Making many assumptions, the human eye has about 500 to 600 megapixels of resolution.
But determining visual acuity is nontrivial. Lots of physics, physiology, and neuroscience enter into it.
Visual acuity depends on a number of physical limitations set by the optics of the lens of the eye as well as the sampling on the retina.
For example, the point spread function of the lens roughly matches the sampling of the retinal mosaic (well, within a factor of 3 or so). A nicely evolved system!
Our eyes' acuity are influenced by
- Refractive error (out of focus lens, often correctable by glasses or contacts)
- Size of the pupil (physical optics tells us that a wide open iris will reduce diffraction)
- Illumination (brighter scenes give more photons, and our neuroprocessing can do more
- Time of exposure to the field
- Area of the retina exposed
- State of adaption of the eye (night [scotopic] vs day [photopic] vision.
I agree with many of the Slashdot posters who've commented on my article of 15 years ago. There's a great deal to munch on - plenty of hilarious mistakes as well as several ideas still worth thinking about.
That 1995 article grew from my questioning attitude. When I hear nearly unanimous commentary without any critical dialog, I become skeptical. Perhaps too skeptical, as that article shows.
At the time, I saw my role as encouraging questions about then-common predictions. As a way of introducing dialog through debate, if not deliberation.
Clearly, I'm no futurist, able to extrapolate across decades. If anyone, I suspect that school teachers are the most in touch with future generations.
Now? Oh, I try to stay away from predictions; two teenagers gleefully keep me informed of my daily mistakes. I teach physics, speak at meetings, and write the occasional article for Scientific American. I make Klein Bottles... and, yes, I sell them online, in obvious contradiction to that 1995 article.
Best wishes to all, -Cliff (in Oakland California, on a Monday afternoon without sunspots)
Perhaps more than 50% - I can imagine meetings amongst the labels to set some number.
Your method is reasonable, discreet, equitable, and practical. It saves face for the labels, sends the proper chill to the downloader, and leaves the door open for further action.
Most air freight shippers use dimensional weight to charge extra on high volume/low mass items. Charging by weight alone won't recover the costs of shipping lightweight items. For example, a pound of popped popcorn will cost more to ship than a pound of popcorn kernels.
Typically, a shipper assumes that typical freight has a nominal density of about 0.2 gm/cm^3... packages with a lower density than this will be charged more.
Of course, humans have a density of about 1gm/cm^3, so we wouldn't be charged extra, should we decide to be shipped airfreight.
For the past 12 years, I've been making & selling Klein Bottles. A quick analysis of online order email addresses (no, I don't spam!) shows the decline in AOL domain for people ordering onesided manifolds:
Before Cuckoo's Egg, I was better known as a planetary scientist. My PhD dissertation relied on polarization data taken by Pioneer 10 & 11 to understand the scattering characteristics of Jupiter's upper atmosphere.
The Pioneer 10 & 11 spacecraft both flew by Jupiter, and Pioneer 11 went on to Saturn encounter.
I remember it well - while a grad student at the Lunar & Plantetary Labs, I helped with the Imaging Photopolarimeter during Saturn Encounter.
The spacecraft, designed in the early 1970's, had essentially no onboard memory, so instructions had to be uploaded in real time. The several hour-long communications delay made for real excitement at encounter (Did the spacecraft survive the ring crossing? Did the instruction arrive? Did the sensor point in the correct direction? Is it returning images?)
We'd spent months in advance, preparing alternative sequences for the encounter. Each sequence was on punched papertape. Then, at encounter in September 1979, we'd pick the tape, mount it on a teletype, and send the data out over the NASA deep space network, then anxiously wait to see if the instructions worked on Pioneer 11.
Want your kids to learn physics? Throw away the computer simulations. Build things with them. Run experiments. Observe and think about the results.
To teach physics, start with things like C-clamps, string, rubber bands, wire, springs, low-friction carts, compasses, magnets, thermometers, balloons, weights, scales, and pulleys.
More advanced stuff: a voltmeter/ammeter (analog stuff), an old oscilloscope, an air table (a kids' hockey table), vacuum pump & bell jar, countdown timer/photogate, etc. Many of these things show up on craigslist for cheap (I picked up two free oscilloscopes and have given them to my sharp high school students).
Computer simulations? Naw. Have your kids do real physics:
A pendulum made of a bowling ball and rope. Time the pendulum swings and then ask: which will change the period - changing the lenghth of the swings, changing the weight, or changing the length of the rope?
Fool around with a signal generator, an oscilloscope, and a microphone. What's a sound wave look like? How is frequency related to period?
