Starship Asterisk*

by **geckzilla** » Sun Jan 12, 2014 5:46 am

deathfleer wrote:is the storm in the sky related to the unusual downpour of the rain in the Earth

No. Also, there is nothing unusual about downpours of rain.

by **deathfleer** » Sun Jan 12, 2014 5:44 am

is the storm in the sky related to the unusual downpour of the rain in the Earth

by **rstevenson** » Fri Jan 10, 2014 12:19 pm

Nitpicker wrote:Imagine Neil Armstrong's quote without a ladder:
That's one small step ... whoa ... actually that was a doosie! ...

That's one slow plunge for a man, one giant face-plant for mankind.

Rob

by **Nitpicker** » Fri Jan 10, 2014 1:54 am

Chris Peterson wrote:
Beyond wrote:Like the warnings pasted all over ladders. IF one paid attention to all of them, one would never, ever use a ladder
Ladders and aspirin. Two products that would never be approved for sale if they were invented today.

Imagine Neil Armstrong's quote without a ladder:

That's one small step ... whoa ... actually that was a doosie! ...

by **Chris Peterson** » Fri Jan 10, 2014 1:42 am

Beyond wrote:Like the warnings pasted all over ladders. IF one paid attention to all of them, one would never, ever use a ladder :!: :lol2:

Ladders and aspirin. Two products that would never be approved for sale if they were invented today.

by **Beyond** » Fri Jan 10, 2014 1:34 am

Like the warnings pasted all over ladders. IF one paid attention to all of them, one would never, ever use a ladder

by **Chris Peterson** » Fri Jan 10, 2014 12:12 am

Nitpicker wrote:Still, there are people out there who do silly things all the time.

That's why there are warning stickers all over things, and manuals are full of them. And smart people generally figure out which warnings matter (and when), and which don't.

by **Nitpicker** » Fri Jan 10, 2014 12:05 am

Chris Peterson wrote:What matters isn't the FOV of the lens, but the lens aperture. You aren't going to set fire to paper with a lens a few millimeters across; one a few hundred millimeters across can melt aluminum. ...
One indicator that having the Sun in the FOV of the camera is generally harmless is the fact that even when the Sun's image isn't on the sensor, there's a good chance it's on some internal surface of the camera. And that doesn't cause any problems.

I suppose if I were silly enough to setup my camera on a tripod, pointed at the midday sun, with my 300mm lens, unfiltered at f/5.6 (aperture ~54mm), it might do some damage to the sensor, shutter or viewfinder. But I certainly wouldn't be able to photograph the sun like that. I'd probably need to set the lens at f/22 (aperture ~14mm) to have a hope of imaging it, which seems pretty harmless. Still, there are people out there who do silly things all the time.

by **Chris Peterson** » Thu Jan 09, 2014 3:16 pm

neufer wrote:
Chris Peterson wrote:
As far as your eye is concerned, magnification is an important factor. A small image of the Sun on your retina doesn't easily cause damage because the heat can be carried away. A large image (with the same surface brightness) can almost instantly cause permanent damage.
I've always thought the primary damage from looking at the (non-sunrise/sunset) sun was due to UV not heat.

UV is associated with long term damage to the retina from simply being outdoors a lot. It causes various retinopathies. Curiously, many older people who wore sunglasses most of their lives are seeing these problems now, because until about 20 years ago sunglasses didn't block UV, but they did block visible light, resulting in larger pupils and more UV exposure than with no sunglasses at all.

But if you stare at the Sun while it's high in the sky, thermal damage will probably kick in before permanent damage from UV (although the latter can cause temporary localized blindness lasting up to several weeks).

A real concern is that a certain percentage of the population have underlying pathologies that predispose them to retinal phototoxicity. Much shorter exposure to bright light can cause permanent damage. It's a major source of complications from otherwise simple eye examinations and procedures that depend on the use of a bright source to illuminate the retina. Most of us just see a bright light for a few seconds; in a few percent of people, there is temporary or even permanent damage. That's not a thermal effect at all.

When I was a laser safety officer at Beckman Instruments many years ago, I saw two kinds of retinal damage from laser exposure- thermal and chemical. The thermal burns were visible on the retina; the photochemical damage wasn't visible, but could only be detected with specialized vision tests.

