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Judd Patterson
08-23-2008, 02:10 AM
http://www.juddpatterson.com/BPN/joshua_stars_bpn1.jpg

Canon 5D, 17-40mm

The North Star (Polaris) sitting steady above a Joshua Tree. The total exposure was approximately 75 minutes (1 minute exposures, stacked). I had to edit out about a dozen airplane trails. Even in a dark spot like this park, you can see the lights from the towns of Joshua Tree/Twenty Nine Palms. The Joshua Tree was lit by moonlight.

Robert Amoruso
08-23-2008, 09:49 AM
Judd,

I would be interested in how you do the exposure stacking.

I like the North Star placement. The two bushes in the lower corners grounds the image well.

Arthur Morris
08-23-2008, 03:04 PM
Sounds like a lot of work but the result is quite dramatic. Could you please explain the basics of stcking 75 one minute exposures? (I have often wonder if the film technique for creating star trail images would work with digital...) Thanks.

Roman Kurywczak
08-23-2008, 04:48 PM
Hi Judd,
I guess I asked for it and got it! Nicely done. I hope you get to do the educational piece on how you did this! Well though out! My guess is that you used this method because the moon would have blown out the Joshua tree.


Sounds like a lot of work but the result is quite dramatic. Could you please explain the basics of stcking 75 one minute exposures? (I have often wonder if the film technique for creating star trail images would work with digital...) Thanks.

Hi Artie,
If you remember.....I used the traditional film technique here; http://www.birdphotographers.net/forums/showthread.php?t=5344
Difference is..........no moon.

Kaushik Balakumar
08-24-2008, 11:37 AM
Love the composition with 2 smaller trees/plants on either sides to balance it.
Wud like to know more about the stacking 75 1-min images...

Mitchell Krog
08-25-2008, 03:36 AM
Very well done Judd, I like ;)

Noel Carboni
08-25-2008, 09:59 PM
Gawgeous! Well done Judd! Wish I had been there with you.

-Noel

Roger Clark
08-25-2008, 11:20 PM
Beautiful!

What was the time interval between 1-minute exposures? What ISO and f/stop.

Here is another star trail image with 1-minute exposures:
http://www.clarkvision.com/galleries/gallery.africa/web/ndutu.night.rnclark.c01.22.2007.jz3f0077-90max.cw-800.html

Roger

David Kennedy
08-27-2008, 11:37 AM
Judd,
Were you layering the photos in Photoshop using Luminosity blending mode, or was this a different technique?

Judd Patterson
08-31-2008, 01:34 PM
35mm
f/4.5, ISO 800
75 one-minute exposures, 1 sec apart

My sincere apologies for the slow response. I am writing an article on night photography for the Everglades Photographic Society but see now that I won't get it finished before some work travel this week...so you more than deserve a reply!!

The way that I see it, there are two major advantages to star trails as a series of stacked, shorter exposures.

1) The shorter exposure helps to keep noise under control. Perhaps this won't be as much of an issue with the newest cameras that seem to have amazing high ISO performance, but in my case a 75 minute exposure at ISO 800 would have been noisy on my 5D.
2) If your battery dies during an exposure, you can still create a final image with what has already been captured and written to your card. I assume (let me know if I am wrong) that if the battery dies on a long single exposure that the camera does not keep enough power in reserve to write the image to your card.

It's also very easy to combine exposures. I batch process a set of RAW files to 16-bit TIFFs. Then it is as easy as starting with your first image, and adding each additional image as a layer with a blend mode of Lighten. Hmmm...that's actually a lot of work with 75 exposures. Never fear, there is a free Photoshop action that does this very efficiently!
http://www.schursastrophotography.com/software/photoshop/startrails.html

Roger Clark
08-31-2008, 03:26 PM
Hi Judd,


35mm
f/4.5, ISO 800
75 one-minute exposures, 1 sec apart

Impressive. Many people (me too) have had issues with cameras having short intervals between exposures adding noise as banding near the bottom of the image. This has been reported on 10D, 20D, 30D, and 1D Mark II canons. Have you seen this on the 5D? On the cameras where it is an issue, one must make the interval longer than the time to write to the card.



