COMPOSIT [NAME] [SIGMA] [ITERATION] [SATURATION] [NUMBER]
COMPOSIT is a powerful command to perform automatic combination of a sequence of images that were registered before. The simplest way to combine the images is of course to add them. COMPOSIT proceeds in that way, but will reject the pixels that have values significantly bad, i.e. for which the difference with respect of the mean of the values in all the images is greater that [SIGMA] times the standard deviation of the values. Moreover, the process may be iterative: at each iteration, a new analysis of the pixels statistics is made with the left ones. This method is called sigma-clipping. To be very efficient, it is necessary to have a large number of images to combine (at least 5). Try [SIGMA] values between 1.5 & 5.
The [NAME] parameter contains the generic name of the sequence, and the [#IMAGE] parameter contains the number of images in the sequence.
The [SATURATE] parameter is a flag. If saturate=1 the max intensity of the coadded image is normaliszed to 32700 if the level is upper to 32767.
This control is not realized if saturate=0. Example:
COMPOSIT M33- 2.5 2 0 7
Combines the images M33-1.PIC, M33-2.PIC...M33-7.PIC with a rejection level of 2.5 sigma. Two iterations are performed.
The COMPOSIT command is a powerful tool that gathers efficiency of the simple addition of images in terms of signal to noise ratio, and the power of median combination in terms of rejection of aberrant pixels (cosmic rays, satellites, ...).
See the general discussion about deep-sky images preprocessing.
COMPOSIT2 [NAME] [FLAG. MAX] [NUMBER]
COMPOSIT2 method use a robust average image using a continuous adaptive weighting scheme that is derived from the data themselves - see Artificial Skepticism Stacking algorithm - Stetson 1989, V Advanced School of Astrophysics [Univerisidade de Sao Paulo], p.1. See also:
http://archive.stsci.edu/hst/wfpc2/pipeline.html and
http://archive.eso.org/archive/hst/wfpc ... letter.pdf.
The given parameters are only the generic name of the input image sequence, the normalized flag (0 or 1, see COMPOSIT) and the number of input image (an unlimited number).
The weights of the pixel values are computed by the equation:
where wi is the weight of the ith pixel value, si is the sigma of the ith pixel in the stack, derived from the readout noise and camera gain. The ri term, which is the residual between the current average pixel value and the value of the ith pixel, is computed at each iteration. This version of COMPOSIT2 use classical and internally coded value for CCD readout noise and camera gain (noise of 15 electrons RMS en 2 e-/ADU). COMPOSIT2 is a simple command to use and efficient for bad pixels rejection.
Important, before use commands like SMEDIAN, COMPOSIT and COMPOSIT2 it is necessary to have the same sky background level for all the images of the sequence. Use the command NOFFSET if is not the case (or NOFFSET2 for select a specific region for the harmonization of the sky level). Similar, is the exposure time is not the same the image should be scaled before stacking (MULT, MULT2, NGAIN2 commands for example).
The choice of the most optimal combining algorithm will depend on the nature of the data and on the exposure type. For produce a clean flat-field or a master dark frame the appropriate command is SMEDIAN (or #SMEDIAN2). For deep-sky imaging the classical sigma-clipping is a good choise for the best conservation of signal to noise (the median lose 30% in signal to noise typically relative to simple sum and the COMPOSIT/COMPOSIT2 commands). The COMPOSIT2 command is now an useful and fast altenative to the sigma-clipping scheme.
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bellissima immagine complimenti Emilio. concordo con il fatto di schiarire l immagine e operare su piu livelli con ps.su iris non so dirti niente.