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FORUMS Photography Talk by Genre Astronomy & Celestial Talk 
Thread started 01 Feb 2016 (Monday) 21:56
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Astrophotography Over the Ocean

 
JMarro
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Feb 01, 2016 21:56 |  #1

Hello everyone,

I am not too familiar with night sky shooting. I have done it maybe a handful of times (see below). I know the basics (focus, 500 rule, etc.), but there is still much to learn. I live in central Florida and have been location scouting for a place by the beach. I have a few questions regarding some of the technical aspects of night time visibility. Light pollution is a major factor where I live. My question is, if I was to wait for the right night, where cloud coverage is low, seeing is optimal, and moonlight is not a factor, would shooting over the ocean reduce the light pollution? I have found an area near Cocoa Beach, which has an interesting foreground element I have been meaning to capture with startrails or possibly the milky way. Although light pollution is bad at Cocoa, I wonder how many stars would be visible overlooking the ocean since there is obviously no light pollution in that direction. Another question I have is whether the humid air from the ocean will create visibility issues? Do any of you have any experience with this?

Thanks,
JMarro

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WildImages
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Feb 02, 2016 09:17 |  #2

I have wondered about the MW with an ocean foreground. Thanks for posting.

That 500 rule you cite is suspicious. Many say one must be even more conservative like a 200 rule. A good source of info is www.clarkvision.com (external link). This guy has lots of experience in night imaging.




  
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JMarro
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Feb 02, 2016 09:36 |  #3

WildImages wrote in post #17882931 (external link)
I have wondered about the MW with an ocean foreground. Thanks for posting.

That 500 rule you cite is suspicious. Many say one must be even more conservative like a 200 rule. A good source of info is www.clarkvision.com (external link). This guy has lots of experience in night imaging.

Yeah, I have read many articles about this rule. Some say 400, some say 500. I haven't seen down to 200, but I will look into it. With the few experiences I have had with star photography, I have gotten by on the 500 rule. Thanks for the info.


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TCampbell
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Feb 02, 2016 10:49 |  #4

Shooting in a direction where you don't have the "light domes" of urban light pollution certainly helps.

You can use websites such as:

cleardarksky.com
clearoutside.com

To check local sky conditions w.r.t. cloud cover, transparency, seeing conditions, etc.

BTW, "seeing conditions" is a term astronomers use to describe the stability of the atmosphere. The best analogy I've come across is the following... suppose you have a pool and you've placed a coin in the bottom of the pool. I had you some binoculars or a telescope and ask you to use it to read the year stamped on the coin. If nobody is making any waves in the pool and the water is absolutely still, then you can probably do it. But if I start making lots of waves in the pool while you try to read the date, you might still be able to tell that there is a coin at the bottom of the pool, but you almost certainly will not be able to read the date. The atmosphere really does work like that pool. Some nights when the seeing conditions are great, I can see amazing amounts of detail... and sometimes nothing will come to sharp focus (as if there is something wrong with the optics on the telescope -- even though it's really a problem with the atmosphere.)

"Seeing" really impacts detail on a very small scale... things that are so tiny that I need to resolve detail on the scale of a few arc-seconds or arc-minutes. When we get to large scale structures it won't be so noticeable. This is why the moon usually looks pretty good (the moon is roughly 30 arc-minutes from edge to edge), but planets are tiny and it's a much bigger issue. (Jupiter's angular width from edge to edge is currently 43 arc-seconds... less than 3/4 of an arc-minute. That means you could line up more than 40 "jupiters" from edge-to-edge in the width of the moon. Tiny amounts of atmospheric distortion will have a much more noticeable and adverse effects on the clarity of small structures (or more accurately, things that seem like small structures because their "angular" size is tiny -- even if the real object is enormous). One other things... wind on the ground is not necessarily an indication of a stable atmosphere. I've had clam days at ground level with lousy seeing. Ideally one would be in a location where the surface of the earth is flat (smooth laminar airflow without geographic creating turbulent air), and you would be at least 200 miles from the jet stream, any warm front, or any cold front.

When using an ultra-wide angle lens, the effects of seeing will not be very adverse on your images.

Moisture, dust, etc. in the atmosphere effect "transparency". The biggest problem with transparency is that when you're near sources of urban light pollution, all those lights have something hanging in the air to light up. A sky with no dust and no moisture in it will be much clearer and there aren't lots of tiny particles that reflect light from urban light pollution. So the sky will seem darker when the atmosphere is clear. This is a big deal for astrophotography because much of what we are trying to detect are faint gray patches of light. If the whole sky background is a faint gray (due to light pollution and poor transparency) then those details won't show up.

