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When is the perfect phase to image Earthshine?

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#1 Borodog

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Posted 14 January 2021 - 09:28 AM

Imaging Earthshine on the dark side of the Moon, particularly without blowing out the illuminated limb, is a challenge. There are a number of competing factors. You'd like the sky as dim as possible, meaning farther from the new Moon, but you want as much reflected Earthshine illuminating the dark side as possible, meaning closer to the new Moon. Somewhere in there is the perfect phase that will allow you to maximize the reflected Earthshine but minimize the scattered sunshine in the atmosphere, which for these purposes just raises the noise floor and obliterates details.

 

Yesterday was the new Moon. Today at sunset the Moon will be 3.5% illuminated at an altitude of 15 degrees above the horizon and have roughly 1.5 hours before Moonset. Tomorrow at sunset the Moon will be 8.5% illuminated at 25.5 degrees above the horizon and have more than 2.5 hours before sunset. The next day, 15.1%, 35 degrees, 3.5 hours. The next, 23%, 43.5 degrees, 4.5 hours. The Earth's phase as seen from the Moon is the opposite of the Moon's phase as seen from the Earth; so as the Moon's illumination is increasing, the available Earthshine is falling, 96.5%, 91.5%, 84.9%, 77%.

 

Somewhere in there is the perfect time to image Earthshine. The exact calculation is beyond the time that I have to put into it at the moment. It would depend on the Earth's illuminated fraction as seen from the Moon, the Moon's albedo and distance, as well as the sky flux of scattered sunlight at the Moon's current altitude as a function of time as the Sun sinks lower behind the horizon, as well as any city light domes in the direction of the sunset/Moonset.

 

But that perfect time is in there somewhere. My challenge to you lunar imagers is to try to go find it. For myself, I may not get it. Trees. I might get a shot between the trees given that it's winter. Tonight the Moon may just be too low. Tomorrow, too cloudy. I suspect the day after will be too late. But I bet somebody can get the shot and post it here.

 

Good luck!



#2 gustavo_sanchez

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Posted 14 January 2021 - 09:37 AM

That’s a good question. I have observed that it’s best for me to see earthshine when the moon is about 4 or 5 days old, with a thin crescent but with a good altitude still to make it easy to observe.

In terms of imaging, I think that the capability to capture earthshine without blowing up the illuminated crescent depends on the dynamic range of the camera too, not only on the phase, so maybe there’s not a “perfect” time for all cameras to achieve the best results. I could be wrong, of course, and look forward for others’ feedback.
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#3 james7ca

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Posted 14 January 2021 - 10:29 AM

Well, winter is generally the best time for the full or gibbous moon (since the moon will be high in the sky). However, for thin crescent moons the best time is probably late spring to summer (for the evening) and late summer and early fall (for the morning).

 

During the summer the sun and crescent moon will set more or less perpendicularly to the horizon and the thin crescent will be higher in the sky.



#4 Stellar1

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Posted 14 January 2021 - 10:32 AM

A thin crescent moon is the best time to see earthshine as looking from the moon to earth, earth would appear as a full earth (earth does have phases as viewed from the moon of course) during this full earth/thin crescent moon phase there is a maximum amount of light being reflected towards the dark side of the moon.

 

correct me if I'm wrong but i think its right lol.


Edited by Stellar1, 14 January 2021 - 10:34 AM.


#5 frank5817

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Posted 14 January 2021 - 12:36 PM

Yes, a thin crescent Moon when it is very cloudy on Earth but not where you are. Since 1958 springtime in the Northern hemisphere has been best for me in evening.

Here is a pretty bad photo I took on May 25th of last year with a hand held camera at the eyepiece of a 6" f/7 scope 1/2 second exposure.

 

DSC00808.JPG


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#6 Tom Glenn

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Posted 14 January 2021 - 03:48 PM

James is correct regarding the seasons and angle of the ecliptic.  In practice, variability in conditions makes it impossible to predict when you best images will occur.  Transparency plays an important role not normally a factor in lunar imaging.  We are currently moving towards a season in which the waxing crescent Moon will provide good Earthshine images.  I took the image below two years ago in February, at about 15% illumination.  

