APOD: Comet ZTF and Mars (2023 Feb 13)

[APOD Robot]

Comet ZTF and Mars

Explanation: No, Comet ZTF is not going to hit Mars. Nicknamed the Green Comet for its bright green coma, C/2022 E3 (ZTF) did, however, pass almost in front of the much-more distant planet a few days ago, very near in time to when the featured picture was taken. The two sky icons were here captured behind a famous Earth icon -- the Matterhorn, a mountain in the Italian Alps with a picturesque peak. Both the foreground and background images were taken on the same evening by the same camera and from the same location. The comet's white dust tail is visible to the right of the green coma, while the light blue ion tail trails towards the top of the image. Orange Mars is well in front of the numerous background stars as well as the dark nebula Barnard 22 to its lower right. Although Mars remains visible in the evening sky for the next few months, Comet ZTF has already begun to fade as it returns to the outer Solar System.

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[AVAO]

Comet ZTF and Mars

The two sky icons were here captured behind a famous Earth icon -- the Matterhorn, a mountain in the Italian Alps with a picturesque peak.

I'm sorry if I have to disagree here as a Swiss native. Two-thirds of the mountain lies in the Swiss Alps. What you see in the foreground of the picture is the swiss side of the massif for the most part. With this "Swiss view", the mountain has also become world-famous as a photo subject.

https://www.myswitzerland.com/en-ch/des ... itzerland/

So the correct description would be:

The two sky icons were here captured behind a famous Earth icon -- the Matterhorn, a mountain in the swiss-italian part of the alps with a picturesque peak.

"The Alps are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately 1,200 km across seven Alpine countries: France, Switzerland, Italy, Liechtenstein, Austria, Germany, and Slovenia."

[Ann]

Comet ZTF and Mars. Image Credit & Copyright: Donato Lioce

---

As I took in this image, a deafening cacophony of cackling voices started to fight for dominance in my mind.

That comet sure is green!!

Hey, I can see its ion tail! Finally!

Why isn't the ion tail blue?

Wow!! Mars is so bright it looks like the Sun!!

What is that dark cloud to the lower right of Mars?

I mean, that dark cloud looks like something a red giant star could have belched out!!

Come on, when it's seen just to the lower right of Mars, it looks like something Mars could have belched out!!!

Come on! What is that thing? And how big is it? I mean, if it's a cosmic cloud of some sort, it must be HUGE!!! To look so large in the sky, I mean, to have an angular size like that!!



Okay, easy, easy now, Ann. We'll try to answer your questions.

So, why does the comet look soooo green?

Well, may this can give you a clue...

APOD 13 February 2023 small detail.png (200.6 KiB) Viewed 8222 times

I very much apologize for the hugely irritating lettering under the picture that I can't get rid of, but if you can disregard that, maybe you can see that the stars leave trails (because the photographer was tracking the comet), and that at least some of the trails are greenish. I take that to mean that the author of the image has "bumped up the green channel" of "enhanced the green signal" in order to make the comet look a little extra green.

That would also explain why the ion tail doesn't look blue, but greenish. It would also explain why the dust tail of the comet doesn't look white, but green, too.

And Mars looks so bright because the comet is faint, while Mars is currently bright in the Earth's skies. In order for the comet to look bright, the picture was produced in such a way that Mars looks brilliant.

And what about the dark cloud? I hope I'm allowed to copy an image by Astro-TAPAS:

Barnard 22 with reflection nebula IC 2087. Image credit: Astro-TAPAS.

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Well, what do you think? It's the same dark nebula, isn't it?

And isn't it interesting that the photographer has managed to bring out this nebula? It sure isn't often that we see dark nebulas showing up in images that are meant to show us something else entirely. I'm not talking about iconic dark nebulas like the Coalsack Nebula or the Pipe Nebula, but anonymous and unfamiliar dark nebulas like Barnard 22.

This certainly says something about how the photographer created his image. But I'll leave that to the many photo experts (Chris?) at Starship Asterisk* to explain.

Anyway. The way Mars posed just above and in front of Barnard 22 made me think of those red giant stars belching out dust clouds and dimming themselves, like Betelgeuse, or of binary stars where one component is dimmed by a companion star which is carrying a giant dust disk along with it!

