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8.5% crescent Moon, April 25, 2020

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#1 Tom Glenn

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Posted 26 April 2020 - 04:08 AM

Each time the crescent Moon comes around, I'm tempted with a challenge of trying to capture both the night and day sides of the Moon in one exposure, without any HDR blends or masks.  It turns out that one of the trickiest aspects of this is simply one of logistics.  I don't have a good view of the Moon once it drops below about 25 degrees, and for the narrow crescents, this means I lose sight of it before the sky gets totally dark.  Tonight, for example, the result would have been much better if I could have imaged about 30 minutes later than I did, but it was not possible.  So this will have to do.  Some considerable downsizing is required so that the image becomes presentable, but I do like that details on both the night and day sides are visible, without any pixel saturation.  To see the faint details on the night side, you may have to dim the lights and crank your monitor, but they are there.  The camera has no trouble capturing the necessary dynamic range in one exposure, at least at this phase, contrary to what you may read elsewhere about this being impossible.  This is also only an 8 bit recording, with 1500 frames stacked.  Exposure was 18ms with 44% gain on the ASI183mm, using a 742nm IR filter, good for an 81% histogram fill.  Scope was a 6" TPO Newtonian, imaged at f/6, 916mm focal length, with a Baader MPCC Mark III coma corrector.  The image is downsized and cropped, but the background is completely intact (no background subtraction).  

 

Moon-04-25-20-TG.jpg


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#2 james7ca

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Posted 26 April 2020 - 06:25 AM

That looks quite good. On the dark side I can definitely see Aristarchus and Grimaldi and a few more maria and also probably Copernicus and Tycho. I can also see a little bit of a halo between the sky background and the sunlit edge of the moon (one fairly narrow and close to the moon and another fainter glow that extends further into the sky). That said, the dark globe of the moon is nicely apparent against the background sky so I think you have a good balance between the dark sky, the dark side of the moon, and the sunlit limb of moon with only a little flare and scatter on the bright limb.

 

Thanks for posting.



#3 aeroman4907

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Posted 26 April 2020 - 08:45 AM

Well done Tom.  I've gone back to a few of my images this past week to see what I could pull from the shadows during a few crescent moon phases.  The best I have been able to see is the edge of the moon in Earthshine, but nothing else.  They are just portions of the moon because I imaged with the 8" scope and my 183 camera, so FOV is limited.  Also, I didn't really try to expose to catch both, but I can tell that is a pretty good feat you have managed.  Well done!  My monitor is good at not crushing blacks, so I can readily see all the different areas in shadows quite easily.  An image like this doesn't readily print, but it does make for an interesting study on a computer monitor.



#4 sunnyday

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Posted 26 April 2020 - 08:47 AM

very successful, very clear.
thank you.



#5 Tom Glenn

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Posted 26 April 2020 - 04:57 PM

Thanks for the comments.  Yes, I think Steve stated things correctly when he said this was an "interesting study", as it's not meant to be much more than that.  Some cosmetic cleanup could be done, of course, as I left the background unaltered to better demonstrate the raw data.  But some masking could be done.  Also, if the goal is to really dig deep into the data and see what lies at the bottom, you can crank the exposure up.  Shown below are two versions of the same image.  The first is actually darkened a bit, and the sky has been evened out.  The second version is a 5 stop exposure increase in Photoshop (on top of what was already done), plus a tweak to the black level.  The detail on the night side becomes evident.  Interestingly, at least to me, is that all of this information lies at the very left of the histogram, and is indistinguishable on a 0-255 histogram, yet becomes visible with the appropriate pushing and pulling.  The current CMOS sensors have a very good ability to lift data up from the very bottom.  They essentially act in an opposite fashion to film, which does very poorly in deep shadows, but is very resistant to overexposure, as I discussed in my Apollo film thread.  CMOS sensors, however, are completely unforgiving of saturation.  

 

Moon-04-25-20-TG-v2.jpg

 

Moon-04-25-20-TG-v3.jpg



#6 lakeorion

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Posted 27 April 2020 - 03:21 PM

Image01.jpg

A non linear stretch.  PixInsight Arcsinh Stretch 6.0x.  Doesn't blow out the highlights.

 

I saw it in the sky that night, with Venus relatively close - I was too lazy to wheel out the equipment to give it a try.


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#7 Tom Glenn

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Posted 27 April 2020 - 03:35 PM

A non linear stretch.  PixInsight Arcsinh Stretch 6.0x.  Doesn't blow out the highlights.

