Friday, July 21, 2017

The last quarter moon

Last week, I set up the Celestron to image the last quarter moon at some of the same places I imaged it two weeks before as well as new locations I have not imaged yet. The images below are the result, using the same camera and Barlow lens, which I am probably going to be upgrading soon. There seems to be a color fringing problem that originates with the Barlow lens, it's most definitely not the telescope itself. They can introduce chromatic and spherical aberrations, which the EdgeHD telescopes otherwise stamp out completely. I'll have to purchase some top shelf Barlow lenses to cure this defect, and even at that the atmosphere can also introduce color fringing because it acts like a weak prism. The only remedy for that is imaging when the moon or planet are as high in the sky as possible.
The Straight Wall or Rupes Recta is an immense thrust fault formed when the surface to the left of this feature dropped downwards relative to the ground to the right. Contrary to the namesake, it's not a vertical cliff, scarp or even a steep slope. The actual slope is about ten degrees, which people can walk up to the top easily over its 1,300 foot height and sixty five mile length. To an astronaut on the ground, it would look like a high hill that extends to both sides as far as he or she could see. To the right is a rille that formed when a lava tube collapses or the ground drops between parallel faults in the lunar crust. When the Sun angle at the site is low, both features are easy to see near the moon's first and last quarter phases through a small telescope. At first quarter, the straight wall is visible as a black line, at third quarter the Straight Wall's steeper face is illuminated and appears white. At other times it's very hard to impossible to see.
Rima Hyginus is an immense rille formed by faulting and collapse of the overlying rock and large scale volcanic eruptions, including fire fountains and pyroclastic flows. The pits along it's length are not impact craters, they are volcanic vents and collapse pits where a large dike channeled magma from the lunar mantle to the surface. From there the lavas spread out over the surround area. The largest pit Hyginus is a caldera. Between Rima Hyginus and the crater Triesnecker are the numerous but much shallower and smaller Triesnecker riles, which cover the lava plains like a fracture pattern in a window across Sinus Medii. They are not related to the crater Triesnecker, but instead are related to the deposition of the mare lavas that formed Sinus Medii. These are likely of tectonic origin.

Near this large and nearly buried by lava flows carter is the strange impact feature known as Davy Catena, a line of small impact craters that formed at the same time. It's thought a small asteroid or comet was disrupted by the Earth's or Moon's gravity before the fragments plowed into the Moon. The impacts occurred after the lavas invaded the region then hardened as evidenced by their fresh appearance, along with most of the other smaller craters in the region.

Near the moon's south pole the surface is entirely composed of lighter aluminum rich rock and is saturated with craters. In other words, the formation of a crater destroys one or more craters already there. Here the large crater Clavius dominates the scene near the center of the photo, with the younger and smaller crater Tycho at the top center. The immense heat of the impacts that formed the larger crater left pools of impact melt that hardened into a smooth crater floor before being crater by smaller and less numerous impactors. Clavius itself has an arc of smaller crater of decreasing size across it's floor while the much younger crater Tycho still has an impressive series of rays extending from it. It was formed about 107 million years ago by an asteroid or comet striking the moon. The 56-mile wide crater has a central peak two miles high in the center that was pushed up by the rebound of the Moon's crust during the impact that created Tycho. Photographs from the Lunar Reconnaissance Orbiter show a 120-foot wide boulder resting on the summit of Tycho's central peak.

The 50-mile wide crater Archimedes lies in Mare Imbrium with the nearby smaller and younger craters Autolycus and Aristillus. After the impact that formed Archimedes but before the impacts that formed the other craters, the lavas that formed Mare Imbrium invaded and flooded the crater floor from below and the surrounding plain. By the time Autolycus and Aristillus formed, Mare Imbrium has solidified into a vast plain of dark basaltic rock. Despite the flooding of the crater by lava, the rim towers over two miles above the lava that forms the present crater floor. Consequently there is a flat crater floor under which the central peak and original crater floor is now buried under lava flows. Luna 2, the first probe to reach the moon crashed into it between the crater Archimedes and Autolycus.
In general, l had a successful run of imaging that lasted until dawn was underway. However, I will clearly need to upgrade the way I'm increasing the magnification to get these lunar close ups and planetary photos. That requires better Barlow lenses, one of which with greater magnification than the 2.8X University Optics Barlow I am using now. When the seeing is very good, I clearly could use a 4X or even 5X to get all the detail I can. That will require careful polar alignment because any error will cause the moon to drift as the telescope tracks. That will make close up imagery much harder, even with stacking video frames. All of these pictures were 75 frames stacked from 20 second AVI video files taken the week before. With the coming favorable apparition of Mars, I'll want to be ready to image this challenging planet before it's out of view for another eighteen months.

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