Telescope Reviews: Wake up for Archimedes

Dear Timo (& Stephen),

Thank you for your enthusiasm! I'm not entirely sure this is the appropriate place to discuss details of a software program, but I'll try to answer your questions (slightly superseded, now, by your success in calibrating your own image -- but perhaps for the benefit of others trying to do the same).

(1) I'm pleased you were able to follow the steps required to make the CLA plates available on your hard drive, but sorry it took so much time and effort. I have some utility programs that are helpful of automatically downloading sequentially numbered images from a website -- so all you have to do is push a button and wait -- and the IrfanView software mentioned in the notes as a handy batch conversion utility that will convert all the images from TIFF to JPG or BMP in a single step (LTVT only knows how to open images in the latter two formats).

(2) The LTVT "Zoom" factor indicates how many windows of the display size (641 pixels wide) would be required to cover the Moon's diameter. As you note, many CLA plates (taken with a 62 inch reflector) hold up well at a Zoom of 20. Many modern amateur photos taken with apertures of 8 inches or so are a bit better, and some of the best taken with larger apertures hold up to a zoom of 50 or more. To go beyond that you want to look at the images taken by spacecraft. The survey images taken by Lunar Orbiter IV, from a distance of about 2000 km, tend to look good at a zoom of 150 or so; while some of those taken at much closer range (<100 km) or other missions can tolerate a zoom of many 1000's, showing very small boulders on the Moon's surface.

(3) Congratulations on locating your "saucer" in CLA Plate B9. The LTVT display has an adjustable "Gamma" value, and setting it to a high number (2 or 3) sometimes helps to bring out detail in the shadows, but not as well as you have done here. In general, the 8-bit format of the images LTVT can read (256 shades of gray) is a distinct limitation. Images captured with greater bit depth have considerably greater flexibility, but LTVT doesn't know how to read them.

(4) Regarding the calibration of your own photos, I'm not sure if you noticed that LTVT has a rather extensive help file that can be accessed either separately (by double-clicking on the Help File icon in the LTVT folder) or from within the program. If you go to "Files...Calibrate a user photo..." and (with the window open) press the "F1" key on your keyboard you should see a fairly detailed explanation of the steps involved (which are actually quite simple). A few users have reported an inability to see the help pages. This seems to be related to their security settings. You may have to manually click on "LTVT_UserGuide.chm" and assure your computer that it's an OK file to open (the extension ".chm" means it's a Microsoft Compiled Help File -- something like a set of off-line webpages).

The required information is the date, time, and location on Earth from which the photo was taken. This is required so that LTVT can know the expected geometry of the Moon as it would have been seen and photographed in the sky. You then have to manually identify the pixel locations of two known points on the Moon and save your result. From this LTVT can determine the scale and orientation of your image relative to the expected one. See also the (slightly out-of-date) User's Guide under Manuals on Henrik's download page. If, after reading the help file, the procedure is still unclear, please let me know. It's not intended to be difficult.

(5) As explained in the help file, you will obtain the most accurate calibrations if you take the time to download and use the optional "1994 ULCN" dot file. That file lets you superimpose the positions of very accurately known control points on the simulated view of the Moon, and it's usually easy to pick two of them as the calibration points for your image. If you superimpose the 1994 ULCN points on one of the "calibrated" CLA plates you will see that most don't line up too well. This is due to some kind of distortion (of unknown origin) in the on-line (and possibly the published) versions of the CLA plates. It's as if someone printed them on curved sheets of paper. Most amateur images (especially those taken with a camera at the focal plane -- as opposed to ones taken afocally through the eyepiece) calibrate much better than the CLA plates, but even with their distortions they provide handy views of the entire nearside at moderate resolution and a variety of sun angles.

(6) Sun angle is the altitude of the center of the sun above the local horizontal as it would be seen by an observer on the Moon's surface at the place where you're pointing the mouse. There is also an "azimuth" involved, which is the clock angle to the Sun from lunar north. The sun angle goes to zero at the theoretical terminator (midway between the red and blue lines), meaning that an observer there would be see the sun at 90° from the zenith, and in some direction indicated by the azimuth. That actual point on the surface may be in light or shadow depending on whether a Sun in that direction and at zero altitude would be visible or not (it might well be hidden by distant terrain). On the sunlit side of the Moon the sun angle is above zero (Sun above horizon) and on the dark side it is negative (Sun below horizon). Each point on the Moon has a specific sun angle at which the Sun rises and sets. There is no way to say the sun angle for an entire photo is "xxx". The value will be different at each point. So you have to say "I saw this when the sun angle at xx West/ yy North was zz degrees" (the azimuth part is usually rather unimportant because it tends to be about the same whenever a given location is viewed at a particular sun angle).

