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Astrophysics > Theory > Flashcards

Theory Flashcards

1
Q

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

A

When there are a large number of counts.

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2
Q

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

A

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

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3
Q

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

A

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

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4
Q

Why do particle physicists use 5 sigma?

A

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

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5
Q

What are 4 sources of photometric extinction?

A

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

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6
Q

Define transmission curve.

A

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

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7
Q

Define wavelength at peak transmission.

A

Wavelength corresponding to the maximum transmission

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8
Q

Define central wavelength.

A

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

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9
Q

Define band pass.

A

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

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10
Q

What do you understand by the term spectroscopy?

A

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

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11
Q

What information can you obtain from stellar spectra?

A 
Emission mechanisms 
Temperatures 
Elemental abundances 
Velocity (including red shift)
Pressure (from line broadening) 
Presence of electric or magnetic fields.
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12
Q

What is free spectral range?

A

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

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13
Q

How would you wavelength calibrate a spectrum?

A

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.

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14
Q

What are the astronomical emitters of radio waves?

A 
Symbiotic stars
Micro quasars 
Pulsars 
Supernova remnants 
Radio galaxies 
Quasars (other AGN)
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15
Q

What are the advantages of radio astronomy?

A

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

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16
Q

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

A

Black body (thermal)

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

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

17
Q

How would you go about measuring RA and dec?

A

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.

18
Q

Why are detectors used in astronomy?

A

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.

19
Q

What do you understand by the term VLBI?

A

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.

20
Q

Why is a reflector better than a refractor?

A

Shorter tubes
Less aberration
Lighter objective
Easier to support

21
Q

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

A

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

22
Q

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

A 
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
23
Q

What do you understand by the term color index?

A

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

24
Q

Why is the color index useful?

A

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.

25
Q

How would you flux calibrate a spectrum ?

A

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.

26
Q

Why do stars appear to twinkle?

A

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.

27
Q

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

A

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.

28
Q

Why do we need to build large telescopes?

A

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

29
Q

Why do all telescopes use mirrors?

A

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

30
Q

What is the Julian day?

A

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

31
Q

What is sidereal time?

A

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

32
Q

What is local sidereal time?

A

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

33
Q

What is a sidereal day?

A

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

34
Q

What are antenna side lobes?

A

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

35
Q

Explain how diffraction gratings work.

A

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

36
Q

What are the noise sources in CCD’s?

A

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

37
Q

What are the disadvantages of radio astronomy ?

A 
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
38
Q

Describe the typical night time procedure for observing.

A

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

39
Q

What are the disadvantages of spectroscopy?

A

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.

Astrophysics (1 decks)

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