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Lunar terminator sequence, 24 hours along the waning Moon

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#1 Tom Glenn

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Posted 31 May 2020 - 04:24 AM

This is a sequence of lunar images from unprocessed data that I recently rediscovered on my old hard drives.  Back in the summer of 2018, I was able to capture data for the entire Moon on two consecutive mornings in August, but the data remained unprocessed because at the time I was working with a lot of planetary data and didn't have enough time.  These days, imaging has been few and far between, so I have been able to go back and complete this data set.  The images come from August 2 and August 3, 2018, at approximately 13:00UT for each image.  Larger images are available through Flickr with the links below.  These are reduced to 70% of the original capture scale, due to personal preferences (IMO almost all images benefit from downsizing...the original capture scale almost always highlights the limitations in the optics).

 

https://flic.kr/p/2j7hdEX

https://flic.kr/p/2j7jRyR

 

The image below is an animated gif (you need to click), with poor quality due to forum restrictions, but which nevertheless nicely shows the change in appearance of the Moon over the course of 24 hours.  To create the animation, the two images were normalized for size due to the changing Earth-Moon distance, and were aligned to illustrate the changing libration.  What appears to be an image shift is actually the change in perspective from our Earth view.  If you look carefully, you will notice that the position of the limb does not change position, but the surface features appear to rotate, and this is because of a change in libration in both latitude and longitude.

 

Click to view animation:

 

Terminator-animation-libration-TG.gif

 

What follows below is a series of cropped images, each at 50% image scale, showing the progression of the terminator in a given field of view over 24 hours.  You will have to follow the original links for higher resolution images of each Moon, but the images below show up nicely at forum size (with no pixel peeping.....for that follow the links above).  


Edited by Tom Glenn, 31 May 2020 - 04:28 AM.

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#2 Tom Glenn

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Posted 31 May 2020 - 04:25 AM

crop1-08-02-18-08-03-18-TG.jpg


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#3 Tom Glenn

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Posted 31 May 2020 - 04:25 AM

crop2-08-02-18-08-03-18-TG.jpg


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#4 Tom Glenn

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Posted 31 May 2020 - 04:26 AM

crop3-08-02-18-08-03-18-TG.jpg


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#5 Tom Glenn

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Posted 31 May 2020 - 04:26 AM

crop4-08-02-18-08-03-18-TG.jpg


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#6 Tom Glenn

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Posted 31 May 2020 - 04:27 AM

crop5-08-02-18-08-03-18-TG.jpg


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#7 aeroman4907

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Posted 31 May 2020 - 10:33 AM

Very nicely captured and processed images Tom, as usual!  I find the amount of libration in one day very interesting.

 

On another note, would you mind posting a native resolution image from this series if you still have one?  I'd be interested in seeing how you come up with that amount of downsizing you end up using.

 

Steve



#8 sunnyday

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Posted 31 May 2020 - 10:35 AM

good work tom .

verry sharps images , thanks 



#9 Tom Glenn

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Posted 31 May 2020 - 08:33 PM

Thanks for the comments and likes.  

 

 

 

On another note, would you mind posting a native resolution image from this series if you still have one?  I'd be interested in seeing how you come up with that amount of downsizing you end up using.

 

Steve, the amount of downsizing is completely random, and actually, I could have done without any, but I don't like the field of view being so restricted when people click on the "original size" image.  Essentially it is too "zoomed in".  This doesn't matter for anyone who downloads the image and opens it in whatever way they want, but I like things to look as good as possible with a minimal amount of work for viewers, because the viewer shouldn't have to optimize anything.  Although admittedly, this is impossible anyway, for the reason described below. 

