APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 3425
Joined: Fri Dec 04, 2009 3:27 am

APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by APOD Robot » Sun Jan 06, 2019 5:20 am

Image A Laser Strike at the Galactic Center

Explanation: Why are these people shooting a powerful laser into the center of our Galaxy? Fortunately, this is not meant to be the first step in a Galactic war. Rather, astronomers at the Very Large Telescope (VLT) site in Chile are trying to measure the distortions of Earth's ever changing atmosphere. Constant imaging of high-altitude atoms excited by the laser -- which appear like an artificial star -- allow astronomers to instantly measure atmospheric blurring. This information is fed back to a VLT telescope mirror which is then slightly deformed to minimize this blurring. In this case, a VLT was observing our Galaxy's center, and so Earth's atmospheric blurring in that direction was needed. As for inter-galaxy warfare, when viewed from our Galaxy's center, no casualties are expected. In fact, the light from this powerful laser would combine with light from our Sun to together appear only as bright as a faint and distant star.

<< Previous APOD This Day in APOD Next APOD >>

heehaw

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by heehaw » Sun Jan 06, 2019 11:01 am

Glad they cleared THAT up! We have enough troubles already!

De58te
Science Officer
Posts: 212
Joined: Mon Sep 30, 2013 6:35 pm

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by De58te » Sun Jan 06, 2019 11:54 am

All that won't be necessary when they finally launch the James Webb Space Telescope. By the way when will they launch it? They have been talking about it replacing the ailing Hubble for the past 15 years. I have even a magazine that says it was scheduled to launch in the year 2018. It is now 2019.

User avatar
orin stepanek
Plutopian
Posts: 4595
Joined: Wed Jul 27, 2005 3:41 pm
Location: Nebraska

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by orin stepanek » Sun Jan 06, 2019 1:34 pm

De58te wrote:
Sun Jan 06, 2019 11:54 am
All that won't be necessary when they finally launch the James Webb Space Telescope. By the way when will they launch it? They have been talking about it replacing the ailing Hubble for the past 15 years. I have even a magazine that says it was scheduled to launch in the year 2018. It is now 2019.

https://www.esa.int/Our_Activities/Spac ... _Telescope

Going to be awhile yet!
Orin

Smile today; tomorrow's another day!

starsurfer
Stellar Cartographer
Posts: 3382
Joined: Thu Mar 15, 2012 7:25 pm

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by starsurfer » Sun Jan 06, 2019 5:22 pm

Click to play embedded YouTube video.

Guest2

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by Guest2 » Sun Jan 06, 2019 11:06 pm

There should be acknowledgement this is a FAKE image (or at least that it's enhanced. )
Laser beams are invisible unless reflected off smoke dust fog or some other particulates.
The light shining on the dome doors is obviously faked.

Boomer12k
:---[===] *
Posts: 2392
Joined: Sun Apr 22, 2007 12:07 am

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by Boomer12k » Mon Jan 07, 2019 1:56 am

Guest2 wrote:
Sun Jan 06, 2019 11:06 pm
There should be acknowledgement this is a FAKE image (or at least that it's enhanced. )
Laser beams are invisible unless reflected off smoke dust fog or some other particulates.
The light shining on the dome doors is obviously faked.
Not entirely true.... https://www.wisegeek.com/what-is-a-visible-laser.htm

:---[===] *

User avatar
alter-ego
Serendipitous Sleuthhound
Posts: 807
Joined: Mon Apr 21, 2008 4:51 am
Location: Redmond, WA

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by alter-ego » Mon Jan 07, 2019 4:35 am

Guest2 wrote:
Sun Jan 06, 2019 11:06 pm
There should be acknowledgement this is a FAKE image (or at least that it's enhanced. )
Laser beams are invisible unless reflected off smoke dust fog or some other particulates.
The light shining on the dome doors is obviously faked.
I don't doubt the visibility of the 589nm beam is more pronounced in the image, but it's certainly not fake. In fact, I'd expect the scattered light from the laser beam itself and that onto the dome opening to be visible by eye in the dark of night. Note that the Milky Way and stars are seen better in the image that with the unaided eye. The image shows stars fainter than mag 8. The image is showing the laser beam brighter that it would appear to the unaided eye.