Betelgeuse belching a hot gas cloud, which grew dark and dusty some distance from the star and dimmed Betelgeuse from the Earth's perspective. Illustration credit: NASA, ESA, and E. Wheatley (STScI)

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Epsilon Aurigae: Bright class F star and companion Class B star surrounded by a dusty disk (artist's impression). Credit: NASA/JPL-Caltech

---

Ann

[Lorenzo Comolli]

This image is clearly striking both for Astronomy and for Image-processing.

Clearly Comet+Mars+Matterhorn is a great idea and for sure the author has invested lot of time imaging and processing. I personally very appreciate the idea.

However the composition is badly done, imho. So badly that it should't be published as APOD.

Astronomy errors.
ZENITH DIRECTION: Matterhorn clearly show that the zenith direction is up, but the stars say it must be 45°rotated up-right (at the time of the image, Mars-Comet direction is nearly toward zenith).
This show that the author probably imaged the star field with equatorial North-up, but the landscape North was up-left.
For a good composition, author should have imaged the comet when it was in that landscape position, and then he should place the long exposure alighed on it.

MOON LIGHT: at the time of Mars+Comet rising, the Moon was in the sky and the mountains should be illuminated. See this webcam at the exact timing of the supposed image.

Image processing errors. (as detailed by Ann)

• green ion tain (should be blue) and green dust tail (should be yellow), this is not realistic

• stars with a trail (rest of comet alignment processing), they can be easily removed with a processing step

• Mars light glare and spikes: they were clearly added in processing, and then the dark nebula has beed added. They must be added as a last processing step, because it is not realistic that the glare is standing behind the dark nebula!

I personally think that night landscape photography must have some ethics behing, even only a little bit.
A fundamental point of a composition is that it must be realistic, it must represent the realty. Ok to add subsequent enhanced images, but unfortunately here realty is not represented.

Regards,
Lorenzo

PS: thanks also to discussions with my friends Edo*, Ale(**) and Ema (*sky simulation by him) (**webcam screenshot by him)

[Chris Peterson]

So, why does the comet look soooo green?

It looks green because of bad image processing. It's as simple as that. The image below was taken the same night, and is spectrophotomerically color calibrated (and consistent with most other images people are making). The coma of this comet is green, the dust tail is yellow-white, and the ion trail is almost the same color as the dust, not the striking blue we sometimes see. And the signal from the dust is a lot brighter than the cyanogen glow of the coma, so it washes out most of the green even close to the head of the comet.
_

[orin stepanek]

Pretty; Be a nice view if your on Mars!

[Michael Teoh]

Astronomy errors.

ZENITH DIRECTION: Matterhorn clearly show that the zenith direction is up, but the stars say it must be 45°rotated up-right (at the time of the image, Mars-Comet direction is nearly toward zenith).
This show that the author probably imaged the star field with equatorial North-up, but the landscape North was up-left.
For a good composition, author should have imaged the comet when it was in that landscape position, and then he should place the long exposure alighed on it.

MOON LIGHT: at the time of Mars+Comet rising, the Moon was in the sky and the mountains should be illuminated. See this webcam at the exact timing of the supposed image.

Image processing errors. (as detailed by Ann)

• green ion tain (should be blue) and green dust tail (should be yellow), this is not realistic

• stars with a trail (rest of comet alignment processing), they can be easily removed with a processing step

• Mars light glare and spikes: they were clearly added in processing, and then the dark nebula has beed added. They must be added as a last processing step, because it is not realistic that the glare is standing behind the dark nebula!

No disrespect, but I think the person behind the photo selection doesn't spend time investigating the technical part of the image. Do we expect them to? On every image?

[Donato]

Hello all,
I am the author of the photo (nice to meet you) and I was alerted that there is some criticism ongoing on this site. So I will try to explain how I took the picture and hopefully address at least some of the criticism.

So the photo was taken the night between the 10 and the 11 of February. What I did was to take the foreground exposure after sunset during the blue hour pointing the camera north west where, as planned with Stellarium, the comet would set early in the morning of the 11. Then, from the exact same tripod position, I tilted the camera up and framed the comet with mars (and part of the dark nebula). I then took a series of exposures until 30 minutes more or less after moonrise… I finally stacked 60 exposures (2 minutes each) to get some good signal to noise ratio. In postproduction, for the sky what I did was to correct the white balance, add contrast by using 3-4 level adjustments, boosted a bit the saturation and further reduce the noise. All of this was done in photoshop. Then I combined blended the sky with the foreground. The spikes were not added in post, they are present on all the raw files. Frankly I am not sure of the reason but this is maybe due to the fact that i used a telephoto lens and not a telescope to capture the images.