Play with thermometers, ice, water, and fire. What's the temperature of ice and water? Can you get water colder than this? How hot is water from the kettle? Can you get water hotter than this?
Get a voltmeter, wire, and some magnets. Can you really induce a voltage by moving a magnet nearby?
Don't sidetrack your kids with simulations & computer graphics. Real physics starts by fooling around with reality.
Obs Feynman quote: "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
In the early 1980's, a group of Santa Cruz physics grad students built a set of computers into their cowboy boots. These timed the spinning of roulette wheels and applied Newtonian physics.
Thomas Bass wrote this up in the 1985 book, The Eudaemonic Pie, and caused the Nevada Gaming Commission to ban the use of these devices.
When a satellite fails, often it cannot be de-orbited. Several failure modes will cause this - the most common is the malfunction of the controller, communications unit, or onboard power system. When any of these fail, there's no way to command the retro-rocket to fire.
Then, too, you need the satellite to be pointed in the correct direction (meaning that its stationkeeping rockets are working), and for it to have enough hydrazine (or whatever) to be deorbited. Near the end of a spacecraft's life, consumables are limited.
And, of course, it takes a lot of energy to de-orbit many satellites. A geostationary comsat needs one heck of a kick motor to get it down. Usually they are not brought down to burnup in the atmosphere. Instead, they are moved a few dozen (hundred?) kilometers inwards from their geostationary slot. This puts 'em well away from the main circle of geostationary satellites.
It's like consumer goods... manufacturers work to make them last long enough to complete their mission; few think about how to get rid of 'em once their purpose has expired.
Most satellites for earth observation use sun-synchronous orbits. These orbits let the satellite's cameras take pictures ob objects at the same solar time. This means that it will pass overhead at the same local time every day... so the images will have the same shadow characteristics.
You accomplish this by making the orbit precess exactly 360 degrees per solar year.
These orbits are typically nearly circular, but needn't be; you can put a spy satellite into a sun-synchronous elliptical orbit, so it'll swoops down and photograph near perigee, then waste a lot of time around apogee.
Since this orbit is around 684 Km, it can be shown that it must be pretty close to circular, has an orbital period of around 100 minutes, and its inclination is probably about 96 to 100 degrees (meaning that the satellite is slightly retrograde - 90 degrees inclination is polar, zero degrees is equatorial) In turn, this means that pretty much all of earth will be seen by the satellite, except for 8 degree circles around the poles.
A 96 minute period means that each successive orbit will look down on a place 15 degrees west... one time zone to the west.
Geosynchronous orbits are pretty useless for this type of work, since they're so far away (you need really big telescopes to get much resolution). Also, you'd only see one hemisphere, and half the year it'd be nighttime over the areas you want to see.
Defining optical resolution from space is a bit tricky, as several generations of optical engineers have discovered.
The main criterion is the telescope's point spread function - this is roughly the angular diameter that a pinpoint star appears to be, as seen through the telescope. We want the smallest point spread function, and it should map onto about one to three sensor pixels. (arguments go here about over/undersampling).
The Fourier Transform of the point spread function is the Optical Transfer Function, which is a graph of the spatial frequencies response of the telescope. It's analogous to a hifi's frequency response... it's an engineering challenge to prevent high frequencies from getting rolled off.
The main limit for high resolution is the diameter of the primary mirror (All mirrors and optical elements, no matter how perfect, have diffraction effects which spread out the light and reduce resolution. The bigger the entrance pupil, the greater the resolution) For the GeoEye, orbiting at 684Km and a resolution of 0.4m, I roughly calculate the primary mirror is somewhere around a half-meter diameter or so, depending on the wavelength of light it's optimized for.
Other things limit resolution - scattering of light in clear air (Rayleigh scattering) screws up the image, especially in the blue. Dust, haze, clouds and urban pollution are a bother, but not as much as you might think. Naturally, there's lots of image processing software... quite compute intensive.
A typical human, seen from above and not casting a shadow, is about 20 to 60 cm across. So someone walking down the street should appear on a few (1 to 5) pixels. Not enough to recognize someone, especially since you're looking down on 'em.
Generally, images taken from aircraft have better resolution (they're closer, and there's less Rayleigh scattering). Perhaps airlines will attach automated, downward looking hires cameras to their daily flights.
Aargh! From these mathematicians grew the "New Math" of the early 1960's.
During the 1950's, high school math was mainly geometry, algebra, trig, and calculus.