There are crazy people who believe there are health benefits from staring at the Sun. They tend to go blind. Luckily, there's a blink reflex that protects us from accidental damage. Unless we're observing with some sort of equipment, we can generally trust that if it's uncomfortable, we shouldn't look long, if it's not there's probably little danger. As with many sunrises and sunsets (the setting Sun is usually more attenuated).

by **neufer** » Thu Jan 09, 2014 3:02 pm

Chris Peterson wrote:
As far as your eye is concerned, magnification is an important factor. A small image of the Sun on your retina doesn't easily cause damage because the heat can be carried away. A large image (with the same surface brightness) can almost instantly cause permanent damage.

I've always thought the primary damage from looking at the (non-sunrise/sunset) sun was due to UV not heat.

by **Chris Peterson** » Thu Jan 09, 2014 2:46 pm

Nitpicker wrote:This is from the safety pages of my DSLR's manual:
...
I think the second warning is a pretty good warning, but the first one may be over the top, even when the sun is high and even if the camera is on a fixed tripod. I wonder if the risk of damage to the camera is greater with the shutter closed, or the mirror down, or even to the perimeter of the sensor? As for damaging the sensor itself, I imagine a wider image might be worse, in terms of concentrating the direct photons from the Sun onto fewer pixels. But I really don't know. I keep thinking about when I was a kid and setting fire to paper with a magnifying glass.

I think that both warnings are exactly what I'd put in a similar product I was selling. And personally, I'd not observe either of them very diligently.

What matters isn't the FOV of the lens, but the lens aperture. You aren't going to set fire to paper with a lens a few millimeters across; one a few hundred millimeters across can melt aluminum. As far as your eye is concerned, magnification is an important factor. A small image of the Sun on your retina doesn't easily cause damage because the heat can be carried away. A large image (with the same surface brightness) can almost instantly cause permanent damage.

One indicator that having the Sun in the FOV of the camera is generally harmless is the fact that even when the Sun's image isn't on the sensor, there's a good chance it's on some internal surface of the camera. And that doesn't cause any problems.

by **Nitpicker** » Thu Jan 09, 2014 2:59 am

This is from the safety pages of my DSLR's manual:

: nanny_state.JPG (34 KiB) Viewed 5450 times

I think the second warning is a pretty good warning, but the first one may be over the top, even when the sun is high and even if the camera is on a fixed tripod. I wonder if the risk of damage to the camera is greater with the shutter closed, or the mirror down, or even to the perimeter of the sensor? As for damaging the sensor itself, I imagine a wider image might be worse, in terms of concentrating the direct photons from the Sun onto fewer pixels. But I really don't know. I keep thinking about when I was a kid and setting fire to paper with a magnifying glass.

by **Chris Peterson** » Thu Jan 09, 2014 2:31 am

Nitpicker wrote:It is always a good idea to exercise caution.

Certainly. Nevertheless, it is somewhere between difficult and impossible to damage your eyes looking at the Sun if it's attenuated below the level of causing discomfort, unless you magnify it.

The few minutes after sunrises and before sunsets are typically quite safe for your unfiltered camera to photograph the Sun, although I recommend not looking through the view finder, if possible, but using the live view screen instead. You can normally glimpse directly at the Sun at these times without doing any permanent damage to your naked eyes, but I'd still recommend you don't do it too much, or too often. And glimpse, rather than stare.

With the Sun higher in the sky, it is a different story altogether. If I happen to look anywhere near the Sun in the middle of the day, my body reacts almost involuntarily by throwing up an occulting hand. It would likely damage an unfiltered camera, too.

It also requires some effort to damage your eyes looking at the full daytime Sun. That's because the blood supply on your retina is very efficient at carrying away heat. So for most people, it requires quite a few seconds of staring at the Sun (which isn't easy) to cause permanent retinal damage or phototoxic retinopathy. Still, there are a variety of otherwise fairly mild pathologies that can amplify that effect. Nobody in their right mind would suggest staring at the Sun when it's high in the sky.

