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FAST and SETI

已有 2851 次阅读 2018-7-20 11:29 |个人分类:总结|系统分类:科研笔记| FAST

 

FAST and SETI

(COSPAR 2018报告讲稿)

FAST stands for Five-hundred-meter Aperture Spherical radio Telescope. It is currently the world largest single dish telescope.

FAST situated in a Karst depression in southwest China. This Karst depression is close to a sphere, which minimize the excavation. It is higher than the Karst depressions around, which makes a good shielding of radio interference. The control room is well behind that hill.

FAST has several science goals. We are interested in the HI both insider and outside of Milky way. Pulsars are also the key science targets. FAST may also contribute a lot in the studies of interstellar molecules. FAST will also join VLBI network and work on fast radio burst (FRB), SETI etc.. I would like to mention, not like FRB, SETI is the science goal of FAST from the beginning. We plan to achieve these science goals simultaneously in a survey called commensal radio astronomy FAST survey (CRAFTS).

The reflector of FAST is active, forming a 300-meter aperture parabola in real time. The feed support system is light-weight, weight only 30 tons. Since there is no solid connection between the feed cabin and the reflector, the measurement and control system play a crucial role.

The feed will illuminate an area with 300-meter aperture at one time. When FAST is tracking a source, the feed and the illuminated area will move accordingly.

In the initial design, there are 9 sets of receivers, covering 70 MHz to 3 GHz. Now it has been reduced to 7 receivers, since now we have a ultrawide band receiver, covering 270 MHz to 1.62 GHz. Now a new ultrawide band receiver is planned, covering about 560 MHz to 3.3 GHz.

These two receivers are the ultrawide band receiver on the left and multibeam receiver with 19 beams on the right. The ultrawide band receiver worked from 2016 to April this year. Now the multibeam receiver has been installed. It is now under testing.

Let’s have a close look at the 19-beam receiver. They are arranged in a hexagon pattern. The beams are numbered in this way. The receiver can rotate, so a uniform scan observation is possible.

If the 19-beam receiver is rotated by 23.413 (about 23.4) degrees, we can make a uniform scan with 2 passes. This would be the scan strategy of the commensal survey.

The backends of FAST are based on ROACH 2 or CRANE. In the future, the backends will be mainly based on ROACH2. The FRB and SETI backends are from Dan’s group.

Since the 19-beam receiver has just be installed for about 2 months, the results we got comes mainly from the ultrawide band receiver. FAST has discovered more than 40 pulsars. Most of these pulsar are discovered during drift scan.

There is a millisecond pulsar discovered by tracking a gamma ray point source discovered by the Fermi satellite. The period is found with FAST observation. Then the Fermi data are reprocessed to confirm this discovery.

The list of new pulsars discovered by FAST can be found here on the CRAFTS website.

Half of the FAST pulsars are discovered by single pulse search. Look at the one single pulse, the signal from pulsar, FRB are similar, maybe the SETI signal will be also similar. At least the data looks similar.

For SETI observation, there is an observing window from 1 to 10 GHz. FAST covers part of this window.

To have an idea of SETI signal, maybe we can have a look at the RFI on our Earth. This is the RFI signal on the FAST site. We can see they appear on some frequencies. This is because there is a committee to allocate the frequencies. Specific activities use specific frequencies.

Let’s compare the pulsar, FRB and SETI signals. In general, the pulsar signal is wide band and periodic, with typical dispersion measure up to several hundred. The FRB signal is wide band and aperiodic, with larger DM up to several thousand. About the SETI signal, we are less certain about their properties. But they may have small dispersion measure, which make it difficult to distinguish them from the interference.

Typically, there are 1 M channels in the SETI backend, with channel width of 5 Hz. It would be hard to cover the whole band of FAST. We have to either increase the channel number of the SETI backend or narrow down the frequency range to be searched.

In the future, we plan to build some small dishes about FAST, expanding FAST to an array called FAST A+. This array is initially intended to localize FRB, but it may also help distinguish and better localize SETI signal.

Although SETI is a science goal of FAST and we will have the SETI backend soon, we have to learn more about the properties of the SETI signal in order to dig it out of the pile of RFI signals.

 




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