(7) For measuring the height of a mountain based on the shadow it casts you first want to make sure you are displaying the photo with the actual illumination under which it was taken. To do so, when loading the photo make sure you select at least the "Overwrite sun angle" option. This will copy the correct values to the "Sub-solar Point" boxes in the main display window, which is very important for correct shadow measurements. In the example you have posted here the caption says you are displaying it in the geometry calculated for your location on May 12, 2008, but it looks like you have managed to manually enter the correct Sun's position ("sub-solar point") for CLA Plate B9 -- so everything is OK (although the software is supposed to suppress the note at the bottom if anything has been altered from the computed values). The next step is to right-click on the image and select "Help" in the pop-up menu. Follow the link to the explanation of "Mouse Options". There are two basic modes for obtaining height differences from shadow lengths:

(a) The "direct" mode measures from the object casting the shadow to the shadow tip.
(b) The "inverse" mode measures from the shadow tip back to the shadow-casting object.

In both modes you initiate the measurement by right-clicking on the appropriate point (shadow-casting object OR shadow tip). The software will then automatically draw a red either in the expected direction of the shadow or in the expected direction back from the shadow tip (the red line is simply the direction of the Sun's rays as they would be seen projected against the Moon and captured on the "film"). As you move the mouse along the line, the software will tell you how far you have moved and what height difference would be required for the shadow to end/start at the place you are pointing to. If you point at the correct endpoint you should have the correct height difference, and by moving the mouse a little you can estimate how much uncertainty there is in the result. In mode "a", the shadow-casting feature is the last bright pixel along the line, and the shadow ends where you encounter the next bright pixel (after an interval of continuous black). In mode "b", the shadow tip is the first dark pixel along the line, and again, the shadow ends where you encounter the first bright pixel (which must be the shadow-producing object).

Important note: The current release of LTVT (version 0_19_1) contains an error which causes the red shadow line to be drawn in the wrong direction if the image is displayed with the "Invert left-right" option selected. I would recommend that you avoid using the "Invert" options if you want to measure height differences using the shadow tool. You can obtain the orientation shown in your attachment simply by setting the "Rot" angle in the main window to 180 (degrees) without using the "Invert" options.

I rarely use the inverted viewing modes myself, so I hadn't noticed this until recently. It must be very frustrating (and confusing) for those trying to measure shadows on inverted images! I'll try to look into what the problem is (early versions worked OK) and fix it in the next release.

I hope this answers your questions,

Jim

P.S.: I've been toying with the idea of creating a free Wiki page for LTVT (and actually "registered" the name yesterday). That might permit some more clearly illustrated and current examples with user interaction, but it's unclear to me there's enough interest in the program to justify the effort.

Hi Jim,
thank you – again – for your most thorough answer!

Also thanks for the info on the additional Lunar texture you provided on the discussions over the Stephen's Rupes Altai pic.

But first I would like to set the record straight about the subsolar point automatic marking of the LTVT on the printout pic. That was my mistake. I was not possibly able to change it manually, as I only now (shame on me) start to learn about these issues at all. So, I simply did not change the illumination conditions data on the program, and therefore it is the same for both of the pics (but fortunately they are very close regarding the terminator, which let me off the hook, I guess).

I would have had to change the ephemeris for the program – to give the exact time of the shooting of the CLA reference plate (which is demonstrated on the photos page as info) – to get the exact (theoretical) terminator (provided the subsolar point on that ephemeris). – Having already loaded the required JPL ephemeris files (all of them, dating back to 1609) this would have been easily done. Then I would have got the printout on the CLA with correct info, I guess. And then I would have repeated the procedure on the correct ephemeris of my own pic on that “Patera Obscurum” saucer.

--- Attached is the CLA b9 that is corrected with regard to the ephemeris 28.4.1966 at 2.37 UT, the angular diameter of the Moon being 1962.6 arc seconds and 49.43 per cent of the Moon being illuminated. Colongitude on that would place the terminator 3.120 degrees west of the Sinus Medii central meridian. The Sun angle on the tip of the Promontorium Agassiz would have been 3.37 degrees.

