3.19 Flip or shift or redshift a spectrum

This window allows you to do three related jobs.

Flipping and shifting a spectrum allows you to assess the symmetry of a line. Shifting alone to determine how well spectra are aligned and redshifting to transform a spectrum or set of line identifiers to guess or confirm the redshift (all line identifier lists are already editable).

The transformation is applied to the spectrum shown in the Spectrum: drop-down menu. Initially this will show any editable spectra displayed in the associated plot (like the line identifiers). If you want an editable copy of the current spectrum just press the Copy button.

One subtlety is that the offset value is either a redshift or a value in the coordinate units of the spectrum being modified, depending on whether the Redshift checkbox is selected, or not. The coordinate units of the spectrum may not be the same as those of the current spectrum in the plot, which can lead to surprising results.

When applying a redshift it is assumed that the spectrum being modified has a wavelength coordinate system, of some kind, unless it has a proper coordinate system defined. In this latter case the only restriction is that you cannot have a standard of rest set to ‘Source’ (as a transformation to and from Source is used to determine the redshift transform). In this latter case use the coordinate system toolbox to transform to another rest frame first.

The Reset button returns a spectrum to an offset of zero.

Visitor lists

It’s often the case that you have a list of lines, in a spectrum, or many spectra, that you’d like to step through to check the radial velocity or redshift estimates, without the bother of reading each spectrum and line, positioning the plot etc. That’s what a visitor list is designed for.

To use this create a simple text file containing either one, two, three or more whitespace separated columns. If one column is present then that is assumed to be the wavelength of the lines. If two columns are present then these are the wavelength and an associated spectrum. If three or more columns are present then the third columns contains the name of the associated spectrum and the second a label to be displayed at the position (when the label is missing it is set to the wavelength). Very short examples of the three types follow:

   6562.8170 spectrum1.fits
   6562.8170 spectrum2.fits
   6562.8170 spectrum3.fits
   6562.8170 spectrum4.fits
   6562.8170 HALPHA    spectrum1.fits
   6583.6000 [NII]     spectrum1.fits
   6716.4700 [SII]6717 spectrum2.fits
   6730.8500 [SII]6731 spectrum3.fits

Visitor lists are a type of line identifier spectrum, so can have fully specified spectral coordinate units written in a header section (see the line identifier lists in the SPLAT distribution themselves for details), so you are not restricted to having all your spectra in the same coordinate systems.

Once the visitor list is loaded File->Read visitor line list, you can start stepping through it using the Visitor controls, or jump to a particular line using the drop-down menu. The default behaviour of this tool, on each visit, is to create a copy of the current spectrum and then flip it centred on the line (the current zoom is also preserved). It you’d like to not create a copy of the spectrum for each visit, or indeed to flip it, then uncheck the Flip checkbox and select the menu item Options->One spectrum for visitor (these choices remain enforced between sessions).

Another way to use the visitor controls is to use a line identifier (either a built in one, or one of your own) as the visitor list. To do this load the line identifiers in the usual way (in the main window) and display them in the plot. You can then make this the visitor list by pressing Set as visitor list. Now using the visitor controls you can step between the lines (remember to de-select Flip and select Options->One spectrum for visitor, otherwise you’ll get the line identifiers copied, flipped and maybe reused!).

SPEFO options

The original motivation for creating this tool (suggested by Petr Skoda, see the acknowledgements section) was really to do flip comparisons of lines to determine radial velocities in the same fashion as the SPEFO program. This has some additional requirements beyond those required for simple shifting in a linear sense, as well as in redshift, and are enabled by selecting the Options->SPEFO options menu item. This reveals two text areas (you need to resize the window to actually see them first time). The upper one shows the flip corrected offset (that’s half the standard shift) and the radial velocity for that flipped shift (calculated as cOffset 0.5/(Flipcentre)), assuming that the coordinate system is some wavelength.

There’s also an editable area for writing some notes about the measurement, both these areas are written the file SPEFO.log by pressing the Save to SPEFO.log file button.

Accelerator keys