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C 2020 F3 NEOWISE Doppler velocities from high resolution spectrum


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About this observation
Robin Leadbeater
Time of observation
10/07/2020 - 23:53
C/2020 F3 (NEOWISE)
Observing location
Three Hills Observatory Wigton Cumbria UK
LHIRES III spectrograph 2400l/mm
ATIK 314 camera
Celestron C11 280mm aperture SCT
3x300 sec
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A high resolution (~0.5A) spectrum covering the yellow part of the spectrum including the sodium D doublet wavelengths.

The Sodium emission dominates the spectrum. (There is as much light from the two narrow sodium lines as in the rest of the light in this spectrum combined.) There are no other strong emission lines in this yellow part of the spectrum (the Swan bands are further to the blue-green) but there are some other faint emission lines though which I have not yet been able to identify.

We can measure Doppler shifts to calculate the comet's motion both relative to us and to the Sun using the sodium light pollution and daylight spectrum as references

The Doppler shift in the sodium emission lines gives the velocity relative to Earth directly = -55 km/s (ie the comet is moving towards us)

We see the Sun's absorption line spectrum in the sunlight scattered from the comet dust. These absorption lines experience two Doppler shifts, first due to the motion relative to the Sun and then after scattering due to the motion relative to Earth. This total shift = -23km/s

The motion of the comet relative to the Sun is therefore -23 +55 = +32 km/s (ie the comet is moving away from the Sun) These are in good agreement with the values given on the  JPL Horizons website ( -54.9km/s and +31.4 km/s for the time of the observation)


nickjames's picture

Robin, I really like this observation and analysis, in particular the velocity relative to the Sun. In my professional life I design equipment that measures the Doppler shift of deep-space spacecraft using coherent RF techniques. This is a bit like your Sun measurement except that the radio signal gets transmitted from the earth, reflected by the spacecraft and then comes back to the earth. We can actually measure the carrier phase to small fractions of a wavelength so the RMS velocity errors are a few tens of microns/s but the principle is the same. Great to see the quality of spectra that very experienced amateurs can produce.

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