 

I saw it in the sky that night, with Venus relatively close - I was too lazy to wheel out the equipment to give it a try.

Thanks, that looks pretty good.  I think it's always tricky to decide upon a display for these types of images.  I like your version, although I think I would downsize by 50% and add some blur to blunt the noise on the night side.  Also, I adjusted the gamma down a bit, because usually we don't see quite so much detail by eye, although it's always fun to play around with these variables.  HDR images (even if only from a single exposure like this) are always nearly impossible to get "right", as it depends not only on the monitor settings, but also personal  perception.  Looks good though.  I've played around with the free trial version of PI before, but could never justify purchasing because I don't do any DSO imaging.  Thanks for your input! 

 

Moon-04-25-20-TG-v4.jpg



#8 james7ca

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Posted 28 April 2020 - 12:04 AM

These are all good and interesting results, but this is also the best time of the year to image a thin crescent moon like shown above (since it is higher in the sky after sunset). Plus, since I live fairly close to Tom I know that on the night he took those shots the sky was very clear.

 

So, you may not get as good of results during other times of the year unless you happen to live in a location that has unusually clear and transparent skies (like on a mountain top). Plus, as the moon phase grows it gets more and more difficult to record earthshine.

 

I do wonder, however, what would be the absolute best setup to record the earthshine. I think you need an optical system that has very little light scatter and flare. That suggests a refractor with very good coatings. Maybe a two-element ED scope without a flattener or reducer. But, I don't know whether you could get a large enough field to cover the entire moon without using some kind of flattener. I know that my Tele Vue NP127is can easily cover a field as large as the moon (using its native four-element optical system), but prior experience shows some fairly significant flare when imaging a bright moon. However, my Stellarvue SV80ST2 does better and that's a three-element APO.

 

With clean optics Tom's Newtonian might be a decent tool for this job (as he has obviously shown), but probably not as well as a good refractor.

 

As always, an interesting topic by Tom.


Edited by james7ca, 28 April 2020 - 12:21 AM.


#9 Tom Glenn

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Posted 28 April 2020 - 01:23 AM

James, thanks for the additional comments.  My natural inclination would be that a refractor would be better, because of no secondary mirror housing to scatter and reflect light.  And my Newtonian doesn't have any baffles on the inside, and I know many refractors do. But your comments about your refractors indicates that it really would be a case by case issue with the coatings and number of glass elements.  Also, I've previously posted an image with a shadow that I later discovered was the secondary mirror.  That link here.  That moon phase was very similar to this one, and the other components were identical, including scope, camera, and filter.  The only significant difference I can think of between the two captures is the gain.  For my image in the other thread, I used a gain of 70, which is only 16%.  This time around, I used a gain of 200, which is 44%.  Why this would effect the ability of the secondary shadow to be detected is unclear, but perhaps it was somewhat like the banding artifacts that we can see in some low gain images, when very dark regions are stretched to the extreme in processing.  It is perhaps counterintuitive to use higher gain for a shot that has higher dynamic range, but what I've discovered is that the dynamic range is not as high as we think, typically less than 10 stops for a thin crescent Moon.  The Earthshine at this phase is actually slightly brighter than during many total lunar eclipses (I made an estimate from photos that I took in January of 2019 that there were about 12 stops difference between the uneclipsed Moon and during the total eclipse....I had ISO 100 exposures of about 4s during the eclipse, and 1/1000s for normal full moon).  So with dynamic range not really being that much of the issue with these Earthshine shots, the difficulty has more to do with poor sensitivity to tiny differences between very low levels of light, and as such, seems to benefit from higher gain because the sampling (e-/ADU) is better.  Stacking the 8 bit frames with higher gain actually produced a better result than my 12 bit stack at lower gain, probably because at the higher gain I was sampling at under 0.5e-/ADU, so-sub electron precision, which would produce less banding.  It's a tricky issue though, because for most "normal" lunar imaging I prefer to use lower gain and longer exposures (in good seeing), to get better SNR in the brighter regions, because those are the more meaningful parts of the image, and I'm not ordinarily going to try and represent Earthshine form the same data.  


Edited by Tom Glenn, 28 April 2020 - 01:28 AM.


#10 Tom Glenn

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Posted 29 April 2020 - 02:00 AM

Here is an interesting paper analyzing Earthshine as a potential source of illumination for scientific exploration.  

 

https://arxiv.org/pdf/1904.00236.pdf

Icarus Volume 321, 15 March 2019, Pages 841-856

 

As stated in the conclusion of the paper:

 

"At most wavelengths, Earth would appear very bright to a lunar observer and could serve as a natural illumination source for future optical measurements of the surface."

