APOD: Comet Lovejoy's Tail (2015 Jan 17)

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APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by APOD Robot » Sat Jan 17, 2015 5:08 am

Image Comet Lovejoy's Tail

Explanation: Sweeping north in planet Earth's sky, Comet Lovejoy's greenish coma and blue tinted ion tail stretched across this field of stars in the constellation Taurus on January 13. The inset at the upper left shows the 1/2 degree angular size of the full moon for scale. So Lovejoy's coma appears only a little smaller (but much fainter) than a full moon on the sky, and its tail is visible for over 4 degrees across the frame. That corresponds to over 5 million kilometers at the comet's estimated distance of 75 million kilometers from Earth. Blown by the solar wind, the comet's tenuous, structured ion tail streams away from the Sun, growing as this Comet Lovejoy heads toward perihelion, its closest approach to the Sun, on January 30. While diatomic carbon (C2) gas fluorescing in sunlight produces the coma's green color, the fainter bluish tail is tinted by emission from ionized carbon monoxide (CO+).

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by Ann » Sat Jan 17, 2015 7:41 am

To me as a Color Commentator, the colors of this picture are interesting. I have long wondered at the relatively non-blue color of the comet's tail. Of course, the ion tail is faint, which means it will show less color.

Rogelio Bernal Andreo's fine picture shows the inner part of the comet's coma to be white and overexposed, while the "middle part of it" is yellow-green. The outer part of the coma is aqua-colored. The tail is faint, and the brightest parts of it are grayish-blue.

At the lower left edge of the picture is a very blue star. Does anyone know what star that is? It would be very interesting to check out its color index.

Ann
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ayeomans

Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by ayeomans » Sat Jan 17, 2015 11:36 am

Great idea putting the size of the moon to indicate scale. Could this be done more often on APOD? Perhaps in a separate overlayor image beneath the main one.

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by rstevenson » Sat Jan 17, 2015 2:49 pm

Ann wrote:Rogelio Bernal Andreo's fine picture shows the inner part of the comet's coma to be white and overexposed, while the "middle part of it" is yellow-green. The outer part of the coma is aqua-colored. The tail is faint, and the brightest parts of it are grayish-blue.
Here's some pixel-level screen grabs, from my DigitalColor Meter...
inner coma.jpg
When you view the overall image, the inner coma does indeed look white, but I think that's just a problem with our eyes. This is the brightest part I could find, and it's still very yellow.
outer coma.jpg
tail1.jpg
The dark parts of the tail are similar in colour balance, but just, well, darker.

Rob
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RBANDREO

Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by RBANDREO » Sat Jan 17, 2015 3:58 pm

Ann wrote:At the lower left edge of the picture is a very blue star. Does anyone know what star that is? It would be very interesting to check out its color index.
Hi Ann!

That star is HIP 18033, spectral class B9 V (blue :ssmile: ).

Thanks!!!
Rogelio

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by Chris Peterson » Sat Jan 17, 2015 4:36 pm

rstevenson wrote:
Ann wrote:Rogelio Bernal Andreo's fine picture shows the inner part of the comet's coma to be white and overexposed, while the "middle part of it" is yellow-green. The outer part of the coma is aqua-colored. The tail is faint, and the brightest parts of it are grayish-blue.
Here's some pixel-level screen grabs, from my DigitalColor Meter...
I'd be very cautious making any rigorous color analysis of this image. First of all, the colors we see in the tail and coma of this nearly dust-free comet are from a handful of C and CN narrowband emissions, which are not recorded accurately in a broadband image. Furthermore, this image has been heavily processed to bring out the tail, which is several orders of magnitude fainter than the coma (the processing is excellent, but you can still see that the stars behind the tail are brighter than the stars in other areas).

Images like this tell us more about the imaging equipment and the processing than they do about real colors (although star colors are generally recorded fairly accurately with broadband imaging, assuming proper processing steps are used).
Chris

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by Boomer12k » Sat Jan 17, 2015 4:47 pm

Wow...awesome shot!

I would get out the scope....but too cold and wet... :o

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by Cousin Ricky » Sat Jan 17, 2015 5:03 pm

I cannot make out the tail in my telescope. I can make out some assymetry in the coma; there seems to be a dark triangular wedge in one direction. I did not make a note of which direction the wedge was in.

My telescope is 200 mm. I'm not sure of the NELM from my yard. It was about 5 a few years ago, but it's certainly gotten worse since then.