First, in "pure" air without aerosols, Rayleigh scattering will make the beam visible. Under a Class 100 HEPA filter, I've seen a green laser beam glow beautifully uniform without bright flashes from particulates. If you're in an unfiltered environment, there'll be varying degrees of particulates and moisture, and there'll be more scattering as you said. The VLT is in such an unfiltered environment.

Second, since these LGS lasers have a Gaussian (continuous) intensity distribution, scattering within the optical tube assembly will occur at some level from one or multiple mirrors. I suspect that what we see is primary or secondary scattering at the exit aperture of that launch tube.

You see, I've spent my career working with and building lasers of all colors and powers.
A pessimist is nothing more than an experienced optimist

User avatar
neufer
Vacationer at Tralfamadore
Posts: 15641
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by neufer » Mon Jan 07, 2019 6:25 am

https://www.nasa.gov/multimedia/imagegallery/image_feature_1768.html wrote:
Observe the Moon
Sept. 28, 2010

<<This photograph shows the Laser Ranging Facility at the Geophysical and Astronomical Observatory at NASA's Goddard Space Flight Center in Greenbelt, Md. The observatory helps NASA keep track of orbiting satellites. In this image, the lower of the two green beams is from the Lunar Reconnaissance Orbiter's dedicated tracker. The other laser originates from another ground system at the facility. Both beams are pointed at the moon - specifically at LRO in orbit around the moon.>>
https://www.skyandtelescope.com/astronomy-news/pesky-problems-for-lunar-reflectors/ wrote:
Pesky Problems for Lunar Reflectors
By: Kelly Beatty | February 21, 2014

<<Astronomers back on Earth periodically fire lasers at these suitcase-size banks of reflectors to determine where the Moon is in its orbit and, specifically, precisely how far it is from Earth. Initially, the laser ranging was good to an accuracy of about 15 cm, 100 times better than any previous method.

But in recent years the accuracy has gotten down to nearly 1 mm, thanks to an effort led by Thomas Murphy (University of California, San Diego) using the 3.5-m reflector at Apache Point Observatory in New Mexico. (Amusingly, Murphy has named the effort the "Apache Point Observatory Lunar Laser-ranging Operation," or APOLLO.) Moreover, thanks to Lunar Reconnaisance Orbiter's images of the long-lost Lunokhod 1 rover (spotted by Russian scientists, by the way), the count of available reflector sites on the Moon now stands at five.

Some years ago, Murphy and his colleagues noticed that the laser pulses were coming back to Earth weaker than expected. "Each shot sends about 1017 [100 quadrillion] photons toward the Moon, and in good conditions we detect about one return photon per shot," the team noted in an article published in Icarus. That's only about a tenth of what should be coming back, leading to suspicion that a microscopic veneer of lunar dust has coated (or abraded) about half the area of the exposed optical surfaces.

But something else is causing weak returns, and the dropoff gets particularly bad (by another factor of 10) around full Moon. In Icarus, Murphy and his team surmised that the problem is due to distortions in the optics due to heat from the Sun. The Apollo program's prisms are recessed into shallow cylinders, so sunlight illuminates them fully only at high noon — that is, around the time of full Moon. The thermal effect is much worse for the Lunokhod arrays — they can't be used at all during the lunar day.

One way to confirm this hypothesis would be to monitor the reflectors' performance during a total lunar eclipse. The rapid plunge into shadow, Murphy reasoned, should allow the optics to cool and relax. Some earlier observations during eclipses already hinted that solar heating was indeed the culprit, but Apache Point was clouded out during lunar eclipses in 2007 and 2008.

Fortunately, clear skies prevailed during the total lunar eclipse of December 21, 2010, and for 5½ hours the team bounced laser pulses off three Apollo reflector arrays and a fourth on Lunophod 1. Sure enough, the efficiency immediately improved, as Murphy and others report in the March 1st issue of Icarus. Interestingly, none of the laser pulses fired at Lunokhod 1 during the eclipse resulted in detections. However, as the team comments, "In general, the Lunokhod arrays are more susceptible to thermal disruptions than the Apollo arrays, so the lack of returns from Lunokhod 1 during the eclipse is not entirely surprising.">>
Last edited by neufer on Mon Jan 07, 2019 2:39 pm, edited 1 time in total.
Art Neuendorffer

User avatar
geckzilla
Ocular Digitator
Posts: 8914
Joined: Wed Sep 12, 2007 12:42 pm
Location: Modesto, CA

