A Dark Sky Over Death Valley (APOD 13 July 2008)

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Expand view Topic review: A Dark Sky Over Death Valley (APOD 13 July 2008)

Re: 2008 July 13 "A dark sky over death valley"

by Chris Peterson » Wed Dec 23, 2009 11:47 pm

matt4444 wrote:I was under the impression that an exposure of a few minutes would result in trails or streaks around the stars. Not so?
It depends on the focal length and where you are pointing in the sky. There is always trailing, it's just a question of how evident it is.

Apparently this image was made with a high quality CCD camera, which probably had a quantum efficiency of around 70%. That being the case, and with a good lens, the exposure for any one tile could have been well under a minute, in which case you'd probably not notice trailing at this image scale.

Re: 2008 July 13 "A dark sky over death valley"

by matt4444 » Wed Dec 23, 2009 11:19 pm

Chris Peterson wrote:
matt4444 wrote:Is that picture of the sky over Death Valley an infrared image? I ask because I'm wondering if that image is representative of what the naked eye would have seen that night at Death Valley. With no moon in the sky, I don't see how the ground could be so illuminated.
It isn't an IR image. This is what you get with an exposure of a few minutes with any camera (film or digital) that can collect photons over that period. The starlit sky is plenty bright enough to illuminate the ground if you expose long enough. That's something that you can't see with the eye, because its maximum exposure time is only about 1/10 second.
By the way, if this image isn't representative of a naked eye viewing of the sky from a really dark place, does anyone have a pic that is?
If you are well dark adapted, you'll see the lake bed as brighter than the background mountains, and you'll see a rich sky, but nowhere near as bright as what the picture shows. You probably won't see clouds, for instance, except as regions without stars.

I was under the impression that an exposure of a few minutes would result in trails or streaks around the stars. Not so?

Geckzilla, the satellite image of the very white appearance of the lake bed was helpful. I didn't realize it was like that.

Re: A Dark Sky Over Death Valley (APOD 13 July 2008)

by neufer » Wed Dec 23, 2009 9:42 pm

bystander wrote:Are the sailing stones why they call it The Racetrack?
Playaing The Racetrack card are we?

http://antwrp.gsfc.nasa.gov/apod/ap020410.html
http://www.urbandictionary.com/define.php?term=playa wrote:
*PLAYA* : A person who is competitive and gregarious by nature. The skill of a playa is measured by the extent of his or her "game." The more "game" a playa has, the more respect they command in their community. A person who has enough game (and hence, enough respect) can do whatever they want, dress however awful (or tacky) they want, say whatever crazy things they want to say, and still win the adoration of others. Often these skills are used to earn sexual or material favors, although not by necessity. Playas are mostly motivated by the ego drive that comes with self expression and self manifestation for its own ends. In many ways, playas are trend-setters rather than trend-followers.

Re: A Dark Sky Over Death Valley (APOD 13 July 2008)

by bystander » Wed Dec 23, 2009 8:43 pm

And now, for something completely different ...

Are the sailing stones why they call it The Racetrack?

Re: A Dark Sky Over Death Valley (APOD 13 July 2008)

by geckzilla » Wed Dec 23, 2009 7:29 pm

Chris Peterson wrote:Picture a more conventional image, with a fully overcast sky and a sheet of paper on the ground. Are there conditions under which the paper will appear brighter than the sky in a wide angle image?
No, I don't think so. As long as the angle is wide enough for the brightest part of the clouds to be included the clouds should be brighter. Now you have me thinking too much into it. It makes sense as it is to me and it does not look like the ground was selectively brightened to me, either. I wonder what a piece of paper laying in the foreground on the clay would do to the image.

Re: A Dark Sky Over Death Valley (APOD 13 July 2008)

by Chris Peterson » Wed Dec 23, 2009 7:07 pm

geckzilla wrote:I'm having a hard time understanding the confusion. The racetrack playa is made of bright white clay. Take a look at a satellite image of it.
I think the reason it seems a bit counter-intuitive is that you have both the illuminated ground and the illuminating sky in the same image. You don't expect the former to appear brighter than the latter.

