My last missive described how we changed zone RFI removal to work for pulses. In this case the "zones" are ranges of pulse period rather than frequency: a lot of the RFI is aviation radar that results in pulses with periods that are multiples of 12 seconds. To figure things out, Eric looked at histograms of pulse counts as a function of period. Since then, he has repeated this exercise for triplets, and then autocorrs. The details were different in each case, but the basic idea was the same.

Once this was done - a couple of weeks ago - I did another Nebula run on the full sky - i.e. finding and scoring multiplets in all pixels. Eric, Dan and I then examined the top-scoring pulse/triplet and autocorr multiplets, looking for RFI that should have been removed by the zone filter. Good news! Although some multiplets still had this sort of RFI, many of them didn't - enough that we'll be able to find plenty of multiplets that are worth re-observing.

So until further notice we're done with computing. No more algorithm-fiddling or scoring runs. Nebula has done its job. This is a huge milestone for SETI@home, and for me personally. I've been working on Nebula for 5 years, and it's been some of the most challenging work - algorithm design, programming, and debugging - I've done.

The immediate next step is for us to go through the top-scoring multiplets - of all the various detection types, bary/non-bary, and scoring variants - and pick 100 or so to re-observe at FAST. That's about how many we'll have time for in the 24 hours of observing time that we've been granted - we'll observe each one for a minute or two, and it can take several minutes to slew from one sky location to the next.

After that, we'll need to adapt our computers at FAST (which currently run SERENDIP spectrometers) to produce the time-domain data needed for SETI@home. Then we have to figure out how to analyze this data; we'll probably do the first part using the existing SETI@home client running on cluster nodes either in China or at the Atlas cluster in Hannover. After that we'll need take these detections and decide whether they "confirm" the corresponding multiplet. We haven't figured the details. In the case of barycentric multiplets - where we know what frequency to look at - this might involve manually looking at waterfall plots of the new detections. For non-barycentric multiplets - where the frequency could be anywhere in a wide range - we could add the new detections to the SETI@home detections, re-run Nebula (at least the multiplet-finding part) and see if it finds the original multiplets with additional detections that increase the score.