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05-11-2008, 05:47 PM   #16
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I have taken 4 minute exposures with my K110D while doing astrophotography. More often, I use 1-2 minutes at ISO 800. I use Noise Reduction, AKA autodark. I have tried dark frames which were subtracted during post processing. The problem is the camera, when taking exposures with only a short gap between them to write to the memory card, heats up. The noise in the light frame increases steadily so subtracting darks later does not work well. I verified this by taking 1 minute dark frames with the camera in the dark (lens well covered). Measuring the noise with Images Plus I found a 7:1 increase in only six frames.

What all this means is I now use the K110D with 1-2 minute photos with NR and even a gap of a minute or so between frames. Sure kills my duty cycle, but it works. See below photo of 13 one minute frames.

As to the mechanics.... To cycle the camera it would be nice to have a portable timer. Unfortunately I have not found a pre-made timer for a Pentax. Some have had success with the Canon timer ($140 US) but you need to change the connector. I use a computer program called DSLR Shutter and a home made relay adapter between the serial port and the camera.

Long exposures require external power, either a Pentax AC Adapter or a large battery with a 12 v to 6.5 v converter. My K110D will kill 4 new alkaline cells in 10 minutes. These guys draw over 1/2 amp with the shutter open!

P.S. If you just want star trails elsewhere on this forum someone mentioned a program called Startrails from Germany. It will stack a series of 30 second shots to make any length star trail photos, say 120 frames for a one hour (15 degree rotation) star trail. Supporters say it is much easier than using general purpose photo software and its free. I have downloaded the program but not tried it yet.

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