For sure many things could have been done better, but I hope I was able to address at least some of the criticism.

[johnnydeep]

So why are there 18 (I think) diffraction spikes on Mars? And why is the ion tail so thin and arrow-straight compared to the broad and curved dust trail? I guess this is a question about comet tails in general now that I think about it. Both tails are made of matter (ions/dust) in orbit while also being pushed by sunlight, so why don't they look roughly similar?

[Michael Teoh]

I think generally it is debatable when a composite image is composed of pictures taken at different angles, orientations, or even locations, rather than different times and/or settings. The orientation of the sky at such altitude is only possible from the southern hemisphere, thus a sky with such orientation does not "belong" to this foreground from this location. Imagine if the landscape part is facing east but the sky part is facing west. It doesn't look right. Anyway, even though you have mentioned that it was a composite image in your post on your Facebook, the APOD editor somehow didn't mentioned it at all.

[Donato]

Hello again,
I am trying, failing, to add a screenshot directly from Stellarium to explain how I imaged and planned the photo…
Anyway you can see from stellarium that the sky in the photo is aligned to the celestial (equatorial) coordinates…I could have used the azhinutal coordinates to align the sky but I preferred to use the equatorial ones

[RJN]

I'm sorry if I have to disagree here as a Swiss native. Two-thirds of the mountain lies in the Swiss Alps. What you see in the foreground of the picture is the swiss side of the massif for the most part. With this "Swiss view", the mountain has also become world-famous as a photo subject.

OK. Sorry, my bad. I just fixed the text on the main NASA APOD. Apologies.
- RJN

[Chris Peterson]

So why are there 18 (I think) diffraction spikes on Mars? And why is the ion tail so thin and arrow-straight compared to the broad and curved dust trail? I guess this is a question about comet tails in general now that I think about it. Both tails are made of matter (ions/dust) in orbit while also being pushed by sunlight, so why don't they look roughly similar?

The 18 spikes on Mars are what you get with a nine-bladed iris on the camera lens, which is a very common number. Instead of a circular aperture you have a polygon with 9 sides (a nonagon). Each straight edge creates a pair of diffraction spikes. So you end up with 18.

Ion tails are often quite straight because they are driven by high speed solar wind and point directly away from the Sun. Dust tails are often much more diffuse because they diffuse evenly away from the comet and then their particles move at different speeds depending on whether they are inside or outside the parent body orbit. And they interact with solar radiation, not the solar wind, which results in a much more scattered direction for these large particles (as opposed to the gas molecules or atoms that make up the ion tail).

[jfgout]

Well, what do you think? It's the same dark nebula, isn't it?

And isn't it interesting that the photographer has managed to bring out this nebula? It sure isn't often that we see dark nebulas showing up in images that are meant to show us something else entirely. I'm not talking about iconic dark nebulas like the Coalsack Nebula or the Pipe Nebula, but anonymous and unfamiliar dark nebulas like Barnard 22.

This certainly says something about how the photographer created his image. But I'll leave that to the many photo experts (Chris?) at Starship Asterisk* to explain.

Ann

While I 100% share your concern about the (excess of) green, this patch of dark nebula is not so difficult to catch in pictures. This being said, there are so many problems with this image (most have already been pointed out, but I'll take a turn at beating the dead horse... and clarifying a few things). I appreciate the author coming here and discussing this, although I note that they did not provide any of the information that could help (or incriminate...) such as: the exact location from where this picture was taken and the focal length of the lens used. Instead, we are served the usual feel good story ("I drove 6 hours to go get this picture, totally worth it!" and the now common but HUGE red flag: "Images taken during the same night, from the same place and with the same camera").

Okay, so here is my list of everything wrong with this picture, in no particular order.

