Photographing star trails ?

Anybody out there in Pentax land ever had a go at photographing star trails ? You know the sort of thing. Set the camera up on a tripod, camera aimed at the north star and take a very long exposure - anywhere between a few minutes to a few hours. There is plenty of good (and sometimes confusing) advice on the web of course, but I was just wondering has anybody here tried it ? There is quite a lot of light pollution where I live, so I wont be able to do it at home. I'm considering using a film camera instead of the K10 as I'm a bit concerned (perhaps unnecessarily ?) about the battery life when taking extremely long exposures.

Any advice or tips from anybody with some first hand experience gratefully received.

Cheers,

/Ian

You can take multiple shots with your k10d for example some off 30 seconds and merge them together on the pc. When taking multiple shots your sensor can also cool down slightly between the shots.

With a full battery there are no worries with the K10D.
Here is a example I did a while ago.

Take into account the exposure time doubles when you have NR switched on.
It takes the same amount of time to process in camera as the actual exposure.

Martin.

Good one Martin, if you use it on a tripod and with the same exposure constantly you only need to make one dark frame extraction.
So if you make 10 shots of 30 seconds you only need one dark frame extraction of 30 seconds for al those shots.
Simply put the lenscap on the lens and make a photo with the same settings.

In this case I did it a bit different After the shot ( I only took one, calculations were made before) I packed up my gear, left the camera turned on, and by the time I got home, the camera was ready processing.
It's all in the timing, right?

Martin.

What gear did you use for that shot Martin, a remote with a Bulb lock?

You also have digital remotes which you can program.
I was thinking of getting one of those and set it to make 10 shots of 30 seconds with an interval of 1 second with the hope that's enough to cool the sensor down.

Just the cable release with the bulb lock. And I would not worry to much about heating up the sensor. I didn't, and the camera didn't exactly go up in flames.

Martin.

My concern is noise with those long exposures, CCD don't like to be turn on for long CMOS are better for that.
A dark frame extraction would remove most if not all of the noise though but that would take another 300 seconds or so

how would you apply the dark frame to the final image?

I only attempted this once from my back yard on a clear night, but my approach was similar to Martins - I used bulb mode with remote trigger lock.

The exposure was about 2 hours if I remember, but my biggest headache was moisture appearing on the lens as the night cooled. A friend of mine overcomes this with a carefully placed fan blowing air over the lens.

Been meaning to try it again one day.

If you process from RAW with UFRAW, you get the option to pick a second RAW file to use as a dark frame. This way the subtraction is done while still at full 12bit colour depth and before the bayer matrix conversion.

Iris is another useful piece of software, but it takes a lot of getting your head around. Free and very powerful, but hard to use.

Did it once with a K100DS and some improvisation....

Took an exposure in northern Vermont for 30 minutes and results came out pretty well. The long exposure NR was irritating though 'cos I had to wait a while for the results!

Improvisation came in because I had no cable release at the time (and the IR remote I subsequently bought was no good for bulb exposures until the K-7 came along).

So I took the camera inside, put it on a tripod and set it to bulb with the lens cap (and viewfinder cap) on. Pressed a large mint to the shutter (a "Mento", from memory) and used a rubber band to keep it there. Carried the whole lot outside and removed the lens cap.

Sat down and became mosquito snack for the next half hour..... Despite that, it was fun and the result was quite good. I should try it again sometime so thanks for starting this thread to remind me.

i like that
made me smile