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Neowise Sky Darker Between Tails - Why?

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#1 Rustler46

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Posted 04 August 2020 - 06:20 PM

I have noticed in a number of photos (including my own) that the background sky is darker between the ion and dust tails compared to the sky exterior to the twin tail's envelope. My visual observations have made note of this along with the dust tail edge being more sharply defined on the ion tail side. The photos show the dust tail fans out on the side away from the ion tail. This side of the dust tail decreases in intensity further from that tail's major axis.

Any idea why these effects take place?

Here are some excerpts from my visual observation notes describing these effects:

2020-7-12 (20X65 binoculars)
Sometimes I get the sense that next to the upper (western) side of the [dust] tail the sky is darker, less so on the right (east) side.

2020-7-14 (20X65 binoculars)
The sky to the left (west) of the [dust] tail seems darker and tail is more sharply defined on that side compared to the right (east) side.

2020-7-17 (20X65 binoculars)
The sky to the left (west) of the [dust] tail seems darker and tail is more sharply defined on that side compared to the right (east) side.

2020-7-24 (20X65 binoculars)
The south [left] side of the [dust] tail that is adjacent to the (unseen) ion tail is sharply bounded into darker sky more so than north side of tail.

On other nights when the comet was observed moon light and local light pollution impacted the view so that the "darker between the tails" effect was not noted.

When discussing this effect with my wife, she wondered if it was similar to the darker sky between primary and secondary rainbows in the daytime sky. The website Atmospheric optics has a discussion on this - Alexander's Dark Band:

Atmospheric Optics - http://www.atoptics.co.uk/
Rainbows - http://www.atoptics.co.uk/bows.htm
Alexander's Dark Band - http://www.atoptics....bows/adband.htm

What are your thoughts on all of this?
I've attached my own photo of the dark between the tails.

Best Regards,
Rusty

C-2020 F3 2020-7-18 17 of 29.jpg

#2 Redbetter

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Posted 07 August 2020 - 02:00 AM

Rusty,

 

I am not sure what to make of this, but I notice in your image that much of the upper/mid left quadrant of sky is quite a bit brighter than elsewhere in the frame.  The darker area between tails seems to be of similar brightness to all of the lower 1/4 of the frame that is not part of the comet/tail.  Unfortunately, with those other dark areas mostly being the edges of the frame, the is latter impression I have might just be an imaging artifact related to field illumination.



#3 Rustler46

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Posted 07 August 2020 - 02:36 AM

Thanks for your comments, Redbetter. They raise some very valid points. My comet photo is not the best example of the effect I had noticed. That photo was shot with some intervening cloud layers, which likely contributed to the uneven background sky.

So I looked for some examples in other photos that seem show the effect. Some of the best are in the DPReview Astrophotography Forum. Here's an example:

https://www.dpreview...s/post/64225647

One of the excellent photos in that thread by Elgol20 is shown below.

Neowise by Elgol20.jpg

Here's another:

Neowise by Elgol20-2.jpg

Maybe it is just a contrast effect. The dust tail is not sharply bounded on the side farthest from the ion tail. But it is quite sharply bounded on the ion tail side.

Whatever the cause of what is seen, it is not a big deal. But it is something I had noticed in visual observations - the sky seemed darker on the left (south) side of the tail, compared to the sky on the north side.

Later in the comet's apparition the separation between dust and ion tail became less. So the effect, if real eventually disappeared.

Best Regards,
Rusty

Edited by Rustler46, 07 August 2020 - 02:39 AM.


#4 Redbetter

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Posted 07 August 2020 - 03:59 AM

It is an interesting visual observation and question.  It is good to put it out there when something like this is seen.  There might be a particular explanation.

 

I only had a night or two where the comet was high enough (out of trees and away from haze/smoke) and in dark enough sky to see the ion tail clearly and extending 5 or so degrees beyond where it separated.



#5 StarBurger

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Posted 07 August 2020 - 07:02 AM

To my eyes this is an optical illusion. On the screen if I cover the ion trail with a finger the sky on either side then looks identical.


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#6 RobertMaples

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Posted 07 August 2020 - 01:58 PM

I think StarBurger is right, it's an optical illusion.  Have you ever seen this one:

220px-Checker_shadow_illusion.svg.png

If not, the blocks marked A and B are the same shade of gray.


Edited by RobertMaples, 07 August 2020 - 01:59 PM.

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#7 BrooksObs

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Posted 07 August 2020 - 04:43 PM

There is absolutely no reason for the sky to actually be lighter to the left of the comet's ion tail as suggested in the images. It has to be either an imaging quirk, or an optical illusion. Nothing but totally empty space occupies the region in question, so it can not be luminous. Neither should the space between the ion and the dust tails be darker that the region left of the ion tail as both essential are devoid of reflective matter.

 

BrooksObs


Edited by BrooksObs, 07 August 2020 - 09:53 PM.


#8 Rustler46

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Posted 07 August 2020 - 04:54 PM

Thanks Redbetter, Starburger and Robert. Your comments are much appreciated. I think you are right that the effect is a contrast thing. Since the ion tail was not seen visually, another contributing factor is how quickly the edge of the dust tail fades away into background. On the ion tail side the fading is abrupt. So the contrast is more easily seen. On the opposite side of the dust tail the lateral fading takes place over a considerable distance away from the longitudinal axis of the tail.

Overall the visual appearance for myself was amazing, particular with unaided eye on the nights of 2020 July 17 & 18. On those nights the comet was still very bright. So the dust tail had a length of around 11°. Also for photographic observations the separation between dust and ion tails was still fairly large. This helped set off each tail as a separate entity. The contrast effect you mention contributed to the "dark between the tails" appearance. But that effect was minor compared to the overall view.

C/2020 F3 Neowise is one many of us will long remember.

#9 Dynan

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Posted 07 August 2020 - 04:58 PM

Plop it in Pixinsight and mouse over a stretched image. It'll give you exact values for any particular pixel.




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