APOD: Comet ZTF and Mars (2023 Feb 13)

[jfgout]

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

Roberto,

The author was fairly clear in their answer on this point:

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…
[...]
Then I combined blended the sky with the foreground.

The implicit part is that combining the sky with the foreground should represent the sky as it was when the comet/Mars were setting behind the mountains (= a few hours after the background sky images were taken). I wont blame the author for not spending the night at the Gornergrat, it is cold up there!!! . The problem is that the result is far from what the sky would have looked like (sure, it could have been worse if the picture had been taken from the North side of the Matterhorn for example...).

Again, when you read the answer from the author, they do stress that the pictures were taken "from the exact same tripod position". What is the point of stressing this, if not to make people believe that this composite represents something real? Based on all the issues that have been pointed out, why bother using a foreground image taken on the same night, or even from the same place?

Cheers,

jf

[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?

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).

Thanks. I get the nonagon aperture explanation. As for dust -vs- ions/atoms being acted upon by radiation -vs- solar wind, does the radiation not affect ions/atoms simply because they are much smaller than (the vast majority) of the wavelengths hitting them? And why doesn't the solar wind exert a force on the dust? Or is it just that the force is tiny relative to the dust grains? [ Bonus question: what's the force of solar radiation compared to that of the solar wind on an aluminum foil square foot at the distance of Earth's orbit? (I assume the material making up that square foot matters a great deal as well?) ]

[stephenramsden]

I just can’t believe they didn’t throw in an ISS transit. Afterall, it’s an APOD, right?

[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).

Thanks. I get the nonagon aperture explanation. As for dust -vs- ions/atoms being acted upon by radiation -vs- solar wind, does the radiation not affect ions/atoms simply because they are much smaller than (the vast majority) of the wavelengths hitting them? And why doesn't the solar wind exert a force on the dust? Or is it just that the force is tiny relative to the dust grains? [ Bonus question: what's the force of solar radiation compared to that of the solar wind on an aluminum foil square foot at the distance of Earth's orbit? (I assume the material making up that square foot matters a great deal as well?) ]

The solar wind consists of charged particles, which interact electromagnetically with the ion tail. Photon pressure is exerted when photons are reflected by a surface (or absorbed and re-emitted), with a partial transfer of momentum. A macroscopic particle is more likely to interact with a photon (or photons) than a small molecule. And the forces generated by the electromagnetic interaction are much larger. So the solar wind dominates the ion tail. As the dust tail is largely neutral, its interaction with the solar wind is much smaller.

[Lorenzo Comolli]

Dear Donato, great to have you here.
Please can you reply to questions posed by others? They whould greatly help to understand how the image was made, than anyone can judge if it is ok or not.

1. Focal lenght of the two images (and eventual crop factor). Can you share 2 originals uncropped?
2. Location (Gornergrat?) How was imaging at night in shuch high altitude in winter?
3. Date time of the two images
4. Did you shot a reference image at the date time of the composition? I.e. at comet mars setting. Can you share?
5. Which processing steps caused the dark nebula stand in front of the mars glare?

Ciao,
Lorenzo

[SpookyAstro]

I've had to give a run down of this image to a few laypeople lately who have asked about it and it has been been pretty difficult.

I won't belabor the point of the technical issues which have already been laid out quite well in this thread and elsewhere on web. But I think it's important to make a correction at this point in the description of the image relating to the 'Both the foreground and background images were taken on the same evening by the same camera and from the same location.' part. I've found it more easy to say that it's an artistic rendering/representation, similar to how APODs in the past have done so for things that don't completely match up with 'reality' or how it would look exactly but may still have value.

I would like to also add that I don't fault the editors who have done such a great job overall considering the task of running APOD and sometimes things go this way, at this point it appears to be just a matter of a simple correction in the description and then call it done IMHO.

Tom Masterson
www.transientastronomer.com

[rcolombari]

Hi Tom!
What you call a simple correction in the APOD description has a deep meaning IMHO and deserves some further toughts.