Then came the New Math. Imported from France, it emphasized set theory, number bases, and abstract number theory. Students learned cardinality, commutative laws, associative laws, and "pure" math, with less applied math and problem-solving.
Many educators (and even more parents) saw the New Math as being too abstract for daily use and undercutting concrete skills such as computation. Physicists, especially objected, when college freshmen could calculate in multiple number bases, but couldn't solve algebraic equations.
Mathematician/singer Tom Lehrer wrote the song,"New Math", with the line, "It's so very simple that only a child can do it!"
One book, Why Johnny Can't Add - the Failure of the New Math, pointed out that in the mid 1970's teachers applauded the death of the New Math. By the late 70's, algebra was back in style, and even trigonometry was being taught. So ended the French invasion of high school math classes.
The latest, of course, is the new-new-math, also called rain-forest math. Don't get me started...
I'm a ham, and I've thought that BPL meant "Brass Pounder's League", an elite group of ham operators who handled a lot of traffic.
I'm also a physicist; I walk out of colloquia when the speaker uses five acronyms I don't know. Seminar cookies aren't worth putting up with small minded arrogance.
I'm also a nonfiction writer. As the the parent posting suggests, acronyms kills your audience. Talk in tongues when you want to exclude others, when you're tight on bandwidth, or when you want to show you're a member of an in-group.
The lithium CR1216 batteries on my shelf started corroding after 4 years. Several of the AG3/CX41 alkaline batteries began leaking after 5 years. Still untouched, in their wrappers.
This article inspired me to dig out my ham radio crystals from the 1960's.
"Rock-Bound" ham radio transmitters could only send messages at one frequency, so amateur operators sometimes modified the crystals to change frequencies. You'd tune the quartz crystals by grinding them with fine powder -- a few swipes would change frequency from 7130 to 7133 KHz (called "KiloCycles" back in the dark ages of the 1960's).
One of the first Cyclotrons is on display, outside in the weather, at the Lawrence Hall of Science in Berkeley, California. The 37 inch Cyclotron is big -- you can climb on it, although you'll get pretty rusty. (In fact, only the magnet's core is on display; the Cyclotron's Dees are missing). For a photo with seven Nobel Laureats standing in front of the ol' beast, see http://dsd.lbl.gov/Seaborg.talks/65th-anniv/27.html
KH-11 series spacecraft were called the Key Hole satellites - they were the first large reconnaissance spacecraft to send images directly to earth; previous spy satellites used film return (clumsy, slow, and unreliable). KH-11's used CCDs - quite advanced for a system developed in the late 1970's.
The seven KH-11 spacecraft had primary mirrors of 2.3 to 2.4 meters. The system provided an ultimate ground resolution between 15 to 50 cm at closest approach (perigee); actual resolution was quite a bit worse.
There's no nuclear battery on board -- power came from 11 unfolded solar panels (which, on the first Key Hole satellites didn't provide quite enough power during downlinks!). I assume the main danger to earthlings is due to the reentry of the main mirror. Since the KH-11s are in polar orbits, the debris could come down anywhere on earth, with a one-in-four chance of hitting land.
The KH-11 spy satellites were developed in parallel with the Hubble Space Telescope, and the same contractors worked on both. In fact, the KH-11 uses much the same hardware (carbon-graphite support system, front door hatch system, data-relay dish through communications satellites). Because of the secrecy surrounding the KH-11 development, the Space Telescope project often saw similar secrecy. Indeed, astronomers were discouraged (or barred) from much of the engineering of the Hubble Space Telescope.
Around 1956, electronics makers began selling hybrid radios with both vacuum tubes and transistors. Emerson's vest-pocket portable model 843 used tubes in the rf stages and a pair of plug-in transistors for audio output. A 6 volt battery lit up the tubes and transistors, while a 67 volt battery kept the tubes' electrons jumping from cathodes to plates.
From Emerson's adverts: "Transistors are so tiny they must be seen to be believed. Transistors are so sturdy they won't break... They will last for life!" and give "greater power without distortion - full reproduction of voice and instruments, balanced tone quality, and greater power output with less distortion, not to mention low battery drain"
What other mixed hybrids have came along? Was there ever a hybrid horse and car?
Curt Herzstark designed the Curta handheld calculator while he was a prisoner in the Buchenwald concentration camp. Upon release in 1945, he started a company to manufacture these mechanical handheld calculators.
Herzstark recognized the importance of user interface... he designed it to be handheld. The main cylinder fits within the hand, and the input sliders were made to be set by fingers. In a foreshadowing of computer architecture, he used complimentary arithmetic to do both subtraction and addition.
