Timescapes - Digital Timelapse Discussion
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Circumpolar Star Trails Vid
http://forum.timescapes.org/phpBB3/viewtopic.php?f=17&t=408
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Author:  yorkie [ Wed Oct 08, 2008 7:19 pm ]
Post subject:  Circumpolar Star Trails Vid

Please check out this my latest vid



Tom left me a message asking how I achieved this result. Well, here goes:

Like any other astro timelapse, I took a series of still images using an EOS 350D - 240 in this case, of 45 seconds each. There was a 55 sec delay between each one because I had the camera set to LE noise reduction - Something I wouldn't bother with in the future.

The first half of the vid is standard stuff - throw all the images into Premier and show for 2 frames each.

The second half is the tedious part. I used a package called K3 CCD Tools (an astronomy image stacking package) to combine images 1 and 2, then saved that result as "result001" for example. Then stacked 1, 2 and 3 and called is "result 002" and so on.

This is easy at first, but becomes more and more tedious and takes longer and longer to do as you get more images in there. But the result is worth the time invested!

The reason I wouldn't bother with noise reduction again is the gaps you get between exposures. The hot pixel problem can be overcome by using a dark field image and subtracting that in K3 too. Simply, that means remembering to take a shot at the same time, and of the same duration as the sky shots, but with the lens cap on. The software then removes all the nasties from the picture.

Once you've done all 240 stacking processes, you throw them in to Premier as before, only this time you get the star trail result instead.

Hope that has clarified it Tom. Have fun trying!

Author:  yorkie [ Wed Oct 08, 2008 7:25 pm ]
Post subject:  Re: Circumpolar Star Trails Vid

I forgot to mention the excessive use of a hairdryer too during exposures. Dewing up was a big problem on this 7 hour stint!

Australia is not as dry as many people think it is!

Author:  Antz [ Wed Oct 08, 2008 7:34 pm ]
Post subject:  Re: Circumpolar Star Trails Vid

Good Stuff!

Author:  rlavoie [ Wed Oct 08, 2008 8:28 pm ]
Post subject:  Re: Circumpolar Star Trails Vid

Very nice! The Magellanic Clouds are beautiful!

Author:  rlavoie [ Wed Oct 08, 2008 8:29 pm ]
Post subject:  Re: Circumpolar Star Trails Vid

Forgot to ask: what was your ISO setting?

Author:  yorkie [ Thu Oct 09, 2008 12:21 am ]
Post subject:  Re: Circumpolar Star Trails Vid

Ok - Tech specs

Canon EOS 350D
Sigma 10-20mm lens set at 10mm at F4.0
ISO 1600 with Long Exposure noise reduction
Exposures of 45 secs with 55 secs between for processing.

Couldn't go much longer with the exposure because the moon was up for the first 3 hours (check the reflection on the right hand side of the obs dome)

Author:  John Jovic [ Thu Oct 09, 2008 3:50 am ]
Post subject:  Re: Circumpolar Star Trails Vid

Nice effect.
JJ

Author:  timescapes [ Thu Oct 09, 2008 8:58 pm ]
Post subject:  Re: Circumpolar Star Trails Vid

So when you use a program for darkframe subtraction, what happens to the spots where the hot pixels were? Does it use some kind of nearest neighbor technique to color in the pixels?

After Effects/Camera RAW seems to automatically remove all my hot pixels when i import RAW image sequences. But sometimes it seems to replace them with generic, light-colored pixels that can appear as "snow" in an HD video.

Author:  rlavoie [ Thu Oct 09, 2008 9:18 pm ]
Post subject:  Re: Circumpolar Star Trails Vid

timescapes wrote:
So when you use a program for darkframe subtraction, what happens to the spots where the hot pixels were? Does it use some kind of nearest neighbor technique to color in the pixels?

After Effects/Camera RAW seems to automatically remove all my hot pixels when i import RAW image sequences. But sometimes it seems to replace them with generic, light-colored pixels that can appear as "snow" in an HD video.

My (limited) understanding is that if your pixel is 100% hot, the program will subtract 100% of the signal and leave the pixel black. But in noise, pixels are not always at 100%. If after say 60 seconds of dark frame, one pixel is 50% hot, the program will subtract that specific level (and in absolute measurement I think, not relative) from your image. If the noise is 50 photons (just to pick a number), the program will cut this amount from the reading on the pixel in your "light" image. But as noise can vary slightly from one dark frame to another, I heard that it is better to average multiple dark frames of same exposure and taken at the same temperature than your good images...

Author:  yorkie [ Fri Oct 10, 2008 4:31 pm ]
Post subject:  Re: Circumpolar Star Trails Vid

timescapes wrote:
So when you use a program for darkframe subtraction, what happens to the spots where the hot pixels were? Does it use some kind of nearest neighbor technique to color in the pixels?

Tom - you've got me worried now, and I had to check it out. Bad news I'm afraid! It seems to just remove them and leave a hole. See the picture here:
Attachment:
dark frame test.jpg
dark frame test.jpg [ 39.6 KiB | Viewed 7005 times ]

On the left is a standard frame. Middle is a simulated dark frame with regular hot pixels (ok I know they're huge, but it's a test) and on the right is the much processed result of subtraction. Big black holes! Oh no!

I've never come across this before, as I've only ever used the package for deep sky imaging through my scope, where usually the subject drifts through the field during exposures. K3 re-aligns them and does the dark field subtraction, but there's plenty of "real" data to fill the gaps.

I'm gonna have to do more experimenting with this!

I'm hoping in the instances where a star crosses over a hot pixel on the frame, the software will only remove the hot pixel element and leave the star there. I'm hoping...

Author:  andrewf1 [ Sat Oct 11, 2008 2:19 am ]
Post subject:  Re: Circumpolar Star Trails Vid

Very nice video, amazing effect...definitely worth the effort as you say. Nice music too :D

Author:  agambaros [ Mon Feb 16, 2009 1:57 pm ]
Post subject:  Re: Circumpolar Star Trails Vid

Part 2 it's very interesting and it's first time that i see it.
Amazing job.
For dark question: with image stacking software you can use a Dark Frame Subtraction for startrails
http://www.tawbaware.com/is_help/imgsta ... .htm#noise
If you record dark frames on star of sequence (with same time of light frame but with cup on lens) there is any timeout.

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