APOD: Comet PanSTARRS with Anti Tail (2013 May 18)

APOD Robot · Post by **APOD Robot** » Sat May 18, 2013 4:25 am

Comet PanSTARRS with Anti Tail

Explanation: Once the famous sunset comet, PanSTARRS (C/2011 L4) is now visible all night from much of the northern hemisphere, bound for the outer solar system as it climbs high above the ecliptic plane. Dimmer and fading, the comet's broad dust tail is still growing, though. This widefield telescopic image was taken against the starry background of the constellation Cepheus on May 15. It shows the comet has developed an extensive anti-tail, dust trailing along the comet's orbit (to the left of the coma), stretching more than 3 degrees across the frame. Since the comet is just over 1.6 astronomical units from planet Earth, that corresponds to a distance of over 12 million kilometers. In late May Comet PanSTARRS will pass within a few degrees of the north celestial pole.

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Ann · Post by **Ann** » Sat May 18, 2013 5:10 am

Anti-tails of comets look fascinating.

This is a fine APOD. Does anyone know what the bright star is?

Ann

alter-ego · Post by **alter-ego** » Sat May 18, 2013 5:58 am

Ann wrote:Anti-tails of comets look fascinating.

This is a fine APOD. Does anyone know what the bright star is?

Ann

Errai ( γ Cep )

Ann · Post by **Ann** » Sat May 18, 2013 6:07 am

Thanks, alter-ego!

Ann

Boomer12k · Post by **Boomer12k** » Sat May 18, 2013 9:06 am

"And it's a high rising, fly ball, AND IT IS HEADING OUT OF THE PARK!!!!! IT'S A HOME RUN!!!!!!"

Wonderful picture, and good example of the types of tails comets can have...

:---[===] *

tekic545 · Post by **tekic545** » Sat May 18, 2013 12:07 pm

Not a bad wide angle view if PANSTARRS, but the color balance is off. The comet submissions on Asterisk contains better images.

Bob

nexstar114 · Post by **nexstar114** » Sat May 18, 2013 1:26 pm

Somebody check the math, please?

Chris Peterson · Post by **Chris Peterson** » Sat May 18, 2013 1:48 pm

tekic545 wrote:Not a bad wide angle view if PANSTARRS, but the color balance is off. The comet submissions on Asterisk contains better images.

Since most astronomical images show color that our eyes are incapable of seeing, there's seldom such a thing as "accurate" color. In the vast majority of cases, the color presented in astronomical images is determined by an aesthetic decision of the image processor. This image was made with a DSLR, a camera using a type of sensor that isn't very well suited to capturing "accurate" color in astronomical objects. With such cameras, it is common to see a different color gamut than we'd see in an image made through carefully chosen astronomical filters. (That is a simple observation, not a criticism.) However, this type of sensor offers the advantage of collecting all the color channels simultaneously, so you don't end up with misregistration between the stars and moving comet.

Understanding the instrument used to collect an image helps us understand the presentation of the image.

Chris Peterson · Post by **Chris Peterson** » Sat May 18, 2013 1:56 pm

nexstar114 wrote:Somebody check the math, please?

Which math? The distance-angle relationship?

distance = tan(3°) / 1.6 AU = tan(3°) / 2.39e8 km = 12.5 million km.

Did you get something different?

neufer · Post by **neufer** » Sat May 18, 2013 2:21 pm

Click to play embedded YouTube video.

Chris Peterson wrote:
Since most astronomical images show color that our eyes are incapable of seeing, there's seldom such a thing as "accurate" color. In the vast majority of cases, the color presented in astronomical images is determined by an aesthetic decision of the image processor. This image was made with a DSLR, a camera using a type of sensor that isn't very well suited to capturing "accurate" color in astronomical objects. With such cameras, it is common to see a different color gamut than we'd see in an image made through carefully chosen astronomical filters. (That is a simple observation, not a criticism.) However, this type of sensor offers the advantage of collecting all the color channels simultaneously, so you don't end up with misregistration between the stars and moving comet. Understanding the instrument used to collect an image helps us understand the presentation of the image.

tekic545 · Post by **tekic545** » Sat May 18, 2013 3:13 pm

Chris,

As to color balance, thank you for the explanation, but I am an imager and I know what a DSLR is. I use two SX OSCs and a mono QSI. The color balance is off. The image is too purple. Stars are generally not purple.

Bob Gillette

Chris Peterson · Post by **Chris Peterson** » Sat May 18, 2013 3:34 pm

tekic545 wrote:As to color balance, thank you for the explanation, but I am an imager and I know what a DSLR is. I use two SX OSCs and a mono QSI. The color balance is off. The image is too purple. Stars are generally not purple.

