Using the Specdre Extension

The demonstration script (demo_specdre) shows the use of the Specdre Extension. There is a tool editext to look at or manipulate the Extension. resamp will usually store some limited amount of information about the interdependence of post-resample pixels in the Extension. If you try to fitgauss such re-sampled data, that application will pick up and use the information in the Extension.

fitgauss or fitpoly will store their fit results in the Extension. In conjunction with the NDF sections you can work your way through the rows of a CCD long-slit spectrum, store each row's fit in the Extension and fill the result storage space. The collection of all fits is again an NDF (located in the Extension) and can be fed into other applications like specplot, ascout, or indeed KAPPA's display. The Extension also provides a place to store a full-size array for the wavelength or frequency etc. of each pixel. This array of spectroscopic values may be created by grow, and is used by arcdisp to apply individual dispersion curves for the rows of an image. The image may be a long-slit spectrum, or the rows may be extracted fibre spectra or échelle orders.

There is some potential for confusion here. You may tend to count pixels in your data starting at 1, you may use NDF pixel indices, NDF pixel coordinates, a vector of pixel centre values, or an N-dimensional array of spectroscopic values.

• Pixel indices along an axis in an NDF are counted from the lower bound to the upper bound. The lower bound is also known as the origin. It is not necessarily 1. Say, if an NDF has bounds 1 ... N, a section of it may have bounds 5 ... (N-6), or -5 ... (N+6) etc. If the section is made permanent in a new file, it will still have origin other than 1. Usually it is these pixel numbers - or the section bounds - that are used to specify NDF sections in parameter values like ndf(-25:32,4:).

• The centres of pixels in an NDF have pixel coordinates that are the pixel number minus 0.5. So if the bounds are 5 ... 95 then the pixels have centre coordinates 4.5 ... 94.5. The beauty of these coordinates is that, if you give each pixel an extent of 0.5 on either side, then the sequence of pixels 1 .... N cover the coordinate range [0;N]. These are only default coordinates, but they are worth mentioning because they are similar, but not equal, to the pixel indices.

• Each axis of the NDF may have an explicit vector with the coordinates of the pixel centres. These can run non-linear, backwards, or in loops. You may find that some software cannot cope with the weirder of these options. These centres can be used to specify NDF sections. You simply give a floating point bound instead of an integer bound. Note that this works only because each axis has a vector of pixel centres as long as the NDF axis, not an N-dimensional array of pixel centres.

• For Specdre it is useful to have in addition an N-dimensional array where for example a wavelength calibration can be stored for each row of an image or a cube individually. For this purpose there may exist an array of spectroscopic values in the Specdre Extension to the main NDF. That array is an NDF with the same bounds as the main NDF. Where the main NDF stores the count rate, brightness etc. of a pixel, the spectroscopic values' NDF stores the wavelength, frequency, radial velocity etc. for that pixel. This information is recognised only by Specdre. You cannot expect Kappa's linplot to use it for axis labelling, Specdre's specplot does use it of course. You also cannot use this array to specify an NDF section in a parameter value.

• There is no rule as to what happens to the centres of the spectroscopic axis when N-D spectroscopic values are created or modified. Both arrays may lead rather independent lives.