Picture a more conventional image, with a fully overcast sky and a sheet of paper on the ground. Are there conditions under which the paper will appear brighter than the sky in a wide angle image?

Re: 2008 July 13 "A dark sky over death valley"

by Chris Peterson » Wed Dec 23, 2009 7:02 pm

neufer wrote:You are probably right again, Chris; however, it does seem counter intuitive to me (and presumably to matt4444) that the ground should, at least, appear to be so much brighter than the "dark sky." Is this an optical illusion of some sort?
I don't think it's an optical illusion. I've seen the effect visually when I'm on desert sand- the ground appears brighter than the sky. In this image, it isn't clear that the ground is really "so much" brighter than the sky; there are clearly parts of the sky that are brighter than the lake bed. There seems to be some horizon light- either from the Sun or Moon below it, or from distant light pollution, as well as the very bright Milky Way.

These shots are normally produced with a high quality CCD camera utilizing a photometric V (green) filter. For purposes of studying light pollution, you would keep the data linear, but this image may have been selectively stretched to make the foreground brighter. The caption says it was processed, but doesn't give any detail.

Re: A Dark Sky Over Death Valley (APOD 13 July 2008)

by geckzilla » Wed Dec 23, 2009 6:52 pm

I'm having a hard time understanding the confusion. The racetrack playa is made of bright white clay. Take a look at a satellite image of it.

Re: 2008 July 13 "A dark sky over death valley"

by neufer » Wed Dec 23, 2009 6:40 pm

Chris Peterson wrote:
matt4444 wrote:Is that picture of the sky over Death Valley an infrared image? I ask because I'm wondering if that image is representative of what the naked eye would have seen that night at Death Valley. With no moon in the sky, I don't see how the ground could be so illuminated.
It isn't an IR image. This is what you get with an exposure of a few minutes with any camera (film or digital) that can collect photons over that period. The starlit sky is plenty bright enough to illuminate the ground if you expose long enough. That's something that you can't see with the eye, because its maximum exposure time is only about 1/10 second.
You are probably right again, Chris; however, it does seem counter intuitive to me (and presumably to matt4444) that the ground should, at least, appear to be so much brighter than the "dark sky." Is this an optical illusion of some sort?

Re: 2008 July 13 "A dark sky over death valley"

by Chris Peterson » Wed Dec 23, 2009 5:19 am

matt4444 wrote:Is that picture of the sky over Death Valley an infrared image? I ask because I'm wondering if that image is representative of what the naked eye would have seen that night at Death Valley. With no moon in the sky, I don't see how the ground could be so illuminated.
It isn't an IR image. This is what you get with an exposure of a few minutes with any camera (film or digital) that can collect photons over that period. The starlit sky is plenty bright enough to illuminate the ground if you expose long enough. That's something that you can't see with the eye, because its maximum exposure time is only about 1/10 second.
By the way, if this image isn't representative of a naked eye viewing of the sky from a really dark place, does anyone have a pic that is?
If you are well dark adapted, you'll see the lake bed as brighter than the background mountains, and you'll see a rich sky, but nowhere near as bright as what the picture shows. You probably won't see clouds, for instance, except as regions without stars.

2008 July 13 "A dark sky over death valley"

by matt4444 » Wed Dec 23, 2009 12:45 am

http://antwrp.gsfc.nasa.gov/apod/ap080713.html

Is that picture of the sky over Death Valley an infrared image? I ask because I'm wondering if that image is representative of what the naked eye would have seen that night at Death Valley. With no moon in the sky, I don't see how the ground could be so illuminated. But then again, unfortunately, I no absolutely nothing about photography. Thanks for any help!

By the way, if this image isn't representative of a naked eye viewing of the sky from a really dark place, does anyone have a pic that is?