The point is, images almost never represent colors as we actually see them. The color balance is not "off", it is simply a case that the imager chose a color balance in this case that isn't the one you would have chosen. Particularly with a OSC sensor, color is always a compromise. It can be impossible to target a particular color (as for the comet) and retain accurate colors elsewhere (the stars).

marshasb · Post by **marshasb** » Sat May 18, 2013 3:43 pm

Distance versus width of comet:
Hello, I also noticed what I thought at first was a math error. 1.7AU should be certainly more than 12 million km: more like 240 million km.
However, after looking at your calculation, it becomes obvious that the 12 million km is the width of the comet tail, allowing for 3 degrees from 1.7AU.
After looking at the original article it was obvious that they were referring to the width of the comet tail in km, not its distance from earth.
marshasb

Joe Stieber · Post by **Joe Stieber** » Sat May 18, 2013 5:41 pm

Chris Peterson wrote:
nexstar114 wrote:Somebody check the math, please?
Which math? The distance-angle relationship?

distance = tan(3°) / 1.6 AU = tan(3°) / 2.39e8 km = 12.5 million km.

Did you get something different?

Chris,

Your equation has the wrong operator; actually: (tan 3°) / 2.39E8 km = 2.19E-10 km !!!

You probably meant: (tan 3°) * 2.39E8 km = 12.5 million km

The latter is not exactly correct though, since the tangent is for a right triangle, but because of the small angle here, the error is not significant.

You could also use the small angle approximation, in which: angle (in degrees) = (size / distance) * 57.3

Therefore: size = 3 * (2.39E8 km / 57.3) = 12.5 million km

Joe

Chris Peterson · Post by **Chris Peterson** » Sat May 18, 2013 5:53 pm

Joe Stieber wrote:
Chris Peterson wrote:
nexstar114 wrote:Somebody check the math, please?
Which math? The distance-angle relationship?

distance = tan(3°) / 1.6 AU = tan(3°) / 2.39e8 km = 12.5 million km.

Did you get something different?
Chris,

Your equation has the wrong operator; actually: (tan 3°) / 2.39E8 km = 2.19E-10 km !!!

You're right. I originally posted this as tan(3°) = distance / 1.6 AU, and then realized it would be better if I just solved for distance, and forgot to change the divide to a multiply when I edited the line.

The latter is not exactly correct though, since the tangent is for a right triangle, but because of the small angle here, the error is not significant.

Exactly. At the accuracy of all the numbers being expressed here, I could have used tangent, sine, or the small angle approximation you suggest. They all yield the result given in the caption, "over 12 million kilometers".

tekic545 · Post by **tekic545** » Sat May 18, 2013 8:06 pm

Chris,

Further on color balance. You're right, I would not have chosen purple stars. Many/most of us, unless we're doing narrowband imaging, color calibrate images on a G2V sun-like star, which produces what the human eye would call a natural color balance with no room for debate, or taste.

Bob

owlice · Post by **owlice** » Sat May 18, 2013 8:31 pm

When one's eye starts calling anything, something is clearly wrong.

Chris Peterson · Post by **Chris Peterson** » Sat May 18, 2013 8:47 pm

tekic545 wrote:Chris,

Further on color balance. You're right, I would not have chosen purple stars. Many/most of us, unless we're doing narrowband imaging, color calibrate images on a G2V sun-like star, which produces what the human eye would call a natural color balance with no room for debate, or taste.

I also use a G2V color calibration process, but it typically doesn't produce the best results with OSC sensors. That's one reason I generally avoid using them, and steer new imagers away from them. It's just so much more work than using separate filters. You need a lot more total exposure time, and the processing effort is significantly greater. But for planets and moving objects, they are often the best way to go.

Personally, I prefer monochrome over color in most cases, and that's how I would have presented today's image, which I think is all about structure, and the color is just distracting.

kfruczko@charter.net · Post by **kfruczko@charter.net** » Sat May 18, 2013 9:30 pm

Guys,

Just looking into your post of Comer PANSTARRS on this day(05/17/2013), I think you goofed on the distance given for the distance of the comet and it's relative distance based on AU's. You state 1.6 AU or 12 million kilometers. In prior post of AU's, you give the distance of an AU as roughly 149 million kilometers +. I think you missed a couple of numbers and zeroes in today's topic.

Just thought I would bring that to your attention. Keep up the good work. I love your website.