Although crank-driven, a Curta is surprisingly fast at the basic four functions. This is because you can rotate the output register to do automatic multiplies by powers of ten.
Made in Lichtenstein, the Curtas were superbly machined, with a feel comparable to a high quality Nikon F camera.
His peppermill calculators were sold from 1947 until 1972; today, they're mostly collectors items. But I use one to run my Klein Bottle business.
Geostationary spacecraft aren't as stationary as we'd like. Due to many forces (the earth's oblateness, tessoral harmonics in the earth's gravitational field, gravity from the moon and the sun), these spacecraft tend to drift, requiring occasional burns of small rockets to keep the spacecraft where it belongs.
In the spacecraft, each of four tanks contains the fuel (hydrazine) and a pressurizing gas (typically helium). There's a system of pipes and valves to allow any tank to feed any of the sets of x-y-z rocket motors. Of course, valves are unreliable, so there's the usual redundancies and crosslinked fuel pipes.
Stationkeeping in geosynchronous satellites requires precisely metered burns at just the right times. Shoot too much hydrazine, and the satellite moves out of the window, and everyone's TV reception goes to pot. Worse, you'll have to fire the rockets again and use more fuel to undo the damage from the previous burn. Too little hydrazine means that you'll need several burns, but these can only be done at certain times. If your first burn is insufficient, you may have to wait for a month (or sometimes six months) before you can fix it. (In fact, you seldom know the exact effects of a burn until doppler & tracking data is analyzed over the next days)
Now, suppose the satellite is low on fuel -- it's near the end of a 15 year lifespan. Three tanks have a little liquid fuel. The fourth tank runs out. If you then simply mix the four tanks, the output fuel line will get a mix of hydrazine and helium. The two phases in the fuel line will cause the motor to sputter, flare, or fizzle. Bad news!
So this is a non-trivial problem. And there's lots of money hanging on the answer.
In the past, the amount of fuel in each tank was determined by simple book-keeping... recording exactly how many grams of fuel was used in each burn. This is imprecise, because of the nature of propellant gauging by measuring pressure and timing burns. So every now and then, the four tanks of hydrozine would be rebalanced by connecting all the tanks together and letting the fuel equilibrate between 'em. Rebalancing the tanks is done by warming a tank and connecting it to the others. The amount of heat to put into a tank depends on how much fuel is in there, but you can't directly measure this... you depend on book keeping.
This paper sounds like they're relating the amount of heat put into a tank, and the tank's temperature. From this relationship, they're getting a better determination of the total hydrazine in each tank, and thus they can better balance the fuel in each tank.
In short, they came up with a nice way to estimate the amount of hydrazine in each tank by measuring the thermal effects. It's a good idea. Might add a few months to the lifespans of some old spacecraft which were launched in the 1990's.
In the 1950's, data was recorded on 7-track tapes at 200 bits per inch. Those 7 tracks recorded 6-bit characters (EBCDIC?) with one bit of parity. A 2400 foot tape might hold some 5 million characters (upper case only, please). By the mid 1960's tape densities more than doubled to 556 bpi.
Sorting algorithms were written to sort information using mag tapes; the speediest would make the tape vibrate in the vacuum columns with a minimum of reel-to-reel motion.
By the 1970's, most shops changed to 800bpi, 9-track tapes, which would happily handle 8-track encoding. Then came 1600 bits per inch -- you could store an amazing 50 megabytes onto a single tape.
There was a constant temptation to compress data so as to stuff as much onto the tape as possible. As a result, many graduate students earned their assistantships by decoding tapes written with oddball parity, density, and encoding combinations.
The scattering matrices from my dissertation are encoded onto 9-track 1600 bpi tapes, carefully stored in my climate uncontrolled attic.
Slashdot Mirror
Making many assumptions, the human eye has about 500 to 600 megapixels of resolution.
But determining visual acuity is nontrivial. Lots of physics, physiology, and neuroscience enter into it.
Visual acuity depends on a number of physical limitations set by the optics of the lens of the eye as well as the sampling on the retina.
For example, the point spread function of the lens roughly matches the sampling of the retinal mosaic (well, within a factor of 3 or so). A nicely evolved system!
Our eyes' acuity are influenced by
- Refractive error (out of focus lens, often correctable by glasses or contacts)
- Size of the pupil (physical optics tells us that a wide open iris will reduce diffraction)
- Illumination (brighter scenes give more photons, and our neuroprocessing can do more
- Time of exposure to the field
- Area of the retina exposed
- State of adaption of the eye (night [scotopic] vs day [photopic] vision.