I wouldn't worry about a camera. If it isn't mounted on a tripod, it isn't going to point to the Sun long enough to cause sensor damage. My allsky cameras (which are fixed mount, of course) run all day, with their lenses open, and the slowly moving Sun tracing a focal line across their sensors. Some of those cameras have been operating more than ten years, and still no damage.

by **Nitpicker** » Thu Jan 09, 2014 1:05 am

Chris Peterson wrote:
Anthony Barreiro wrote:I wouldn't encourage people to try to see sunspots with their naked eyes. Even when the Sun is low to the horizon and the visible light is significantly attenuated, there's still a lot of dangerous infrared radiation. It would be safer to say something like, "visible without magnification through a safe solar filter."
This is a bit of a myth. In fact, the cornea and lens pass nothing longer than 1200 nm. Except for a transmission peak around 1100 nm, nearly the entire IR range shorter than 1200 nm is highly attenuated. And even at sunset, the IR component of sunlight has an irradiance less than half of that of the peak visible light.

The reality is, if the Sun is attenuated enough in brightness by the atmosphere that you can look at it comfortably, it is going to be very difficult to damage your eyes. There simply isn't enough UV or IR making it to the back of the eye. You actually have to stare at the full, unattenuated Sun for quite a while (and quite painfully) to damage the retina if you aren't using any magnification.

It is always a good idea to exercise caution. I was just reading in the news about a whole bunch of North Americans burning themselves with boiling water yesterday, after throwing it (poorly) into the freezing air and trying to make snow, after watching a broadcast of someone else doing it (safely). There's nothing like a good example!

The few minutes after sunrises and before sunsets are typically quite safe for your unfiltered camera to photograph the Sun, although I recommend not looking through the view finder, if possible, but using the live view screen instead. You can normally glimpse directly at the Sun at these times without doing any permanent damage to your naked eyes, but I'd still recommend you don't do it too much, or too often. And glimpse, rather than stare.

With the Sun higher in the sky, it is a different story altogether. If I happen to look anywhere near the Sun in the middle of the day, my body reacts almost involuntarily by throwing up an occulting hand. It would likely damage an unfiltered camera, too.

by **Nitpicker** » Thu Jan 09, 2014 12:39 am

FloridaMike wrote:
Nitpicker wrote:<animated gif>
Animated gif of the sunset on Jan 5 from my backyard in Brisbane, seen through smoky bushfire haze.
Great work. I'd love to do something like that. Is the reddened tree shadow the most distant of three trees between the camera and the sun? The red effect is unexpected.

Thanks Mike. The gum tree in the foreground is less than 100m from the camera. The gum trees on the ridgeline behind which the Sun sets, are about 1km away. The reddening of the tips of these more distant trees, is merely the effect of some of the sunlight passing through the thinner parts of the foliage.

The shots were taken with no filters whatsoever (apart from the smoke, but even that may not have been necessary, it just made it easier for the exposures to capture both the sunspots and the trees) with the sun about 6° above horizontal.

Chris Peterson wrote:I'm not sure it could be called a flash. Any image of the Sun near the horizon (or stars, for that matter) will show the effects of dispersion, with a red fringe at the bottom and a blue fringe at the top. That effect is stable, lasting for many minutes.

I'd say I've never noticed a green flash since I've lived most of my life on the east coast of a continent, or nowhere near the ocean. Green flashes seem most common over the ocean. I've only ever actively observed a handful of sunsets over the ocean in other parts of the world, and (prone to a nice sleep in) only a handful of sunrises anywhere.

Prismatic dispersion of light does make more sense in this APOD (thanks Chris). I've never noticed it in any of my sunset or sunrise shots, but that's not that many. However, every single crepuscular colour shot of Mercury and Venus I've ever taken, shows it to some extent. And I can often see it if I bother to observe stars low on the horizon, too. Initially I thought this was an aberration in my optics, but it is really an aberration of the sky (which some people seem to like). Anyway, it just made me revisit the theory on this topic, which immediately confused me, as my initial reckoning put red on the top and blue on the bottom. It was not until I read the last section -- "A Seeming Discrepancy Explained" -- in the following link, that it suddenly made sense:
http://www.astropix.com/HTML/L_STORY/At ... ersion.HTM

by **Chris Peterson** » Wed Jan 08, 2014 8:22 pm

Ron-Astro Pharmacist wrote:I was about to ask a dumb question such as, “Why is the landscape to dark?” when it dawned on me that this photo must have been taken through a solar filter. Not exactly naked-eye but I got that point. It might have more accurate to say "was visible but taken with magnification through a safe solar filter." That was probably obvious to most.

Nope. I don't think this image was made through any filters. Solar filters attenuate the light so much that you'd see nothing but the Sun- no landscape, no clouds. And at this time of day, you'd barely even see the Sun itself.

