Theory

Question 1

Under what circumstances can you use a Gaussian to approximate a Poisson distribution?

Answer 1

When there are a large number of counts.

Question 2

Why is Poisson statistics the correct one to use for counting photons?

Answer 2

Poissons distribution has no negative counts which of course would be unphysical.

Question 3

Why do scientists use 3 sigma as the lowest level of significance which they consider a detection?

Answer 3

Chance of finding a 3 sigma detection event randomly is extremely unlikely. 3 in 1000.

Question 4

Why do particle physicists use 5 sigma?

Answer 4

There are tons of collisional events taking place which means that there will be a significant amount of 3 sigma results by pure chance.

Question 5

What are 4 sources of photometric extinction?

Answer 5

Rayleigh scattering by molecules
Absorption by ozone
Molecular absorption by
Scattering by aerosols

Question 6

Define transmission curve.

Answer 6

Transmission curve/ response curve, is the percentage of incident light transmitted as a function of wavelength.

Question 7

Define wavelength at peak transmission.

Answer 7

Wavelength corresponding to the maximum transmission

Question 8

Define central wavelength.

Answer 8

Lamda central = midpoint between lamda 1 and lamda 2 where s(lamda1) = s(lamda2) = 1/2 s(lamda peak)

Question 9

Define band pass.

Answer 9

Range of transmitted wavelengths, often measured as FWHM of the transmission curve.

Question 10

What do you understand by the term spectroscopy?

Answer 10

Spectroscopy is the process of dispersing light as a function of wavelength.

Question 11

What information can you obtain from stellar spectra?

Answer 11

Emission mechanisms 
Temperatures 
Elemental abundances 
Velocity (including red shift)
Pressure (from line broadening) 
Presence of electric or magnetic fields.

Question 12

What is free spectral range?

Answer 12

The free spectral range of a diffraction grating is the largest lamda range for a particular spectral order that does not overlap with an adjacent order

Question 13

How would you wavelength calibrate a spectrum?

Answer 13

Observe a source with features (usually emission) at well known wavelengths. Based on this, work out the conversion from pixel number to wavelength. Apply conversion to the observation.

Question 14

What are the astronomical emitters of radio waves?

Answer 14

Symbiotic stars
Micro quasars 
Pulsars 
Supernova remnants 
Radio galaxies 
Quasars (other AGN)

Question 15

What are the advantages of radio astronomy?

Answer 15

Can observe in both day and night
Can penetrate clouds
Radio interferometry can give excellent resolution
Only stopped by lightning and strong wind.

Question 16

What are the three sources of radio emission? Give example:

Answer 16

Black body (thermal)

Continuum sources (quasars,AGN, pulsars. Synchrotron radiation and braking radiation)

Spectral line sources (H1 spin flip transition, recombination lines, molecular lines)

Question 17

How would you go about measuring RA and dec?

Answer 17

Wait for object to cross the meridian:

Dec- measure the altitude of the object and take into account the latitude.

RA - sidereal time when object is on the meridian.

Question 18

Why are detectors used in astronomy?

Answer 18

They have increased sensitivity (more wavelengths accessible, can detect individual photons an exposures can be done to increase signal).

There is a permanent record, data which can be stored and used later.

Respond linearly or semi linearly whereas the human eye responds logarithmically.

Question 19

What do you understand by the term VLBI?

Answer 19

VLBI sands for very long base-line interferometry. This is when a signal from an astronomical radio source is collected by multiple radio dishes on earth. The distance between these dishes are calculated by the time delay in the signal. This allows the observation of an object made simultaneously to be combined. Emulating a telescope with a dish size equal to the maximum seperation between the telescopes.

Question 20

Why is a reflector better than a refractor?

Answer 20

Shorter tubes
Less aberration
Lighter objective
Easier to support

Question 21

Why is an alt-az mount better than an equatorial mount?

Answer 21

No counter weight
More compact design, build smaller domes therefore cheaper.
Much more stable structure.

Question 22

What do you look for when thinking about building a telescope?

Answer 22

Close to the equator (see more stars) 
Large proportion of clear nights and stable atmosphere 
Isolated and away from light pollution 
High up a mountain less ATM
Dry => better seeing

Question 23

What do you understand by the term color index?

Answer 23

Color index is simply the difference in magnitudes of a source as measured in two different filters. CIab= Ma-Mb

Question 24

Why is the color index useful?

Answer 24

Can be useful to help constrain temperatures, meal abundances and redshift

Color - color plots can be useful In identifying the ages and evolutionary stages of stellar populations.

Question 25

How would you flux calibrate a spectrum ?

Answer 25

Observe a source (usually featureless astronomical object) with a well known flux spectrum. Determine the conversion from count-rate to flux. Apply this to the data.

Question 26

Why do stars appear to twinkle?

Answer 26

In space, light of the stars appears steady and images are sharp but in passing through the atmosphere, narrow shafts of starlight are constantly shifted due to turbulence in the atmosphere. The image dances around in the eyepiece for people on the ground. This is known as atmospheric seeing.

Question 27

What are the advantages and disadvantages of putting a telescope in space?

Answer 27

Advantages: above atmosphere so not affected by seeing or absorption. Can make round the clock observations. Disadvantages: hella expensive, inaccessible if something goes wrong, size restriction and possible damage by cosmic rays.

Question 28

Why do we need to build large telescopes?

Answer 28

To gather as much light as possible and achieve the highest angular resolution.

Question 29

Why do all telescopes use mirrors?

Answer 29

Reflecting telescopes are easier to design, build and maintain in comparison to refracting telescopes.

Question 30

What is the Julian day?

Answer 30

Number of days since noon at Greenwich on 1 jan 4713 bc

Question 31

What is sidereal time?

Answer 31

Sidereal time is the time relative to the stars (instead of the sun).

Question 32

What is local sidereal time?

Answer 32

Local sidereal time is the right ascension of a star on the observers meridian.

Question 33

What is a sidereal day?

Answer 33

One sidereal day is the time it takes the earth to rotate once with respect to the stars: 23:56 minutes.

Question 34

What are antenna side lobes?

Answer 34

Essentially diffraction pattern of a telescope. Sidelobes are smaller beams which are away from the main beam.

Question 35

Explain how diffraction gratings work.

Answer 35

Works on the principle of interference. Two parallel rays of light have slightly different path lengths as they are reflected off the grating. Constructive interference occurs where the difference in path length equals to the the wavelength of light or multiple integers of the wavelength. Different wavelengths will experience constructive interference at different reflection angles

Question 36

What are the noise sources in CCD's?

Answer 36

Dark current- signal from unexposed CCD Read noise - uncertainty in counting electrons in pixels Background - diffuse light from bright sky Photon counting - intrinsic uncertainties in reliably counting photons

Question 37

What are the disadvantages of radio astronomy ?

Answer 37

``` Radio pollution is bad hey Poor single dish resolution Low power signal from photons Means we need big dishes Need interferometry so can be very expensive ```

Question 38

Describe the typical night time procedure for observing.

Answer 38

At twilight we take flats of the sky We take biases too Dark are done by keeping the lenscap on the telescope for the same amount of time as your exposure Take multiple images of an object

Question 39

What are the disadvantages of spectroscopy?

Answer 39

Slow and more expensive faintest objects in the universe are to faint for spectroscopy. Object needs to be bright enough. Subtraction from sky signal is very poor in spectroscopy.