 

I've noticed that despite the fact that high resolution monitors are now common, images are not displayed at 1:1 pixel ratios.  For example, my MacBook Pro Retina display is 2880x1800 pixels.  If a large image is displayed with 1:1 pixel mapping from image to screen, it looks amazing.  Sadly, that's not the way images are displayed by default.  Apple lets you choose the screen resolution, but in all cases the screen is rendered to simulate a lower resolution screen.  They do this because text and graphics would be tiny if web pages displayed at 1:1.  This does give a realistic look at what lower resolution monitors will show.  Those monitors might actually be displaying true 1:1 pixel images, but because they have larger pixels, the image is very zoomed in and looks poor.  So, downsizing the image helps keep the panorama from becoming overly zoomed in when opening the "full resolution" image.  So really, this is simply a problem with web display.  Flickr is particularly bad, as are most of the other photo sharing sites.  There aren't many good ways to showcase 10,000 pixel images on the web, but a smooth pan and zoom feature would be ideal, similar to what the LRO Quickmap offers.  But ultimately, I chose the 70% scale because it retained most detail, but kept the size under control.  Honestly, a 50% scale looks even better on low resolution monitors (or my MacBook's simulation of such), but then you do start to lose significant detail.  So it's a bit of a game.  Also worth noting is that the same image looks different depending on which program you use to view it (on the web browser, Photoshop, Preview, etc), because they all resample the image to different degrees.  

 

The other issue here, although of lesser significance, is that when we capture images at the diffraction limits of our equipment, we are also fully sampling diffraction artifacts, as well as a host of other things that conspire to degrade the image.  Notably, if you look at the image scale produced by the Narrow Angle Cameras (NACs) onboard the Lunar Reconnaissance Orbiter (LRO), you find some interesting information.  Each NAC utilizes a Ritchey-Chretien telescope of focal length 700mm, aperture 198mm, f/3.5.  The pixels of its digital camera are 7um.  Now, if you follow the general guidelines of amateur imaging, you would image 7um pixels at f/35, which is 10x higher than what the LRO is doing.  Imaging at f/3.5 gives them very clean images.  The resolution of those images comes from the extreme proximity to the Moon, not from the power of the telescope used.  Unfortunately, there are no real lessons we can apply here as amateurs, because we have no choice but to image at longer focal lengths.  But it is true that downsampling images can make them cleaner, at the expense of resolution.  I've mentioned this before in the context of LRO images, in which downsampled WAC mosaics that produce similar image scales to modest amateur images have far more detail than those images.  

 

All of that said, my decision was arbitrary and not based upon any specific measurements.  Below are 100% reproductions of a section of each image.  They look amazing at a true 100% rendering on my monitor, but slightly too large at the "fake 100%" scale that is typically shown on the web.  The extra pixels aren't very helpful for web display, but would come in handy for making large prints at 300dpi.  

 

08-02-2018-TG-Plato-100percent.jpg


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#10 Tom Glenn

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Posted 31 May 2020 - 08:34 PM

08-03-2018-TG-Plato-100percent.jpg


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#11 lakeorion

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Posted 31 May 2020 - 08:56 PM


The resolution of those images comes from the extreme proximity to the Moon, not from the power of the telescope used.

Yeah, they cheat.  Also don't have to worry about seeing.



#12 Tom Glenn

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Posted 01 June 2020 - 02:32 AM

Very nicely captured and processed images Tom, as usual!  I find the amount of libration in one day very interesting.

 

Thanks, Steve.  Yes, the libration is fascinating, and can be observed in very short time intervals.  Also apparent in the raw images was the difference in apparent size between the two images, with the Moon approaching closer to the Earth during this interval (for the animation I normalized this, however).  There are a number of ways that one could choose to align the images to create an animation.  The NASA "Dial a Moon" simulator (link here) shows how the Moon would appear to a fictitious geocentric observer as it crosses the local meridian at the indicated time.  This takes into account the position angle, which effectively describes whether the polar axis of the Moon is tilted to the left or right of vertical when you look at the Moon.  This angle, relative to the phase, changes with the seasons.  For my animations, I attempted to align the visible profile of the sunlit limb, which then creates an appearance of rotation.  Between the first and second images (24 hours apart), the libration in latitude and longitude each changed by less than 0.5 degrees, but this is enough to create a visible effect.  

 

Shown below is a rotated version of the same animation.  Between the August 2 and August 3 image, the libration was increasing to north and to the west, which effectively causes surface features on the Moon to appear to rotate to the south and east, or down and to the left in this animation.  This is approaching the best possible scenario in which to observe features on the far western limb, or near the North Pole.  In the next post, I will show a cropped region of the animation focusing on Mare Orientale, which was well placed on this occasion.  