The foreground and sky are dark simply because of the limited dynamic range of the camera. Getting the proper exposure for the Sun meant that the rest of the scene was underexposed. This is pretty much how any ordinary, unfiltered camera will show the setting Sun.

by **Ron-Astro Pharmacist** » Wed Jan 08, 2014 8:09 pm

Anthony Barreiro wrote:
APOD Robot wrote:... A sunspot so large it was visible to the naked eye ... .
I wouldn't encourage people to try to see sunspots with their naked eyes. Even when the Sun is low to the horizon and the visible light is significantly attenuated, there's still a lot of dangerous infrared radiation. It would be safer to say something like, "visible without magnification through a safe solar filter."

I was about to ask a dumb question such as, “Why is the landscape to dark?” when it dawned on me that this photo must have been taken through a solar filter. Not exactly naked-eye but I got that point. It might have more accurate to say "was visible but taken with magnification through a safe solar filter." That was probably obvious to most.

by **Chris Peterson** » Wed Jan 08, 2014 7:08 pm

Anthony Barreiro wrote:
APOD Robot wrote:... A sunspot so large it was visible to the naked eye ... .
I wouldn't encourage people to try to see sunspots with their naked eyes. Even when the Sun is low to the horizon and the visible light is significantly attenuated, there's still a lot of dangerous infrared radiation. It would be safer to say something like, "visible without magnification through a safe solar filter."

This is a bit of a myth. In fact, the cornea and lens pass nothing longer than 1200 nm. Except for a transmission peak around 1100 nm, nearly the entire IR range shorter than 1200 nm is highly attenuated. And even at sunset, the IR component of sunlight has an irradiance less than half of that of the peak visible light.

The reality is, if the Sun is attenuated enough in brightness by the atmosphere that you can look at it comfortably, it is going to be very difficult to damage your eyes. There simply isn't enough UV or IR making it to the back of the eye. You actually have to stare at the full, unattenuated Sun for quite a while (and quite painfully) to damage the retina if you aren't using any magnification.

by **Anthony Barreiro** » Wed Jan 08, 2014 6:52 pm

APOD Robot wrote:... A sunspot so large it was visible to the naked eye ... .

I wouldn't encourage people to try to see sunspots with their naked eyes. Even when the Sun is low to the horizon and the visible light is significantly attenuated, there's still a lot of dangerous infrared radiation. It would be safer to say something like, "visible without magnification through a safe solar filter."

by **FloridaMike** » Wed Jan 08, 2014 6:32 pm

Nitpicker wrote:
Click to view full size image
Animated gif of the sunset on Jan 5 from my backyard in Brisbane, seen through smoky bushfire haze.

Great work. I'd love to do something like that. Is the reddened tree shadow the most distant of three trees between the camera and the sun? The red effect is unexpected.

by **Rothkko** » Wed Jan 08, 2014 6:10 pm

este flash duró unos pocos segundos, y parece tan verde como azul
this flash lasted a few seconds, and it looks as green as blue

http://asterisk.apod.com/viewtopic.php? ... 50#p217555

by **Ron-Astro Pharmacist** » Wed Jan 08, 2014 5:41 pm

CPatriot wrote:It takes light 8 minutes to reach earth from the sun.? what would be the speed of the energy released by these events.? Occurred on the 7th effects on earth on the 9th.?

Lots of information at this site.

http://www.spaceweather.com/

by **CPatriot** » Wed Jan 08, 2014 5:24 pm

It takes light 8 minutes to reach earth from the sun.? what would be the speed of the energy released by these events.? Occurred on the 7th effects on earth on the 9th.?

by **geckzilla** » Wed Jan 08, 2014 4:41 pm

This page at AtOptics discusses the difference between green rim and green flash.

by **Chris Peterson** » Wed Jan 08, 2014 4:11 pm

Ray-Optics2 wrote:I'd call it a green flash (although where people draw their line between blue and green is fluid.) I've seen scores of them - they're very common from Southern California beaches. They are NOT stable over time, usually lasting no more than a second.

The green flash is certainly not stable. But the red and blue fringes seen above and below any body near the horizon are certainly stable (and virtually always present, under all conditions).

No green flash is evident in this image.

Starship Asterisk*

APOD: Sunspot at Sunset (2014 Jan 08)

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