You don't want any moonlight in the sky because that will add light pollution and reduce contrast. Check your calendar and plan to shoot when it's near the "new moon" or even the "3rd quarter" moon (because the third quarter moon doesn't rise until a few hours before sunrise -- so it won't be in the sky after sunset and you'll have hours to work.)

The part of the Milky Way that faces the center of the galaxy (the direction of the constellation Sagittarius) is the most dramatic. The part that faces away (mostly what we see after sunset in the winter months) is much less dramatic (that's facing the direction going toward the outer edge of the galaxy). If you get up in the pre-dawn hours (but now you want to shoot at a 1st quarter or new moon -- not 3rd quarter) then you get a preview of the skies in the coming months and the summer Milky Way will be rising. Unfortunately right now it's rising so close to sunrise that by the time it's really "up" you're probably betting some pre-dawn twilight interfering with the image. Each month the rise & set times shift forward by 2 hours. At around 4am in April you can catch the "summer" Milky Way rising high enough to be dramatic. In May it's even better.




  
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JMarro
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Feb 03, 2016 12:10 |  #5

TCampbell wrote in post #17883042 (external link)
Shooting in a direction where you don't have the "light domes" of urban light pollution certainly helps.

You can use websites such as:

cleardarksky.com
clearoutside.com

To check local sky conditions w.r.t. cloud cover, transparency, seeing conditions, etc.

BTW, "seeing conditions" is a term astronomers use to describe the stability of the atmosphere. The best analogy I've come across is the following... suppose you have a pool and you've placed a coin in the bottom of the pool. I had you some binoculars or a telescope and ask you to use it to read the year stamped on the coin. If nobody is making any waves in the pool and the water is absolutely still, then you can probably do it. But if I start making lots of waves in the pool while you try to read the date, you might still be able to tell that there is a coin at the bottom of the pool, but you almost certainly will not be able to read the date. The atmosphere really does work like that pool. Some nights when the seeing conditions are great, I can see amazing amounts of detail... and sometimes nothing will come to sharp focus (as if there is something wrong with the optics on the telescope -- even though it's really a problem with the atmosphere.)

"Seeing" really impacts detail on a very small scale... things that are so tiny that I need to resolve detail on the scale of a few arc-seconds or arc-minutes. When we get to large scale structures it won't be so noticeable. This is why the moon usually looks pretty good (the moon is roughly 30 arc-minutes from edge to edge), but planets are tiny and it's a much bigger issue. (Jupiter's angular width from edge to edge is currently 43 arc-seconds... less than 3/4 of an arc-minute. That means you could line up more than 40 "jupiters" from edge-to-edge in the width of the moon. Tiny amounts of atmospheric distortion will have a much more noticeable and adverse effects on the clarity of small structures (or more accurately, things that seem like small structures because their "angular" size is tiny -- even if the real object is enormous). One other things... wind on the ground is not necessarily an indication of a stable atmosphere. I've had clam days at ground level with lousy seeing. Ideally one would be in a location where the surface of the earth is flat (smooth laminar airflow without geographic creating turbulent air), and you would be at least 200 miles from the jet stream, any warm front, or any cold front.

When using an ultra-wide angle lens, the effects of seeing will not be very adverse on your images.

Moisture, dust, etc. in the atmosphere effect "transparency". The biggest problem with transparency is that when you're near sources of urban light pollution, all those lights have something hanging in the air to light up. A sky with no dust and no moisture in it will be much clearer and there aren't lots of tiny particles that reflect light from urban light pollution. So the sky will seem darker when the atmosphere is clear. This is a big deal for astrophotography because much of what we are trying to detect are faint gray patches of light. If the whole sky background is a faint gray (due to light pollution and poor transparency) then those details won't show up.

You don't want any moonlight in the sky because that will add light pollution and reduce contrast. Check your calendar and plan to shoot when it's near the "new moon" or even the "3rd quarter" moon (because the third quarter moon doesn't rise until a few hours before sunrise -- so it won't be in the sky after sunset and you'll have hours to work.)