1) The orientation of the sky relative to the horizon is wrong. On this picture, tau Tau (that's funny...), 95 tau, Mars, and the top part of IC2087 (the dark nebula) form more or less a horizontal line. Another way to say that is that the line joining tau tau to Mars is ~parallel to the horizon. But from this part of the world, shortly before setting, this line should form an ~45 degrees angle with the horizon. Anyone can use STellarium (or other) to check this.
In other words: the sky orientation is rotated ~45 degrees relative to reality.

2) I have strong doubts about the location the sky relative to the mountain. This one is tricky to check without knowing the exact location of where the picture was taken. I'm going to assume that it was taken from the Gornergrat observatory area. This would put the summit of Matterhorn at a 265 degrees heading. When Mars was ~5 degrees above the horizon on that day, its azimuth was 300 degrees. So (assuming that I did not mess up too much my guess about the location from where the photo was taken), not only is the orientation of the sky not correct, it is also not placed correctly relative to the foreground. Mars should be on the other side of the Matterhorn.

3) Sky and foreground were most likely taken with different sampling sizes (= different scale). A quick plate solving of the image indicates an image scape of 7.09"/pixel (on 4229x3838 full resolution image). On that same image, the height of the edge above Hörnlihütte (the shoulder on the right side of the mountain) is 591 pixels. These 591 pixels represent 1200 meters (the difference in elevation between Hörnlihütte and the summit) or 4190 arcseconds (591*7.09), using the previously obtained sampling scale. For 1200 meters to cover 4190 arcseconds, one would need to be standing ~50 km away from the Matterhorn. That is ~5x the distance from the Gornergrat. So, unless there is a secret spot to take this picture of the Matterhorn from 50 km away (unlikely), or the foreground (mountain) is simply 5 times smaller than what it should be if it had been take with the same sampling size (=same camera and same focal length) as the background sky.


4) As previously mentioned, the comet is waaaayyyyy too green. We can see that the leftover trails made by the brightest stars (trails from staking/tracking on the comet, which moves fast relative to the background stars) are all green. This clearly tells us that the author turned the color balance/saturation to the top in the green for the layer that contains the image tracked/stacked on the comet.


5) At the time Mars and the comet were that low in the sky, the foreground would have been inundated by the light from the gibbous Moon.


6) Absolutely zero trace of atmospheric extinction. Even in the transparent skies of high elevation Gornergrat, there should be some level of atmospheric extinction visible. On this part, the author was honest in their follow up, explaining that the background sky was captured shortly after nightfall (and before moonrise), when the Mars/Comet duo was high in the sky (and nowhere near the horizon).


So, to summarize, this image shows a comet with an absurdly green color, in the wrong location relative to the foreground, with a sky at a 45 degrees angle from reality and with a factor 5 difference in the scale between the foreground and the sky. As someone suggested, at this point, why not just use a picture of the Eiffel tower in the foreground? Or a macro shot of a beautiful mountain flower (okay, not many flowers in February in the Alps...)

What really bothers me is the use of the this: "Both the foreground and background images were taken on the same evening by the same camera and from the same location." This sentence is clearly there to let us believe that this picture represents some sort of reality. For all the reasons that I have detailed above, it is clearly NOT the case.

APOD has an ethic statement which reads: "APOD accepts composited or digitally manipulated images, but requires them to be identified as such and to have the techniques used described in a straightforward, honest and complete way." In my view, the description for this image should include the fact that different focal lengths were used for the sky and the foreground, the fact that the sky was positioned on the left of the mountain and rotated 45 degrees because it looked nicer and that the colors of the comet had been boosted in the green through the roof.

I know that the authors of the APOD do this on their free time and I certainly appreciate their work. It might be nice to have a group of "reviewers" (experienced astrophotographers) who could share their opnion with the APOD's editors and share their concerns before a photo with extensive digital manipulation is published. The APOD used to be some sort of holly grail for amateur astronomers, but more and more people in the hobby are having a negative view of it because of these images that can be considered "fake". I still love the APOD, but I would love to see a better/strict implementation of the ethics statement mention earlier.

jf

[Ann]

There is a great photo of Comet ZTF, Mars and Barnard 22 (and the Pleiades) by Luca Fornaciari in the Recent Submissions thread.

Do look at the full size image.