 

https://www.cloudyni...ebruary-8-2019/

 

A smaller crescent will have more Earthshine, and less flare from the sunlit side, but it will also be lower in the sky, and the sky will be brighter.  Probably something between 10% and 20% illuminated will consistently provide good opportunities.  To maximize Earthshine details, the sunlit side will necessarily be overexposed.  You can (and I have posted many times) pull out faint hints of Earthshine details from a "normal" exposure, but this will not provide many details.  There are perhaps 10-12 stops difference in exposure between the sunlit and Earthlit sides of the Moon, depending on the phase.  The overexposed sunlit side will always flare out into the Earthshine side, obscuring details near the terminator.  This is more limited at narrow crescent phases.  You also have to be mindful of the fact that many "Earthshine" images you see on the web are in fact composites from different dates, and therefore are simulated images that don't represent the Moon at any actual moment in time, nor in what is possible from a single capture.  And some of these simulated images don't use Earthshine at all, but a reduced opacity Full Moon in the background.  Unfortunately, not all images are associated with a description of the methods used, and can perpetuate some misunderstandings about what is possible.  


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#7 Borodog

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Posted 14 January 2021 - 11:48 PM

Good discussion. As for imaging Earthshine without blowing out the illuminated limb, I have a plan. If it works, I'll post it here. If it doesn't work, I'll post it here anyway.


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#8 Borodog

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Posted 15 January 2021 - 12:06 AM

Tom, I took the liberty of practicing my intended technique on your excellent images as a proof of concept. It's a bit rough and ready, but it will definitely work. The idea is simply to make an HDR composite of 2 images at two different exposures, just like your two images. If we can stack a thousand frames to make an image, I figure stacking 2 is fair game.

 

 

Attached Thumbnails

  • Moon_HDR_practice.jpg


#9 Tom Glenn

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Posted 15 January 2021 - 04:10 AM

Tom, I took the liberty of practicing my intended technique on your excellent images as a proof of concept. It's a bit rough and ready, but it will definitely work. The idea is simply to make an HDR composite of 2 images at two different exposures, just like your two images. If we can stack a thousand frames to make an image, I figure stacking 2 is fair game.

HDR is certainly fair game, although this is quite tricky on the Moon, and to be honest, I find very few examples of HDR Earthshine images that I think look very good.  A general problem with HDR is that the image becomes very flat, without contrast, because you are trying to compress too many tones into too few spaces.  Your rendition above for example, suffers from this, with the sunlit side of the Moon not having nearly enough brightness (or contrast) compared to the Earthlit side.  I have come across a few examples of very nice HDR images that have used half a dozen or more exposures blended with tone mapping software.  But even these generally have defects along the terminator, and so have to be reproduced at very small image scales to look good.  And there are some images out there that look quite good at first glance, but that I wouldn't consider HDR at all, because they are actually composited across different dates, and so fall under more of a definition of simulated images (albeit with real data) rather than HDR.  Not that there's anything wrong with that.  But I guess my point is that it is surprisingly difficult to render something that looks anything like what we see by eye, which is a testament to the processing power of the human brain.  


Edited by Tom Glenn, 15 January 2021 - 04:20 AM.

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#10 james7ca

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Posted 15 January 2021 - 05:37 AM

Imaging the crescent with earthshine can be done while the sky is still blue, very shortly after sunset. I kind of like that effect because you can capture the blue sky, the sunlit portion of the moon, and enough earthshine to show (faintly) some of the larger features on the moon (like the maria). Then, if you wait until it gets a little darker you can record stars along with the moon.

 

Here are examples of both techniques:

 

  https://www.cloudyni.../#entry10661572

 

  https://www.cloudyni.../#entry10280288

 

And here is an HDR that captured both the dark side and the illuminated crescent:

 

  https://www.cloudyni...e/#entry7736169


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#11 Borodog

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Posted 15 January 2021 - 09:56 AM

HDR is certainly fair game, although this is quite tricky on the Moon, and to be honest, I find very few examples of HDR Earthshine images that I think look very good.  A general problem with HDR is that the image becomes very flat, without contrast, because you are trying to compress too many tones into too few spaces.  Your rendition above for example, suffers from this, with the sunlit side of the Moon not having nearly enough brightness (or contrast) compared to the Earthlit side.  I have come across a few examples of very nice HDR images that have used half a dozen or more exposures blended with tone mapping software.  But even these generally have defects along the terminator, and so have to be reproduced at very small image scales to look good.  And there are some images out there that look quite good at first glance, but that I wouldn't consider HDR at all, because they are actually composited across different dates, and so fall under more of a definition of simulated images (albeit with real data) rather than HDR.  Not that there's anything wrong with that.  But I guess my point is that it is surprisingly difficult to render something that looks anything like what we see by eye, which is a testament to the processing power of the human brain.  