The part of the Milky Way that faces the center of the galaxy (the direction of the constellation Sagittarius) is the most dramatic. The part that faces away (mostly what we see after sunset in the winter months) is much less dramatic (that's facing the direction going toward the outer edge of the galaxy). If you get up in the pre-dawn hours (but now you want to shoot at a 1st quarter or new moon -- not 3rd quarter) then you get a preview of the skies in the coming months and the summer Milky Way will be rising. Unfortunately right now it's rising so close to sunrise that by the time it's really "up" you're probably betting some pre-dawn twilight interfering with the image. Each month the rise & set times shift forward by 2 hours. At around 4am in April you can catch the "summer" Milky Way rising high enough to be dramatic. In May it's even better.

Thanks for your detailed response. It helps a lot. So what do you suggest would be the minimum time before sunrise that you won't see the effect of light from the sun. 1.5 hrs? When I checked a few weeks back, the milky way was rising about an hour before sunrise.


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TCampbell
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Feb 03, 2016 15:47 |  #6

JMarro wrote in post #17884440 (external link)
Thanks for your detailed response. It helps a lot. So what do you suggest would be the minimum time before sunrise that you won't see the effect of light from the sun. 1.5 hrs? When I checked a few weeks back, the milky way was rising about an hour before sunrise.

If you don't want any light from the sun, then the answer is to look up the "astronomical twilight" and use that.

There are three twilights... "civil", "nautical", and "astronomical".

Civil twilight lasts from sunset until the sun drops 6º below the horizon. That's when most people think it is dark enough that they want lights to see.

Nautical twilight lasts until the sun drops to 12º below the horizon. There is enough light scatter in the sky for boats to navigate.

Astronomical twilight lasts until the sun drops to 18º below the horizon. During astronomical twilight there is still some light scatter from the sun. After the sun drops beyond 18º below the horizon you've reached the point where there is no sunlight scatter in the sky at all... it's finally "dark" (barring artificial light pollution).

The next question is "when" or "how long" do you have to wait for astronomical twilight to end. The answer is.. it depends on the time of year and where you are.

On the day of the equinox when you get 18º is 1/10th of 180º (half the rotation of the Earth) and that means the sun will be 18º below the horizon in just 72 minutes. But that's not going to work for everyone.

If, for example, you live just out of the Arctic or Antarctic circles, then the Sun is traveling nearly parallel to the horizon as it sets and even after sunset, the sun might *never* get to 18º below the horizon (depending on time of year and your location).

From my latitude (about 42.5º North) it's currently taking just about 1.5 hours ... but near the summer solstice it takes about 2 hours and 15 minutes.

Check on almanac and it should provide the times of "civil", "nautical", and "astronomical" twilights for both morning and evenings of any day you might be planning to shoot.

It's nice to be able to set up while there's still light -- so I usually plan to arrive at my shooting site a little before sunset, setup the gear, and then wait for dark to start imaging.




  
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Niteclicks
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Feb 05, 2016 09:07 |  #7

Go to http://www.usno.navy.m​il/ (external link) you can get everything and anything related to time. Print charts for a week ,month or year. Very useful site.




  
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Feb 05, 2016 09:18 |  #8

TCampbell wrote in post #17883042 (external link)
The part of the Milky Way that faces the center of the galaxy (the direction of the constellation Sagittarius) is the most dramatic. The part that faces away (mostly what we see after sunset in the winter months) is much less dramatic (that's facing the direction going toward the outer edge of the galaxy). If you get up in the pre-dawn hours (but now you want to shoot at a 1st quarter or new moon -- not 3rd quarter) then you get a preview of the skies in the coming months and the summer Milky Way will be rising. Unfortunately right now it's rising so close to sunrise that by the time it's really "up" you're probably betting some pre-dawn twilight interfering with the image. Each month the rise & set times shift forward by 2 hours. At around 4am in April you can catch the "summer" Milky Way rising high enough to be dramatic. In May it's even better.

Thanks for posting this info. I have an iPhone app called Photo Pils that gives info about the rise and fall of the MW plus gives an indication of where the "bulge" is, but I have never figured out how to predict when the best time to go looking would be.

Are there any other tips or resources on predicting MW position you could point out for us?


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Mar 05, 2016 03:43 |  #9

JMarro wrote in post #17882958 (external link)
Yeah, I have read many articles about this rule. Some say 400, some say 500. I haven't seen down to 200, but I will look into it. With the few experiences I have had with star photography, I have gotten by on the 500 rule. Thanks for the info.

just don't forget, the 500 rules to a full frame

if you are using a crop sensor, then you need to multiply the lens f/l by the crop factor, then dividing 500 by that figure

Dave


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