Star Trail Photography - Again

[duranash]

I'm interested in trying some star trail photos with my Canon 5DII.  I'm thinking of using the technique mentioned here.....http://www.naturephotographers.net/articles0509/fv0509-1.html
For me, leaving the shutter open and exposing the sensor for 5 minute intervals bothers me.  Will those long exposures cause any damage to the sensor or camera?  I believe I read that long exposures causes the sensor to get hot.  What do you think about 30, 5 minute exposures, one second apart in terms of sensor heat?  It's an expensive camera (at least on my budget) and I don't want to do anything to damage it or shorten its useful life span.  Thanks

[KeithB]

"Hot" is a relative term here.  Sensor heating during long exposures is strictly a *noise* problem, not a *reliability* problem.  The semiconductors I work with can easily last 1,000,000 hours at 200 degrees C, and the reliability doubles for every 10 degrees lower in temperature.

The problem, though, is that noise is a function of temperature, so if the sensor gets warm, there is more electrical noise added to the image.  This is why astronomical sensors are cooled with LN2.  Canon uses CMOS, which runs even cooler than the CCD's common in other cameras.

In any case, I don't think that reliability should be a concern, here.  Battery life is more of a concern, it might be wise to externally power the camera.

[klindup]

I would not expect any problems with a long exposure to capture star trails.  Many people use much longer exposures (30 minutes +) with a tracking mount to photograph the night sky.  The problem with longer exposures is that you also capture satellite tracks, aircraft and the odd meteor.

Ken Lindup

[Bob Atkins]

The camera may well have built in protection against over heating. Don't forget that the 5D II is designed to shoot video and in video mode the sensor is fully active all the time. A long static exposure is no more taxing than shooting video.

I'm pretty sure that if the camera overheats in video mode a message gets displayed and you have to wait a few minutes for it to cool down.

Whether or not it over heats depends on the ambient temperature. At 120 degrees in the middle of the day in death valley, I'm sure you'd run into problems. Shooting at night when it's cool you should be OK for hours.

I can't see any reason why 20 5min exposures  with 1 second between them would be any better than one 100 min exposure as far as sensor temperature goes. Nothing is going to appreciably cool in that one second.

[duranash]

Thanks all for your replies.  You have all increased my comfort level

Bob - In reading several web tutorials, many folks seem to prefer doing multiple shots and merging them in Photoshop.  The stated basis for that technique is to reduce sensor noise by using shorter rather than longer exposures.  Several folks discuss using a continuos burst of 30 sec or so exposures, with 30 sec between them.  They end up with a LOT of images to merge.  Others, like the link I posted, use longer exposures & fewer images...but with the same rationale...lower noise?  As you said, one wonders if noise is REALLY less with lots of exposures versus one long one?  One thought that may have some validity is the idea that multiple, shorter exposures may tend to keep the sky darker in each image...so the trails are displayed on a darker background?

Anyway, thanks for the previous input.  I shall just play around with some shots & see where I end up.

[Bob Atkins]

30 seconds on and 30 seconds off will probably keep the temperature (and hence noise) down, However 300 seconds on and 1 second off probably won't do anything for you in that regard. For star trails you really can't 30s on and 30s off because you won't have continuous trails. It's fine if you have a tracking mount of course - but then you don't get trails at all!

There are a number of reasons to stack multiple images. If the noise was random it would tend to lower it, though I think that much of the noise at high iso settings with a hot sensor may be pattern noise and that's not going to go away by averaging images. You probably do get some noise reduction when stacking, since some of the noise will be random. You may also get less "sky fog" with stacked shorter images.

At high magnification (planetary photography) stacking multiple very short exposure images can reduce blurring due to turbulance. If fact people shoot video and then stack the frames. 

[KeithB]

Just make sure that you have the "long exposure noise reduction" [or whatever it is called!] turned OFF.  While it sounds like it will help you, what it does is take *two* exposures, one with the shutter open and one with the sutter closed and "subtracts" the two images.  This removes the effects of any hot pixels in the sensor.  Fowever, for a 30 sec exposure, the shutter-closed exposure is *also* 30 seconds, so the sensor does not get a chance to cool.

[klindup]

There is another trick you can do to capture the path of the brightest star in the sky, namely the sun.  You need to set a camera up pointing at the sun and over a 12 month period , each month you capture a picture of the sun from the same point at the same time to record a series of 12 or more pictures.  Combine the images and you will have a photograph that records the course of the sun's passage around the sky in a figure of 8.  The result is called an analemma. 

Ken Lindup