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Дата изменения: Mon Dec 23 18:23:59 2013 Дата индексирования: Fri Feb 28 09:06:23 2014 Кодировка: Поисковые слова: comet |
Pipeline processing |
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The data contained in the FITS archive can be processed on-line with pipeline procedure.
This page introduces the syntax of the pipeline processing and describes the commands implemented on the pipeline. You may also be interested to see examples of pipelines.
The pipeline processing is divided in three parts:
z=d
Display entry from the HFA index catalogue (n=11), no
data processing
z=p
Input an image into the pipeline and close the pipeline
(output). This is used to download data as archived.
z=s
Statistics
z=vg
Gray scale display of an image
z=vp
View the profile along the slit (long slit spectra)
z=vs
View a spectrum
The output of the pipeline can be directed to an Hypertext page, in such case the effect of the pipeline processing can only be evaluated through the visualisation and data evaluation functions. It can also be downloaded as a FITS file, either to a file or to a fits viewer.
The pipeline functions may be selected from the HTML editor displayed when opening a file form the fits archive. They can also be easily accessed with a utility program like wget. The parameters to the HyperLeda request are normally passed with the GET method, and you simply have to learn the syntax and the meaning of the arguments to make requests more elaborated than what permet the editor. We are trying to keep the editor simple enough to allow an easy and fast evaluation of the content of the archive.
The evolution of the system has been very rapid in the recent months and in many places the documentation is out of date. We are currently working to remedy this point. We do not foresee important change in the pipeline software before the end of this year (1999).
The request is composed by the name of the request, here fG, and the command
located after the "?". The command consists in different parameters separated
by the character "&". The first parameter c=i says the "type" of
request.
The second parameter fa:L93111HP1/00147,
a shortcut for o=fa:..., identifiy the file in the archive to
enter in the pipeline. The third parameter, z=..., is the pipeline.
The last parameter a=html says that the output is an html page.
The general rules are that (1) the order of the parameters is unsensitive
(2) each parameter should appear only once (3) most
of the parameters have default value, eg. the default for c is
c=o.
The pipeline z=... described above apply the flat-fielding correction (ffc), resample into wavelength with a linear wavelength scale, default wavelength range and oversampling by a factor 2 (wrs[1,-1,-1,-2]), then the sky is subtracted, a 10 arcsec region around the obhject's center is extracted and then averaged.
Lsetp
: Type of resampling: Lsetp=1
: Linear;
Lsetp=2
: Logarithmic
Wstart
: Wavelength of first pixel in output spectra (in Angstrom). If Wstart=-1
the first pixel in output spectrum will
corresponds to the firts pixel in input spectrum.
Wend
: Wavelength of last pixel in output spectra (in Angstrom). If Wend=-1
the last pixel in output spectrum will
corresponds to the last pixel in input spectrum
Wstep
: Step in Angstrom between two pixels in the output
spectrum. If Wstep < 0
, it is the resampling factor.
Scaling
: The flux scale: Scaling=1
, linear;
Scaling=2
, decimal logarithm; Scaling=2
, magnitude.
Type
: Nature of the resulting flux. Type=ins
,
instrumental flux. Output of instrument, ie. detected flux corrected for
detector effects (bias subtraction, flatfielding...);
Type=obs
, Observed flux, below the earth atmosphere (corrected
for instrumental response)
Type=obs
: Normalized to continuum (spectrum) or background (image).
The flux can be returned in a linear scale (this is the default: Lfc=1) or in decimal logarithm Lfc=2. The second argument Syst indicates the coordinate system used for the last two arguments. The value can be m (default) n or w meaning respectively: maximum range, world coordinates or natural coordinates. W1 and W2 are these coordinates.
The vector used to normalize the flux is stored in an HDU appended to the current FITS file.
e1 and e2 are the extraction sizes along each axes, if there is only one spatial axis, e2 may be omitted and otherwyse it is ignored. system stand for n,w,w or a, with the meaning:
It is advised to insert this function on the pipeline before any resampling since spikes contain information at the cut-off spatial frequency (by definition) and hence are poorly interpolated. Interpolation usually result in a smearing (making their afterward detection harder) or worse in oscilations (eg. with splines interpolator) producing some "replicas" of the spike in its wings.
The convolution proceeds in the Fourier space from spectra sampled in log-wavelength. Hence, if necessary (ie. if the spectra is calibrated in wavelength), this function calls wrs before the convolution to resample in logarithm (oversampling by a factor 2) and calls it again after the convolution to restore the sampling. This is time consuming and resampling is a source of degradation, so, try to minimize the number of resampling.
vsg cannot be performed if the spectrum has not been calibrated in wavelength.
repval is the replacement value . If it is not given the pixels will be set to the usual "not a number" value.
This procedure may be used, for example, to mask the regions of telluric lines in a spectrum.
Available filters are (i) gaussian convolution and (ii) gaussian unsharp masking.
gaussian convolution: The image is convolved by a gaussian of standard deviation
p1 (in pixels) along both axes.
gaussian unsharp masking: The image is convolved by a gaussian of standard deviation
p1 (in pixels) along both axes and the function returns the difference between the
original image and the convolved image.
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