Ann

[Donato]

Dear @Jfgout and @Ann,

Thanks for your review even though frankly I believe you are a bit too strong in your words and frankly, especially from professionals or anyway experts, I would expect a bit less aggressive/offensive tones and more constructive words… anyway let me please try again

I clearly mentioned that this is a composite image. Please check yourself my IG page… so the whole presumed root cause of hiding things etc is a bit difficult to understand for me as from the very beginning I have not hidden it is indeed a composite image.
Please check yourself in stellarium (I hope to find a way to paste a screenshot) and you will see that the sky is aligned using celestial/equatorial coordinates. I could have used azimuthal coordinates but I preferred to use celestial ones when taking the sky… shall I be killed for this ? Not sure frankly
Indeed I augmented the saturation in post processing as already explained: it is my personal opinion (and as such I might be totally wrong) that in 100% of astro photography pictures the saturation is increased in post…maybe I could have done it better but, once again, shall I be killed for this ? Not sure


Bottom line is at least for me: as a non professional who is indeed prone to errors (even though I giorni was able to explain myself and how I imaged the picture), I would have expected a significant more constructive criticisms and not attacks like I have read… or at least this is what I do in my profession/hobbies when I see some opportunity for improvements

[Roberto Colombari]

Now the question is how the comet ended up with a further north declination compared to Mars while it's exactly the opposite.
I really would remove this image from APOD. I am not talking about the quality of the processing, anyone has his/her own style.
In this case this image is simply wrong.

[donato]

Dear @Roberto Colombari,

can you please tell me how can I attach an image when posting a reply so that i can attach my screen shot? you rotated the celestial coordinates by ~180-45=135 degrees instead of rotating by 45 degrees...so what you write I am sorry but I think it is misleading... please tell me how can I add my screenshot to the thread make copy and paste but it is not working...shall I register or something? It is a bit unfair if I cannot show you exactly what I mean frankly speaking

[Roberto Colombari]

> can you please tell me how can I attach an image when posting a reply so that i can attach my screen shot

You have to upload the screenshot on postimage.org, for instance, get the direct link to the image and put it here.

> you rotated the celestial coordinates by ~180-45=135 degrees instead of rotating by 45 degrees

I've simply aligned the sky of your picture with Stellarium. If you look at it both the screenshots have the same position angle, more or less.
The point that I have probably missed is that you shot the comet the night between 10 and 11 of February, right? When it really had further north declinations than Mars. I thought you shot it the next night when it got southern.

[Roberto Colombari]

2) I have strong doubts about the location the sky relative to the mountain. This one is tricky to check without knowing the exact location of where the picture was taken. I'm going to assume that it was taken from the Gornergrat observatory area. This would put the summit of Matterhorn at a 265 degrees heading. When Mars was ~5 degrees above the horizon on that day, its azimuth was 300 degrees. So (assuming that I did not mess up too much my guess about the location from where the photo was taken), not only is the orientation of the sky not correct, it is also not placed correctly relative to the foreground. Mars should be on the other side of the Matterhorn.

jf

From Gornergrat you spot Monte Cervino a few degrees South when looking to straight to West; let it be, as you said, azimuth between 260º and 265º.
Now, reading what they guy wrote ("I shot Mars and the Comet till the Moon rise" [i.e., ~22:45 local time]), it's not so unlikely to have Mars on that side of Monte Cervino.

Cheers

[donato]

> can you please tell me how can I attach an image when posting a reply so that i can attach my screen shot

You have to upload the screenshot on postimage.org, for instance, get the direct link to the image and put it here.

> you rotated the celestial coordinates by ~180-45=135 degrees instead of rotating by 45 degrees

I've simply aligned the sky of your picture with Stellarium. If you look at it both the screenshots have the same position angle, more or less.
The point that I have probably missed is that you shot the comet the night between 10 and 11 of February, right? When it really had further north declinations than Mars. I thought you shot it the next night when it got southern.

Dear @Roberto Colombari,

You can find the screenshot here: https://i.postimg.cc/yWqkfDRt/screenshot-stellarium.jpg
And thanks for the tip on how to add an image in the thred.
You will see in the screenshot that the position of mars and comet is indeed correct. The photos were taken during the nught between the 10th and 11th of Feb, as written in some other replies. The screenshot I linked is according to this.

I hope I clarified...

