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Author Topic: Star trails can you help??? Have I got the facts straight.  (Read 3752 times)
sanfairyanne
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« on: May 06, 2011, 06:56:01 PM »
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I am off to Easter Island and notice that my visit coincides with a new moon, unfortunately right on my second to last day. I hope to do a star trail around the southern star i.e, all the stars will revolve around the southern star. I intend to put one of those stone Maui close to the centre and light it up with a flashlight.

Naturally Ill try a few days prior to the new moon but unfortunately I cant take a computer with me to see my results so Im hoping to get it right on one of those occasions.

Ill use a Canon 5DII with a 16-35mm, Im predicting I can get between 10 - 30 feet away from the stone figures which are somewhere in the region of 8 feet tall. Ive got my depth of field worked out so that should be ok.

Im expecting very little light polution, theres a town 15km away but its small and of course other than that Easter Island its way out in the Pacific.

Can anyone help with suggestions for settings, I envisage using the lens at its widest focal length of 16mm and an aperture of f4 gives hyperfocal distance around 8.5 feet to infinity so no trouble there. So my main concerns are centred around iso and time of exposure. Given the wonders of noise reduction software and the low light capabilities of the 5DII is it fair to say if I shoot a test shot at say one minute at 800iso f4 and that test shot is not wildely over or under exposed then I can just stick with that.
Then if I tack together 40 exposures along with a second or two of flashlight painting on each exposure into the relevant software and all should be ok. I realise I should be aware of condensation. Shoot in RAW. White Balance at daylight. Camera noise reducion off. IS off.   Am I on the right track?  Many thanks.
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Marlyn
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« Reply #1 on: May 07, 2011, 03:47:26 AM »
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This should help - http://roaminwithroman.com/ebook.html  'A Digital guide to Photographing the Night Sky'  for US$23
I don't have any affiliation with the author other than reading about it on www.birdphotographers.net


You can easily do 1 hour exposures at 200 ISO (the best ISO for that camera IMO),  with a 5D and deal with any noise issues.

Regards

Mark

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sanfairyanne
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« Reply #2 on: May 07, 2011, 07:00:35 AM »
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Mark,

Thanks, Im a bit dubious though, I mean isnt the whole point of stacking digital files so that you can have the shot (end result stacked and merged 40 odd shots) with minimal noise reduction.

Again as I understand things theres a trade off with these kind of shots. Too low an iso and you get low star contrast, too high and you get too much contrast and too much noise.

Thanks though and Ill look up that book.

Andy
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bill t.
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« Reply #3 on: May 07, 2011, 03:13:12 PM »
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Even my moderately light polluted urban house is a reasonable test location to approximate star photography exposures.  I have used my trashbin-on-wheels to stand in for rock-like shapes.

As already said, 40 stacked exposures will forgive a LOT of noise!  But those wide angle lenses have gobs of chromatic aberration, I think that would be the main thing to test for.  Nothing shows up optical aberrations of any kind more than point light sources.  You might be better off with something more like f8, and testing several focal lengths on the lens to find the optically best one.

I'm sure you're aware that the Milky Way can become a major player in astro landscapes.  While star trails are pretty cool, the first thing that jumped into my head is a shot of a statue with a huge, sharp Milky Way in the background as a kind of counter monolith.

Anyway, I'm envious!  Be sure to post some pictures.
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sanfairyanne
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« Reply #4 on: May 07, 2011, 03:41:23 PM »
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I honestly dont know what to think, Ill try the shot and see what happens, every shot Ive taken this year has turned to crap so at least I cant be too disappointed.

Ill be sure to bring my results to the forum, though itll be early July before I can process the results.
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Jonathan Ratzlaff
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« Reply #5 on: May 07, 2011, 11:27:28 PM »
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As was said before, you may want to use a different aperture to ensure depth of field.  You need to do some tests ahead of time, however even at 30' feet away with 16mm your foreground may not be critically sharp.  As was said in some other posts, you may be astounded at how much the milky way shows up in your image and while you are doing star trails, you may also want to do other experiments as well before you go if you can.
Sharp star exposure limit is 600/focal length which in your case is about 35 sec.  This is where hight ISO can come in handy.  There is always noise reduction later.

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Peter McLennan
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« Reply #6 on: May 08, 2011, 11:51:54 AM »
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Sharp star exposure limit is 600/focal length which in your case is about 35 sec.

Can you clarify this rule of thumb? It seems I may have stumbled upon it by accident.

I tried some long exposure starfield images in Death Valley last month and decided on location that any exposure longer than about 30 seconds resulted in stars producing lines, not dots.  12mm lens on a Nikon D300

« Last Edit: May 10, 2011, 10:34:20 PM by Peter McLennan » Logged
kevk
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« Reply #7 on: May 10, 2011, 03:07:12 AM »
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"I hope to do a star trail around the southern star i.e, all the stars will revolve around the southern star. "

By the way - there is NO "southern star" equivalent to the North Star, there is only dark space there. You can use the Southern Cross constellation as a guide to finding the south celestial pole - google "southern cross" for details.

Kevin
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sanfairyanne
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« Reply #8 on: May 10, 2011, 08:07:29 AM »
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Yes youre right about that dark space, you have to use the length of the cross and multiply it by four and a half to find the approximate spot to aim the centre of your lens.

