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Flashcards

ASTR211 Flashcards

Liam C English 45 cards

Cards in this deck (45)

  1. 1What is convolution?Convolution is the use of a kernel to modify an image.
  2. 2What are uses of convolution?Convolution is often used to blur or soften images, however there are also sharpening kernels
    Convolution kernels can also be used for edge detection
  3. 3What are some common convolution kernels?The bas relief kernel, and the Sobel filter are common edge detection convolutions.
  4. 4What is deconvolution and why is it used?When an image is taken through the atmosphere, the atmosphere effectively acts as a convolution kernel.
    Deconvolution is commonly used to try and reverse this effect.
  5. 5What are some issues with deconvolution?A convolution of two very different images can often give a very similar resultant image. As a result of this, deconvolutions often don't have unique solutions.
  6. 6What is a bias frame?A bias frame is used to find the fixed pattern noise of a CCD
  7. 7What is the exposure time of a bias frame?0 seconds
  8. 8What is dark current?Dark current is caused by thermal agitation of materials in a CCD, which can cause electrons to be captured in the CCD pixels. These values are then read out as 'dark noise'.
  9. 9How is dark noise managed?A frame is captured and the master-bias is subtracted. Multiple of these frames are then averaged to remove random noise, giving the average dark noise.
  10. 10What are the sources of random noise?Readout noise from the ADC (Analog to Digital Converter)
    Poisson noise due to photon arrival being a random event
  11. 11What are flat fields?Flat fields are exposures done on a white surface (often the dome wall or sky), to identify the non-uniform response of the detector due to pixel sensitivity variations and shadowing
  12. 12How are flat fields used?Flat fields are normally averaged to a PV (Pixel Value) of 1, and the resultant image is divided by the flat-field to remove effects of the flat field.
  13. 13How are images calibrated?CCD Images are calibrated by subtracting the master-bias and bias-subtracted master-dark (dark frames must be scaled to correct exposure time), and then dividing through by the flat-field. This gives a calibrated image, free of any systematic noise.
  14. 14What is photometry?Photometry is the science of measuring how bright things are
  15. 15What is aperture photometry?Aperture photometry is done by placing an 'aperture' (circle) around a star, and adding up the PV's from inside the aperture. Typically there are two rings, the second ring being used to measure the background light so that it can be subtracted.
  16. 16What are causes of uncertainty in photometry?Readout noise, poisson noise of photon arrivals and dark current.
  17. 17How large should a photometry aperture be?The inner aperture must be large enough to include all the light from the star, but if it is too large the noise becomes a significant factor.
    The aperture is typically set at three times the full-width half-maximum
  18. 18There is a situation where aperture photometry cannot be used. What is this situation?Aperture photometry is unsuccessful in crowded star fields, as aperture photometry requires stars to be relatively isolated
  19. 19What is an alternative photometry method to aperture photometry?PSF photometry, which involves fitting a model PSF to the flux recorded in each pixel. The stellar flux is then determined by integrating the fitted PSF model.
  20. 20What is difference image photometry?It is a relatively new area of photometry, which takes two images taken at different times. The sharpest image is blurred to the same PSF as the other image, and they are subtracted from each other. This should result in any constant-brightness objects disappearing, and variable objects remaining.
  21. 21What was the original astronomical filter system?Johnson-Cousins UBVRI system
    UBV invented in 1953 by Johnson
    RI invented by Cousins in 1960s-1970s
  22. 22What are narrow-band filters used for?These are typically used to sample specific spectral lines. They are normally designed in pairs, so that one filter is placed on the spectral line, with another just nearby for comparison.
  23. 23What can colour indices, such as B-V be used for?The colour of a star is directly proportional to it's temperature, so colour can be used as opposed to temperature
  24. 24What is the purpose of interferometry?Interferometry can be used to identify angular sizes of objects in the sky, by combining light from two different sources
  25. 25What is one of the main uses of prism spectroscopy?Objective prism spectroscopy can be used to take low resolution spectra of multiple subjects simultaneously.
  26. 26What is the blaze angle of a reflection grating?The blaze angle is the angle between the normal lines of the facets and the grating
  27. 27What is a facet in a reflection grating?Facets are the grooves on the surface of reflection gratings
  28. 28How is overlapping of colours in different diffraction orders avoided?Colour filters can be placed in the path to filter out light which may overlap
  29. 29What is the difference between raw and extracted spectrum?Raw spectrum are simply the image which is displayed on the CCD.
    Extracted spectrum is a graph generated from the raw spectrum, which plots wavelength against intensity
  30. 30What is the resolving power of a spectrograph?The resolving power of a spectrograph is a measure of it's ability to separate parts of a spectrum.
    It is given by the formula R = wavelength/resolution element
    For a diffraction grating R = Nm where N is the number of lines and m is the order
  31. 31What defines low, medium, and high resolution spectrographs?A low resolution spectrograph has resolving power less than 2000
    A medium resolution spectrograph has resolving power between 2000 and 10000
    A high resolution spectrograph has resolving power over 10000
  32. 32How is the wavelength determined for a spectrograph?An arc-lamp with known emission wavelengths is normally built into astronomical spectrographs, allowing a wavelength scale to be assigned
  33. 33When are arc-exposures normally taken?Arc-exposures are normally taken before and after each stellar spectrum
  34. 34What is Rayleigh's Criterion for resolution?Rayleigh's criterion for resolution states that object are resolvable if their angular separation is such that the intensity maximum of one objects airy disk falls on the intensity minimum of the other objects airy disk. If the angular separation is less than this, the two objects are indistinguishable.
  35. 35What is a typical incident angle for a diffraction grating in an echelle spectrograph?Greater than 60 degrees
  36. 36What orders of light does an echelle spectrograph measure? Why, and what is a consequence of this?Echelle spectrographs typically measure between diffraction order 30 and 100.
    This is due to the high angle of incidence of the grating.
    At orders this high, there is typically a lot of overlapping orders.
  37. 37How are the overlapping orders of an echelle spectrograph dealt with?Overlapping echelle orders are typically separated with a 'cross-disperser' (typically a prism or low-order grating), which splits the orders up.
    This results in a large, very high resolution continuous spectra which runs left to right, top to bottom
  38. 38Spectrographs are typically not mounted to the instruments they receive the light from. Why is this, and how do they receive the light?The light is fed to the spectrographs through fibre optic cables. This is because the spectrographs are quite heavy, but also very delicate and precisely calibrated.
  39. 39What are the three common approaches of astronomical colour imaging?1. Natural colour processing, which aims to provide colours which we would observe if the objects were bright enough
    2. Representative colour processing, where colours from outside the visible spectrum are assigned visible colours
    3. Enhanced colour processing, used to show fine details such as spectral emission lines
  40. 40How are colour astronomical images made, if most CCD's capture in black and white?RGB filters are used, to get light levels for each colour, then recombined using software to provide the colour
  41. 41How is ground level gamma ray astronomy done, if gamma rays do not reach the Earths surface?Some gamma rays get very close to ground level, and then decay into a shower of subatomic particles. These particles can then be detected by their Cherenkov Radiation
  42. 42What is the wavelengths range of X-Rays?1.24nm to 0.00124nm
  43. 43What is the wavelength range of UV?100nm to 300nm
  44. 44What are the typical targets of UV astronomy?Hot stars and ionised gases
  45. 45What is the wavelengths range of infrared?1µm to 100µm

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Flashcards on ASTR211 Flashcards, created by Liam C on 05/11/2015.
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