question sheet

Question 1

dynamic range

Answer 1

brightest/faintest source that CCD can image

Question 2

charge transfer efficiency

Answer 2

fraction of charge lost when pixel moved to next pixel in sequence

Question 3

max cte

Answer 3

moving pixel furthest from readout

Question 4

charge transfers for max cte for a 2048x2048 ccd

Answer 4

moves along 2047 rows
down 2047 columns

so 4094 charge transfers

Question 5

how to work out maximum total cte

Answer 5

if ie 99.95%

(0.9995)^charge transfers

eg(0.9995)^4094

Question 6

reasons for CCD readout noise

Answer 6

due to random fluctuations introduced by the amplifier, readout electrons and analogue to digital converter

associated with converting e- to DN for analysis

Question 7

flat field

Answer 7

represents the response of each pixel to illumination

Question 8

obtain a flatfield by

Answer 8

exposing CCD to uniform illumination

then normalise each pixel value by dividing by the average value over all pixels

Question 9

corrected counts per pixel

Answer 9

N=X-B/F

Question 10

how would you take a flat field image for a CCD onboard a telescope in space

Answer 10

can use the earth as the flat field source

can fix the telescope’s pointing on a distant source such that the earth would eclipse the telescope for part of orbit

Question 11

if pixel in flatfield has a value >1

Answer 11

pixel is ‘overdetecting’

Question 12

if pixel in flatfield has a value <1

Answer 12

it is ‘underdetecting’

Question 13

mean filtering

Answer 13

pixel replaced by mean value of neighbours

ie 4 nearest

Question 14

why is median filtering preferred for cosmic ray removal

Answer 14

cr pixel value&raquo_space;> rest so mean would not be good representative

Question 15

CCD or CMOS?
A satellite imaging the whole Sun, monitoring it for the bright compact emission from
solar flares

Answer 15

CMOS
trying to observe bright features at same time

want to avoid blooming of saturated pixels

Question 16

CCD or CMOS
A ground-based telescope with a large-aperture, that produces wide-field optical images of the faint night sky

Answer 16

CCD
faint source so want highest quantum efficiency and low readout/dark current noise

need uniformity across large image

Question 17

CCD or CMOS?
A ground-based telescope studying the very rapid and bright changes in the solar atmosphere with both high time and spatial resolution

Answer 17

CMOS
rapid variations so need high readout rate

Question 18

non-uniform background across an image implies

Answer 18

there is a non-uniform sensitivity across the CCD

need a flat filed image

Question 19

how to correct for cosmic rays

Answer 19

median filtering

Question 20

how to correct for blooming

Answer 20

median filtering
or use shorter exposure time
swap to CMOS

Question 21

blooming is due to

Answer 21

the overflow of photoelectrons from one potential well to the next along the conduction path

Question 22

power on pixel

Answer 22

illumination x pixel area

Question 23

for uniform source, theta=

Answer 23

y/f

where y is the pixel size

Question 24

what is the psf

Answer 24

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

Question 25

psf arises due to

Answer 25

a variety of effects poor focusing, diffraction, scattered light

Question 26

the recorded intensity distribution is the

Answer 26

convolution of the true intensity distribution with the PSF

Question 27

if the psd is known, then Itrue can be calcualted via

Answer 27

inverse F of (F(iobs)/F(PSF)) thanks to convolution theorem

Question 28

With a point source, aperture photometry can be used to

Answer 28

measure the total source intensity without correcting for the PSF

Question 29

aperture photometry

Answer 29

place apertures of different sizes around the source and measure total intensity intensity vs aperture radius should plateau after certain aperture size is reached

Question 30

time series analysis approach when you have poorly sampled data

Answer 30

phase folding

Question 31

phase folding process

Answer 31

for a range of guess periods T, the data is reorganised into ins within the trial period then averaged if no pattern, try different T

Question 32

what extra information does wavelet analysis give about a time series compared to the auto-correlation approach

Answer 32

eg when oscillations changing in time (amplitude, frequency, when) example: gravitational wave signal of merging blackholes

Question 33

when would wavelet analysis be used

Answer 33

changing amplitude and period

Question 34

when would phase folding be used

Answer 34

non-sinusoidal and irregularly sampled data

Question 35

when would power spectrum or autocorrelation be used

Answer 35

sinusoidal components of constant period

Question 36

cone of influence

Answer 36

region of wavelet spectrogram where edge effects can be a significant issue wavelet powers calculated from these regions are unreliable and my be false signals

Question 37

significance

Answer 37

signal expressed as number of standard deviations =N/σs

Question 38

equivalent width

Answer 38

width (expressed in wavelength units) of a rectangle extending from B=0 up to the continuum level such that the area of the rectangle matches the area of the absorption line

Question 39

assumptions made when estimating true intensity

Answer 39

symmetric profile linear background gaussian profile so using intensity << core intensity

Question 40

gain

Answer 40

gain [e- Dn^-1] relates the number of photo-electrons n to the detected counts N [DN] via N=n/g

Question 41

two main types of instrumental noise present in CCD detectors

Answer 41

1. dark current due to thermally generated electron-hole pairs in CCD material 2. readout noise due to moving stored charge, readout amplification and a to d converter