[Roberto Colombari]

Yeah, that's right. At that time the Comet had a declination far northern than Mars. I read 11th of February meant as the night of the 11th of February

[donato]

Thanks Roberto for the feedback...so I hope the topic is now closed and I can relax...it was actually a bit frustrating (and stressful) for me because I felt attacked (too much frankly speaking)...and I was not expecting that, especially considering that nothing at all like this happened for my other apod. Now I know what can be expected

Ciao,
Donato

[jfgout]

Dear @Jfgout and @Ann,

Thanks for your review even though frankly I believe you are a bit too strong in your words and frankly, especially from professionals or anyway experts, I would expect a bit less aggressive/offensive tones and more constructive words… anyway let me please try again

I clearly mentioned that this is a composite image. Please check yourself my IG page… so the whole presumed root cause of hiding things etc is a bit difficult to understand for me as from the very beginning I have not hidden it is indeed a composite image.
Please check yourself in stellarium (I hope to find a way to paste a screenshot) and you will see that the sky is aligned using celestial/equatorial coordinates. I could have used azimuthal coordinates but I preferred to use celestial ones when taking the sky… shall I be killed for this ? Not sure frankly
Indeed I augmented the saturation in post processing as already explained: it is my personal opinion (and as such I might be totally wrong) that in 100% of astro photography pictures the saturation is increased in post…maybe I could have done it better but, once again, shall I be killed for this ? Not sure


Bottom line is at least for me: as a non professional who is indeed prone to errors (even though I giorni was able to explain myself and how I imaged the picture), I would have expected a significant more constructive criticisms and not attacks like I have read… or at least this is what I do in my profession/hobbies when I see some opportunity for improvements

Dear Donato,

I'm sorry if you've found my words too strong or my tone aggressive, but all I did was demonstrate, using math and basic astronomy/photography knowledge all the manipulations in your image which I think should have been made clear from the start. In my book, that's constructive criticism. But sure, you can reply with things like "shall I be killed for this?" while ignoring all the important details (was the picture taken from the Gornergrat?, which focal lengths were used?) to pretend that you are a victim of extremists like me...

So, yes, you did mention from the start that this was a composite image (I mean, no one in their wright mind would try to pass this as a one shot) but failed to mention any of the many issues that I have mentioned. Then, why go through the trouble of specifying that the different parts of the composite were taken on the same day with the same camera? If you change the lens, paste the sky background in an impossible position relative to the foreground and at a 45 degrees angle after saturating the greens to the max (I mean, the white stars leave a green trail on your picture!!!!), why not go all the way and also add the Eiffel tower or the leaning tower of Pisa in the image?

Please check yourself in stellarium (I hope to find a way to paste a screenshot) and you will see that the sky is aligned using celestial/equatorial coordinates. I could have used azimuthal coordinates but I preferred to use celestial ones when taking the sky…

This does not mean anything. An alignment is relative to something. In your case, it seems logical to use the foreground (the mountains) as the baseline. Again, if you do this, why stop there and not use an image taken at a different time and from a different place for your foreground?


After you mentioned it, I searched for your previous APOD. Well, it's a very nice image (congratulations). It might would have been nicer with a mountain in the foreground

From Gornergrat you spot Monte Cervino a few degrees South when looking to straight to West; let it be, as you said, azimuth between 260º and 265º.
Now, reading what they guy wrote ("I shot Mars and the Comet till the Moon rise" [i.e., ~22:45 local time]), it's not so unlikely to have Mars on that side of Monte Cervino.

Of course, at the time he took the background sky pictures, Mars was close to the meridian (azimuth ~180 degrees). But at that time, it was also 60 degrees above the horizon!!! So, a picture with the Matterhorn in it taken at this time would have a LOT of sky between the mountain (horizon) and Mars/Comet (constellations of Pisces and Cetus, Jupiter would be visible, ...). If you paste the sky in the location where it would have been at the time the comet/Mars were approaching the west horizon, try to do it reasonably correctly.

Now, there are lots of good things: there is plenty of signal, the dark nebula is nice. So, I'll end my message on this positive note.

jf

[Roberto Colombari]

There remain few points that are really a little bit confusing at least for me.
You shot the Comet-Mars conjunction from sunset 'till 22:45, local time approximately.
At this time there's still a good amount of sky (Jupiter, Cetus, etc...) between them and the horizon where they'll set later one. Why can't we see them in the image?

Cheers