è OPT:
OPT [DARK FRAME NAME]
In long exposure CCD imagery, one of the major difficulties posed during preprocessing is the dark current correction. This interference signal, due to thermal charges, is added to the signal produced by the observed objects. The problem is to suppress this interfering component of the image because it is noise that impairs the detectability.
A classic solution is to take an exposure of the object under study with an integration time T, then to take another exposure of length T while placing the detector in total darkness. This last exposure is called the dark current map. This map is a constant, to within a coefficient, for a given CCD. To first order, the coefficient is proportional to the temperature of the CCD and to the integration time. In the preceding procedure, the dark current map is simply subtracted from the image of the object.
This is, however, far from the ideal solution. In fact, this procedure implies that a dark current map must be taken after each image of the object. This is very constraining when the exposure time is several minutes or more.
Things seem to go better if the temperature of the CCD is perfectly stable. In this case, a priori only one map is necessary. It can be taken, for example, at the beginning of the observing session, and can be used to correct all the images. If the exposure time is not the same on the dark map and the image to be processed, the dark map must be multiplied by a coefficient before the subtraction. The coefficient is the ratio of the exposure time of the image to the exposure time of the dark map.
Besides the fact that is a delicate matter to maintain the detector at a fixed temperature, this method has the following inconvenience - the dark map has its own noise (readout noise) and when the map is subtracted from the images, this noise is actually added to the images.
There is a more efficient method:
- Take several (5 to 10) dark images with integration times that are not necessarily equal, but long enough to be sensitive to the dark current (they should have the same duration as the observation exposures). The CCD should be cooled to reduce the readout noise as much as possible.
- For each exposure in darkness, another one with a minimal integration time is taken. This provides the offset map.
- For each of the dark images, subtract the corresponding offset map. The resulting images then contain only the thermal component of the signal.
-Sum all the images from the previous step to obtain the dark map. This adds the thermal contributions from each image, but averages the readout noise. This map can be considered constant for a given CCD (it is still good to redo this procedure every 1 or 2 months to allow for any possible change in the electronic characteristics of the detector).
- Now a given image must be corrected. The difficulty is in finding a multiplicative coefficient, to apply to the dark map, which will optimally correct this image. This is what the command OPT does.
Iris solves this problem nearly instantaneously using an analytical approach.
You have to select an area typically 30 to 100 pixels in width with the mouse. Then run the command. [dark frame name] is the name of the dark current map. The program will return the coefficient to apply to the dark current map to create a new map that is optimal for the image to process. Example:
OPT DARK
calculates the optimal coefficient for the dark map DARK.PIC to correct the current image.
The OPT command should be systematically used to preprocess deep sky images. With this command in hand, you do not need to worry about taking dark images during the night. The result is quite good because the criteria chosen minimize the noise.
(scusate l'inglese, io che ho fatto parte del team di traduzione non ho la versione in italiano
)
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