- Eye motion & object motion in scene
See http://www.clarkvision.com/imagedetail/eye-resolution.html
For a good review of visual acuity, see:
http://webvision.med.utah.edu/KallSpatial.html
I agree with many of the Slashdot posters who've commented on my article of 15 years ago. There's a great deal to munch on - plenty of hilarious mistakes as well as several ideas still worth thinking about.
That 1995 article grew from my questioning attitude. When I hear nearly unanimous commentary without any critical dialog, I become skeptical. Perhaps too skeptical, as that article shows.
At the time, I saw my role as encouraging questions about then-common predictions. As a way of introducing dialog through debate, if not deliberation.
Clearly, I'm no futurist, able to extrapolate across decades. If anyone, I suspect that school teachers are the most in touch with future generations.
Now? Oh, I try to stay away from predictions; two teenagers gleefully keep me informed of my daily mistakes. I teach physics, speak at meetings, and write the occasional article for Scientific American. I make Klein Bottles ... and, yes, I sell them online, in obvious contradiction to that 1995 article.
Best wishes to all,
-Cliff (in Oakland California, on a Monday afternoon without sunspots)
Well spoken, PFI_Optix!
Perhaps more than 50% - I can imagine meetings amongst the labels to set some number.
Your method is reasonable, discreet, equitable, and practical. It saves face for the labels, sends the proper chill to the downloader, and leaves the door open for further action.
Most air freight shippers use dimensional weight to charge extra on high volume/low mass items. Charging by weight alone won't recover the costs of shipping lightweight items. For example, a pound of popped popcorn will cost more to ship than a pound of popcorn kernels.
Typically, a shipper assumes that typical freight has a nominal density of about 0.2 gm/cm^3 ... packages with a lower density than this will be charged more.
Of course, humans have a density of about 1gm/cm^3, so we wouldn't be charged extra, should we decide to be shipped airfreight.
For the past 12 years, I've been making & selling Klein Bottles. A quick analysis of online order email addresses (no, I don't spam!) shows the decline in AOL domain for people ordering onesided manifolds:
1997 13% (about)
1998 15% (about)
1999 16% (about)
2000 14.8%
2001 13.4%
2002 12.2%
2003 10.0%
2004 4.8%
2005 4.4%
2006 4.7%
2007 3.9%
2008 3.5%
2009 2.5%
Cheers, -Cliff
Yep, same guy.
Before Cuckoo's Egg, I was better known as a planetary scientist. My PhD dissertation relied on polarization data taken by Pioneer 10 & 11 to understand the scattering characteristics of Jupiter's upper atmosphere.
Cheers,
-Cliff
Pioneer 10 and 11, of course (not 11 and 12)
The Pioneer 10 & 11 spacecraft both flew by Jupiter, and Pioneer 11 went on to Saturn encounter.
I remember it well - while a grad student at the Lunar & Plantetary Labs, I helped with the Imaging Photopolarimeter during Saturn Encounter.
The spacecraft, designed in the early 1970's, had essentially no onboard memory, so instructions had to be uploaded in real time. The several hour-long communications delay made for real excitement at encounter (Did the spacecraft survive the ring crossing? Did the instruction arrive? Did the sensor point in the correct direction? Is it returning images?)
We'd spent months in advance, preparing alternative sequences for the encounter. Each sequence was on punched papertape. Then, at encounter in September 1979, we'd pick the tape, mount it on a teletype, and send the data out over the NASA deep space network, then anxiously wait to see if the instructions worked on Pioneer 11.
A prime number can represent information which is forbidden to possess.
See http://en.wikipedia.org/wiki/Illegal_prime
This goes back about a decade to the AACS encryption key controversy.
Want your kids to learn physics? Throw away the computer simulations. Build things with them. Run experiments. Observe and think about the results.
To teach physics, start with things like C-clamps, string, rubber bands, wire, springs, low-friction carts, compasses, magnets, thermometers, balloons, weights, scales, and pulleys.
More advanced stuff: a voltmeter/ammeter (analog stuff), an old oscilloscope, an air table (a kids' hockey table), vacuum pump & bell jar, countdown timer/photogate, etc. Many of these things show up on craigslist for cheap (I picked up two free oscilloscopes and have given them to my sharp high school students).
Computer simulations? Naw. Have your kids do real physics:
A pendulum made of a bowling ball and rope. Time the pendulum swings and then ask: which will change the period - changing the lenghth of the swings, changing the weight, or changing the length of the rope?