 

Also, my investigations of some Apollo image archives showed that several Earthshine photographs were taken by the astronauts using a Nikon 35mm film camera while in orbit.  One example of such, taken of the crater Riccioli by Apollo 16, is shown below.  Interestingly, you can clearly see shadows in the craters, despite this region being completely in lunar night.  Riccioli lies on the western limb of the Moon, and so light from Earth arrives at a very low angle, hence the shadow profile on the craters.  

 

AS16-127-20019_SML.jpg



#11 IR656nm

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Posted 02 May 2020 - 05:59 AM

21_06_15_g5_ap128-UM-PS1-R6-PS.jpg Hi Tom,

 

Nice shot of the moon ! I have been imaging sun and moon during lockdown with my ASI120mm-S. WIth a 70mm refractor (no barlow) - the moon on 26th April 2020. Clear skies, little wind. I think you have to do two exposures and blend those using separate layers to get the earthshine. Sometimes the earths atmosphere lights up the dark side of the moon, but with the clearer, less dusty polluted skies now that seems less apparent. If you do a long exposure to get the dark side of the moon, its not that difficult, but hard to get perfect, if you blend in the short and long exposure one. Here is one oversharpened (Unsharp mask, wavelets, levels, contrast etc) in Photoshop and Registax 6. 

 

https://astrobin/uyuwti/0/

 

 

Magnus

London


Edited by IR656nm, 02 May 2020 - 06:04 AM.


#12 Tom Glenn

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Posted 02 May 2020 - 11:26 AM

 I think you have to do two exposures and blend those using separate layers to get the earthshine. 

Thanks for your comments.  My post here, as well as others I have made, show that you don't need two exposures to detect Earthshine.  However, you are correct that you need to overexpose the sunlit portion of the Moon if you wish to obtain good details of the Earthshine side.  The post below shows one example I have made in the past.  

 

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



#13 IR656nm

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Posted 02 May 2020 - 12:02 PM

Thanks for your comments.  My post here, as well as others I have made, show that you don't need two exposures to detect Earthshine.  However, you are correct that you need to overexpose the sunlit portion of the Moon if you wish to obtain good details of the Earthshine side.  The post below shows one example I have made in the past.  

 

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

Hi, What I meant, is that in the past you can sometimes easily see the dark side of the moon visible with the naked eye; I think this depends on atmospheric conditions. Could be due to dust and other factors.  If you can see it with your eyes, it should be much easier to capture it with a camera. However, to get high quality, low noise, you probably need 2 separate exposures. Nowadays, maybe its harder because there is less pollution in the air.

 



#14 Tom Glenn

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Posted 02 May 2020 - 12:10 PM

Hi, What I meant, is that in the past you can sometimes easily see the dark side of the moon visible with the naked eye; I think this depends on atmospheric conditions. Could be due to dust and other factors.  If you can see it with your eyes, it should be much easier to capture it with a camera. However, to get high quality, low noise, you probably need 2 separate exposures. Nowadays, maybe its harder because there is less pollution in the air.

Actually, less pollution is helpful.  The Earthshine is caused by light from the Sun reflecting off Earth and illuminating the night side of the Moon.  The intensity of Earthshine is related to the phase of the Earth, as seen from the Moon, which is the reciprocal of the lunar phase that we see.  Earthshine is brightest during thin crescent phases.  But because the thin crescent Moon is close to the horizon at sunset, it is sometimes difficult to see.  Pollution and haze reduces our ability to see it.  Clear, clean air, actually helps significantly.  Additionally, Spring is the best season to look for Earthshine on a waxing crescent Moon, because the angle of the ecliptic is steep where it meets the horizon, so the crescent Moon is actually higher above the horizon shortly after sunset.  The opposite is true for the waning crescent Moon before the new moon, which is best viewed in the Fall (for the northern hemisphere).  


Edited by Tom Glenn, 02 May 2020 - 12:10 PM.


#15 lakeorion

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Posted 05 May 2020 - 03:21 PM

I've played around with the free trial version of PI before, but could never justify purchasing because I don't do any DSO imaging.  Thanks for your input! 

 

attachicon.gifMoon-04-25-20-TG-v4.jpg

Tom,  I greatly prefer PixInsight's Multiscale Linear Transform over other implementations of starlet sharpening.  Similar concept, far more flexible and real time preview windows to see the effect of each individual change.

Might be worth it for another trial.




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