First Attempt

Discussion forum for Tawbaware's Star Tracer and Image Stacker software
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luigi
Posts: 21
Joined: Mon Apr 06, 2009 6:34 pm

First Attempt

Post by luigi » Fri May 21, 2010 12:51 pm

First of all thanks Max for this wonderful piece of software!

I started with this image I took recently at the beach:

Image

I masked the ground and created a Tiff with the alpha layer

Since the photo is the result of a run of startrails.de to stack several shorter exposures it didn't have exif info.
I unchecked FOV and distortion from the optimize column. (ok?)
I entered the vertical FOV for the lens as it was taken in portrait orientation (is this ok?) 70.4 degrees for a 17-40 lens in a FF camera.

I created the lines and got a nice average 0.00/1.21/0.57

First I tried a single 24000 seconds rotation output as a single layer tiff the result is:


Did it work? I guess not and I'm worried about the effect of uneven sky.

I'm sure I'm doing something wrong so let's better ask :)

Image


Luis[/img]

luigi
Posts: 21
Joined: Mon Apr 06, 2009 6:34 pm

Post by luigi » Fri May 21, 2010 2:36 pm

Without changing anything a 30 seconds x 100 times rotation worked as a single JPEG:

Image

luigi
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Post by luigi » Fri May 21, 2010 2:42 pm

10 rotations of 3000 seconds and now I'm puzzled :)

Image

maxlyons
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Re: First Attempt

Post by maxlyons » Fri May 21, 2010 7:23 pm

Luis,

Those look reasonable given your inputs. Star Tracer just rotates the image by the amount you specify (in seconds), the number of times you specify. The reason why the rotation is specified in seconds (rather than degrees) is that it makes it much easier for the average person to figure out how much rotation to perform. For example, if you know that your camera pauses for 2 seconds between shots, and you just want to fill in those 2 second gaps between star-trail segments, then a single rotation of 2 seconds should be enough (or maybe 3 seconds, just to be safe). If you had to specify the rotation amount in degrees, then you'd have to calculate that you want to rotate the image by 0.00833 degrees, which isn't quite as intuitive!

Perhaps you were assuming that if you asked Star Tracer to perform one rotation with a time of 24000 seconds, then you'd end up with an image containing star trails that looked like they were taken over a 6-7 hour period? If so, then that isn't correct.

As for the sky, that is also expected...the "flattened" output formats just perform a simple layer blending like Photoshop's Lighten mode. So, the brighter parts of the sky end up dominating over the darker parts if you use this output mode.

In general, when trying to perform very long star-trail extension (where you are trying to extend the trails to several multiples of their original lengths), I think Star Tracer will output the raw materials necessary to create interesting images, but you'll probably need to perform some manual blending/editing to create something aesthetically pleasing. You may find that outputting a multilayer Photoshop file or multiple TIFF files allows more control that the flattened output formats.

Max

luigi
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Post by luigi » Fri May 21, 2010 7:36 pm

Thanks Max, I just don't understand what a rotation of 24000 seconds does... Can you further explain to me how it works? What is the difference between 10 rotations of 30 seconds and 1 of 300 seconds ?

dsp
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Location: Hoboken, NJ

Post by dsp » Sun May 23, 2010 8:57 am

I haven't used it yet, but this is how I interpret Max's answer. The time entered as rotation just gets converted to a single angular measure (well, probably the remainder of the computed angular measure/360).

As for the angle per second, think 360/(24*60*60)---> ~.00417 degrees per second. So a single 300 second "rotation" takes the original image and rotates it 1.25 degrees, and blends it with the original If your original exposure was 30 seconds, you will now have a photo that looks like you took around a 300 second exposure, but blocked the camera during the middle 240 seconds. It only took the original 30 second image, and shifted it in time to 300 seconds later - it doesn't "fill" in the time, only shifts.


So, now you do your 10 x 30 second shifts. It takes the original image, shifts it first 30 seconds, then again, then again, and so on. So the first 30 second "rotation" blends with the original exposure, and the trails meet - that is the end of the original star trails merge with the beginning of the first shifted image. This process would then be repeated 9 more times, giving a continuous star trail exposure of the 300 seconds.

Essentially, you don't want your shift to be longer in time than your exposure, or you will get gaps. If you want to have a synthetic 6 hour exposure from a 6 minute capture, you will want to do sixty rotations of 360 seconds, not one rotation of 21600 seconds...

best regards, Darcy

maxlyons
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Post by maxlyons » Mon May 24, 2010 7:40 pm

luigi wrote:Thanks Max, I just don't understand what a rotation of 24000 seconds does... Can you further explain to me how it works? What is the difference between 10 rotations of 30 seconds and 1 of 300 seconds ?
I think Darcy beat me to it!

Max

luigi
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Post by luigi » Wed May 26, 2010 1:48 pm

Yes, thanks for the help!

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