Fool around with a signal generator, an oscilloscope, and a microphone. What's a sound wave look like? How is frequency related to period?
Play with thermometers, ice, water, and fire. What's the temperature of ice and water? Can you get water colder than this? How hot is water from the kettle? Can you get water hotter than this?
Get a voltmeter, wire, and some magnets. Can you really induce a voltage by moving a magnet nearby?
Don't sidetrack your kids with simulations & computer graphics. Real physics starts by fooling around with reality.
Obs Feynman quote: "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
In the early 1980's, a group of Santa Cruz physics grad students built a set of computers into their cowboy boots. These timed the spinning of roulette wheels and applied Newtonian physics.
Thomas Bass wrote this up in the 1985 book, The Eudaemonic Pie, and caused the Nevada Gaming Commission to ban the use of these devices.
See http://en.wikipedia.org/wiki/The_Eudaemonic_Pie
When a satellite fails, often it cannot be de-orbited. Several failure modes will cause this - the most common is the malfunction of the controller, communications unit, or onboard power system. When any of these fail, there's no way to command the retro-rocket to fire.
Then, too, you need the satellite to be pointed in the correct direction (meaning that its stationkeeping rockets are working), and for it to have enough hydrazine (or whatever) to be deorbited. Near the end of a spacecraft's life, consumables are limited.
And, of course, it takes a lot of energy to de-orbit many satellites. A geostationary comsat needs one heck of a kick motor to get it down. Usually they are not brought down to burnup in the atmosphere. Instead, they are moved a few dozen (hundred?) kilometers inwards from their geostationary slot. This puts 'em well away from the main circle of geostationary satellites.
It's like consumer goods ... manufacturers work to make them last long enough to complete their mission; few think about how to get rid of 'em once their purpose has expired.
http://www.sciplus.com/
Most satellites for earth observation use sun-synchronous orbits. These orbits let the satellite's cameras take pictures ob objects at the same solar time. This means that it will pass overhead at the same local time every day ... so the images will have the same shadow characteristics.
You accomplish this by making the orbit precess exactly 360 degrees per solar year.
These orbits are typically nearly circular, but needn't be; you can put a spy satellite into a sun-synchronous elliptical orbit, so it'll swoops down and photograph near perigee, then waste a lot of time around apogee.
Since this orbit is around 684 Km, it can be shown that it must be pretty close to circular, has an orbital period of around 100 minutes, and its inclination is probably about 96 to 100 degrees (meaning that the satellite is slightly retrograde - 90 degrees inclination is polar, zero degrees is equatorial) In turn, this means that pretty much all of earth will be seen by the satellite, except for 8 degree circles around the poles.
A 96 minute period means that each successive orbit will look down on a place 15 degrees west ... one time zone to the west.
Geosynchronous orbits are pretty useless for this type of work, since they're so far away (you need really big telescopes to get much resolution). Also, you'd only see one hemisphere, and half the year it'd be nighttime over the areas you want to see.
Defining optical resolution from space is a bit tricky, as several generations of optical engineers have discovered.
The main criterion is the telescope's point spread function - this is roughly the angular diameter that a pinpoint star appears to be, as seen through the telescope. We want the smallest point spread function, and it should map onto about one to three sensor pixels. (arguments go here about over/undersampling).
The Fourier Transform of the point spread function is the Optical Transfer Function, which is a graph of the spatial frequencies response of the telescope. It's analogous to a hifi's frequency response ... it's an engineering challenge to prevent high frequencies from getting rolled off.
The main limit for high resolution is the diameter of the primary mirror (All mirrors and optical elements, no matter how perfect, have diffraction effects which spread out the light and reduce resolution. The bigger the entrance pupil, the greater the resolution) For the GeoEye, orbiting at 684Km and a resolution of 0.4m, I roughly calculate the primary mirror is somewhere around a half-meter diameter or so, depending on the wavelength of light it's optimized for.
Other things limit resolution - scattering of light in clear air (Rayleigh scattering) screws up the image, especially in the blue. Dust, haze, clouds and urban pollution are a bother, but not as much as you might think. Naturally, there's lots of image processing software ... quite compute intensive.
A typical human, seen from above and not casting a shadow, is about 20 to 60 cm across. So someone walking down the street should appear on a few (1 to 5) pixels. Not enough to recognize someone, especially since you're looking down on 'em.
Generally, images taken from aircraft have better resolution (they're closer, and there's less Rayleigh scattering). Perhaps airlines will attach automated, downward looking hires cameras to their daily flights.
