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C2020 F3 NEOWISE - A spectrum of the tail

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#1 robin_astro

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Posted 19 July 2020 - 10:59 AM

Here is an emission line spectrum taken in the tail using an ALPY 600. (It is much fainter than the coma. The absolute intensity in the spectrum tail is only ~1% of that in the coma spectrum)

 

_c2020f3neowise_nucleus_lowertail_220_30_sky_dust_sub_20200713.png

 

The spectrum is very different from the coma normally seen in amateur spectra and along with CN, NaD is dominated by CO+, the double lines which dominate the ion tail.  No sign of the illusive source of the red component in the ion tail yet but I have other spectra from different positions in the tail to process so I am still looking :-)

 

 

 

Cheers

Robin


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#2 stefan822

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Posted 20 July 2020 - 07:13 AM

Hi Robin,

 

thanks for the the measurement.

 

There is indeed a peak near 6200 A. In that region are some H2O+ lines. Hale-Bopp has been photographed with a 6199A +/- 7nm filter: https://agupubs.onli....1029/97GL03704

 

But my camera is blind in that region.

 

I derived some values from my pictures. Raw frames where only dark-current and flat-field corrected, stacked, downscaled by factor two an 5x5 pixel median filtered, i.e. 10x10 pixel noise filtered. (No background correction)

 

Brightness was measured on 12 coordinates in 3 groups. Each group (red, green, blue frames in the image below) consists in 4 coordinates which lay approximately on a line. The outer points where used for background measuring. Between the outer points background is approximated linearly. The frame depicts the area where the values where median filtered.

 

Here here the background corrected results of the inner points (#2 left inner frame which seems to be bluer in the image):

 

Red frame #2:     red: 1.2     blue: 31.9

Red frame #3:     red: 20.9   blue: 62.2

 

Green frame #2:  red: 4.0    blue: 43,4

Green frame #3:  red: 25.3  blue: 90,3

 

Blue frame #2:     red: 4.1     blue: 47.0

Blue frame #4:     red: 34.3  blue: 107.0

 

Units are e- per pixel an per second. Btw, background flux was 2903 e-/s for blue and 1350 e-/s for red.

 

So the red component is quite significant and not distributed equally in the ion tail. (Was not sure about that because the shape of the reddish component can also come from erroneous black level calibration)

 

Regards Stefan

Attached Thumbnails

  • rgb3-2-mp.jpg


#3 robin_astro

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Posted 20 July 2020 - 12:33 PM

Here is a zoom on the red region of the tail emission line spectrum. 

 

_c2020f3neowiselowertail220_30_20200714_007_leadbeater_g2vsub_dustabssub_red_annot.png

 

The most likely candidate for the red colour in your NaD blocked image looks to be the triplet at ~ 6543,6562,6594 but I don't have an identification for the ion. This includes H alpha at 6563 so a wide H alpha filtered image might be interesting 

 

 

Cheers

Robin



#4 stefan822

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Posted 25 July 2020 - 04:14 AM

Hi,

 

last night I was able to capture the comet again. Weather condition where very poor (bad transparency, some clouds).

 

I attached a RGB image with white balance to ion tail, red component and blue component (in that order).

 

Color factors vs. neutral white balance where 2.0 for red and 1.5 for green. In the image I shot on Jul 14 color color factors where 3.9 for red and 2.2 for blue. Width of the RGB image is 5.6°, width of the color components is 11.1°. The same Na-blocking instrument was used.

 

So the red feature is still present and it seems to be even stronger than before. The structure is the same: i.e. the reddish part of the ion tail is on the inner side which is closer to dust tail. I did not made the same measurement as earlier in this thread, but the red component is now about half as bright as the blue component.

 

The question arises: Why is the red component not visible in the spectra taken close to the core? Where are the molecules ionized? If the ionization would occur in the dusty region near to the coma that would explain the position (inner side) in why it is not visible in the spectra above.

 

Currently the reddish feature is probably best observed in a distance of about 1.5° to 2° from the core. I also shot photos with a larger instrument in that distance. But processing will take a while because the core which I usually use con alignment was out of view.

 

Regards Stefan

Attached Thumbnails

  • neowise-04-rgb3-crop-half.jpg
  • neowise-04-r-quart.jpg
  • neowise-04-b-quart.jpg


#5 robin_astro

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Posted 25 July 2020 - 05:26 AM

Hi Stefan,

 

I would not expect the lines seen in the tail spectrum to be detectable in the the coma, or or least only weakly. For example the main CO+ lines seen clearly in the tail spectrum are not detectable in the coma spectrum.  (There is approximately a 100x difference in intensity scale between the coma and tail spectra shown above.) The coma spectrum is dominated by the much stronger dust and C2 etc components. 

