Definitions and Explanations Flashcards
CCDs readout the image by
moving the stored charge across the device
CMOS readout the image by
reading out each pixel individually
CCDs and CMOS detectors work via the
photoelectric effect in a semiconductor
full well capacity
maximum number of electrons that can be held before pixel saturates
CCD readout method
Applying sequences of voltages along the columns and down the rows of the CCD, transferring charge from one pixel to next
Charge transfer efficiency
describes the fraction of charge transferred per pixel within the semiconductor
Analogue to digital convertor
converts voltage to ‘data numbers’
3 phase readout scheme
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
Thermal Noise / Dark Current arises from
thermal energy in the CCD material, leading to lattice vibrations called phonons
Dark frames
are exposures with no illumination falling on the CCD
Electronic Noise
Each stage of the photo-electrons to DN conversion can contribute noise
Electronic noise can arise in
transfer of charge from pixel to pixel
amplification of readout voltage
measurement of amplified voltage
quantisation noise
conversion of the analogue voltage into a digital signal in the ADC
Bias frames
are exposures of zero duration without light falling on the CCD
Bias frames are needed to
quantify the effect of the ADC offset
ADC offset voltage
the CCD output voltage is compared to a steady reference voltage, and the small difference is amplified.
Flat field
represents the response of each pixel to illumination
corrects for non-uniform CCD response,
Taking a flat field
exposing the CCD to a uniform light source, then normalising each pixel value by dividing by the average value over all the pixels.
underpreforming
pixels producing < 10 DN
Cosmic Ray Spikes
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
Correcting for Cosmic Ray Spikes
Mean or Median filtering
CCD quantum efficiency peaks
in the optical, but the wavelength response can be broadened into the UV by coatings
Anti-reflection coatings improve
QE down to about 350nm
Rear-side illumination
gives increased sensitivity at λ < 400nm
the limited full well of a CCD pixel limits
CCD dynamic range and can lead to blooming
dynamic range
is the ratio between the brightest and faintest sources that can be recorded
Blooming
photo-electrons overflow from one potential well to the next along conduction paths leading to bright streaks which cannot be corrected.
Active Pixel Sensor
an APS detector is a detector in which individual pixels contain the photosensitive material and an amplifier
CCD vs CMOS Electronic Noise
CMOS preferred with low electronic noise as each pixel has its own amplifier -> low bandwidth -> low noise
CCD vs CMOS Quantum efficiency
CCD preferred for operation at low light levels
CCD vs CMOS Readout rate
CCD slower
CMOS faster
usually unimportant
CCD vs CMOS Blooming
CMOS preferred but anti-blooming techniques help in CCDs
CCD vs CMOS Flat Field
CCD preferred can be made very uniform.
CCD vs CMOS Dark Curent
CCD preferred
CCD vs CMOS Spectral Coverage
CCD better outside the optical range
CCD vs CMOS Flexibility
CCD readout needs circuits
CMOS readout needs software and computing power
CCD vs CMOS Power
low power means CMOS preferred for space
In a lab you can take a flat field image using
an artificial light source
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.
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.
mean-filtering
pixel replaced by mean value of neighbours
Mean is not a good representative of this cosmic ray pixel and the neighbouring pixels.
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
bright features cause
blooming of saturated pixels
faint source need
highest quantum efficiency
wide-field optical imaging needs
low readout/dark current
Convolution
Describes the effect of one signal on another
cross-correlation
Describes the similarity of two signals
Auto-correlation
Measures how well a signal matches a time shifted version of itself
Point spread function
is the distribution of intensity in the image plane when a point source is viewed through a telescope.
the point spread function arises
because of a variety of effects: e.g. poor focusing, diffraction, scattered light.
can correct for effects of the PSF by
deconvolving using the convolution theorem
PSF can be measured
by observing a bright point source near the target object
Aperture photometry
place apertures of different sizes around the source and measure the total intensity.
plot curve of growth
Lines recorded have a profile of
intensity versus wavelength that is a combination of the true profile and the instrumental line profile
Phase folding
is used for irregularly and poorly sampled data.
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.
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.
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
if the spectral velocity is dominated by a single v(LOS) we can use
cross-correlation to find it
if v(LOS) does not align
the two signals will be small -> CCF is small
if v(LOS) does align
the two signals will be large -> CCF is large
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
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
high pass filter
cuts off low frequency signals
low pass filter
cuts off high frequency signals
how to obtain a cleaner time series
inverse fourier transform
example of phase folding
extra-solar planet transits
cone of influence
the region where the boundary effects are important, resulting in unreliable wavelet power
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
Extracting spectrum
often CCD is a 2D image of (λ,y)
get spectrum for particular y or spatially integrated Σy
Correcting for tilt
when the spectrum not aligned to CCD x,y
(i,j)tilted ->(rotated) (i’,j’)
Interpolation
after tilt correction the pixel values might not be integrs
-> interpolate tilt corrected values into int pixel locations
Sky background
Background sky sources due to atmospheric scattering.
sky background unlikely constant with λ.
Spectral Diagnostics
properties of remote source from spectral measurements
spectral lines are produced by
a hot tenuous gas i.e. gas clouds
significance of an observation
is defined as the signal expressed as a number of standard deviations
Equivalent width
is a way to measure the total absorption in a spectral line
optical depth
Describes the absorption of photons
atomic line diagnostics
involve measuring the ratio of intensities between two emission lines from the same element
term or Grotrian diagrams
Illustrate the possible transitions in an element
most common spectroscopic diagnostics rely on
assuming that the radiating gas/plasma is in local thermodynamic equiliibrium
in local thermodynamic equilibrium (LTE)
the matter is in thermal equilibrium in some small neighbourhood around a point in space
assumes slow changes
LTE also means that the system reaches equilibrium via
collisions between particles
optically thin approximation
the radiation escapes the gas/plasma without interaction
a collision
a free electron perturbs an orbital electron
the population ratio of two levels depends on the
temperature
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
ionisation potential, χ
the energy required to remove an electron
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
the statistical weight of a particle
is the number of states that it can occupy
metastable level
the upper level of a forbidden transition
finite lifetime of electrons in the excited states
Lead to natural broadening and collisional broadening
doppler/motion on micro or macroscale
thermal, turbulent and rotational
particle distribution f(v)dv for LTE
is a Maxwell-Boltzmann distribution
Ionisation balance curves
Shows T range each ion is most abundant