Kenny Ruczko

geckzilla · Post by **geckzilla** » Sat May 18, 2013 9:39 pm

kfruczko@charter.net wrote:Just looking into your post of Comer PANSTARRS on this day(05/17/2013), I think you goofed on the distance given for the distance of the comet and it's relative distance based on AU's. You state 1.6 AU or 12 million kilometers. In prior post of AU's, you give the distance of an AU as roughly 149 million kilometers +. I think you missed a couple of numbers and zeroes in today's topic.o

The wording was somewhat ambiguous but I think that it was meant to state that the comet itself is 12 million kilometers wide, not 12 million kilometers away from Earth.

Chris Peterson · Post by **Chris Peterson** » Sat May 18, 2013 10:15 pm

kfruczko@charter.net wrote:Just looking into your post of Comer PANSTARRS on this day(05/17/2013), I think you goofed on the distance given for the distance of the comet and it's relative distance based on AU's. You state 1.6 AU or 12 million kilometers. In prior post of AU's, you give the distance of an AU as roughly 149 million kilometers +. I think you missed a couple of numbers and zeroes in today's topic.

You need to look at the preceding sentence. Together, what both sentences are saying is that at a distance of 1.6 AU, the field subtended by 3° corresponds to a linear distance of over 12 million kilometers- the apparent size of the comet's tail (it's actually longer, since our view is foreshortened).

JohnD · Post by **JohnD** » Sun May 19, 2013 4:32 pm

The very name "antitail" implies that this is a comet tail that appears to precede the comet in its orbit.
Previous comets have had antitails that were more obvious and anomalous than Pan STARRS'
This is difficult to understand, yet the explanation to this APOD offers only, "It shows the comet has developed an extensive anti-tail, dust trailing along the comet's orbit (to the left of the coma), stretching more than 3 degrees across the frame" "Trailing"? Then it must be behind the comet? I'm even more confused.

The "antitail" link to http://www.daviddarling.info/encycloped ... itail.html which is almost as brief, "Part of the dust tail of a comet that seems to point, often like a spike, toward the Sun. This rare phenomenon is an illusion caused by the viewing geometry and typically occurs when Earth crosses the plane of a comet's orbit when the comet is relatively close to the Sun. Under these circumstances, the cometary dust, which lies in a thin sheet and lags behind the comet, may be seen edge-on as an antitail. One of the most prominent antitails ever seen was that of comet Arend-Roland during its perihelion passage in 1957."

For those of use without an insight into orbits and astronomical points of view, please may we have some better explanation of this apparently bizarre phenomenon? A diagram would be good!

Thanks
John

owlice · Post by **owlice** » Sun May 19, 2013 4:49 pm

"anti" means against or in opposition; you've confused that with "ante," which means before. So no, the name "antitail" does NOT imply that the comet tail "appears to precede the comet in orbit."

Googled "antitail diagram" and found this; maybe it'll help.

Chris Peterson · Post by **Chris Peterson** » Sun May 19, 2013 4:59 pm

JohnD wrote:The very name "antitail" implies that this is a comet tail that appears to precede the comet in its orbit.

The term refers to a tail that appears opposite the "normal" dust trail, not to the orientation of the tail with respect to the direction the comet is moving.

alter-ego · Post by **alter-ego** » Mon May 20, 2013 5:34 am

owlice wrote:"anti" means against or in opposition; you've confused that with "ante," which means before. So no, the name "antitail" does NOT imply that the comet tail "appears to precede the comet in orbit."

Googled "antitail diagram" and found this; maybe it'll help.

Chris Peterson wrote:
JohnD wrote:The very name "antitail" implies that this is a comet tail that appears to precede the comet in its orbit.
The term refers to a tail that appears opposite the "normal" dust trail, not to the orientation of the tail with respect to the direction the comet is moving.

Without disagreement, the "anti-tail" link describes it correctly:
"Part of the dust tail of a comet that seems to point, often like a spike, toward the Sun. This rare phenomenon is an illusion caused by the viewing geometry and typically occurs when Earth crosses the plane of a comet's orbit when the comet is relatively close to the Sun. Under these circumstances, the cometary dust, which lies in a thin sheet and lags behind the comet, may be seen edge-on as an antitail"
It is not a separate tail at all, and as stated, not related to orientation wrt comet's direction of motion. It is a relatively rare illusion having to do with the geometry of the comet's orbit wrt Earth. The additional picture below may help visualize the origin of the antitail

: Antitail Geometry

Starship Asterisk*

APOD: Comet PanSTARRS with Anti Tail (2013 May 18)

APOD: Comet PanSTARRS with Anti Tail (2013 May 18)

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