SOFIA Science Data Archive: Spectra

Visualization of SOFIA spectra use capabilities of Tables, (image) Visualization, and Plots. Generic help on those capabilities can be found in those other sections; since spectra are a special case of plots, this section focuses on spectra specifically found in the SOFIA Science Data Archive.

Contents of page/chapter:
+Introduction
+Image Planes
+Data Tables
+Plots (Charts)
+Choosing among Table, Chart, or Image
+File Contents Menu
+Wavelengths

 

Introduction

Some of the spectral files in the SOFIA Science Data Archive show the spectrum as an image, some as a table, and some as a plot. In many of the more processed data (e.g., levels 3 and 4), you can interact with the spectrum as rendered in any of these ways. Some of the data products are multi-plane files, and you can scroll through them and/or pick the plane to visualize. Some of the data products, if you view the original planes, have data that may be hard to interpret at first glance. In those cases, the SOFIA Science Data Archive does some simple extraction and presents that extracted data to you in a form (converting units and presenting as plots or tables) that make more scientific sense for quick assessment of the data; these are labeled "extracted data" and can be thought of as sort of a "value added" view of the data.

To visualize spectra, you must be on the search results page, with an instrument tab in the foreground on the left (not the AOR tab) and have the "Data" tab in the foreground on the right. (And, of course, your search must have returned spectra.)

Image Planes

Images can be interacted with as described in the (image) Visualization section. However, zooming has little/no meaning in particular for the extracted spectrum.

When there are multiple image HDUs or planes (see images section for more on HDUs and planes), you can click on the arrows in the top of the image to scroll through the planes.

If the data are stored as a multi-plane FITS file, you may be able to scroll through the planes, as in this EXES Level 2 "raw" product:

If the data are stored as a multi-plane FITS file, but where each plane is a separate, extracted spectrum, it may look like this EXES Level 3 "spec" product:

If there are additional HDUs, sometimes there is a "File Contents" menu that appears at the top of the data window, from which you can select a different HDU image (this is an EXES "readouts_coadded" file):

By default, it shows you all the images in the file and in this case, you can navigate using arrows as in the examples above, but you can also navigate to different HDUs using the "File Contents" menu.

Data Tables

Each of these tables can be interacted with as described in the Tables section. Note that you can copy or view the contents of a table cell.

Some FITS files are (nearly unreadable) tables, as in the case of this FIFI-LS Level 1 product:

Most of the really unreadable ones are lower-level data products.

Many of the higher level tables have useful information but may still be cryptic, such as this EXES level 3 "sky_orders_merged_1d" product:

Note that this table has a choice at the top to view it as a table, a spectrum, or an image. In this specific case, the 'table' view is the most human-readable, but see the next section.

Plots (Charts)

In some very specific cases like the one immediately above, where the data as shown seem nearly-but-not-quite human readable, the tool is smart enough to do better.

Some FITS files can be rendered as plots (sometimes called charts), as in the case of this EXES Level 3 "mrgordspec" product:

Note that the label of this plot is "extracted data." This means that the data shown here are converted in some way compared to the original data in the FITS file, and if you use the "File Contents" menu at the top left (more on this below), you can access the original data as well as these extracted (or "value added", if you will) data. The tab at the top says "spectrum" as well -- if you want to see the constituent data, you can click on "table" to get it.

Each of these plots can be interacted with as described in the Plots section. The axes labels and units are extracted (interpreted) from the FITS file itself, and may be cryptic, but are meant to give you some indication of the file contents before you download it to work with on your own disk.

Choosing among Table, Chart, or Image

For some products, you can choose whether to view the data product/data product plane as a table, chart, or image. The data come up with a default choice, but you can pick a different option. In this: , "Image" is selected; to pick another option, just click on your choice.

File Contents Menu

Some products have a "File Contents" menu: You can use this menu to select which part of the data product to view.

You can have something that is all images, as in this EXES Level 2 product:

You can have something that is mostly tables, as in this GREAT Level 4 product:

You can have these options, as for this EXES Level 3 product:

In this case, you can view the original data, seen here as "HDU #0 (2D image - show as table or chart or image)." Or, you can view the "Extracted Data." Note that this option is indented on the menu. This view of the data is extracted from the original FITS data, and is sort of a value-added view of the data that the tool is presenting you, and this view should be the default for data products with this option. The "Extracted Data" view will combine information so as to, e.g., plot flux as a function of wave number. If you go back to the "HDU #0..." view, and view that as a table or plot, you will find data with columns like "naxis1_data_0," which is what is provided by the data file itself, but the "Extracted Data" view interprets naxis1_data_0 properly as wavenumber. Here is an example for this EXES Level 3 product; the first is the "Extracted Data" table view and the second is the "HDU #0" table view. You can see how the "Extracted Data" view is easier to scientifically interpret.

This long example File Contents menu is from a HAWC+ Level 4 product, and nearly all the image planes are true images.

In this case, the selected option is "All Images in File," and it becomes a view like the multi-plane images described above, where you use the arrows to page through the image planes. However, this menu can also be used to select the image you would like to view, or view the last three planes (which are tables).

This example File Contents menu from a FIFI-LS Level 3 product:

shows a mixture of 2D images and tables, including one that contains a single table cell. This menu also has an "All Images in File," and you can scroll through all the images. But the data products in this file are far more diverse than just images, and the File Contents menu is the most efficient way of exploring these other data planes.

Wavelengths

Some data products provide wavelengths, but those wavelengths may be in unexpected places. Here are the two most unusual.

This FIFI-LS Level 4 product shows wavelength as one of the planes within the HDU, and you can scroll through the wavelengths using the second set of blue arrows.

This FIFI-LS Level 3 product shows the wavelength of the pixel under your cursor when you stop moving your mouse (look lower center of this screen snapshot). Note that this only appears when viewing HDUs from the File Contents menu (as opposed to "all images in file").