Aargh! From these mathematicians grew the "New Math" of the early 1960's.
During the 1950's, high school math was mainly geometry, algebra, trig, and calculus.
Then came the New Math. Imported from France, it emphasized set theory, number bases, and abstract number theory. Students learned cardinality, commutative laws, associative laws, and "pure" math, with less applied math and problem-solving.
Many educators (and even more parents) saw the New Math as being too abstract for daily use and undercutting concrete skills such as computation. Physicists, especially objected, when college freshmen could calculate in multiple number bases, but couldn't solve algebraic equations.
Mathematician/singer Tom Lehrer wrote the song,"New Math", with the line, "It's so very simple that only a child can do it!"
One book, Why Johnny Can't Add - the Failure of the New Math, pointed out that in the mid 1970's teachers applauded the death of the New Math. By the late 70's, algebra was back in style, and even trigonometry was being taught. So ended the French invasion of high school math classes.
The latest, of course, is the new-new-math, also called rain-forest math. Don't get me started...
Well put!
I'm a ham, and I've thought that BPL meant "Brass Pounder's League", an elite group of ham operators who handled a lot of traffic.
I'm also a physicist; I walk out of colloquia when the speaker uses five acronyms I don't know. Seminar cookies aren't worth putting up with small minded arrogance.
I'm also a nonfiction writer. As the the parent posting suggests, acronyms kills your audience. Talk in tongues when you want to exclude others, when you're tight on bandwidth, or when you want to show you're a member of an in-group.
The lithium CR1216 batteries on my shelf started corroding after 4 years. Several of the AG3/CX41 alkaline batteries began leaking after 5 years. Still untouched, in their wrappers.
This article inspired me to dig out my ham radio crystals from the 1960's.
... all the more so for having fooled with quartz crystals forty years back.
"Rock-Bound" ham radio transmitters could only send messages at one frequency, so amateur operators sometimes modified the crystals to change frequencies. You'd tune the quartz crystals by grinding them with fine powder -- a few swipes would change frequency from 7130 to 7133 KHz (called "KiloCycles" back in the dark ages of the 1960's).
I just photographed a couple such crystals and put them at http://picasaweb.google.com/BoomingHand/HamRadioCrystalsFromK7TAOnceWN2PSX where you can see the primitive technology. The quartz itself is about a square centimeter and maybe a half millimeter thick.
All of this was made obsolete by quality variable frequency oscillators, often using phase-locked loops.
I'm thrilled to see the 4.5 GHz resonators at Cornell
One of the first Cyclotrons is on display, outside in the weather, at the Lawrence Hall of Science in Berkeley, California. The 37 inch Cyclotron is big -- you can climb on it, although you'll get pretty rusty. (In fact, only the magnet's core is on display; the Cyclotron's Dees are missing). For a photo with seven Nobel Laureats standing in front of the ol' beast, see http://dsd.lbl.gov/Seaborg.talks/65th-anniv/27.html
KH-11 series spacecraft were called the Key Hole satellites - they were the first large reconnaissance spacecraft to send images directly to earth; previous spy satellites used film return (clumsy, slow, and unreliable). KH-11's used CCDs - quite advanced for a system developed in the late 1970's.
The seven KH-11 spacecraft had primary mirrors of 2.3 to 2.4 meters. The system provided an ultimate ground resolution between 15 to 50 cm at closest approach (perigee); actual resolution was quite a bit worse.
There's no nuclear battery on board -- power came from 11 unfolded solar panels (which, on the first Key Hole satellites didn't provide quite enough power during downlinks!). I assume the main danger to earthlings is due to the reentry of the main mirror. Since the KH-11s are in polar orbits, the debris could come down anywhere on earth, with a one-in-four chance of hitting land.
The KH-11 spy satellites were developed in parallel with the Hubble Space Telescope, and the same contractors worked on both. In fact, the KH-11 uses much the same hardware (carbon-graphite support system, front door hatch system, data-relay dish through communications satellites). Because of the secrecy surrounding the KH-11 development, the Space Telescope project often saw similar secrecy. Indeed, astronomers were discouraged (or barred) from much of the engineering of the Hubble Space Telescope.
Around 1956, electronics makers began selling hybrid radios with both vacuum tubes and transistors. Emerson's vest-pocket portable model 843 used tubes in the rf stages and a pair of plug-in transistors for audio output. A 6 volt battery lit up the tubes and transistors, while a 67 volt battery kept the tubes' electrons jumping from cathodes to plates.
