency signals
65
low pass filter
cuts off high frequency signals
66
how to obtain a cleaner time series
inverse fourier transform
67
example of phase folding
extra-solar planet transits
68
cone of influence
the region where the boundary effects are important, resulting in unreliable wavelet power
69
wavelength calibration
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
Extracting spectrum
often CCD is a 2D image of (λ,y) get spectrum for particular y or spatially integrated Σy
71
Correcting for tilt
when the spectrum not aligned to CCD x,y (i,j)tilted ->(rotated) (i',j')
72
Interpolation
after tilt correction the pixel values might not be integrs -> interpolate tilt corrected values into int pixel locations
73
Sky background
Background sky sources due to atmospheric scattering. sky background unlikely constant with λ.
74
Spectral Diagnostics
properties of remote source from spectral measurements
75
spectral lines are produced by
a hot tenuous gas i.e. gas clouds
76
significance of an observation
is defined as the signal expressed as a number of standard deviations
77
Equivalent width
is a way to measure the total absorption in a spectral line
78
optical depth
Describes the absorption of photons
79
atomic line diagnostics
involve measuring the ratio of intensities between two emission lines from the same element
80
term or Grotrian diagrams
Illustrate the possible transitions in an element
81
most common spectroscopic diagnostics rely on
assuming that the radiating gas/plasma is in local thermodynamic equiliibrium
82
in local thermodynamic equilibrium (LTE)
the matter is in thermal equilibrium in some small neighbourhood around a point in space assumes slow changes
83
LTE also means that the system reaches equilibrium via
collisions between particles
84
optically thin approximation
the radiation escapes the gas/plasma without interaction
85
a collision
a free electron perturbs an orbital electron
86
the population ratio of two levels depends on the
temperature
87
detailed balance
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
ionisation potential, χ
the energy required to remove an electron
89
collisional ionisation
collisions between fast electrons and atoms in a hot gas can result in the ejection of a bound electron into a free state
90
the statistical weight of a particle
is the number of states that it can occupy
91
metastable level
the upper level of a forbidden transition
92
finite lifetime of electrons in the excited states
Lead to natural broadening and collisional broadening
93
doppler/motion on micro or macroscale
thermal, turbulent and rotational
94
particle distribution f(v)dv for LTE
is a Maxwell-Boltzmann distribution
95
Ionisation balance curves
Shows T range each ion is most abundant

Astronomical Data Analysis 2 (11 decks)

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