Star Trails Problem: Is it the kit, the CF card or me??

[eyetranced]

I tried my first hand at some star trail photography a couple of nights ago having read a recent “Try This...” article in a magazine that, er, competes with AP (sorry guys!).

As recommended in the article I was shooting from a tripod in RAW using continuous frames via a remote lock at 30 sec each on an ISO 100 setting. However, I noticed that every time each frame was completed, it seemed to take as long for the camera to process the image to CF card at the shot it self, before it then took the next 30 second frame.

I combined all the frames using a piece of software from startrails.de (recommended in the article) which progressively combines the frames much more quickly than having to manually combine them in Photoshop.

The results were pretty good but the actual star trails look slightly odd. The results show slightly ‘peppered’ star trails with noticeable gaps, like the each trail is made up of close white dots.

I’m using a Canon 400D with a SanDisk Ultra II 4Gb CF card. I ran a test on a second 400D I own and exactly the same thing happens; take a 30 sec shot, wait the same time while the camera stores the image to CF. Each 30 sec shot is the same file size as if I’d taken a photo at say 1/60 sec.

I wondered if the ‘dot effect’ is a result of the time it’s taking to process each frame to CF card. Has anyone experienced this problem with star trails too? Is it the technical limitations of my camera or the CF card I’m using? Or am I missing something? Over to you guys.

[Fen]

Have you got 'Noise Reduction' (or similar wording) turned on? That can increase the save time for the images if they are noisy or long exposures.

[eyetranced]

Hi Fen,

Yes as am matter of fact, I did have noise reduction turned on primarily to tone down the light effects coming from buildings in the shot. I'll run some tests with the NR turned off and let you know the results. Thanks for pointing this one out. Much appreciated.

[Nod]

Try turning off "Long Exposure Noise Reduction" (or its equivalent in Canon speak). Have a look in the manual to see how to do this.

[eyetranced]

To both Fen and Nod

Thank you both so much. Turning off Noise Reduction reduced the save-time to CF to almost an instant. I guess my next problem is how to get the shots without the risk of excessive long-exposure light noise. Filters perhaps?

[AlexMonro]

The noise (as opposed to flare or ghosting from bright light sources just outside the frame) is probably mainly from thermal noise or "hot pixels" within the sensor itself. The Long Exposure Noise Reduction attempts to reduce this by performing Dark Frame Subtraction - the camera effectively makes another "exposure" of equal duration with the shutter closed, which should theoretically have the same thermal noise as the main exposure, and then subtracts this from the main shot, theoretically subtracting out the thermal noise. This is why you get the long delay with this noise reduction turned on.

You can get a similar effect by making your own dark frame and doing the subtraction manually in your favourite image editor. Take an exposure equal to the total duration of your real exposures with the lens cap on, then subtract from the final image. If you can't do a single exposure of long enough duration, you can subtract shorter exposures multiple times for a close approximation. (Ideally you would reproduce the thermal profile of the sensor as accurately as possible, by switching the camera off, allowing time to cool to the same ambient temperature, then taking the dark frames at exactly the same intervals as the original shots, but that's a lot of faffing about for probably minimal advantage).

[eyetranced]

Thanks AlexMonro. Will give it a go but I'm pretty basic when it comes to using something like Photoshop (actually all I know is Photoshop, and even that is on the rookie scale).

Can you point me to any web links that describe how to do this subtraction thingy or what is the best 'search phrase' in Goole that will find me the info I need?

[beejaybee]

Turning off Noise Reduction reduced the save-time to CF to almost an instant. I guess my next problem is how to get the shots without the risk of excessive long-exposure light noise. Filters perhaps?

Filters won't work, the noise is generated in the camera, nothing to do with the illumination of the scene.

The correct solution is what astronomers call "dark framing". Take an exposure with the lens cap on & the same settings as the "light frames" (the ones with the scene on). It's vital that the dark & light frames be taken at the same operating temperature, as the noise generated in the camera doubles with every 7C increase in temperature. Having got the dark frame & the light frame set, subtract the dark frame from each light frame, this cancels the noise plus the effects from "stuck" pixels.

The usual "long exposure noise reduction" is just an automated way of doing this - the camera takes a dark frame (keeping the shutter closed, not fitting the lens cap!) after each light frame & does the subtraction itself, that's why it appears to be "busy" between exposures. If you don't want the timeouts, just take responsibility for taking the dark frames and doing the subtractions yourself. Of course, the auto method is easier & makes sure that the temperature of the dark & light frames match reasonably well.

If you're going to use dark frame subtraction, a marginal improvement usually results from taking a sequence of five and processing them to take the median value from each pixel position to create the dark frame to be used. This helps remove random variations, resulting in less noise in the finished image.

For star trails, you may want to experiment with using a low ISO speed and a small aperture - this works well with digital as there is not the reciprocity failure problem that afflicts films with exposures running into many minutes. Try about 1 hour at f/11, ISO 100. The main problems with very long exposures are:

clouds blowing across

interference from e.g. bright aircraft lights

dewing of the lens - a lens cap may help but a dew heater (a band which fits around the lens, containing low power heating elements), or occasional blasts of warm air from a hairdryer, is more effective.

[eyetranced]

Just a note to thank everyone who contributed their know how to this problem. Went back out last night an a 3-hour shoot, Noise Resuction turned off and a carefully placed and shaded Tilley Lamp under the camera to stop lens dewing as the frost began to fall.

The results look a lot better than before thought I must admit I'm not sure I understand what it is I'm actually doing with this adding/subtracting pixel stuff but it's enjoyable learning.

Must appreaciated. Thank you all again.

[Larry Shone]

I've been reading about this method of noise reduction and I just realised why the panasonic takes so long when making a long exposure. I noticed when making say a 5 second exposure it then takes 5 seconds to do something. I now know what that something is! Don't think i can turn it off with the panasonic tho

[tonycro]

this is worth a read www.liquidinplastic.com/2008/06/startrails/, from this thread on flckr http://www.flickr.com/groups/startra...7608415392833/