From Emerson's adverts: "Transistors are so tiny they must be seen to be believed. Transistors are so sturdy they won't break... They will last for life!" and give "greater power without distortion - full reproduction of voice and instruments, balanced tone quality, and greater power output with less distortion, not to mention low battery drain"
What other mixed hybrids have came along? Was there ever a hybrid horse and car?
Curt Herzstark designed the Curta handheld calculator while he was a prisoner in the Buchenwald concentration camp. Upon release in 1945, he started a company to manufacture these mechanical handheld calculators.
... he designed it to be handheld. The main cylinder fits within the hand, and the input sliders were made to be set by fingers. In a foreshadowing of computer architecture, he used complimentary arithmetic to do both subtraction and addition.
Herzstark recognized the importance of user interface
Although crank-driven, a Curta is surprisingly fast at the basic four functions. This is because you can rotate the output register to do automatic multiplies by powers of ten.
Made in Lichtenstein, the Curtas were superbly machined, with a feel comparable to a high quality Nikon F camera.
His peppermill calculators were sold from 1947 until 1972; today, they're mostly collectors items. But I use one to run my Klein Bottle business.
Geostationary spacecraft aren't as stationary as we'd like. Due to many forces (the earth's oblateness, tessoral harmonics in the earth's gravitational field, gravity from the moon and the sun), these spacecraft tend to drift, requiring occasional burns of small rockets to keep the spacecraft where it belongs.
... recording exactly how many grams of fuel was used in each burn. This is imprecise, because of the nature of propellant gauging by measuring pressure and timing burns. So every now and then, the four tanks of hydrozine would be rebalanced by connecting all the tanks together and letting the fuel equilibrate between 'em. Rebalancing the tanks is done by warming a tank and connecting it to the others. The amount of heat to put into a tank depends on how much fuel is in there, but you can't directly measure this ... you depend on book keeping.
In the spacecraft, each of four tanks contains the fuel (hydrazine) and a pressurizing gas (typically helium). There's a system of pipes and valves to allow any tank to feed any of the sets of x-y-z rocket motors. Of course, valves are unreliable, so there's the usual redundancies and crosslinked fuel pipes.
Stationkeeping in geosynchronous satellites requires precisely metered burns at just the right times. Shoot too much hydrazine, and the satellite moves out of the window, and everyone's TV reception goes to pot. Worse, you'll have to fire the rockets again and use more fuel to undo the damage from the previous burn. Too little hydrazine means that you'll need several burns, but these can only be done at certain times. If your first burn is insufficient, you may have to wait for a month (or sometimes six months) before you can fix it. (In fact, you seldom know the exact effects of a burn until doppler & tracking data is analyzed over the next days)
Now, suppose the satellite is low on fuel -- it's near the end of a 15 year lifespan. Three tanks have a little liquid fuel. The fourth tank runs out. If you then simply mix the four tanks, the output fuel line will get a mix of hydrazine and helium. The two phases in the fuel line will cause the motor to sputter, flare, or fizzle. Bad news!
So this is a non-trivial problem. And there's lots of money hanging on the answer.
In the past, the amount of fuel in each tank was determined by simple book-keeping
This paper sounds like they're relating the amount of heat put into a tank, and the tank's temperature. From this relationship, they're getting a better determination of the total hydrazine in each tank, and thus they can better balance the fuel in each tank.
In short, they came up with a nice way to estimate the amount of hydrazine in each tank by measuring the thermal effects. It's a good idea. Might add a few months to the lifespans of some old spacecraft which were launched in the 1990's.
Yep, same guy.
In the 1950's, data was recorded on 7-track tapes at 200 bits per inch. Those 7 tracks recorded 6-bit characters (EBCDIC?) with one bit of parity. A 2400 foot tape might hold some 5 million characters (upper case only, please). By the mid 1960's tape densities more than doubled to 556 bpi.
Sorting algorithms were written to sort information using mag tapes; the speediest would make the tape vibrate in the vacuum columns with a minimum of reel-to-reel motion.
By the 1970's, most shops changed to 800bpi, 9-track tapes, which would happily handle 8-track encoding. Then came 1600 bits per inch -- you could store an amazing 50 megabytes onto a single tape.
There was a constant temptation to compress data so as to stuff as much onto the tape as possible. As a result, many graduate students earned their assistantships by decoding tapes written with oddball parity, density, and encoding combinations.
The scattering matrices from my dissertation are encoded onto 9-track 1600 bpi tapes, carefully stored in my climate uncontrolled attic.