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The lack of understand of what this instrument does compared with the slitless (Star Analyser?) spectrum of M42 shown there was amusing considering the ~30x difference in cost.
That spectrograph (in non automated form) has been around a few years (Maurice Gavin did a youtube review on it). I’ve yet to see a published spectrum using one though. There have been quite a few failures by companies bringing instruments into this field in the past few years. I would say to succeed it needs to get into the hands of a few observers who can show what it can do and get some results out there. That is why the Star Analyser and the instruments from the leading supplier in this high end field succeeded where others have struggled. Their latest UVEX spectrograph, also using reflective optics is coming out in commercial form later this year but is already well known from the results produced using 3D printed kit built prototypes. Getting potential customers directly involved in product development has proved to be a good way of ensuring a market for the final products.
Stunning indeed. The sodium emission is dropping away fast now. I wonder how far out from the sun we will be able to detect it? Here is a quick raw spectrum image (yet to be reduced) of the NaD line region from 2020-07-20.
Compared with the same 10 days earlier, the sodium lines are now almost superimposed on the sky lines here as the Doppler shift drops to zero as the comet passes us.
Cheers
Robin
Hi Hugh,
It was done rather crudely in this example. The spectra are flux calibrated using a nearby reference as normal. I then just subtracted the Pickles G2v spectrum scaled relative to the comet spectrum in a region where there is just the dust continuum. It is pretty good at removing the solar absorption lines and most of the dust continuum but leaves a residual shape in the continuum dependent on the broad reflectance spectrum of the dust so not good for quantitative measurements. It would be better to modify the G2v spectrum first to better fit the dust continuum leaving just the emission.
Cheers
Robin
A spectrum of the emission lines from the comet tail.
It is very different to those of the coma usually seen in amateur spectra and is dominated by CO+ emission lines (the absolute intensity in the spectrum is only ~1% of that in the coma spectrum)
Cheers
Robin
It looks like it has finally reached maximum (Vmag 12.2) 3 weeks after the very early discovery. Curiously, the classification spectrum submitted to TNS has still not been made public though.
That’s great. There is definitely no Na extending out that far sideways in my image so combining the two images we see just Na near the centre of the coma and streaming out in the tail direction.
Cheers
Robin
Hi Hugh,
Interesting. This cross section through the coma appears to show a different story to this vertical section.
https://britastro.org/comment/8942#comment-8942
Perhaps there could be some directional effects in the differences between Dust,Na D and other emission bands with the gas emission moving off the slit to the left relative to the dust. I have some cross sections across the tail still to analyse which might shed more light.
Cheers
Robin
A (sky subtracted but uncorrected for instrument response) spectrum image showing a cross section through the coma, taken 20200713.985 with the ALPY600 (An extreme non linear gamma contrast stretch had been applied to reveal the outer regions while not saturating the inner coma) Full size image attached
Although very intense, the sodium emission is confined to a narrow region of the central coma, particularly compared with the CN band which is visible well beyond the dusty region.
Robin
I’ve finally got round to analysing the high resolution spectrum from 10th July. The velocities relative to sun and earth are satisfyingly close to those published by JPL Horizons
https://britastro.org/node/23284
Robin
I have indeed. (Taken with the ALPY)
I just need to get round to reducing them (I am currently just finishing off the Doppler shift calculations on the high resolution spectrum)
I have checked with the SPI team who have confirmed that their published image was mirrored so their observation now agrees with Torsten’s ie the sodium tail is anticlockwise relative to the dust tail
Robin
There’s now a NEOWISE image showing the sodium tail embedded within the dust tail.
https://psi.edu/news/neowisesodiumtail
This image is close in though so perhaps the sodium tail emerges from the dust tail further out to produce the narrow reddish tail. Not sure why it is at a larger angle than the ion tail rather than between them like Hale Bopp but perhaps that is a geometry effect ?
There has been some discussions about this reddish tail on Cloudy Nights.
https://www.cloudynights.com/topic/716780-comet-neowise-high-resolution-spectrum/?p=10332533
I wondered about sodium but the neutral sodium tail seen with Hale Bopp fell between the dust and ion tails rather than at a larger angle as here. I am wondering if it could be from NH2, the emission bands of which fall in the red region of the spectrum. Perhaps some mechanism separating ions by molecular weight, CN, C2 being heavier ?
Robin
Hi Mike,
Yep definitely a little orange image of the comet complete with tail in the spectrum there.
Hi Nick,
The sodium emission is so intense in the spectrum that I wonder if there is any sign of a sodium tail like in Hale Bopp? I hope to take a spectrum of a cross section through the tails if the weather cooperates but a narrow band image though an Na D pass filter could be interesting. Not a common filter among amateurs though I guess. The inverse filter response is more common !
Robin
This is what the spectrum in the region near the Na D lines looked like tonight at ~1A resolution. (The inset is the guider image showing the position of the slit). Note the Doppler blue-shift in the comet Na D lines relative to the sky lines
Robin
Not visible for a few days yet from the observatory so I rigged up a portable setup last night using a Star Analyser in front of a 50mm lens on an AS120mm camera, normally used as a guide camera on the ALPY spectrograph.
There is not much contrast between the spectrum and the bright sky but the sodium emission is clear.
Cheers
Robin
