Comet NEOWISE has been the naked eye comet that many astronomers have been eagerly waiting for. It has been a month-long marathon of observations, early wake ups and nights out.
July 6th 2020 morning (0.30 AU from the Sun)
2.5 days after perihelion. Very welcome visual sight and with binoculars, 7 years after my last naked-eye comet in 2013 (C/2011 L4 PANSTARRS). The comet appears definitely yellowish, probably due to atmospheric extinction and blue background sky).
The image with 190mm diameter Maksutov-Newton telescope (about 10 minutes total exposure) shows a dust tail about 40 arcmin long 2.5 days after perihelion, with a nice parabolic shape around the nucleus, something typically seen on very active comets near the sun.
July 7th 2020 morning (0.31 AU from the Sun)
3.5 days after perihelion. Again imaged with the 190mm diameter Maksutov-Newton telescope (about 10 minutes total exposure), but in much better conditions (dark sky and no louds). The comet shows a dust tail more than 1.5° long, as well as a red tail, the sodium tail.
Enhancing the area around the nucleus allows to reveal well defined shell structures, as well as what looks like 2 forward jets. Trailing the nucleus is a narrow, dark line, exactly in the antisolar direction, likely the shadow of the dust-dense enveloppe of the nucleus projected inside the tail of the comet. The bottom edge of the dust tail shows periodic enhancement of density, that will become the future synchronic striation of the dust tail.
July 8th 2020 morning (0.32 AU from the Sun)
Wonderful sight of the comet NEOWISE with bright noctilucent clouds! The color contrast between the electric blue noctilucent clouds and the yellow comet was striking!
Stacking around 20 minutes of exposure at 200mm f/2.8 allows to reveal details of the comet despite the strong interference of the dawn. Dust tail is 4.5° long, with the first signs of synchronic striation. The red sodium tail is 4° long. Despite the blue sky background, the ionic tail can be detected and is already 4.5° long. It is interesting to see that the sodium tail is much brighter than the ionic tail.
July 11th 2020 morning (0.37 AU from the Sun)
NEOWISE with “sodium” tail and ion tail at 200mm focal length. Good data set, the sodium tail is wider than on the 12th of July. To be processed.
observation with 100mm binoculars.
July 12th 2020, morning (0.39 AU from the Sun)
Excellent set of images. The observation was made from the Northern coast of Finistère with the comet above the ocean, and absolutely no light pollution. The view was just spectacular to the eye, and with the 100mm binoculars. The tails of comet NEOWISE go out of the frame, which means the tails longer than 10°.
The image reveals the complex tail structure of the comet: a large, fanned dust tail with beautiful synchronic striation, the blue ion tail tails with blue gaseous wisps, and the rare red “sodium” tail.
If you are not familiar with the anatomy of comets, below is a labeled image to help identifying the main features of the comet
The image is acquired with the 70mm-200mm focal length zoom, set at 200mm and at aperture of f/2.8. Altogether, the total exposure is a bit less than 1 hour. In order to avoid blur due to the motion of the fine structures within the tails, a registration on the details of the tails was made. It allowed to keep all the finest details visible. Being able to obtain images with stars subtracted well enough to allow to register on the tail structures is clearly the most difficult part of the processing.
The excellent signal to noise ratio of the images with zero light pollution interference allows to reveal unusual, discrete features, like the ion tail beneath the dust tail, color variations of the dust tail with larger dust, located on the bottom edge of dust tail being slightly redder, or the green C2 coma signature visible very near the nucleus. Even color variations within the synchronic bands structure seems visible on the enhanced color image below.
In order to help seeing the structure of the different, overlapped, tail components, I did some color subtraction to help distinguish the different tails. Such channel subtraction increases the noise, but with the very good signal to noise ratio of the data, it is possible to isolate meanignfully the different components of the comet using this technique.
- Subtracting the blue channel from the signal of the “sodium” tail (Red and Green channels) allows to show the actual structure of the sodium tail: a straight and mostly featureless tail, extending up to the edge of the frame.
- Subtracting the mostly white dust tail from the ion tail signal reveals the ion tail even beneath the brighter dust tail. The ion tail comes from the right hand side of the nucleus mostly on this day. This subtraction also shows that the upper part of the dust tail is bluer, as well as possibly two different dust populations organized in syndynes.
