Long star trail exposures with digital good or bad?

[juliehodgins]

<p>I was wondering what the opinion is about extremely long exposures with digital cameras. I am heading out to a family reunion in Saskatchewan on the weekend and I really wanted to take some star trail shots just like I used to with my film camera. The sky is so amazing and clear out there! No city light pollution!</p>

<p>I have read a few things about it not being very good for your sensor, it can overheat and damage the sensor causing dead pixels? Is this true? I am not worried about battery life, I do have plenty of backups. I just don't want to run the risk of damaging my sensor. </p>

<p>Is it worth going out and purchasing a remote to do long exposures of about an hour or so, should I do several 30 second shots and just merge them in photoshop(that would be a lot of photos!)? Or should I just dust off my old film camera and use that? I've gotten so used to seeing my results right away over the past several years that I might get a little impatient using my film camera! Lol.</p>

<p>Also I have both the Canon 5D and the Canon 5D MarkII.. It says you can use both the RC-1 and RC-5 remotes, but do you have to sit there and hold the button down on the remote for the full hour? Or once in bulb mode is it press once to open the shutter and twice to close the shutter, just like on my old film camera? Sorry I really don't have much experience with remotes, and I really want to do these shots just for the fun of it since I will have the pleasure of being under the clear prairie skies.</p>

<p>Any advise would be great!<br>

Thanks!<br>

Julie</p>

[ben_goren]

<p>Julie,</p>

<p>I haven't done star trail pictures, but I can answer at least a couple of your questions.</p>

<p>All the remotes will have a way of locking the shutter open so you can walk away. On mine, the RS-80N3, there's a plastic plate that slides to lock down the button. Obviously, a timer remote can be programmed for whatever you need. </p>

<p>The Web research I've done indicates that the preferred method for start trails on a DSLR is multiple stacked short exposures. The standard technique seems to be an exposure that's proper for 30 seconds (generally the longest non-bulb shutter speed), set the camera in ``burst'' mode, lock the shutter on the remote switch, and walk away. The camera will keep taking 30-second exposures as fast as it can until it runs out of storage space or battery.</p>

<p>Then, you use something like one of these:</p>

<p>http://www.schursastrophotography.com/software/photoshop/startrails.html</p>

<p>http://www.startrails.de/html/software.html</p>

<p>to combine the (possibly hundreds) of exposures (after first doing any necessary batch processing, such as conversion from RAW, color adjustment, etc.).</p>

<p>I've never heard of any dangers from damaging the sensor from long exposures. I don't think it would be a problem, but this technique obviously avoids it. It also offers other advantages: if you're set for a four-hour exposure but your battery dies after two, this way you've still got the equivalent of a two-hour exposure. If some idiot shines a flashlight into the lens in the middle of the shot, you only lose that one frame, and the gaps of one missing frame probably won't even be visible on the final stacked version. You could also edit out interloping aircraft without much trouble, and so on.</p>

<p>I might add -- with digital, ``film'' is cheap. Practice in your backyard before you leave. The shots may be completely blah, but it's better to ``waste'' the time figuring out what you're doing at home than on location.</p>

<p>Cheers,</p>

<p>b&</p>

[rivi]

<p>These are done digital, usually stacked: http://www.astrosurf.com/sguisard/</p>

<p>Long exposures don't damage the sensor, but a digital sensor at room temperature it will "overexpose itself" into saturation through the dark current if integrating to long, rendering the image worthless. Just make sure the camera between two exposures does not do anything else, like taking a dark frame, as this would cause visible gaps in the star trails.</p>

[juliehodgins]

<p>Thanks everyone! This has all been extremely helpful!</p>

[ben_goren]

I just did my first real attempt at wide-field astrophotography last night, and I'll add a few bits of

advice.

 

First, absolutely most certainly without a doubt give it a try before you go. I didn't get it right last

night, and I wouldn't be surprised if I don't get it right the next time, either.

 

Second, do all your setup, including composition *AND FOCUS* before it gets dark. Even with a

decent flashlight it was nearly impossible. I suppose a helper who could get right up to the things I

was trying to focus on might have helped....

 

Closely related would be a suggestion to only plan on getting one ``shot'' per night, even if it's

composed of multiple exposures. I had in mind about three or four I wanted to do, but gave up after

the second. Of course, that partly had something to do with the approaching dust storm that I

spotted as it was almost on top of me, but still....

 

Anyway, I'm sure it's perfectly doable, but you'll need some experience.

 

Cheers,

 

b&

[rivi]

<p>As focusing before is sometimes not possible, here's a trick (good ol' focus sequence, astronomers did that with photoplates in the past):</p>

<ul>

<li>Set up zoom</li>

<li>Take single shot of bright star in frame center.</li>

<li>Look at the image on the LCD back with highest magnification, see how star looks</li>

<li>Adjust focus slightly by defined amount if not sharp, repeat above sequence.</li>

<li>If more blurry, move focus back and repeat.</li>

<li>If less blurry, move focus forward and repeat.</li>

</ul>