1) The shorter exposure helps to keep noise under control. Perhaps this won't be as much of an issue with the newest cameras that seem to have amazing high ISO performance, but in my case a 75 minute exposure at ISO 800 would have been noisy on my 5D.

Actually the multiple exposures created more noise, and the method of stacking enhances it even more. You are always better off taking as long of an exposure as you can get away with (usually limited by overexposing something like the sky background or stuff lit up by the moon or local lights).

Noise sources are: thermal noise which accumulates proportional to the square root of the total exposure time, so with multiple exposures you are simply adding the thermal noise. Also for star trails, you are not averaging, so the lighten step enhances noise, and each lighten step compounds the problem. Another major noise source is read noise from the sensor and downstream noise in the electronics. With many short exposures, you are adding read noise, and that too gets enhanced with each lighten step. In cases where you average (say a galaxy photo), you average the noise, so read noise from multiple exposures has less impact.



2) If your battery dies during an exposure, you can still create a final image with what has already been captured and written to your card. I assume (let me know if I am wrong) that if the battery dies on a long single exposure that the camera does not keep enough power in reserve to write the image to your card.


I agree with this. I've had my battery die multiple times, but at least with shorter exposures, I got something, just not as much as I wanted ;-).

Roger

Judd Patterson
08-31-2008, 07:45 PM
Thanks for the reply and extra information Roger. No I haven't noticed any issue with banding near the bottom of the image. I seem to recall reading somewhere that it was most pronounced when the affected cameras were in continuous shooting mode. If that is true, that might be a factor. My 5D was in single shot mode and being run with a TC-80N3.

I enjoyed reading through your information on noise. I learned a few new things that are definitely good to know. I also did a couple of tests and saw first hand how the lighten mode did not help reduce noise (unlike some averaging I've tried in the past with Milky Way images). On the image that I posted above I also did do one manual dark frame subtraction to keep noise under control. I simply added a single exposure (1 minute, lens cap on) and set the blend mode to Difference.

Here is a 100% crop from the final image (you can see that even at 1 second, there are small gaps in the star trail). The noise doesn't seem bad considering the 74 lighten blends...hard for me to believe it would be even better in a single 75 minute exposure. I'm going to need to test that one in order to convince myself! :)
http://www.juddpatterson.com/BPN/joshua_closeup.jpg

This was my first ever exposure over 15 minutes, so I'm definitely still learning...and found that image stacking worked alright. Thanks again for the help!

Roger Clark
08-31-2008, 10:21 PM
Hi Judd,

People who have had issues with banding caused by short intervals between frames typically use single shot mode. I know for sure that I had the problem on my 1DII in single shot mode, and I used a TC-80N3.

75 minutes is certainly a long exposure for a digital camera. Many astrophotographers use 5 to 10 minute exposures. In this article,
http://www.clarkvision.com/photoinfo/night.and.low.light.photography
I show an exposure of 623 seconds (10.4 minutes) at ISO 1600, which I think looks pretty good, so 20 minutes at ISO 800 should be pretty good also.

In my Ndutu star trails image,
http://www.clarkvision.com/galleries/gallery.africa/web/ndutu.night.rnclark.c01.22.2007.jz3f0077-90max.cw-800.html
I did 1 minute exposures with 15 second gaps. I selected the stars and rotated the stars in another layer to fill in the gaps.
Next trip I plan on longer exposures, 5 to 10 minutes each.

Roger

Judd Patterson
09-01-2008, 12:54 AM
Thanks again for all your information Roger. I was also thinking that next time I should try 5-15 minute exposures. I'm out the door right now to catch a flight, but I intend to go through those pages that you put together on your website...good stuff!