My photo was taken on the ephemeris 12th of May 2008 at 20.43 UT. Moon's angular diameter was 1871,9 arc-seconds, and 57.5 per cent of the Moon was illuminated.

Given the colongitude on the ephemeris of my pic, the terminator was 1.508 degrees west of the Sinus Medii central meridian. The Sun angle on the tip of the Promontorium Agassiz was 3.49 degrees at azimuth of 91.08 degrees.

The difference between the terminator location of the two pics would be (in colongitude) 3.120 - 1.508 degrees, that is 1.612 degrees. That would mean that my pic is taken about... (1.612 difference in colongitude / 0.5079 colongitude movement per hour) ... i.e about 3.17 hours earlier?

The Sun angle being (on my photo) measured by the LTVT on the Promontorium Agassiz tip at 3.49 degrees, versus the 3.37 degrees on the CLA B9 (three hours later), the shadows are that much longer as the Sun is lower...

Hope I did not mess things up in this. - And can I measure the Sun angle from the mountaintop, actually?

Incidently, regarding the illumination of the terminator zone on the CLA reference plates, many of them seem to be quite much on the dark side. Also the twisting of the pics you mentioned may come from the same source…

Namely, the downloaded TIFF –file info tells that the pics are shot with Cybershot camera!! This was a big surprise to me, if it really is so. This (afocal) shooting would bring along two distinctive effects.

The most apparent would be (for an expert like you to notice) the systematic (or random) tilting of the pics axis – as if the plates were curved somehow. I don’t know how controlled a procedure it was when these plates were shot, if it really was done by camera. To elaborate upon the effects one should know whether there was a measured exact distance between the camera and the plates all the time, and how the plates were accurately positioned for the digital reproduction to maintain this constantly enough.

The best way of accomplishing decent results would obviously have been through scanning the originals. – But is it really possible that these were not scanned – but shot afocally? It is actually hard to believe. The second consequence for this would be, that each plate would have been interpreted by the camera with regard to grayscale adjusting, brightness and contrast. The fact that I was able to pull that much out of the CLA plate was surprising. That info must have been on the original plate, and the digital reproducing procedure has at some point simply hideen that pixel info of the lightning conditions on the terminator. The postprocessing brought that info back for us to see.

Learning all these things about selenography, it is actually funny how difficult it is to start to think to a certain degree independently of what one observes from the Earth’s surface. I think I start to understand why folks should always give the UT of the shooting on the photo they provide.

Despite I still have “a few things” to learn on selenographic coordinates, colongitude, Lunar equator and Sun’s selenographic longitude, in the following I would like to wrap up about what I learned going through the excellent pages you provided for. Those who are familiar with these, please stop reading…. NOW… (no... please check these ...)

As the Sun (due to the Earth-Moon system movements) is percepted to wander over the Moon skies, the Sun appears to shine from the Lunar equator. Only a shiftment (or an inclination) of 1.6 degrees north or south occurs. In angular diameter, this is only about 3 times the apparent angular diameter (0.5 degrees) of the Sun, as seen from the Moon. – By a coincidence, this is same as the apparent Lunar angular diameter observed from the Earth.

The location where the observer on the Moon would see the Sun shine directly above his or her head, is called the subsolar point. The location on the Moon, where the Sun is observed rising, is called the morning terminator.

As the sphere of the Moon is 360 degrees, having travelled the distance of 180 degrees, one would be on the location of the evening terminator, where the Sun would be observed setting. The (morning) terminator moves westward (towards decreasing selenographic latitude) along the Lunar surface about 12.191 degrees a day and 0.5079 degrees an hour.

This causes the phases of the Moon to be observed from the Earth. As the Moon is interlocked with the Earth (having only minor shiftments of its axis called libration, that can bring interesting features to be seen from the farside of the Moon), we cannot observe the Moon at all when complete shadow – or the 180 degrees darkness between the morning and evening terminators – falls completely over the nearside of the Moon.

The concept of colongitude tells where the morning terminator falls on the Moon. It is the location of the Sunrise on the Moon.

Colongitude is measured west from the central meridian (crossing Sinus Medii) of the Moon. Thus, colongitude is 90 degrees minus the selenographic longitude of the subsolar point (where the Sun is directly above the observer). Colongitude increases with time, being near 0 at the first quarter, near 90 at the full moon, near 180 at the last quarter and near 270 at the last quarter. The whole cycle of colongitude is 360 degrees, and it is called the synodic month.