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by geckzilla » Mon Jan 07, 2019 7:45 am

Guest2 wrote:
Sun Jan 06, 2019 11:06 pm
There should be acknowledgement this is a FAKE image (or at least that it's enhanced. )
Laser beams are invisible unless reflected off smoke dust fog or some other particulates.
The light shining on the dome doors is obviously faked.
you are not the only person to call one of these laser images fake
it happens every time one gets posted to APOD
but you are still wrong.
Just call me "geck" because "zilla" is like a last name.

starsurfer
Stellar Cartographer
Posts: 3382
Joined: Thu Mar 15, 2012 7:25 pm

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by starsurfer » Mon Jan 07, 2019 11:20 am

First there was fake news and now there's fake APODs? What would Donald Trump think?

User avatar
neufer
Vacationer at Tralfamadore
Posts: 15641
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by neufer » Mon Jan 07, 2019 2:45 pm

starsurfer wrote:
Mon Jan 07, 2019 11:20 am

First there was fake news and now there's fake APODs?

What would Donald Trump think?
My eldest daughter lives just a mile away from NASA's Goddard Space Flight Center in Greenbelt, Md and I have never ever seen the Milky Way from her house :!:

IMO the actual fake part of the Jan 6 APOD is the green screen display of the Milky Way.
Art Neuendorffer

Piffle

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by Piffle » Mon Jan 07, 2019 4:58 pm

geckzilla wrote:
Mon Jan 07, 2019 7:45 am
Guest2 wrote:
Sun Jan 06, 2019 11:06 pm
There should be acknowledgement this is a FAKE image (or at least that it's enhanced. )
Laser beams are invisible unless reflected off smoke dust fog or some other particulates.
The light shining on the dome doors is obviously faked.
you are not the only person to call one of these laser images fake
it happens every time one gets posted to APOD
but you are still wrong.
Thanks Geck. Why educate when you can cast blame? Boomer, alter-ego and Neufer got nothin' on you.

User avatar
Chris Peterson
Abominable Snowman
Posts: 14142
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by Chris Peterson » Mon Jan 07, 2019 5:17 pm

alter-ego wrote:
Mon Jan 07, 2019 4:35 am
Guest2 wrote:
Sun Jan 06, 2019 11:06 pm
There should be acknowledgement this is a FAKE image (or at least that it's enhanced. )
Laser beams are invisible unless reflected off smoke dust fog or some other particulates.
The light shining on the dome doors is obviously faked.
Second, since these LGS lasers have a Gaussian (continuous) intensity distribution, scattering within the optical tube assembly will occur at some level from one or multiple mirrors. I suspect that what we see is primary or secondary scattering at the exit aperture of that launch tube.
The laser launch telescope is a 40 cm refractor (acting as a 20x beam expander) with a 97.7% throughput. That telescope is baffled and coated with a light absorbing material. So I'm guessing we're not seeing any scattered light from the optical train at all. Most likely, the telescope and dome structures are simply illuminated by the primary atmospheric scatter. I'm not sure if that would be visible to our eyes or not, but it's not in the least surprising it would be captured by an exposure long enough to record this much depth in the sky field.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
neufer
Vacationer at Tralfamadore
Posts: 15641
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by neufer » Mon Jan 07, 2019 6:37 pm

Chris Peterson wrote:
Mon Jan 07, 2019 5:17 pm

The laser launch telescope is a 40 cm refractor (acting as a 20x beam expander) with a 97.7% throughput. That telescope is baffled and coated with a light absorbing material. So I'm guessing we're not seeing any scattered light from the optical train at all. Most likely, the telescope and dome structures are simply illuminated by the primary atmospheric scatter. I'm not sure if that would be visible to our eyes or not, but it's not in the least surprising it would be captured by an exposure long enough to record this much depth in the sky field.
https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/4lgsf/ wrote:

<<April 2016 saw the arrival of four new stars above the Paranal skies. After years of development, ESO has completed the installation of the 4 Laser Guide Star Facility or 4LGSF, a new subsystem of the Adaptive Optics Facility (AOF) at the Very Large Telescope (VLT).

