SD1100 shuts down during long exposure

[bobg]

My SD1100 shuts down when I do very long exposure. I'm trying to take 30-60 minute or longer exposures of the night sky rotation. 10-20 minutes works fine (here's a photo... http://tinyurl.com/longexpsky ), but with 30 minutes or more, the camera eventually shuts down with the lens out and no photo saved. I thought it was a battery problem, so I bought a power cord on Ebay (model ACK-DC10), and the same problem occurs when it's plugged in instead of running on a battery. Any tips? Is the SD1100 programmed to shut itself down, and is there a CHDK setting to avoid shut down that I'm just missing, or might a script be necessary? Thanks!

[reyalp]

See this post http://chdk.setepontos.com/index.php/topic,3461.msg39209.html#msg39209

2147 seconds (~35 minutes) is the current limit.

[bobg]

Yup, saw that, I guess it's the same for the SD1100. Bummer.

Does anyone know if there's a way to keep it from turning itself off then?

[reyalp]

Yup, saw that, I guess it's the same for the SD1100. Bummer.

Does anyone know if there's a way to keep it from turning itself off then?

Use an exposure shorter than 35 minutes.

[barberofcivil]

Yup, saw that, I guess it's the same for the SD1100. Bummer.

Does anyone know if there's a way to keep it from turning itself off then?

Use an exposure shorter than 35 minutes.

From what I've seen and read, for long night-time exposures, it's better to use a series of shorter exposures and stack them to get less noise.

[bobg]

A series of short exposures won't work, unfortunately, since it takes as long to process as it does to shoot -- I'd miss half of the pattern during processing periods, getting a dotted line from each star instead of a continuous line.

Any other ideas? Are longer exposures impossible?

[reyalp]

A series of short exposures won't work, unfortunately, since it takes as long to process as it does to shoot -- I'd miss half of the pattern during processing periods, getting a dotted line from each star instead of a continuous line.

You can force dark frame subtraction off, and do your own dark frame processing later.

Any other ideas? Are longer exposures impossible?

Might be possible with additional code (figure out where the value from apex2us goes and override it there) but figuring that out would require someone with good reverse engineering skills to spend some  time (quite possibly a lot) on it. Realistically, I suspect there are very few people with a use for more than 30 minutes exposure on these cameras.

[barberofcivil]

A series of short exposures won't work, unfortunately, since it takes as long to process as it does to shoot -- I'd miss half of the pattern during processing periods, getting a dotted line from each star instead of a continuous line.

You can force dark frame subtraction off, and do your own dark frame processing later.

Yes, you would really only need one dark frame to remove dark frame noise.
All the other random noise from frame to frame should get minimized by the stacking process.  In fact, from what I have seen, this is one of the advantages of it, the noise i slargely removed compared to a single long frame.

[bobg]

That would be a great fix, making it so that there's only 1 frame of dark frame subtraction at the end of a long exposure. It doesn't exist already, does it? I wait 20 min after a 20 min exposure now... I tried just turning off the dark frame subtraction, but the pixel artifacts are pretty bad then.

[barberofcivil]

That would be a great fix, making it so that there's only 1 frame of dark frame subtraction at the end of a long exposure. It doesn't exist already, does it? I wait 20 min after a 20 min exposure now... I tried just turning off the dark frame subtraction, but the pixel artifacts are pretty bad then.

You wouldn't necessarily want it this way.  What you want to be able to do is take all the pictures you've taken and average them into one exposure (using Registack or whatever), then do a dark frame subtraction.  What you can do is take your own dark frame at the end of your series: put the lens cap on or something that completely blocks the light and take a photo with the same exposure settings as the others and use that as you DF.

Note that the individual pictures may have a lot of noise in them, but when you average them together, the noise should largely disappear (as it is usually different from frame to frame and would get average weighted out).  What you would be left with for noise or artifacts would only be that for the dark frame (hot pixels, amp glow).

-tgq

[reyalp]

That would be a great fix, making it so that there's only 1 frame of dark frame subtraction at the end of a long exposure. It doesn't exist already, does it? I wait 20 min after a 20 min exposure now... I tried just turning off the dark frame subtraction, but the pixel artifacts are pretty bad then.

Many programs support dark frame subtraction, so you just have to make your own dark frames. I suggest you google around a bit on this topic, it's very common in astrophotography.

Note that this does require that you work in raw (CRW or DNG) to get the best results. The jpeg process would spread the noise around a lot in unpredictable ways.

Hmm, it would be possible to modify CHDK to do dark frame subtraction on the RAW buffer using pre-existing dark frames stored on the camera. These could be looked up by exposure time and sensor temperature. This would be slow, but it would be constant time so for exposures that run many seconds it would be a pretty big win.