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Lecture 19
| Question | Answer |
|---|---|
| The first step after aperture is chosen for digital aperture photometry is to | add all the pixel contributions that are located within the aperture, multiplying by the fractional area A[x,y] of a given pixel within the aperture |
| The second step after aperture is chosen for digital aperture photometry is to | Estimate the per-pixel contribution from the background by either selecting a range of pixels on some source-free area of the image or defining an annulus with inner radius and outer that samples the background immediately surrounding the source |
| The third step after aperture is chosen for digital aperture photometry is to | subtract the sky/background emission from the total so that the remainder is the emission from the source alone. |
| Photometry is | the study of the brightness properties of astronomical sources |
| Photometric observations are almost always | done in a way that we throw out or ignore the vast majority of photons incident on our telescope |
| It is generally more scientifically useful to only collect | photons within a restricted wavelength range (bandpasses) and compare colours than to just collect photons irrespective of their wavelength |
| The wavelength ranges are known as bandpasses, and we use | filters with a defined photometric response function to collect only these photons that fall within the bandpass of interest |
| But photon detectors do not measure | energy directly. They are counting devices, so are only sensitive to overall photon flux. We can define the monochromatic photon flux |
| One must take care to compare magnitude apples to apples. Two sources may have the same | brightness overall, but if one produces proportionally more low-energy photons than the other then our photon counting system will report a brighter magnitude simply because it has to count more photons |
| Modern photometric systems are almost all defined using | bandpass filters with thin optical coatings that define the cut-on and cut-off wavelengths |
| A neutral density filter | simply limits transmission at all wavelengths equally. Especially useful for observing bright targets that would otherwise saturate too quickly |
| Atmospheric transmission Satm(wavlength) also plays a key role in photometry | at short wavelengths the atmosphere becomes opaque beyond around 320nm which is the blue optical cutoff. It is also opaque in certain wavelength ranges in the NIR. Z, Y, J, H, K designed to be sensitive in the atmospheric transmission windows |
| Multi-band photometry in essence is just | very low resolution spectroscopy |
| A photometric measurement is basically sampling | the monochromatic flux of a smoothed spectrum at the filter's effective wavelength. |
| A feature like a break in a spectrum can be relatively easily measured by | using two bands centered on either side of the break (in photometry). Filters need to be close enough to sample spectral regions near the break but avoid crossing it. Similarly sampling too far risks contamination from unrelated features |
| The narrow filters will be very sensitive to the | line strength, while the broadband will be relatively insensitive |
| The filter response function is well specified | This determines the instrumental system, so that measurements taken with the same instrumental setup can be directly compared. Useful for time series. Limited usefulness if you want to compare to other instrument measurements |
| A method of standardizing measurements across passbands is well specified | the constant Cp is known and can be used to ensure that measurements made by different observers agree. Differences in response function arises from manufacturing and can be corrected in a standard way |
| Combining the fact that the filter response function and a method of standardizing measurements across passbands are well specified then | provide a method for measurements taken at different telescopes, times, (i.e. independent measurements) to be compared directly |
| Many different photometric systems have been defined, each for their own reasons | These systems are standardized by a set of agreed upon, well measured, non-variable standard stars that are located across the sky. |
| By measuring stars from the photometric systems, and noting differences between instruments, | systematic effects can then be corrected for. |
| The ZYJHKLMNQ system | Shares a common zeropoint with UBVRI system in that all magnitudes for a A0V star are zero. Most famous IR telescopes are UKIRT and IRTF and VISTA |
| Disadvantage of ZYJHKLMNQ system | Atmospheric water vapour can significantly affect the exact bandpass definitions; most stable in high altitude, dry locations |
| The airmass tells us how much more | atmospheric absorption will occur along a given line of sight with respect to the zenith. |
| Bouger's law is a linear function of airmass, so | gives us a method of converting an apparent magnitude measured from inside the atmosphere to one taken outside the atmosphere |
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