 

Click for animation: 

 

Terminator-animation-libration-TG-horizontal.gif


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#13 Tom Glenn

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Posted 01 June 2020 - 02:39 AM

Here is a closer look at the Mare Orientale region.  For anyone who wants a bit more description of this region, see my previous post on the topic.  Sorry for the small size and poor quality of the animation, but significant modifications have to be made to compress this into 500kb.  What may appear as an image shift is actually an apparent rotation, which can easily be observed near the limb.  The mountains on the limb, and Mare Orientale itself, are rotating into view, ever so slightly.  This represent less than 0.5 degrees of apparent rotation, but on a larger scale, with libration in longitude ranging from approximately +/- 8.5 degrees, you can easily see how this dramatically affects the visibility of Mare Orientale.  Note that I altered the gamma in this cropped animation to better highlight the mare regions.  I should also add that in all of these animations, I attempted to process the images in a way that consistently reflects the tonal changes that occur during the lunation.  This is actually quite difficult, as each image is independently captured, and therefore reflects a different set of conditions.  

 

Click for animation:

 

Mare-Orientale-animation-libration-TG.gif


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#14 aeroman4907

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Posted 01 June 2020 - 06:54 AM

Thanks for the comments and likes.  

 

 

Steve, the amount of downsizing is completely random, and actually, I could have done without any, but I don't like the field of view being so restricted when people click on the "original size" image.  Essentially it is too "zoomed in".  This doesn't matter for anyone who downloads the image and opens it in whatever way they want, but I like things to look as good as possible with a minimal amount of work for viewers, because the viewer shouldn't have to optimize anything.  Although admittedly, this is impossible anyway, for the reason described below. 

 

All of that said, my decision was arbitrary and not based upon any specific measurements.  Below are 100% reproductions of a section of each image.  They look amazing at a true 100% rendering on my monitor, but slightly too large at the "fake 100%" scale that is typically shown on the web.  The extra pixels aren't very helpful for web display, but would come in handy for making large prints at 300dpi.  

Tom, thanks for posting the native resolution portions of these images.  I am glad you retained the original full size image even if not posted here or even on Flickr.  I see your perspective was based upon people using other types of displays rather than limitations of your images.  When I was looking at the 8-3-18 'full' sized image on Flickr, I could tell there was more resolution captured than presented.  I have a '4k' monitor which is great for viewing at high resolution with large areas still visible.  My general rule for evaluating images (my own or others) is if it can withstand the 200% zoomed view in PS and still look respectably sharp.  This is the pixel peeping level, so some slight loss in sharpness is acceptable to me at this level of zoom.  Viewed at 100% zoom, of course the image is tack sharp.  I would also say the images with this criteria also look tack sharp at 150% zoom.

 

When I do my own evaluation of what is a proper presentation resolution for an image, I use the mouse wheel to zoom in and out of an image.  If I naturally want to zoom in to see more details, this typically means that focus is still rendered adequately sharp.  For images that are presented at too high a resolution, I need to zoom out.  When I know my images don't make the 200% zoom test, I zoom out to establish when the image begins to look acceptably sharp.  I then resize my image accordingly.

 

For images that are sharp and are presented at a resolution lower than what they 'deserve', I find myself zooming in further, possibly 300% or 400% because my eye registers the image as sharp and I want to see more.  This was indeed the case regarding the images you posted here and on Flickr.  One other dead giveaway to me that the images were more resolute than you initially presented was the signs of resolving the doublet crater in Plato (see below).

 

Great job and thanks for the explanation!

 

Doublet-Crater.jpg


Edited by aeroman4907, 01 June 2020 - 06:55 AM.

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#15 james7ca

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Posted 01 June 2020 - 01:25 PM

The image showing the animation of the lunar limb (post #13) gives a definite 3D feel to the mountains along the very edge of the moon. Well done.



#16 Tom Glenn

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Posted 01 June 2020 - 04:46 PM

James and Steve, thanks for the additional nice comments.  Much appreciated!




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