K20D Interval shooting............

OK think this is the right area to post this question in, but apologies if not.........

I have a pentax K20d and want to do some star trails........... rather than having 1 x 60min exposure, I want to do someting like 80 x 25 secs with a small interval in between........... less noise - less light pollution etc - and then stack in Ps.

So had a play indoors with the interval shooting mode.....

For example if I set it on M with a 10 sec exposure and an interval of 11 secs is it

a) 0.00.00 shutter open, 0.00.10 shutter close, 0.00.11 shutter open etc

or

b) 0.00.00 shutter open, 0.00.10 shutter close, 0.00.21 shutter open etc


For ref i have selected 10m Jpeg only with 3 out of 4 levels for quality, but i find if setting up for say 9 shots, using a class 10 SD card, It's getting about 6 of them............

which is odd as in rapid fire mode at that level i struggle to hit the buffer on jpeg only.



any help much appreciated

Get a cable interval timer (cheap(ish) on eBay) as they are more flexible then the build in timer and offer you a bit more.
Just as an example have a look at this one link

The theory is fine, but unless you run hacked firmware you'll struggle with this technique for star trails using the K20D. That particular camera does dark frame subtraction noise removal for anything over about 8 secs exposure...

Use anything under 8 secs and you'll probably not pick up sufficient star detail, too long an exposure and because of the DFS you'll have dotted lines for star trails. Long exposures were the reason I kept hold of my K10D after 'upgrading'

Hopefully someone can come up with something more constructive to help solve this.

It's (a) on a K5 so imagine it's the same on a K20d. digitalCG is right about the dark frame subtraction, but if you keep the exposure down you can do stuff like this


M31-128 by neal_harman, on Flickr

which is 100 3 second exposures stacked, K5 with sigma 50-150, ISO 128K, cropped a fair bit, with 20 dark frames. The K20d won't do such high ISO, but a shorter lens will obviously let you use longer exposures (8-10 sec for 50mm) before you get trailing.

If you're going to do it, you're probably better off with something like Deep Sky Stacker - there's a tutorial
here - I didn't use the capture software he talks about but just took multiple dark frames (identical to the image frames but with the lens cap on) and loaded them directly into DSS. It's not brilliant, but I'm pretty happy given it needs no special equipment - just stuff a lot of us have and some free software.

thanks for that, I'll try that out when I get access to the big 'scope.

Bret