The 4LGSF complements the Laser Guide Star Facility (LGSF). Instead of one laser, the 4LGSF sends four laser beams into the skies to produce four artificial stars by exciting sodium atoms located in the atmosphere at an altitude of 90 kilometres. Each laser delivers 22 watts of power — about 4000 times the maximum allowed for a laser pointer — in a beam with a diameter of 30 centimetres.

Lasers can excite sodium atoms in the mesosphere, which is located 90–110 kilometres above the Earth’s surface. The fluorescent light that is emitted by the sodium atoms and collected by the telescope is affected by the atmosphere in the same way as the light emitted by real stars is. So, the fluorescent light from the sodium atoms can be used by the adaptive optics system to measure and compensate for the distortions introduced by the atmosphere.

ESO and several European institutes and industries, including TOPTICA, Germany; TNO, the Netherlands; MPB Communications, Canada; Optec, Italy; Astrel, Italy; and Laseroptik, Germany and INAF–Osservatorio di Roma, have made significant contributions to the project. The new 4LGSF is based on several new technologies. One of these is fibre Raman laser technology, which was developed by ESO and transferred to industry, resulting in a Raman fibre amplifier laser that is lighter, more rugged, and represents a breakthrough in size and stability.

All the operations at the 4LGSF will follow a protocol to avoid any risk to aircraft. The laser system is equipped with an automatic aircraft avoidance system that shuts down the lasers if an aircraft ventures too close to the beams.>>
Art Neuendorffer

User avatar
alter-ego
Serendipitous Sleuthhound
Posts: 807
Joined: Mon Apr 21, 2008 4:51 am
Location: Redmond, WA

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by alter-ego » Tue Jan 08, 2019 4:48 am

Chris Peterson wrote:
Mon Jan 07, 2019 5:17 pm

The laser launch telescope is a 40 cm refractor (acting as a 20x beam expander) with a 97.7% throughput. That telescope is baffled and coated with a light absorbing material. So I'm guessing we're not seeing any scattered light from the optical train at all. Most likely, the telescope and dome structures are simply illuminated by the primary atmospheric scatter. I'm not sure if that would be visible to our eyes or not, but it's not in the least surprising it would be captured by an exposure long enough to record this much depth in the sky field.
I agree, illumination from the light saber could very well dominate. However the shadow line on the inside of the dome appears to sharply defined. It appears to be limited by the edge of the tube in a cone-like fashion. Scattered light from the beam should illuminate the structure below that line which must be the case because the camera is well below the lip of the tube and it sees the beam just fine.

In any case, the details matter here. Where's vendetta when you need him.
A pessimist is nothing more than an experienced optimist

User avatar
MarkBour
Subtle Signal
Posts: 814
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by MarkBour » Thu Jan 10, 2019 3:21 am

Aha! So, we're pushing the moon away, Mr. Heisenberg?
Mark Goldfain

User avatar
neufer
Vacationer at Tralfamadore
Posts: 15641
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by neufer » Thu Jan 10, 2019 3:54 am

MarkBour wrote:
Thu Jan 10, 2019 3:21 am

Aha! So, we're pushing the moon away, Mr. Heisenberg?
  • Not necessarily.
If the lasers are pointing to the West then they will impart added angular momentum LE to the Earth's rotation of
up to: LE = REPlt/c where RE = the Earth's radius, Pl is the laser power & t is the duration time of the lasing.

If these laser beams intercept the Moon (and are essentially absorbed)
then the Moon must lose an equal amount of angular momentum (thereby bringing it closer).
Art Neuendorffer

User avatar
MarkBour
Subtle Signal
Posts: 814
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by MarkBour » Fri Jan 11, 2019 5:23 am

neufer wrote:
Thu Jan 10, 2019 3:54 am
MarkBour wrote:
Thu Jan 10, 2019 3:21 am

Aha! So, we're pushing the moon away, Mr. Heisenberg?
  • Not necessarily.
If the lasers are pointing to the West then they will impart added angular momentum LE to the Earth's rotation of
up to: LE = REPlt/c where RE = the Earth's radius, Pl is the laser power & t is the duration time of the lasing.

