NEOWISE with the Star Analyser

[robin_astro]

The Comet 2020 F3 NEOWISE should make a great target for high resolution spectroscopy once it becomes visible from my observatory in  a few days but I tried it a couple of days ago with a simple portable rig consisting of the Star Analyser 100 in front of 50mm SLR lens mounted on an ASI120MM, normally used as a guide camera on the observatory spectrographs

There is little contrast between the spectrum and the bright sky but the Sodium emission is obvious as a tiny image of the comet in the spectrum.  This is quite unusual I think, the first comet I have measured showing this. 

As a bit of fun I followed in the footsteps of Bundsen and Kirchoff and took a spectrum of sodium (salt in a flame)  using the same setupand

and compared them, proving it really was sodium !

[robin_astro]

Now visible from the observatory. This is what the raw spectrum image looked like lat night in the region around the Sodium D lines at ~0.4A resolution (LHIRESIII slit spectrograph with a 2400l/mm grating covering ~150 A).  The inset image is from the guide camera to show the position of the slit.



I have over exposed the coma in this image to bring out the detail in the spectrum of the tail.  The sodium emission is very intense but there is also some weaker molecular emission lines present. (To be identified once I have wavelength calibrated using the  spectrograph internal Ne/Ar lamp). Note the blue Doppler shift  in the Na D emission lines relative to lines in the sky background due to the comet's motion.

Cheers
Robin

[andrew s]

Nice work Robin. Let us know how fast it is moving relative to earth once it's  calibrated.

Regards Andrew 

[markse68]

it’s frozen sea water?

[tooth_dr]

1 minute ago, markse68 said:

it’s frozen sea water?

Its losing 10 tonnes of this salty water per second.  Phenomenal

[markse68]

3 minutes ago, tooth_dr said:

Its losing 10 tonnes of this salty water per second.  Phenomenal

that is incredible! 😳 just how big is it?

[robin_astro]

47 minutes ago, andrew s said:

Nice work Robin. Let us know how fast it is moving relative to earth once it's  calibrated.

A quick back of envelope calculation off the image gives ~60km/s towards us.

(A note for all doom mongers and conspiracy theorists who might be lurking - This does not mean it is going to hit us, it is also moving sideways relative to us !)

The shift is ~6 pixels and the spacing between the two Na D lines (6A) is ~32 pixels so   

6* 6/32  ~ 1.125 Angstrom blue shift. 

The Na D doublet wavelength is 5893 A so 

velocity = c * 1.125/5893   = 57km/s

It will be interesting to see if the velocity relative to the sun can also be picked from the shift in the solar absorption lines from the scattered sunlight (a combination of both the velocity relative to the sun and us.)

Cheers

Robin

Edited July 11, 2020 by robin_astro

[robin_astro]

7 hours ago, markse68 said:

that is incredible! 😳 just how big is it?

From NEOWISE observations the nucleus is ~5 km diameter apparently which is on the large size according to the S&T website

https://skyandtelescope.org/astronomy-news/comet-neowise-delights-at-dawn/

but we don't see the nucleus as it is completely shrouded in dust

Robin

[CraigT82]

Fascinating thread. Would the Sodium be responsible for the golden/orange apparent colour of the comet? 

[andrew s]

14 minutes ago, CraigT82 said:

Fascinating thread. Would the Sodium be responsible for the golden/orange apparent colour of the comet? 

Yes, think sodium street lights.

Regards Andrew 

[robin_astro]

I have now processed and calibrated the high resolution spectrum.  See the attached poster.

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 the spectrum combined.)

There are no other strong lines in this yellow part of the spectrum (the Swan Bands are further to the blue-green) 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 us 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 us.  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 results are pleasingly close to the figures given by the JPL Horizons website for the time of the observation (-54.9km/s and 31.4 km/s)

https://ssd.jpl.nasa.gov/horizons.cgi

Robin

Edited July 15, 2020 by robin_astro

[andrew s]

Very good result Ri in, well presented.

Regards Andrew 

[saac]

This is astronomy for sure - amazing work, thanks for sharing.  Do you know if this level of analysis would be accessible using the Low Spec Spectrometer (1200 lines per mm grating) . 

Jim 

[robin_astro]

2 hours ago, saac said:

 Do you know if this level of analysis would be accessible using the Low Spec Spectrometer (1200 lines per mm grating) . 

Hi Jim

The resolution of the LHIRES with a 2400 l/mm grating is about 4x higher than the LowSpec with a 1200 l/mm grating. (~0.5A compared with 2A. An 1800l/mm grating would be better).  You would be able to detect  Doppler shifts of this magnitude (It is relatively straightforward to detect shifts of say ~1/20 of the resolution) but the shifts would be less than the resolution so not resolved  like in this example.

Cheers

Robin