Ideally the nighttime timelapse would run from dusk until dawn.
So yes in such case some exposure control is necessary.
Once darkness has come, the exposures could be done with constant settings (e.g. 20 seconds at ISO800, open aperture, focus at infinity).
In this case, rawopint may be useful. You can set the night exposure just using the Max Tv and Max ISO values, once the scene gets dark enough it will just stay on those values.
If the moon will be up, you have to decide how you want to handle it. If the moon is properly exposed, the rest of your scene will likely be pitch black. If you want lots of stars, the moon will be totally blown out. For a wide field of view, I'd suggest it's better to let the moon be blown out, but it's a matter of preference.
In rawopint, you can control how the moon is handled by setting the over "overexp thresh". If it's significantly smaller than the fraction of pixels occupied by the moon, then it will try to keep the moon from being blown out. If it's significantly larger, then the moon will be ignored. Clouds will complicate this. If sunrise or sunset are also in the field of view, you'll have to consider that too.
This discussion is diverting from the small powerbank mod but for me it is very interesting.
I will split it into a thread in the scripting forum to make it easier for others to find.
In-camera noise reduction doubles the time to take a picture, would the frequency of once every ~45 seconds be sufficient?
Probably the best way to tell is to do some tests. However, I would say that for a 30 second exposure on a reasonably new camera like the sx260, you can probably just disable it and still get acceptable quality.
(If not: how to do black frame noise reduction externally on the PC in batch?)
You need software that supports it. raw therapee and dcraw are examples of programs that do. I don't know off hand if they will let you do it using jpegs.
You may also find useful resources on astro-photography sites. Here's an example of subtracting dark frames with imagemagick
https://astrofloyd.wordpress.com/2013/10/27/astrophotography-with-imagemagick/Here is one I did using fixed exposure on D10.
Note some of the jerkyness is due to the video being 15 FPS, just because there were so few frames. Settings were
Shutter: 32 seconds
ISO 80
F/2.9, 35 mm (35 mm equivalent, really)
CHDK raw
+2 Ev in raw processing
Dark frame disabled, subtracted in raw therapee batch
The actual time between exposures was ~36 seconds (32 for the exposure 4 to save the raw)
Note I used ISO 80, because adding the equivalent number of stops Ev compensation in raw processing gives pretty much the same result as high ISO, but avoids the jpeg processing time associated with using high ISO (this may not apply to all cameras). The high ISO processing time is separate from the canon dark frame. If you don't use raw, you'll want to use a higher ISO.
