Euclid Data Explorer at IRSA: Spectra

Visualization of spectra use capabilities of Tables and Plots. Generic help on those capabilities can be found in those other sections; this section is a generic help page for spectra, which are a special case of both tables and plots, and collects all the spectra-specific information in one place.

Contents of page/chapter:
+Plotting Spectra
+Redshifting Spectra
+Viewing as a Table
+Combining Spectra

 

Plotting Spectra

Spectra are plotted by default as signal (in 1e-16 erg/Angstroms/s/cm^2) vs. lambda in Angstroms, with connected points. These plots, while they look like other kinds of plots in IRSA tools with this look-and-feel, have different choices than regular plots, because the tool understands that it is plotting a spectrum.

Changing what is plotted by clicking on the gears is similar to, but not quite the same as, the generic case. Now, because it knows it is plotting a spectrum, you can select the x- and y-axis columns and units from a pre-defined set of choices in the drop-down menus, where it will convert the units when necessary.

In this example, the tool has identified the wavelength axis as 'WAVELENGTH', understood the units (Angstroms), and is showing them in the observed reference frame. It has identified the flux axis column as 'SIGNAL' and the corresponding error as 'UNCERTAINTY', and understood the units as 1e-16 erg/Angstroms/s/cm^2. From the drop-down menus, you can choose to convert the wavelength to Angstroms, nanometers, microns, millimeters, centimeters, or meters. It is plotting the spectrum as connected points, with error bars.

You don't have as much flexibility in these plots as you do for plots in general, but the options you do have are highly customized to spectra, such as redshifts. See the next section!

Redshifting Spectra

When the tool recognizes the wavelength axis, it offers you a choice of redshifting the spectrum. By default, it assumes you want to plot the data as observed:

but if you pick "Rest Frame":

you can enter a redshift, and it shows you how it adjusts the wavelength axis accordingly:

Click 'Apply' to implement these changes in the plot. The axis labels on the plot correspondingly change.

To change back to the data as observed, simply pick "Observed Frame" from the drop-down menu.

Viewing as a Table

Near the top left of the spectrum plot, you can find this choice:
The default view is to show the spectrum as a plot. If you click on "Table", it will show you the spectrum as a table. The table view has more information than you get from just the plot, such as the pixel location and the time of observation. You can impose filters on the spectrum from this table, because it behaves just as all other tables in this tool.

⚠ Tips and Troubleshooting: Since several of the values in most Euclid spectra are zero to indicate "no data", you probably want to impose a "> 0" filter on the "SIGNAL" column right away to make the plot look better.

Combining Plots

Pinning plots is covered in the generic plots section, but in brief, pinning allows you to temporarily 'save' a given plot.

When you have more than one plot pinned, this icon may appear at the top of the plot pane: This means "Combine Chart".

This option only appears if you have pinned at least two plots, and it will only let you combine plots if it recognizes that you have spectra loaded.

To start this process, click to select the first chart you want to combine, then click on the "combine chart" icon. You get a pop-up like this:

All of the remaining pinned charts that can be combined appear as a list at the top. Once you select them via the tickboxes on the far left of the list (the first one in the list is selected here), they appear as options on the bottom of the pop-up window. For this example, I am combining three Euclid spectra.

Continuing through this pop-up, you can choose to set the title of the new plot you are about to create -- the default is "combined".

The next choice is "Apply cascading." Here is what this is and why it matters. If you are combining spectra that are nearly all the same brightness, the spectra will be plotted on top of each other. Sometimes that is what you want, and sometimes that is not. If you click on the "Apply cascading" option, you have an additional choice:

This is telling you how it is going to stack the spectra on the final plot. See below for examples with and without cascading. You can adjust the amplitude of the cascade by changing the size of the padding, as shown.

Finally, you can change the name of the trace as displayed on the plot (and in the pull-down menus in the tool) for each of the spectra you are combining.

Click "OK" to actually make the new plot.

Here are some examples of combined spectra, both observed and rest frame. All are useful, but in different contexts.

Note that after you combine a plot, there is a new drop-down at the top of the plot that controls which trace is in the 'foreground' for changing plot parameters or selecting points, but you can also simply click on points in the plot to bring that trace to the foreground.

⚠ Tips and Troubleshooting