As we pointed out the comet is at the right place for the date and time declared by the author but there's a whole amount of sky that doesn't show up to the West. At the time the comet has been imaged, ZTF and Mars were around 50º-60º up in the sky while in the image they look pretty low.
Moreover, as Lorenzo highlighted, the glare of Mars is behind the dark nebula. How could it be possible?

All these considerations lead to just a conclusion: copy and past of 2 different images resulting in something that never happened. The sky simply wasn't in this configuration.
How can such an image be APOD worthy?
It's true that, besides what many out there think, APOD doesn't mean that the images are chosen by NASA or pass through a peer review before getting published or must have a specific scientific meaning (NASA is just the hosting as far as I know, nothing less or more than this). Nevertheless, since the APOD audience is really wide, I think that the images should at least meet some basic astroimaging criteria.

[johnnydeep]

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).

Thanks. I get the nonagon aperture explanation. As for dust -vs- ions/atoms being acted upon by radiation -vs- solar wind, does the radiation not affect ions/atoms simply because they are much smaller than (the vast majority) of the wavelengths hitting them? And why doesn't the solar wind exert a force on the dust? Or is it just that the force is tiny relative to the dust grains? [ Bonus question: what's the force of solar radiation compared to that of the solar wind on an aluminum foil square foot at the distance of Earth's orbit? (I assume the material making up that square foot matters a great deal as well?) ]

The solar wind consists of charged particles, which interact electromagnetically with the ion tail. Photon pressure is exerted when photons are reflected by a surface (or absorbed and re-emitted), with a partial transfer of momentum. A macroscopic particle is more likely to interact with a photon (or photons) than a small molecule. And the forces generated by the electromagnetic interaction are much larger. So the solar wind dominates the ion tail. As the dust tail is largely neutral, its interaction with the solar wind is much smaller.

Ah, yes, I completely forgot about the charged particles! Makes sense now - thanks!

[SpookyAstro]

Hi Tom!
What you call a simple correction in the APOD description has a deep meaning IMHO and deserves some further toughts.

As we pointed out the comet is at the right place for the date and time declared by the author but there's a whole amount of sky that doesn't show up to the West. At the time the comet has been imaged, ZTF and Mars were around 50º-60º up in the sky while in the image they look pretty low.
Moreover, as Lorenzo highlighted, the glare of Mars is behind the dark nebula. How could it be possible?

All these considerations lead to just a conclusion: copy and past of 2 different images resulting in something that never happened. The sky simply wasn't in this configuration.
How can such an image be APOD worthy?
It's true that, besides what many out there think, APOD doesn't mean that the images are chosen by NASA or pass through a peer review before getting published or must have a specific scientific meaning (NASA is just the hosting as far as I know, nothing less or more than this). Nevertheless, since the APOD audience is really wide, I think that the images should at least meet some basic astroimaging criteria.

Hi Roberto!! Hope all is well with you

Right I read about the orientation of the sky being incorrect based on what it would look like, I for sure agree with you and others that have stated this 100%. This image doesn't represent reality.

I guess my point is that it is very difficult to explain how this astroimage was assembled to match up with how it would look if you were there and were viewing it with a good pair of binoculars because it wouldn't look like this at all since the sky is not orientated correctly in this image. So what at this point I think is most important is to modify the description of the image at the very least to say that it's an 'artistic interpretation' or something like that as it doesn't qualify under the commonly understood definition of a composite astroimage since the sky is registered using a different coordinate system as the author stated in an earlier comment. I'm going to tread lightly here but I would kind of consider this as something like the artistic renderings of what it would look like if you were on an exoplanet looking at the night sky, it's not really based on reality it's an artistic expression type deal. Like this APOD and others like it: https://apod.nasa.gov/apod/ap050805.html

I'm not sure what qualifies any image to be published as APOD, I'll leave that up to the editors and I think that their freedom/flexibility to publish a varying array of different types of images/renderings/art etc is what makes APOD great in my opinion. Are their better more accurate examples of what this event looked like in our night sky, I would say yes. Should any of those images replace this one? That's totally up to the editors.