 

I believe the source of the red component may be visible in the tail spectrum (The  ~ 6543,6562,6594 triplet) but we need someone to identify it. (I have not been able to)

 

Cheers

Robin



#6 robin_astro

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Posted 25 July 2020 - 05:31 AM

I will try to take some more tail spectra if  get the chance but with my setup I can only take them within a few arc minutes from the coma as I need the coma to be visible in the field to guide the spectrograph. (Can you let me know the scale and orientation of your image so I can identify the best place to look?) 

 

Thanks

Robin



#7 robin_astro

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Posted 25 July 2020 - 05:38 AM

Have you confirmed there is no leakage of your sodium block (published curves do not always match reality.)

The  position of the red tail could be consistent with the sodium tail (It is produced by a different mechanism to the ion tail so points in a different direction.)  

Having said that, the relative strengthening of the red tail now does seem to confirm that it is not sodium as the sodium production is much lower now.

 

Cheers

Robin



#8 stefan822

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Posted 25 July 2020 - 08:52 AM

Hi Robin,
 

I would not expect the lines seen in the tail spectrum to be detectable in the the coma, or or least only weakly. For example the main CO+ lines seen clearly in the tail spectrum are not detectable in the coma spectrum.  (There is approximately a 100x difference in intensity scale between the coma and tail spectra shown above.) The coma spectrum is dominated by the much stronger dust and C2 etc components.


Your spectrum of the tail close to coma in post #1 shows these blue ion emissions. But almost no red ion emissions.

But the red:blue ratio i measured on Jul 14 was about 1:3 to 1:4, see post #2.
 
 

I will try to take some more tail spectra if  get the chance but with my setup I can only take them within a few arc minutes from the coma as I need the coma to be visible in the field to guide the spectrograph. (Can you let me know the scale and orientation of your image so I can identify the best place to look?)

I sent you a link with an linearly scaled fits image which you can use to check the situation. (Brightness! Is it worth it to try?)

Dust tail and ion tail become clearly separated at a distance of >1°. At this position it should be quite sure to catch the red source.

 

Image width of the full view (fits image per PM) is 11.1°, width of the cropped RGB image is 5.6°.

The direction is easy: use a ruler to draw a line from sun to comet. (Projection center should be the center between these two objects) (That's how I captured the detail shot (not processed yet) with a larger instrument at 2° distance)

Exact guiding is not necessary because it is a larger area. You just need to avoid to catch a star.
 

Have you confirmed there is no leakage of your sodium block (published curves do not always match reality.)
The  position of the red tail could be consistent with the sodium tail (It is produced by a different mechanism to the ion tail so points in a different direction.)  
Having said that, the relative strengthening of the red tail now does seem to confirm that it is not sodium as the sodium production is much lower now.


I used two cameras in parallel which are a part of an array. I also processed images of these cameras separately: both results look exactly the same. So, if filters are erroneous both of them are erroneous in the same way, which is unlikely because the second filter was purchased about one year later.

 

Lets say the filters leak 10% Na light. If I measure a red to red:blue ratio of 1:3 images without Na-blocking would have a red:blue ratio of about 3:1.

 

Regards Stefan

 



#9 robin_astro

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Posted 25 July 2020 - 10:49 AM

 

Exact guiding is not necessary because it is a larger area. You just need to avoid to catch a star.
 

 

Hi Stefan,

 

Thanks for the image. My drive is sidereal and the total exposures are over an hour. I doubt that the tail will be visible in the guider that far out and the comet moves a long way in that time. (I also need to confine the comet to a particular part of the slit so I can subtract the sky background spectrum.)  Fingers crossed for a clear night in the next few days (The forecast is not good here)

 

Cheers

Robin



#10 Octans

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Posted 26 July 2020 - 03:18 PM

The band near 650-660 nm pretty closely overlaps a known H2O+ band. It might also be the normally stronger H2O+ band near 700 nm. An ion tail very weakly shows up in my i' images which just covers that band, although I haven't done a proper comparison of the width with g'. This is likely a different tail from the far stronger red tail earlier that appeared in the r' band (which also now shows a weak ion tail) that was almost certainly Na, but has since vanished.

 

 


Edited by Octans, 26 July 2020 - 03:30 PM.


#11 robin_astro

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Posted 27 July 2020 - 08:04 AM

The tail emission spectrum posted at the top of the thread was cropped at the long wavelength end because of the difficulties of accurately subtracting the tellurics at this low altitude but I do have data at longer wavelengths. (The suspect strong telluric regions have been cropped)

 

_c2020f3neowiselowertail220_30_20200714_007_leadbeater_g2vsub_dustabssub_red2_telcrop.png

 

There is no sign of any other strong emission lines in this region. 




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