The Nebula web interface

Nebula has a web interface showing the results of the most recent run of the Nebula pipeline. We use this ourselves to investigate our algorithms, and it is also designed to be explored by SETI@home users and the public.

For the most part the interface is self-documenting. It shows you a list of the top-scoring multiplets of various types. If you click on a multiplet ID it shows you the details, including a list of the signals that comprise it. If you click on a signal ID it shows you a "waterfall plot" of the signal's neighborhood in time/frequency space. These are a bit complex and requires some additional explanation.

Waterfall plots

A "waterfall plot" is a graph of the signals in a particular range of times and frequencies. Waterfall plots let us visualize RFI, helping us study and improve our RFI algorithms.

What the graphs mean

We graph frequency horizontally and time vertically. Thus, for example, a vertical stripe in a waterfall plot represents a group of signals detected at the same frequency over a time interval. If the telescope pointing changed over this interval, the group of signals is almost certainly RFI. (If a source is celestial, you won't see it when the telescope moves.)

When you click on a signal ID, you'll see a waterfall plot with that signal in the center (indicated by a small square). The frequency width is 2000 Hz, and the time span is 1728 seconds, or about a half hour. Each signal type is shown with a different symbol.

The right part of the graph shows how the telescope moved over the time interval, expressed as angular separation from the sky postion of the central signal, measured in beam widths. This is shown on a log scale for dynamic range.

The telescope movement graph has one point per signal, so if there are few signals the graph will be sparse. The symbol used for each indicates with beam the signal is from; at a given time the beams point at slightly different sky positions.

The key point is that if two signals are separated by more than a few beam widths, they aren't from the same celestial source. So if you see a vertical band of signals, and the telescope moved by 10 beam widths over that interval, you know it's RFI. If the telescope didn't move during that interval - which you can see from the telescope movement graph - it's not necessarily RFI.

Changing the window

The green buttons above and to the left of the waterfall plot let you change the time and frequency size of the window by factors of 2 and 10, and let you move the window by one half-width in either dimension. This lets you zoom into to examine a few signals, or zoom out and see a large span of time or frequency.

RFI removal

By default, the waterfall plot shows all signals, including ones flagged as RFI. If you click the "Standard" button below the graph, you'll see a waterfall plot with RFI removed. Flipping back and forth between the two pages shows what was flagged as RFI.

The web interface also lets you experiment with the RFI removal algorithm. Scroll down the page and find "Custom RFI removal". Below that you'll see checkboxes for the various RFI algorithms, which you can turn on or off. This lets you, e.g., see the effects of one algorithm in isolation. You can also set the parameters of particular algorithms, namely multibeam and drifting.

When you click OK, RFI removal with your choice of algorithms and parameters will be applied to the signals in that window, and you'll see a waterfall plot of the results.

We used this tool to find parameters values that work well across a range of examples.

Note: Custom RFI removal with the default settings should produce the same results as Standard. If not, please let me know.

Other controls

The "Show signal list" button shows a textual list of the signals in the window and their parameters.

If you do custom RFI removal, you'll see an additional "Show stdout" button under the graph. If you click on this, you'll see the diagnostic output of RFI removal for your custom settings. This will be voluminous and probably incomprehensible. To make sense of it, you'll need to read the C++ source code of the RFI removal program.


The Nebula web pages for signals, multiplets, and pixels have a "Bookmark" button (you'll need to log in to the SETI@home web site to use this). Use this to mark things that are interesting for any reason, such as:

When you record the bookmark, write a comment saying what's interesting. If you like, send me (David Anderson) an email so that I can check it out.

Scoring details

A final thing of note: on the page for a pixel, you'll see a "Scoring details" button. This runs the multiplet finding/scoring program for that pixel in a verbose mode, and shows you the output. This will be voluminous and mostly incomprehensible. To make sense of it, you'll need to look at the code.

©2019 University of California
SETI@home and Astropulse are funded by grants from the National Science Foundation, NASA, and donations from SETI@home volunteers. AstroPulse is funded in part by the NSF through grant AST-0307956.