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Дата изменения: Thu Aug 7 20:42:29 1997 Дата индексирования: Tue Feb 5 12:01:16 2013 Кодировка: Поисковые слова: ultraviolet |
The solution adopted to this problem for NICMOS is the provision of a bright object mode which enables targets to be observed which are ~600 times brighter than is possible in the other modes without saturating. In BRIGHTOBJ mode, an ACCUM sequence of operations is performed on one pixel in each quadrant at a time. That is, the pixel is reset, read, integrated, and read again with the difference between the final and initial readouts being stored as the measured signal and the interval between the reads being the exposure time. This process is repeated sequentially for all pixels in each quadrant. Users can think of this as integrating on a single pixel at a time. The smallest integration time which can be used is 1.024 milliseconds. Figure 15.1 illustrates the operation of bright object mode. Initially the detector is reset and the first pixel (solid shading) in each quadrant is read. A reset is then made and the second pixel in each quadrant is read. The process continues until all 16,384 pixels in each quadrant have been read.
Figure 15.1: Bright Object Mode Operation
The time required to take a BRIGHTOBJ mode exposure can be rather long. Since photons are only collected in one pixel per quadrant at an time, the time associated with obtaining the frame is where EXPTIME is the integration time per pixel (i.e. the observation time is approximately (1282) x the exposure time). For example, if an integration time of 0.1 seconds is used to observe a bright target then the actual time required to complete the observation would be around 27 minutes! This means that allowing for acquisition time only two such exposures could be obtained in a single target visibility period. However, it is not always so serious. In the case of Jupiter for example the integration times required per pixel are only of the order of milliseconds and so the total integration time will only be around 20 seconds.