- Subtracting both the white dust tail and the ion tail from the Red channel isolates the “sodium” tail. It shows that, contrary to the ion tail that comes mostly from the right hand side of the nucleus, the “sodium” tail comes straight at the back of the nucleus. The fact that the two tails do not seem to come from exactly the same area probably explains why, on this day, the two tails only partially overlapped, which help in distinguishing individually both of them.
- Eventually, subtracting the dust tail from the Green channel in the inner area reveals a green coma around the nucleus hidden beneath the bright dust tail.
Analysis of the color signal of the comet to reveal independently the different components of the comet (open in high resolution here).
July 13th 2020 morning (0.41 AU from the Sun)
NEOWISE with sodium tail and ion tail at multiple focal lengths. Excellent data set, the sodium tail is seems to have multiple streamers. To be processed.
July 18th 2020 evening (0.54 AU from the Sun)
On July 18th, NEOWISE has definitely dimmed significantly. Yet, this day is the one when the dust tail was visually the longest, going beyond 15° length ! Ion tail seems to be at the threshold of visibility. Photographically, the tails are even longer, with the dust tail reaching about 25° long, and the ion tail reaching around 40° long!
Although the sodium tail seems to have totally disappeared, using image subtraction it seems the sodium tail can still be detected: it seems to be totally hidden beneath the ion tail, and quite more narrow that the ion tail. It seems to be about 15° long.
July 22th 2020 evening
Visually, the comet has dimmed again since the 18th of July, making it a pale sight compared to what it was 10 days before. For imaging NEOWISE, I used 14mm focal length in order to capture the whole tail length, with a total exposure of 75 minutes. Yet, the choice of focal length was not the best, as using a 35mm focal length would have given a deeper image and more length probably. The tails lengths are indeed similar to images of the 18th of July, with dust tail reaching 25° length, and the ionic tail going up to 40° or 45°. The sodium tail is not be detected, probably both from the fact it dimmed, and also because of the use of a shorter focal reduced too much the signal to noise ratio.
July 28th 2020 evening
NEOWISE with Maksutov Newton telescope, imaged during 1h30minutes, showing a spiral jet rotating around the nucleus of the comet, and structures in the ion tail drifting along the tail, blown by the solar wind.
July 30th 2020 evening
NEOWISE with Celestron11 telescope
Comparison of ionic and sodium tails
Even though the data from the 11th and 13th of July are not processed with optimal workflow, the differences in the structure of the tails is already worth having a look before the data are fully processed. Especially, the changes of structure of the sodium tail, which has been not detailed before is very interesting.
July 7th morning: this day, only dust tail and sodium tail were visible. With the longer focal of the Maksutov Newton, channel subtraction allows to see the shape of the sodium tail beneath the dust tail. The sodium tail appears to be quite different in shape than the dust tail, even near nucleus, being much more “triangular” than the parabolic shape of the dust tail. The shape of the sodium tail in the subtracted image looks very similar to what has been observed near the comet head by the Planetary Science Institute, where the sodium tail is also more triangular and narrow than the dust tail.
July 8th morning: the sodium tail is quite narrow, while the ionic tail appears broader and mostly separated from the sodium tail. The signal to noise ratio is limited by the strong interference of the dawn light. Although the images are not very clear, the configuration seems very similar to the one of July 12th.
July 11th morning: the sodium tail is quite broad, with a rather sharp left hand side edge, and a soft right hand side edge. The ion tail shows a main streamer coming from the right hand side of the nucleus and another streamer coming from the left hand side of the nucleus.
July 12th morning: the sodium tail is quite narrow, and appears as a single streamer, quite well defined. The ion tail comes mostly from the right hand side of the nucleus. The fact that the two tails are spatially separated was a great chance for making images where they can be individually identified. The overall combination is very similar to the one of the 8th of July.
July 13th morning: the sodium seems to show two separate streamers, coming from both side of the nucleus. Although the ion tail seems to come mostly from the right hand side of the nucleus, it is notably more divergent than the previous days and thus broader.
July 18th evening: the images being taken with a shorter focal length, they are less detailed and more irregular due to the effect of removal of background stars. The sodium tail shows up as a single narrow streamer, located beneath the main streamer of the ionic tail, which is also quite narrow on this day.
HDR astrophotography by Nicolas Lefaudeux 