Most of what we do in taking pictures of objects in the sky is make things that don't look like they do when we look at them by eye. We magnify them, we color saturate them, we integrate up thousands of times more light than our eyes can pick up, we stretch them, poke them, prod them, and otherwise massage them until they look pleasing to us.

 

HDR does indeed compress the dynamic range; that's exactly what HDR is for. It's the only thing that it's for, and it's the only way to take the picture that I want: dynamic range compression. Otherwise the Earthshine is lost in the darkness and noise or the illuminated limb is blown out.

 

And as I said, my test with your images was rough and ready. I spent all of about 30 seconds making it. :O) I think what I will do if I can get a shot on the Moon (stupid trees) is actually shoot a whole range of exposures from the right exposure for the illuminated limb to the right exposure for the Earthshine and stack them all. That should hopefully help with the problem at the terminator that you mentioned, which is absolutely real and caused by the blowout in the illuminated limb capturing exposure. Masking of whites and blacks may help preserve some dynamic range as well, but that will take some experimentation, and my tools there are limited (I don't have photoshop and I know practically nothing about GIMP).

 

Regardless, I am sure I will get an image that I find aesthetically pleasing, and that's all I care about. :O)


Edited by Borodog, 15 January 2021 - 03:45 PM.


#12 Tom Glenn

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Posted 15 January 2021 - 03:07 PM

Most of what we do in taking pictures of objects in the sky is make things that don't look like they do when we look at them by eye. We magnify them, we color saturate them, we integrate up thousands of times more light than our eyes can pick up, we stretch them, poke them, prod them, and otherwise massage them until they look pleasing to us.

I agree, and I bolded your last statement that I think sums everything up.  It has to look pleasing.  Most HDR images I see of the Moon do not look pleasing (to me).  But if you generate one that looks pleasing to you (or your target audience), then by definition you have succeeded.  

 

 

HDR does indeed compress the dynamic range; that's exactly what HDR is for. It's the only thing that it's for, and it's the only way to take the picture that I want: dynamic range compression. 

Compression is required, of course.  In general, the purpose of HDR is to produce a composite photo that looks better than the original component images, because it looks more faithful to what we see by eye with an enhanced dynamic range.  If the HDR composite actually looks worse than the component images, then it has failed in its purpose.  And therein lies the challenge with the Moon. 


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#13 Borodog

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Posted 15 January 2021 - 03:48 PM

Won't happen tonight. Socked in with clouds, as predicted.



#14 aeroman4907

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Posted 15 January 2021 - 03:53 PM

Now having witnessed and studied a crescent Moon with Earthshine in binoculars, I will attempt to create such an image, although I will be utilizing the "simulated" approach as Tom mentioned above with data taken on different days.  It is a laborious process and I will post separately sometime in the future.

 

Steve


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#15 Borodog

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Posted 15 January 2021 - 04:19 PM

I look forward to seeing it! Personally I have no problem with images combining data from different days. People do it in deep sky AP all the time. ;O)


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#16 Tom Glenn

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Posted 15 January 2021 - 05:14 PM

I look forward to seeing it! Personally I have no problem with images combining data from different days. People do it in deep sky AP all the time. ;O)

There's no problem with any of it.  The only problem would be if someone was passing off an image as representing something that it wasn't.  The main difference in DSO imaging is that the objects don't change over time.  You can combine data over many years.  The Moon is constantly changing phase and libration, and so combining data from different days is more problematic.  And then, some people take a Full Moon and reduce the opacity and layer it behind a crescent Moon to simulate Earthshine.  There is nothing wrong with doing this, again assuming it is correctly represented, but the composite image then becomes an artistic creation, arguably more so than the average astro photo.  But again, the main problem is if images like this get passed around online, as they sometimes do, with misrepresented methods, and so people aren't aware of the methods used.  Somewhat like superimposing a star field behind a landscape that wasn't actually captured as such.  At best, if these are properly labeled as "impossible" composites then people can decide for themselves whether they like the work.  At worst, such images can become fraud.  But fortunately that is somewhat rare (although when it does happen it's a high profile disaster, like what happened a few years ago with National Geographic).  


Edited by Tom Glenn, 15 January 2021 - 05:16 PM.



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