Clear skies bud!

Tom

[Michael Teoh]

Dear Donato, great to have you here.
Please can you reply to questions posed by others? They whould greatly help to understand how the image was made, than anyone can judge if it is ok or not.

1. Focal lenght of the two images (and eventual crop factor). Can you share 2 originals uncropped?
2. Location (Gornergrat?) How was imaging at night in shuch high altitude in winter?
3. Date time of the two images
4. Did you shot a reference image at the date time of the composition? I.e. at comet mars setting. Can you share?
5. Which processing steps caused the dark nebula stand in front of the mars glare?

Ciao,
Lorenzo

I would add:

6. Did you mention it being a composite in your submission to APOD? You did in your FB/IG post, but in the submission?

I have seen the APOD authors mentioning an image being composite when it was, so I'm sure they won't miss it if you did mentioned.

As Tom suggested, a revised description would at least settle the part on misleading information. As for whether it was a badly done composite or not, that most likely will continue to be debated.

[jfgout]

I note that the author is in no hurry to answer problematics questions such as: what focal length was used for the two different images. Before this thread gets completely forgotten, I'm going to write my own "honest" version of this APOD's description:

Reading back the description for this image, I'm amazed by this part: "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." Well, there is no white nor blue in this image of the comet, it is all green! It made me want to write a "honest" description of the picture (in the same spirit as honest trailers for movies...):

Donato went to the magnificent Swiss Alps to capture this beautiful conjunction of C/2022 with Mars. Shortly after sunset, he captured a wide-angle view of the the Matterhorn and surrounding mountains. He then used a telephoto lens with a much longer focal length to record images of the comet while it was almost directly overhead and recorded 60 frames of 2 minutes each while tracking the sky with an equatorial mount. Because the comet's motion relative to the background sky was consequent in 2 hours (~15' or half the size of the angular size of the full moon), the final composite requires mixing together a layer of stacked images tracked on the comet with another layer of stacked images tracked on the stars. Succumbing to the constant buzz about the GREEN comet, Donato could not resist and he moved the white balance cursors through the roof in the comet layer to make it appear as green as possible. Unfortunately, by doing so, he removed the subtle white/yellowish color of the dust tail and the delicate blue colors in the ion tail.
After merging the comet and background sky layers, the author decided that it would look great with the Matterhorn in the foreground and blended in the landscape picture taken earlier, positioning the sky in a way to produce the best possible looking composition.

For reasons that escape our comprehension, the author resisted the temptation to also include a view of the Eiffel tower in the foreground, but he insisted on the fact that both the sky and the landscape images had been taken during the same night, from the same camera, as if this was a token of authenticity.

Maybe I should offer my services to the APOD as a text writer

jf

PS: Just to be clear, I totally understand that the authors of the APOD do not have the time to investigate the many images submitted. That's also why I think it is important that the astrophotography community does this job of pointing out data manipulation problems that do not respect APOD's ethical statement.

[Lorenzo Comolli]

I think that the APOD caption should include a sentence like:

This image is an artistic composition of images taken from the same location but at different sky positions, times and focal lengths respect to the final composition. This is not representing the real sky.

The actual sentence informing about the image tecniques is

Both the foreground and background images were taken on the same evening by the same camera and from the same location.

but I think it is misleading, implying that taking the images in this way is sufficient to give a realistic view.

Regards,
Lorenzo

[MelvzLuster]

Great & wonderful scenario of ZTF Comet & Mars! A sign that we have a spectacular healthy space in a Universe full of wonders & mystery. Yes, we have an Infinite Cosmos ever expanding directly proportional to the increase in population growth of our planet Earth. Our eternal Universe approves and favors our worthwhile endeavor to explore & inhabit Mars & other habitable exoplanets. For these reasons, we are going to live forever conquering the infinity & vastness of the Universe for all eternity! And so it is!