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by geckzilla » Sat Jan 17, 2015 5:11 pm

Do astrophotographers ever image comets using narrowband filters? I guess not because the fast-changing nature of comet tails would make this difficult since narrowband imagery requires significantly longer exposure times. You could do it but the result would probably be rather blurry. I suspect the filters are also pretty rare to come by.
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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by Chris Peterson » Sat Jan 17, 2015 5:18 pm

geckzilla wrote:Do astrophotographers ever image comets using narrowband filters? I guess not because the fast-changing nature of comet tails would make this difficult since narrowband imagery requires significantly longer exposure times. You could do it but the result would probably be rather blurry. I suspect the filters are also pretty rare to come by.
I have. I've got a lot of narrowband filters for non-astronomical purposes. Long exposure isn't a problem if you track on the comet, which is what I normally do.
Chris

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by geckzilla » Sat Jan 17, 2015 5:32 pm

Track the comet, yes, that is very important. But with a comet like this, the tail is changing fast enough that even if it's tracked a long exposure will be blurred. Imaging would also optimally have be done simultaneously rather than one filter at a time.
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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by geckzilla » Sat Jan 17, 2015 5:54 pm

Jose just posted a cool animation of the comet's tail flowing away from the comet in the submissions thread. Here's a link to better illustrate what my previous post was concerned with.
http://www.astrobin.com/full/147512/0/
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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by Chris Peterson » Sat Jan 17, 2015 6:36 pm

geckzilla wrote:Track the comet, yes, that is very important. But with a comet like this, the tail is changing fast enough that even if it's tracked a long exposure will be blurred. Imaging would also optimally have be done simultaneously rather than one filter at a time.
This comet is somewhat unusual in that its ion trail is extremely faint compared with the coma emission, and in having almost no dust to reflect light. But a great many comets are stable enough to get an hour or more of exposure without any significant change in tail structure. Even Lovejoy comes close.

Simultaneous imaging is quite practical these days. We're doing it with meteors. It can be very difficult to observe a meteor spectroscopically, so we aim 8 or 16 cameras at the same spot, each with a different filter, and collect all the video simultaneously. Completely practical with cheap cameras and lenses, cheap multichannel frame grabbers, and cheap, fast computers.
Chris

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by Ann » Sat Jan 17, 2015 7:58 pm

RBANDREO wrote:
Ann wrote:At the lower left edge of the picture is a very blue star. Does anyone know what star that is? It would be very interesting to check out its color index.
Hi Ann!

That star is HIP 18033, spectral class B9 V (blue :ssmile: ).

Thanks!!!
Rogelio
Thank you, Rogelio! Yes, HIP 18033 is pretty blue, with a Tycho color index of -0.07. Its spectral class is B9p Si, which means it is silicon-enhanced. It also means it is a little bluer than we would normally expect from a star of spectral class B9.

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by Ann » Sat Jan 17, 2015 7:59 pm

rstevenson wrote:
Ann wrote:Rogelio Bernal Andreo's fine picture shows the inner part of the comet's coma to be white and overexposed, while the "middle part of it" is yellow-green. The outer part of the coma is aqua-colored. The tail is faint, and the brightest parts of it are grayish-blue.
Here's some pixel-level screen grabs, from my DigitalColor Meter...
inner coma.jpg
When you view the overall image, the inner coma does indeed look white, but I think that's just a problem with our eyes. This is the brightest part I could find, and it's still very yellow.
outer coma.jpg
tail1.jpg
The dark parts of the tail are similar in colour balance, but just, well, darker.

Rob
Thanks, Rob, very interesting!

Ann
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same old idiot

Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by same old idiot » Sun Jan 18, 2015 3:54 am

Not a comment. A question.
Photo says head is about as large as moon in inset.
I am NOT Astronomical.
I saw Orion's belt. I went outside at 20:30 on Jan 17th. Location for me is Arizona.
Yes I was in the desert.
Away from Phoenix and Tucson .
I could see very good, but I did not see any comet.
I had a scope, but could not find it through my 50MM Nightsight Binoculars .
Nothing. :oops: So is there a time to go look?
I was halfway between Orion's belt and the Pleiades. Nothing. :oops:
Someone want to clue me in as I shall try again with some info.
PLEASE no star chart references outside of what I just gave. I need something like a position azimuth wise :?:

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by geckzilla » Sun Jan 18, 2015 3:58 am

same old idiot wrote:A question.
Your question wasn't clear if you mistakenly think it will look as big as the moon to your eyes, so I will say this: It looks more like a dim, fuzzy star. It's not as big as the moon to your eyes. If you're looking around where it should be seen and you see a star that is slightly fuzzy, then you probably found it. If all the stars are slightly fuzzy to your eyes then I don't know how to help you. I can't see it at all where I live. :(
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RBANDREO

Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by RBANDREO » Mon Jan 19, 2015 4:22 pm

Chris Peterson wrote:I'd be very cautious making any rigorous color analysis of this image. First of all, the colors we see in the tail and coma of this nearly dust-free comet are from a handful of C and CN narrowband emissions, which are not recorded accurately in a broadband image.
Any color analysis has to always be based on at least two variables ... more actually, but these two at the very least: 1) bandpass filters used and 2) the criteria used by the person producing the image. The first is known, L+RGB, and I wouldn't expect anyone doing an analysis ignoring this fact. The image is presenting the broadband colors that were recorded. Recording at any other bandpass and produding a (RGB) image is a translation/interpretation. Neither is accurate or inaccurate, they're just different representations, and that's the only "caution" required :-)

Whether RGB are the best filters for this object, that's a different question I think. In this case, color saturation has been enhanced, absolutely, but there's no color shift (intentional at least!).
Chris Peterson wrote:(the processing is excellent, but you can still see that the stars behind the tail are brighter than the stars in other areas).
I know many people - particularly those with some experience processing astroimages - think that the brighter stars behind the tail "reveal" that the area of the tail has been stretched selectively. However, if you pay very close attention, you may notice that the reason the stars are brighter in the tail has nothing to do with that.

The stars behind the tail appear brighter (they certainly do) because after most of the post-processing, the stars all over the image were too rough (no question processing was aggressive overall), so, using a star mask, I toned them all down to bring back a bit of their usual Gaussian profile, EXCEPT those in the area of the tail. I could not include the stars behind the tail without producing dark ringing around them, so I left them out. This process does not affect background sky, comet or tail, only the stars. Experienced eyes always see more in an image than most people, but sometimes their interpretation can too be inaccurate. I should know, it's happened to me before! 8-)

Cheers,
Rogelio

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Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by Chris Peterson » Mon Jan 19, 2015 4:31 pm

RBANDREO wrote:Whether RGB are the best filters for this object, that's a different question I think. In this case, color saturation has been enhanced, absolutely, but there's no color shift (intentional at least!).
Right. Not suggesting intentionality. It's just that broadband filters virtually never capture the apparent color of narrowband emissions very well (and sometimes disastrously). That is, if you look at the emission for C2 or CN in a reference tube, it will look significantly different from what we see in a broadband image displayed on a typical monitor. Just the nature of the beast. And it's not a problem, just an observation that we can't compare the color of a narrowband source against the color of stars (blackbodies) and take away much useful information.
RBANDREO wrote:
Chris Peterson wrote:(the processing is excellent, but you can still see that the stars behind the tail are brighter than the stars in other areas).
I know many people - particularly those with some experience processing astroimages - think that the brighter stars behind the tail "reveal" that the area of the tail has been stretched selectively. However, if you pay very close attention, you may notice that the reason the stars are brighter in the tail has nothing to do with that.

The stars behind the tail appear brighter (they certainly do) because after most of the post-processing, the stars all over the image were too rough (no question processing was aggressive overall), so, using a star mask, I toned them all down to bring back a bit of their usual Gaussian profile, EXCEPT those in the area of the tail. I could not include the stars behind the tail without producing dark ringing around them, so I left them out. This process does not affect background sky, comet or tail, only the stars. Experienced eyes always see more in an image than most people, but sometimes their interpretation can too be inaccurate. I should know, it's happened to me before!
Thanks for the processing details. One thing I do in this situation is to stack a processed and unprocessed layer and then use the Lighten method of combining them. This knocks out dark ring artifacts. It also softens the processing, but usually leaves enough behind that the result is more aesthetic than no processing at all.
Chris

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RBANDREO

Re: APOD: Comet Lovejoy's Tail (2015 Jan 17)

Post by RBANDREO » Tue Jan 20, 2015 6:16 am

Chris Peterson wrote:Thanks for the processing details. One thing I do in this situation is to stack a processed and unprocessed layer and then use the Lighten method of combining them. This knocks out dark ring artifacts. It also softens the processing, but usually leaves enough behind that the result is more aesthetic than no processing at all.
Yes, there are several ways to reduce or eliminate ringing. I usually try to avoid it rather than fix it afterward but I certainly wouldn't shy away from correcting it if I felt I had to. I admit if I didn't go for it this time it was mostly because I just didn't want to bother :wink:
Thanks!
Rogelio