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Observatory astro
Not on formula sheet
| Question | Answer |
|---|---|
| As early as the 1940s the photomultiplier tube was able to | electronically make precise brightness measurements that could out-perform photographic technology |
| Photomultiplier tubes (PMTs) were replaced by | Charge-coupled devices starting in the 1970s, but still have specialized applications today ("photon counting" devices, rapid variability) |
| At the front of a PMT is a | photocathode, held at a constant large negative voltage, that will release a single electron when a photon is incident |
| In a PMT, a series of n dynodes are placed along the PMT at | increasingly less negative voltages. Each can easily release electrons when struck by an energetic particle |
| In a PMT, the initial electron is accelerated towards | the first dynode, which releases 2-10 more electrons, which get accelerated toward the next dynode and so on. Finally, a pulse of (2-10)^n electrons are incident on an anode, that can be recorded |
| Most charge-coupled devices are built upon | the metal-oxide semiconductor (MOS) capacitor |
| MOS capacitor in charged coupled device | "sandwich" semiconductor material and positively charged metal wafer as bread, and oxide insulator as the filling |
| Charged coupled devices and MOS capacitor | photon ionization produces electron hole pairs in the semiconductor. Electrons get drawn toward metal wafer, but blocked by insulator; holes get sept to ground. Results in charge build up in depletion region |
| The charge in charged coupled devices scale with | the number of electrons, which scales with number of incident photons |
| In charged coupled devices, each pixel has its own | MOS capacitor, which after receiving photons, can be read-out through an amplifier and analogue to digital converter (ADC).Amount of charge recorded tells how many photons produced the signal in a given pixel |
| Charge-coupled device works as | a bucket-brigade |
| Bucket brigade | During exposure, incident photons release electrons that begin piling up the MOS capacitor depletion regions. Exposure is stopped, and first row from parallel registers shifts down into serial register, and everything else shifts down |
| Once in the serial register each pixel | gets passed through an amplifier, and the total charge (and thus number of photons incident) is converted to a voltage that gets recorded as a number |
| Channel stops | are used to prevent electrons from diffusing along columns prior to readout. Essentially high negative voltage Si barriers that electrons cannot cross |
| If enough electrons build up so that their potential reaches their full well (channel stops) | they can spill over into adjacent row pixels, resulting in blooming. The channel stops mean this can only occur in one direction. This is saturation |
| What is saturation | enough electrons have gathered in the depletion region to counteract the potential well. The bucket overflows |
| Detector noise usually limits | the lowest useful intensity that will produce a response |
| At very high intensities, a detector can | saturate, meaning that increased numbers of photons no longer produce a predictably increasing signal |
| Between saturation and detector noise is | the linearity region that define the dynamic range (Phigh/Plow) of the detector |
| With digital detectors, we are limited by | the number of bits that we can record |
| The process of converting a continuous stream of photons to digitalized data is called | analogue to digital conversion (ADC) |
| With a 16 bit system, the minimum and maximum signals are | minimum is 0 and max is (2^16)-1 |
| gain | changes the amount of input required to produce a single unit of output to modify digital sensitivity |
| ADC reports | integer numbers stored in computer memory |
| ADU | analogue to digital unit is the integer value of a CCD pixel reported by the ADC. The number of electrons required to increase an ADU by a single unit is called the gain |
| gain is measured in | electrons/ADU |
| What does lowering the gain do? | increases sensitivity. Then the pixel will digitally saturate more quickly (without reaching full-well), but the non-linear shoulder in the response is avoided |
| What does a higher gain mean? | the full-well is reached prior to digital saturation, but at the expense of decreased sensitivity |
| What is a fundamental limitation of CCD sensitivity at low light levels | readout noise |
| To avoid the statistical inevitability of the ADC producing negative values of output, we apply | a bias voltage to every pixel in the detector |
| The bias level is | a pedestal voltage applied to all pixels so that the ADU's in the raw images are always >0 (overcoming the statistical fluctuations in the readout noise) |
| After applying bias level, we then take | bias frames, or images with exposure times of 0s, to capture the signal caused by the bias voltage and subtract that signal off in post-processing to leave only the signal caused by the photons |
| CCD sequencer controls what | the shifting of voltages in the register. It has to be finite and precise. |
| What contributes to readout time | shifting, setting voltages, opening/closing shutters, data transfer, gain and bias level |
| Fast readout times | sample pixels faster and incur larger readout noise (but takes less time) |
| Long readout times | have cleaner signals but incur more overhead |
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