If these laser beams intercept the Moon (and are essentially absorbed)
then the Moon must lose an equal amount of angular momentum (thereby bringing it closer).
Ha! Fascinating point. I was thinking only of those Apollo reflectors that apparently only will be useful if the Moon is almost exactly overhead (right?) But if the lasers are fired at other things, such as the LRO, I guess that can happen in whatever direction. (Well, they can certainly fire whenever they want, but to get any useful data, they'd have to at least hit an appropriately-inclined reflective surface on the object.) I guess folks in multiple nations fire range-finding lasers into space all the time and in all sorts of directions:
https://www.youtube.com/watch?v=u1guaP6e8os

I don't know if they have to know what they're aiming at and if they have to get specific about it, or if they can just aim at a typical metal satellite whenever it passes and assume they'll get a usable reflection. Anyway, if they hit the LRO, they would alter its orbit around the Moon, rather than the Moon's orbit around us.

But back to the Moon. I suppose if they hit it at all times of evening, it would average out to the same as hitting it when it is straight overhead (actually I should be saying "at zenith"). I'm actually thinking that there would be some tendency (for psychosocial reasons) to hit it more often when it is in the East, but maybe not.

But what you're telling me is that I can bring the Moon down, if I shoot at it when it is going down.
Mark Goldfain

User avatar
Chris Peterson
Abominable Snowman
Posts: 14142
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by Chris Peterson » Fri Jan 11, 2019 5:48 am

MarkBour wrote:
Fri Jan 11, 2019 5:23 am
I was thinking only of those Apollo reflectors that apparently only will be useful if the Moon is almost exactly overhead (right?)
Why? The reflectors are made up of an array of corner cube prisms. They always reflect light back in the same direction of the incoming light. So it doesn't matter where the Moon is, other than atmospheric extinction possibly being an issue once it gets fairly close to the horizon.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
neufer
Vacationer at Tralfamadore
Posts: 15641
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by neufer » Fri Jan 11, 2019 3:13 pm

MarkBour wrote:
Fri Jan 11, 2019 5:23 am
neufer wrote:
Thu Jan 10, 2019 3:54 am
MarkBour wrote:
Thu Jan 10, 2019 3:21 am

Aha! So, we're pushing the moon away, Mr. Heisenberg?
  • Not necessarily.
If the lasers are pointing to the West then they will impart added angular momentum LE to the Earth's rotation of
up to: LE = REPlt/c where RE = the Earth's radius, Pl is the laser power & t is the duration time of the lasing.

If these laser beams intercept the Moon (and are essentially absorbed)
then the Moon must lose an equal amount of angular momentum (thereby bringing it closer).
Ha! Fascinating point. I was thinking only of those Apollo reflectors that apparently only will be useful if the Moon is almost exactly overhead (right?) But if the lasers are fired at other things, such as the LRO, I guess that can happen in whatever direction. (Well, they can certainly fire whenever they want, but to get any useful data, they'd have to at least hit an appropriately-inclined reflective surface on the object.) I guess folks in multiple nations fire range-finding lasers into space all the time and in all sorts of directions:
https://www.youtube.com/watch?v=u1guaP6e8os

I don't know if they have to know what they're aiming at and if they have to get specific about it, or if they can just aim at a typical metal satellite whenever it passes and assume they'll get a usable reflection. Anyway, if they hit the LRO, they would alter its orbit around the Moon, rather than the Moon's orbit around us.

But back to the Moon. I suppose if they hit it at all times of evening, it would average out to the same as hitting it when it is straight overhead (actually I should be saying "at zenith"). I'm actually thinking that there would be some tendency (for psychosocial reasons) to hit it more often when it is in the East, but maybe not.

But what you're telling me is that I can bring the Moon down, if I shoot at it when it is going down.
Yes, you "can bring the Moon down, if I shoot at it when it is going down" (for "psychopathic reasons?") thanks to photons hitting the Moon and being absorbed and/or randomly reflected. (Note: the laser beams at the Moon are hundreds of meters wide so hardly any of the beam gets intercepted by artificial objects.) Likewise, shooting lasers to the East slows the Earth's spin and sends the Moon into a higher orbit.

