Euclid Data Explorer at IRSA: Spectra

Visualization of spectra uses 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
+Potential confusion: Why is it reloading?

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 shifting the spectrum to correct for cosmological redshift. 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. HOWEVER, this filter will only 'persist' (stick around after you navigate away from the spectrum and return to it later) if you pin the spectrum and filter on the pinned spectrum's table.

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 "Normalize & shift traces before overplotting." 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 "Normalize & shift" 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 these offsets. You can adjust the amplitude of the offset 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

The plot you have selected when you click on "Combine Chart" is implicitly part of the combining process, so it's not available to select with a tickbox in the pop-up.
At this time, you can't combine charts that the tool doesn't recognize as spectra, even if they are in the same parameter space.
To delete a combined plot that you have created, just click on the 'x' in the upper right of the plot.

Potential confusion: Why is it reloading?

Once you start playing around with the Euclid spectra, you may discover some apparent inconsistencies with how the tool is behaving. There is method to the madness.

After a search, you have a list of sources on the left and spectra on the right, which change based on what you have selected on the left.

When you view a spectrum, you might find that there are some values in the spectrum that you'd prefer that the tool not plot, such as zero values. You can view the spectrum as a table, and use the properties of the table to impose filters on the spectrum to remove the outliers, and then the plot looks better. So far, so good.

If you then click on a different row on the left, a new spectrum appears. You might impose the same filter there. But then, you want to go back to the first spectrum to compare the spectra. You do that ... and the filter you imposed is gone!

Each time you change what you've clicked on the left, it reloads the entire data product on the right. So, if you impose a filter on the plot or the data table, that filter is removed when you go back and reload the data product. In order for the filter to "stick", you need to first pin the spectrum (), and then filter the table associated with that pinned spectrum. Then the filter persists even if you click away from that spectrum and then come back to it.

When you pin the spectrum, it also makes viewing the spectrum faster. Every time you reload the data product, the tool is re-pulling the spectrum out of a large file of many spectra. By pinning the spectrum, you're temporarily saving the data in your browser, so it works faster all subsequent times you want to view the same spectrum.