Death Valley

by jackturner1abc » Tue Jul 15, 2008 1:37 pm

I have been viewing APOD since 2003 and I tell everybody about it. It is educational and my favorite eye candy. I actuall like seeing the same picture more than once, but one that has shown technological advances in lenses. Example the "man in Mars". Originally thought to be a structure turned out to be shadowing; good revisit. The Death valley shot shows nothing new, was done within the last year and there are just too many other fantastic shots not being shown. There should be more photos from the satellites we have around Saturn, on Mars, around the Sun and Mercury. Nonetheless, Death Valley at night is one I kept.

by DavidLeodis » Mon Jul 14, 2008 12:07 pm

I have no problem with repeating an APOD image, but the explanation should be updated. In that to the APOD of July 13 2008 it states its image is a "composite of 30 images taken two years ago", which is the same as that given in the APOD of May 8 2007.

Death Valley

by orin stepanek » Sun Jul 13, 2008 12:56 pm

They put shielded lights near airports; so I don't think it would be much of a problem to do this on new street lights.
Orin

Re: Death Valley

by apodman » Sun Jul 13, 2008 12:48 pm

wolfie138 wrote:they'd had this one before, 8 may 2007
Yes, thank you, here's the link ...

http://antwrp.gsfc.nasa.gov/apod/ap070508.html

... and a link to the brief discussion ...

http://asterisk.apod.com/vie ... hp?t=11432

... and a link to a close-up of a (different?) moved rock ...

http://antwrp.gsfc.nasa.gov/apod/ap020410.html

Look at the hand in the rock's track in the link above, then look at the rock in the rock's track for a sense of scale. If this rock is the one they're talking about that weighs 300 kg (660 pounds for the metrically challenged), it's awful dense because it isn't very big.

From http://geosun.sjsu.edu/paula/rtp/ ...

Image

This animation reminds me of the crabs moving the ship across the desert in Pirates of the Caribbean: At World's End.

The caption from 070508 is almost the same as 080713:

08 May 2007: "Light pollution is threatening dark skies like this all across the US"

13 July 2008: "Light pollution is threatening dark skies like this all across the US and the world"

Looks like this thing is spreading. Maybe Mars is next. And it must be really bad on the Sun. 8)

Seriously, according to http://www.sciencemonster.com/planets_sun.html the Sun's brightness is equal to 4,000,000,000,000,000,000,000,000 light bulbs. On Earth, that would be about 700,000,000,000,000 light bulbs (a 4-mile cube of them) per person. The same site says I would weigh 5040 pounds on the Sun (2290 kg for the English-units challenged), and there's nothing "light" about that. But I think "kids" science sites like this are great.

Death Valley

by wolfie138 » Sun Jul 13, 2008 6:56 am

boo, they'd had this one before, 8 may 2007 :-(

glorious pic tho. totally agree re light pollution. seems to be getting harder and harder to just go out in the back garden to see stars/meteors etc - even w/out the goddamn council having put a goddamn streetlight right outside the house >:-(

A Dark Sky Over Death Valley (APOD 13 July 2008)

by apodman » Sun Jul 13, 2008 5:04 am

http://antwrp.gsfc.nasa.gov/apod/ap080713.html

As previously stated, I'm against light pollution.

Among all the problems in the world, this is one where we can actually improve the condition through easily understandable actions and conventional methods and materials.

What we have to do is basically aim our lights where we need the light and not elsewhere, especially up into the sky where it is wasted anyway and washes out the wonders of nature.

The technology we need is no more than shielded light fixtures. Most light fixtures are shielded anyway, they just need to use shields of the right shape and orientation. But somebody has to buy and install all these fixtures, so somebody has to keep pestering architects, designers, planners, and governments to remind them to do it right. Doing it right saves money on electricity, too. You don't need to start by replacing all the old fixtures at great expense, but at least do new installations right.

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