26 July 2020 at 11:46 am #582928Hazel McGeeParticipant
Brief gap in clouds last night (25/07/2020) at around 23:00 (BST). At present we can just catch it (sideways) through our west-facing bedroom window. Not naked eye from here (Surrey) at this time, although we saw it clearly naked eye at about 02:30 in a crisply clear but brightening NNE sky on July 11. But last night it was still good in binoculars (8×50) and very easy to locate beneath the Plough. Faint and fairly diffuse dust tail, better with averted vision, but any ion tail not seen.26 July 2020 at 11:53 am #582929Hazel McGeeParticipant
Fabulous sequence David!26 July 2020 at 1:11 pm #582930Grant PrivettParticipant
Really nice to see this.
The comet is approx 30 degrees from the sun, so the background should display Rayleigh scattering induced polarisation of a few percent. From memory, the orientation in the first image looks roughly right given the suns position pretty much below the north horizon. The comet should have its own dust response – as the Zodiacal light does – and certainly appears to here. I’m a bit perplexed by the orientation of the background polarisation in the second image though. Was the amplitude the same?
Your amplitude measure, was that a Stokes parameter or the degree of total linear polarisation or similar?
I imagine perfect alignment of the 4 polariser images is essential to the values measured when the target has fast changes in brightness. I have always had greatest problems getting good results in parts of the image with steep gradients – subpixel registration is important (I found to my cost).
I assume you only used linear polarisers.26 July 2020 at 3:16 pm #582932David StrangeParticipant
Thanks Hazel, cloudy here for the next few days as well.
David27 July 2020 at 11:27 am #582934Bill WardParticipant
No, it was done with a circular polariser in front of the lens, I don’t have a large enough linear polariser for those lenses.
Anyway, one image is taken as the zero degree image, then the pol is rotated through 45 deg, then another 45 deg, then one more. Giving 0, 45, 90 and 135 degree images. Even that’s not going to be very accurate!
The polarisation functions in IRIS are used to produce the various maps.
Oddly enough on the second night, you could easily see the difference in the various frames due to the sky pol on the camera LCD display. IRIS offers several pol functions but I don’t know how it’s actually determining the various vectors to produce the images.
Everybody seems to get obsessed taking images I just wanted to try something different and see if it would work! The fact that the Rayleigh scattering/polarisation is perpendicular to the solar direction would seem to suggest it was successful after a fashion… but how accurate, who knows! That’s why I say the stronger pol nearer the nucleus could still be an artifact.
Perhaps someone else can try it…?, the comet is still around!27 July 2020 at 2:57 pm #582936Nick JamesParticipant
I’ve only just got around to processing my images of C/2020 F3 (NEOWISE) from July 22. An animation is here:
This shows a field of view of 33×22 arcmin processed using a Larson-Sekanina filter with r=2, th=10 deg. There are 9 frames each of around 330s duration (from 2143 – 2228). You can clearly see motion in the tail and material spiraling out from the centre of the coma. The small black dot at the centre of the coma is the reference pixel for the filter.
I have done quite a few experiments with this data and I think the parameters I have chosen are the best compromise to show detail and motion (i.e. around 300s integrations and L-S with r=2, th=10).
It always amazes me that so much relative motion is visible in active comets over such a short period of time.27 July 2020 at 6:03 pm #582938Nick JamesParticipant
Thanks to Hazel for pointing out that we need a new chart for this comet. Here it is: https://britastro.org/wp-content/uploads/sites/2020f3_Aug.pdf1 August 2020 at 2:22 pm #582959Grant PrivettParticipant
Could you tell me what sort of circular polariser it was please? Am sort of surprised it was so successful against the linear signal from the comet.
Is it one that normally fits on a DSLR camera lens?6 August 2020 at 2:12 pm #582975Robin LeadbeaterParticipant
I have finally completed the analysis of my spectroscopic cross sections through coma and tail taken 2020-07-13 and added a poster to my members page
It shows some interesting differences in the distribution of the various components which make up the cometary material.
The CN and Sodium components are seen in both coma and tail but the C2 component which dominates the coma emission spectrum is absent in the tail, replaced by the CO+ component.
The spatial distributions are significantly different for the various components. In particular the sodium emission in the coma is confined to a narrow central region but the CN component is widespread in both the coma and tail. The Sodium distribution in the tail is skewed anticlockwise relative to the dust, consistent with the presence of a separate sodium tail.
Robin6 August 2020 at 3:02 pm #582977David SwanParticipant
Very nice, Robin. Is there very much in the literature on the spatial distribution of the various components in comets longitudinally through an apparition? If only bright-ish comets coming close-ish to Earth were more frequent…6 August 2020 at 8:34 pm #582979Robin LeadbeaterParticipant
I approached this not knowing much about it just to see what might be possible to measure but think there is other pro work like this around, though most of it seems to be concentrated on the coma rather than the tail. I’ve not seen anything like this by amateurs though. In fact I cannot recall seeing amateur tail spectra of any comet and it was quite a surprise when I recorded it as it looked so different to the coma spectra you usually see, with CO+ in place of C2. The Sodium distribution looks similar to what the PSI team got which is what I was mainly interested in trying to pick up.
How far the CN spreads surprised me. I suspect the distribution across the tail actually spreads much further even than my plot suggests. The measured distribution approaches zero steeply rather than gradually tailing off suggesting it is clipped. It likely extended beyond the ends of the slit so was clipped when I used the edges of the field to subtract the sky background.
It would be nice to measure further out along the tail but I already had to stitch 2 slit lengths together to cover the width of the tail even this close in.
Robin19 September 2020 at 9:29 am #583128Bill WardParticipant
I was perusing this thread looking for another comment when I saw your question, apologies for the late answer…
The filter was a HOYA HMC circular polariser and yes it’s just an ordinary photographic type as recommended for DSLR’s using autofocus.
It was a fun experiment. Nice to see a bit of theory in practice!
Bill.4 May 2021 at 7:33 pm #582774Mike HarlowParticipant
I did try a transmission grating spectrum this morning just for fun. Maybe shows the image of the tail in sodium light??? A bit faint but I’ll try again next time I get the chance. Larger image on my members page,
Details: Thor labs 300l/mm transmission grating in front of 55mm lens on un-modified Canon 550D DSLR.
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