Shooting lasers with the Moon at zenith has no effect on angular momentum. However, if one only shoots when the Moon is approaching (i.e., from apogee to perigee) then one can slow the Moon until it has the least Energy for its fixed angular momentum (i.e., a circular orbit of radius Rc= 2/[1/apogee + 1/perigee] = 382800 km versus the current semi-major axis of 384400 km. If one only shoots when the Moon is receding (i.e., from perigee to apogee) then one can speed up the Moon until it has the most Energy for its fixed angular momentum (i.e., a parabolic orbit of Perigee = Rc/2 = 191400 km.
Art Neuendorffer

User avatar
MarkBour
Subtle Signal
Posts: 814
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by MarkBour » Fri Jan 11, 2019 5:44 pm

Chris Peterson wrote:
Fri Jan 11, 2019 5:48 am
MarkBour wrote:
Fri Jan 11, 2019 5:23 am
I was thinking only of those Apollo reflectors that apparently only will be useful if the Moon is almost exactly overhead (right?)
Why? The reflectors are made up of an array of corner cube prisms. They always reflect light back in the same direction of the incoming light. So it doesn't matter where the Moon is, other than atmospheric extinction possibly being an issue once it gets fairly close to the horizon.
I see that's another thing I had wrong. I don't know much about them, but was imagining something about their nature from this one line in one of Art's quotes (https://www.skyandtelescope.com/astrono ... eflectors/) above:
The Apollo program's prisms are recessed into shallow cylinders, so sunlight illuminates them fully only at high noon.
Now that you questioned this, I looked a bit at:
https://en.wikipedia.org/wiki/Lunar_Las ... experiment
So I guess they can reflect as well at any of a range of angles. I now wonder if the whole configuration is nicely set up to work at any angle that we would ever see from the earth. (I had never even heard of corner reflector prisms before this -- they're pretty neat optics. Shows how little I know about this subject, since I get the impression they are in common use for a number of applications.)
Mark Goldfain

User avatar
MarkBour
Subtle Signal
Posts: 814
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by MarkBour » Fri Jan 11, 2019 6:05 pm

neufer wrote:
Fri Jan 11, 2019 3:13 pm
Shooting lasers with the Moon at zenith has no effect on angular momentum. However, if one only shoots when the Moon is approaching (i.e., from apogee to perigee) then one can slow the Moon until it has the least Energy for its fixed angular momentum (i.e., a circular orbit of radius Rc= 2/[1/apogee + 1/perigee] = 382800 km versus the current semi-major axis of 384400 km. If one only shoots when the Moon is receding (i.e., from perigee to apogee) then one can speed up the Moon until it has the most Energy for its fixed angular momentum (i.e., a parabolic orbit of Perigee = Rc/2 = 191400 km.
... And then there is no apogee.
Thanks for the analysis. That's what I call top-notch conic relief.
Mark Goldfain

User avatar
Chris Peterson
Abominable Snowman
Posts: 14142
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by Chris Peterson » Fri Jan 11, 2019 6:15 pm

MarkBour wrote:
Fri Jan 11, 2019 5:44 pm
(I had never even heard of corner reflector prisms before this -- they're pretty neat optics. Shows how little I know about this subject, since I get the impression they are in common use for a number of applications.)
Yeah. Like every one of those little molded plastic reflectors that you find on cars and trucks and driveway markers.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
neufer
Vacationer at Tralfamadore
Posts: 15641
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: A Laser Strike at the Galactic Center (2019 Jan 06)

Post by neufer » Fri Jan 11, 2019 6:20 pm

MarkBour wrote:
Fri Jan 11, 2019 5:44 pm

(I had never even heard of corner reflector prisms before this -- they're pretty neat optics.)
Many clothing stores used to have the 2D version: non-reversing mirrors so that
you could see yourself as others see you (e.g., with your right hand still a right hand.)

Parity is conserved with an even number of reflections (but not with an odd number).
https://en.wikipedia.org/wiki/Non-reversing_mirror wrote:
<<A non-reversing mirror (sometimes referred to as a flip mirror) is a mirror that presents its subject as it would be seen from the mirror. A non-reversing mirror can be made by connecting two regular mirrors at their edges at a 90 degree angle. If the join is positioned so that it is vertical, an observer looking into the angle will see a non-reversed image. This can be seen in places such as public toilets when there are two mirrors mounted on walls which meet at right angles. Such an image is visible while looking towards the corner where the two mirrors meet. The problem with this type of non-reversing mirror is that there is usually a line down the middle interrupting the image. However, if first surface mirrors are used, and care is taken to set the angle to exactly 90 degrees, the join can be made almost invisible.>>
It was always fun to find buildings with 90 degree angle corners that one could hit tennis balls against and have them come back at the same angle.

(And with a single bounce off of the ground one had one's own 3D corner reflector.)
Art Neuendorffer