Class Note for ASTR 518 at UA
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This 4 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Arizona taught by a professor in Fall. Since its upload, it has received 18 views.
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Date Created: 02/06/15
gt Differential Photometry Spatial differential variability can be measured by monitoring nearby stars simultaneously with the measurement target This gain comes for free with imagers but can be extended to aperture photometers using more than one detector for example Atmospheric effects filter leaks and time variability of the system can all be made to cancel to first order Spectral differential narrow spectral features can be measured by comparing signals from a narrow and broad filter both centered on the feature there are other strategies too o o 9 F N 80 r J i I 60 r I B 60 60 2390 0 I 40 O o 40 40 W c 60 20 20 8 0 O l l 1 l 6 o 0 8 8 8 8 8 lo 0 39 L o m a r m m m n q 339 V V v Passbands Of the H photometric system Equivalent Width W VS ratio of signals gt High speed photometry With arrays the readout time is fixed by the speed of the transfer of signals and the resulting charge transfer efficiency for a CCD plus the speed of the output amplifier and the receiving electronics However with nearly all arrays one can just get signals from a subsection which can be read out at the optimal speed per pixel If only a small fraction of the pixels in the array are being read out then this subsection can be read much faster than the entire array while still maintaining the good performance for the subsection only of course For example a CCD can have the clocks run fast to shift the charge out over the quotunwantedquot area dumping it by resetting When you get to the subarray the clocking is slowed down and the pixels are read out at the standard rate When you get past the subarray it is usually desirable to clock through the entire array so charge does not build up and bleed into regions where you do not want it In theory infrared arrays allow full random access you could address just the subarray However since usually shift registers are built in to shift the entire array out serially usually one has to use a fastadvance strategy similar to that with the CCD to get to a subarray and a similar readout strategy also A few arrays have extra logic to allow direct addressing of a subarray eg the Teledyne arrays for NlRCam One can also use a custommanufactured small array if one wants fast readout without the advancing over pixels gt High speed photometry with detectors with gain Another way to get high speed is to use gain so the effective read noise is kept small CCDs are now available where the voltage on the output register is increased so there is mild avalanching so as the charge is passed from one gate to another the amount of charge grows Since the fast transfers in a CCD are all in the output register this strategy lets one move the charge quickly and get enough signal that the residual extra noise from the degradation in CTE is not a problem It also allows faster operation of the output amplifier Avalanche gain can also be used in silicon PIN diodes that are appropriately designed These devices are used in a photoncounting mode and for low enough input photon rates can have noises ofjust a few electrons With higher input rates the avalanche gain adds considerably to the noise however In the infrared the solid state photomultiplier essentially an impurity band conduction IBC detector can be modified in construction to have a gain region With appropriate adjustment of both the bias voltage and the temperature of the detector this device gives an avalancheassisted large pulse out for each photon detected analogous to the PIN avalanche pulse counting photodiode gt High accuracy eg planetary transits 0 Array detector virtually required because an aperture would modulate the signal with tiny telescope motions 0 Can mitigate pixel nonuniformity by putting many pixels across the point spread function HD 209458 transits Mauna Kea Observatory 1 j xi 099 7 7 098 r 15 Nov 1999 6 7 1 4 0997 3 098 7 12 Nov 2000 relative flux 099 098 r 19 Nov 2000 quoti 39 l l l l l l 5 6 7 8 9 10 UT hours Transit systems are based on small telescopes eg telephoto lenses for cameras to study bright stars 0 Space telescopes are also used for transits very successfully Although the man ixel rule cannot be imposed high quality data are being obtaIned by careful modeling of the results taking advantage of the steady pointing and lack of effects from the atmosphere
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