For instance, if the Moon would have been observed (as in CLA plate e12), on the 19.1.1967 at 2.41 UT, the angular diameter of the Moon would have been (on that ephemeris) 1810.7 degrees, and 52.62 per cent of the Moon would have been illuminated. The colongitude would have been 7.130. This would mean, that the terminator of the rising Sun (morning terminator) would be located 7.130 degrees west of the Sinus Medii central meridian on the Moon.

A quite similar pic (of the crater Ammonius) would be taken (for instance) on the ephemeris 28.2.2004 at 18.50 UT. Then the angular diameter of the Moon would be 1794.6 degrees and 55.94 per cent of the Moon would have been illuminated. Colongitude would have been 7.667.

The difference beteween these two shots would be in colongitude 7.667 - 7.130, which is 0.537 degrees. Thus, the terminator would have moved about an hour westward, determining from that difference in colongitude - and knowing that the terminator moves about 0.5079 degrees an hour westward.

On the bank of Ammonius, then, the LTVT gives that the Sun angle would have been on the 19.1.1967 (at 2.41 UT) 6.58 degrees, and on the same location on the 28.2.2004 (at 18.50 UT) 7.11 degrees. Thus, the shadows would be about an hour "longer" in the 1967 photo as the Sun would be that much lower...

Now I also think I start to understand the meaning of the red and blue stripes on the LTVT, representing the position of the terminator on the Moon.

Between the lines must be the “theoretical terminator”, as you explained. One should actually say like “the Central peaks of Copernicus first become visible when the morning terminator at 10 degrees North is between 22.9 and 23.3 degrees West.”

You pointed out, that given the same colongitude (stating the locationg where the morning terminator is crossing the equator), the inclination of the Sun from the Lunar equator will cause counter-clockwise twist, when the Sun is north of the equator. The twist would be clockwise, when the Sun is south of the equator (withing the boundaries of 1.6 degrees into both directions). For the evening terminator, the twisting directions would be the opposite. Thus, as you wrote, “this twisting of the terminator can cause features north and south of the equator (that might otherwise be observed when the Moon is seen at the same colongitude in another lunation) to be thrown in or out of sunlight”.

Instead of colongitude, given the exact UT time of observing, the subsolar point becomes calculated. It is then possible (for the LTVT) to estimate correctly the Sun’s angle for different locations, actually according to the mouse movement on the calibrated map. The terminator must then “fall” between the red and blue lines, in the middle of which is the theoretical terminator. Where exactly this falls, is determined on the Sun’s inclination north or south from the Lunar equator (within the boundaries of 1.6 degrees).

So, if a “non-twisted”, decent pic is correctly calibrated into LTVT, the terminator – if I understand correctly – should “fall” somewhere between these two colored lines. Of course, many times a local terrain factor is playing in into this equation. Single or a set of illuminated high mountain peaks can be on the terminator, or a crater or lowlands of some sort can be on the shadows. Or maybe a mountain, obstructing the Sunlight and causing a shadow to cast from the Sun’s direction towards the terminator.

Speaking of shadows, that shadow tool you mentioned I have to try out! It is amazing how much one can find with just the right click of the mouse on this program!

This is a fantastic learning journey, which is only just beginning! I like to share my learning, so folks might get some ideas that would help to understand the selenography. – Hope those who already know these issues (and have read this far) can bear with me on this!

Jim, I would strongly vote for you establishing that Wikispace for the LTVT. It is a most powerful tool for a Lunar enthusiast interested in terminator issues. – And that batch-loader of the web pics would be nice to have downloadable there, too. I have to admit it came to my mind as an idea that somebody must have developed a batch tool – or something – for this purpose. You know the feeling of washing dishes manually – only there is the pleasure of warm water there. Downloading, my only pleasure was to glance through the pics, which actually was interesting.

That JPL Horizons is also a very interesting link.

Reading your text giving and example on the JPL Horizons... what would the Moon have looked like, observing over a certain man's shoulders, on a location in Padua, Italy (11 degrees, 52 minutes, 17,4 seconds Eastern longitude, 45 degrees, 24 minutes, 00,7 seconds Northern latitude at the elevation of 43,7 meters from the sealevel)... as the Sun was gradually setting on the 30th of November 1609…

Be well!

EDIT: added an example on colongitude, provided new reference pic of the CLA b9 plus tried to calculate the Sun angle and the difference in hours between the pics using colongitude...

Edited by Mare Nectaris (06/02/08 10:26 AM)