Lots of Google about this.
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EricV
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« Reply #9 on: May 11, 2011, 09:16:18 PM »
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For the geeks out there who like calculating sugh things ....
Star trails revolve 360 degrees times sine(angle from earth axis) in 24 hours, which works out to 73 micro-radians per second (for stars farthest from the pole).  Star trail on sensor will have length proportional to this angle times lens focal length.  For a 16mm lens, start trail length will therefore be 1.2 microns per second on the sensor.  For comparison, the star itself will have a size given by the diffraction spot, which is around 10 microns for a perfect lens at f/8.  So a very critical observer might be able to notice elongation of star images in an exposure longer than about 10 seconds.  How long a trail you can see in practice depends on other factors like print size and viewing distance, which you can read about if you look up "circle of confusion".
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NortheastBay
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« Reply #10 on: May 11, 2011, 09:44:19 PM »
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This was my first attempt at star trails while on Lake Powell. A couple of things working against me, full moon and a popular flight path.  This shot consist of 167 images, 30-second exposures each for a total of almost 2-1/2 hours.  Stacked in CS3 Extended.

5D2
Zeiss 21mm @ 2.8
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sanfairyanne
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« Reply #11 on: May 11, 2011, 10:53:49 PM »
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You say the moon worked against you but youve got nice light on the foreground yet still clarity in the stars. Heres a shot I found taken by someone else (I hope they dont mind) from Namibia. I love the light on the foreground and wonder if it, like yours was taken with a bit of moon or maybe an exposure blend.

I guess its probably that it could be either.
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Lonnie Utah
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« Reply #12 on: May 12, 2011, 11:19:19 AM »
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If you are stacking exposures anyway, why not just paint the statue 1 time and call it good?

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bill t.
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« Reply #13 on: May 12, 2011, 01:10:26 PM »
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A large Moon makes the sky too bright for extremely long exposures, star trails wash out.  However, it does produce very magical looking landscapes with shorter exposures.

Many of the scenes in this timelapse video are Moon lighted, and there's a good comparison of dark sky and moonlight sky when the Moon rises at about 2:00.

http://vimeo.com/6686768

I like the Lake Powell shot a lot.  Astronomers probably dislike horizon glows, but for most folks it adds a certain desirable interest.  And likewise the plane trails add an interesting dynamic.  There probably won't be the typical "sodium vapor" horizon glow at Easter Island, but rather a much lighter shade of very pretty blue.  You're killin' me with this Easter Island thing... I wanna go!
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sanfairyanne
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« Reply #14 on: May 12, 2011, 05:27:48 PM »
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Lonnie

That is a very good point. It probably is possible to just paint the foreground once but I have only ever done this sort of shot by painting each time. Does anyone know if it is indeed possible to do this, it would be nice to be able to just paint once and just sit back.

Bill thanks for the video link I will have to look at this once I am back in humanity.

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bill t.
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« Reply #15 on: May 23, 2011, 12:04:35 AM »
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the first thing that jumped into my head is a shot of a statue with a huge, sharp Milky Way in the background as a kind of counter monolith.

Proving once again that every picture than can be imagined has already been taken, check out the picture behind the guy's head in this link somebody just posted on another thread!  The image is shown prominently in the video.  I swear I never saw that picture before today...that I consciously remember.

http://www.epson.com/cgi-bin/Store/jsp/Pro/FocalPoints/Story/Epson4900Series.do?BV_UseBVCookie=yes
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sanfairyanne
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« Reply #16 on: June 01, 2011, 09:44:28 PM »
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Bill,

I just got back from that very spot in the video, the picture in the video was taken at Tongariki on Easter Island. I was at the island for eleven days and on the day before the new moon I got my shot. At least I think I have, I wont be able to process it until I get to a computer with CS5. In all I was four hours in the dark as it took three attempts to get the shot I wanted. Seeing that Epson video makes me wish Id spent the extra time and stayed out longer at night but then one has to sleep some time.

I took 55 shots at 1 minute 20 seconds. 16mm on full frame. iso 1600 f4. I expect to have a shot of the stars making a rotating star trail with the Moai (stone statues) to the left of the image.

Ill get my shot on here in July. Thanks again for your help.
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bill t.
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« Reply #17 on: June 01, 2011, 09:52:45 PM »
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Well that sounds exciting!  Don't let us down, we're waiting.

Sometimes when you're facing a pile of shots like it can look like you're never gonna get anything out of it.  But just hang in there through the darkest hours, and something great will emerge, I promise.
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Lonnie Utah
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« Reply #18 on: June 02, 2011, 09:12:15 AM »
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That is a very good point. It probably is possible to just paint the foreground once but I have only ever done this sort of shot by painting each time. Does anyone know if it is indeed possible to do this, it would be nice to be able to just paint once and just sit back.

Since you will be using a tripod, and the image will not change other than the position of the stars, my assumption is that it would be just like replacing the sky in CS5, just in reverse.  Or, you could create an image where you composite your sky and then replace that sky in the image where you have your foreground light painted.  It could easily be done using layer masks.

http://www.youtube.com/watch?v=DA8TgyTruV8

Given the expense of this trip, I would for sure practice at home before I tried this technique on vacation.  Good luck.
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Lonnie Utah
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« Reply #19 on: June 02, 2011, 09:16:30 AM »
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Can you clarify this rule of thumb? It seems I may have stumbled upon it by accident.

http://vimeo.com/16833554

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