Comète C/2021 A1 Leonard

Hamish Barker
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Re: Comète C/2021 A1 Leonard

Post by Hamish Barker »

Took this one last night. 4x300s with 175mm f7 newtonian and L200 with 180lpmm grating.

I think my wavelength calibration might be slightly off, if the guessed-at lines are correct.
leonard2021a1_20211225_426_Hamish Barker.png
Robin Leadbeater
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Re: Comète C/2021 A1 Leonard

Post by Robin Leadbeater »

Hi Hamish,

Is it bright enough for a spectrum of the tail? That can show a very different spectrum. Measuring the spectrum at different points across the coma can also be interesting. Here is an analysis I did of 2020 F3 (NEOWISE) for example.
https://britastro.org/observations/obse ... 992eb70386

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
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Hamish Barker
Posts: 226
Joined: Thu Jun 20, 2019 12:11 am

Re: Comète C/2021 A1 Leonard

Post by Hamish Barker »

that one was right across the nucleus, so I also tried selecting the binning region above and below the nucleus so see the difference with less dust reflection. That was just 4x300s, so with more exposure, I think further along the tail should be possible with longer exposures. Also I have just discovered that I had been using the more noisy high speed readout mode on my camera (a very old QHY2Pro). The slow readout seems much less noisy. Yet I did proper measurements of readout noise and gain in high speed read mode and came up with 0.198e-/ADU, 3.8e- read noise, and 0.003e-/pix/sec at -5 degrees. I checked it twice as this seemed a little better than spec (expected 5-6e- read and 0.01e-/pix/sec), but maybe I got a good example. So with slow readout maybe it will be even better, which is great, since it means low motivation to upgrade.

The challenge to distant tail spectrum on the comet may be getting enough exposure time on it before it's too near the horizon. Also, it's raining today, and we are going away on holiday until the 9th from tomorrow, so by then I guess it may be diving towards the sun. At the moment it's reasonably high at end of twilight, about 28 degrees. I haven't looked ahead to see if it will start diving back towards the sun.
Hamish Barker
Posts: 226
Joined: Thu Jun 20, 2019 12:11 am

Re: Comète C/2021 A1 Leonard

Post by Hamish Barker »

Hi Robin,

How did you remove the dust component? Did you just divide by a solar spectrum?

Cheers,
Hamish
Hubert Boussier
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Re: Comète C/2021 A1 Leonard

Post by Hubert Boussier »

Hello Hamish,

Some answers in these topics, but mainly in French.

http://www.spectro-aras.com/forum/viewt ... =41&t=2872

http://www.spectro-aras.com/forum/viewt ... 6&start=10



Hubert
Robin Leadbeater
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Re: Comète C/2021 A1 Leonard

Post by Robin Leadbeater »

Hamish Barker wrote:Hi Robin,

How did you remove the dust component? Did you just divide by a solar spectrum?

Cheers,
Hamish
Hi Hamish

Dividing but the solar spectrum is sometimes used but it does not work correctly as it affects the strength of the emission lines. You have to subtract the solar spectrum, adjusted for the broad continuum shape of the scattered light. I first calibrated the spectrum for instrument and atmosphere as normal using a reference star. I then subtracted the dust using a G2v template modified to fit the underlying comet continuum shape.
There is more information in the JBAA article
https://britastro.org/jbaa/pdf_cut/jbaa_24438.pdf

Note even then this is not exactly the emission spectrum as it will be modified (reduced in intensity and reddened) by any cometary dust along our line of sight in front of the emission source. I don't know how this could be corrected for though

Cheers
Robin
Last edited by Robin Leadbeater on Wed Dec 29, 2021 2:21 pm, edited 1 time in total.
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
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Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
Contact:

Re: Comète C/2021 A1 Leonard

Post by Robin Leadbeater »

Hubert Boussier wrote:Hello Hamish,

Some answers in these topics, but mainly in French.

http://www.spectro-aras.com/forum/viewt ... =41&t=2872

http://www.spectro-aras.com/forum/viewt ... 6&start=10



Hubert
Hi Hubert,

Division by the solar spectrum does not give the correct intensity of the emission lines (for example where the solar continuum is weak, this will exaggerate the intensity of the emission.) To obtain the correct emission line spectrum, the solar spectrum (corrected for the broad shape of the scattering) must be subtracted

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
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Joined: Mon Sep 26, 2011 4:41 pm
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Re: Comète C/2021 A1 Leonard

Post by Robin Leadbeater »

Robin Leadbeater wrote: I then subtracted the dust using a G2v template modified to fit the underlying comet continuum shape.
To produce the template:

I first divided the background subtracted and response corrected comet spectrum by the Pickles G2v spectrum, filtered to match the resolution of the comet spectrum
I then produced a smoothed curve passing through the continuum points between the emission lines/bands. (The wavelength dependent scattering coefficient of the dust)
I then multiplied the Pickles spectrum by this smooth curve to produce the template of scattered sunlight.
This template was scaled to match the comet spectrum continuum and then subtracted to leave just the emission component.

To produce the cross sections of emission intensity I had to use a different technique to remove the dust component. Here I started with the dark, flat and background corrected spectrum image and took profiles across the comet at the emission lines/bands and at the dust continuum at wavelengths either side close to the emission. The matching average dust continuum profile was then subtracted from each emission profile.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
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