SPHEREx Data Explorer: 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. Specific help on the Spectrophotometry Tool can be found elsewhere.

Contents of page/chapter:
+Plotting Spectra
+Redshifting Spectra
+Viewing as a Table
+Exploring Dates of Observation

 

Plotting Spectra

Spectra are plotted by default as F-sub-nu vs. lambda in microns, 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 'lambda', understood the units (microns), and is showing them in the observed reference frame. It has identified the flux axis column as 'flux' and the corresponding error as 'flux_err', and understood the units as uJy. 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. When you save the extracted spectrum, all of the additional information (time of observation, bitflags, etc.) beyond just the wavelength and flux comes with it.

Exploring Dates of Observation

Because of the way that SPHEREx observes, it builds up each spectrum relatively slowly over time. This might matter if the targets you care about vary with time. This example demonstrates how to explore the dates of observation in a SPHEREx spectrum.

First, use the spectrophotometry tool to obtain a spectrum and plot it. Because you are constrained by the fact that the tool knows that this is a spectrum, you are limited in how you can change the plot.

Following the basic properties of plot manipulations, click on the gears to change what's plotted. Expand "trace options." Under "Color Map", select the date column: "mjd." Don't forget to select the color scale you prefer under that. For this example, the color table is "Rainbow." (The string "#this" is a strange-looking IVOA term that means "the main data product.")

Now the points are colored corresponding to the date of observation. When you move your mouse over any point, the pop-up tells you the wavelength, flux, and date corresponding to that point, enabling a point-by-point exploration. You can see more explicitly how it's built up this spectrum in chunks. Possibly more interestingly, however, all the red points are systematically lower at shorter wavelengths than the purle/blue points, suggesting that the star itself has changed brightness in the ~6 months between observations.