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okay, so the camera does cool some if you give it a break, the question now remains, does the improved performance from giving it a break to cool acutally outweight the time you loose collecting signal. In terms of your specific example, here's what I'm asking: which would give the best SNR, 360s @43 C or 300s @ 39 C?

Good question. I don't know the answer, I'd make an experiment.

To be picky, teoretically it may be problematic calibrating and stacking several light frames with different temperatures. Anyway, modern processing software, like Pixinsight, adopt approaches that mitigate a lot this problem.

Pragmatically, I'd do pauses between frames anyway, since they fit in my workflow, and help me avoiding errors and problems, or at least spot them easily (I would be afraid to stop a long sequence to check the setup). 
I consider the fact that pauses give me more stable temperatures as a positive side effect ;-)

btw, I'm really enjoying this conversation, this is something I had not considered this carefully before.

I'm enjoying it too, but maybe this OT is going for too long, don't know if moderators could complain about it.

Regards
Gerlos

By experience, with 30s breaks between 600s subs I can see CMOS temp becomes stable after 3~4 shoots. And without breaks CMOS temp does not stop to rise, even after 1h.

I confirm this. Here's my figures:

• 6 shots, 300s, no pauses: T1=37°C, T2=39°C, T3=40°C, T4=41°C, T5=42°C, T6=43°C
• 6 shots, 300s, 60s pauses: T1=39°C, T2=38°C, T3=36°C, T4=39°C, T5=39°C, T6=39°C

Did this experiment at home, with higher temps compared to usual astro photography conditions.
For the first group, I set the intervalometer to 5 min and bulb exposure to 5 min. For the second group, I set the intervalometer to 6 min and bulb exposure to 5 min.
Between first and second group I did a pause of ~15 min.

Later I downloaded the shots, and read the temperatures using exiftool.
60 seconds seems to me a reasonable interval also for (manual) dithering and occasional checks to see if everything is running fine.

Usually the next day after a night of shooting I look at the temperature in the exif data and shoot darks for every unique temp and shutter length that I see. I get my camera to the temps it was at by putting it in the fridge to cool it down or running liveview to warm it up.

Really smart. Thanks for sharing.
So far I used a simpler workflow, since I take dark frames during twilight and during dawn, when the sky is brighter, and sometimes a couple of them in the middle of the night, when my subject changes side of the meridian and I have to rotate the mount.
I'll try your approach, maybe this way I'd get better calibration.

But if you have some evidence that giving the camera a break to cool is actually better, I'd certainly amend my current process.

It works. I've somewhere the results of an experiment done during a cloudy night, when I find them I'll post them.

You can send out data over the USB with PTP, also for something more simple, turning on/off the audio preamp causes a detectable signal in the audio line in.

Good to know. Maybe we could program and arduino board to capture such signal and do something cool. ;-) Need to tinker a little with it...

I cannot afford an autoguide setup, so my cheap eq mount's periodic error does this for me :p The draw back is that I could never hope to get 600s like you mention, 120s is my practical limit and that is usually only for small declinations, and if I spend a lot of time getting the polar alignment perfect.

Similar performance on my CG5 before I plugged in the auto guider... My limits were between 200 and 300s. Now I do polar alignment using a polar finder and then using goto software in less than half an hour (made some marks on the mount for easy weight balancing), and the Synguider do the rest.
The problem is that it's "too perfect" ;-) But once I tell it to stop guiding, periodic error and everything else do their job, moving my frame in unpredictable way.

You will have to sync dithering between shoots, so you will need a special firmware for the SW SG (Synguider) and add a plug in the SW SG to drive the bulb of your camera.
Other option, set the SW SG as a slave device, with a mod FW too, and use a master device like Arduino to drive all the setup.

And in all options, I don't see how make a script in the camera to drive something outside with in/out informations (sync)... Maybe with flash on second curtain + timming (?). It looks hard.

It's true, it's hard, so it's not my priority at the moment.

At the moment, I just start autoguiding, take a shot, manually stop auto guiding, move slightly the mount, so the star on the screen of the guider moves from one side of the frame to the opposite side, start guiding again, and take another shot, and so on. Sometimes I don't need to even manually hit buttons, since when I stop guiding the star starts to oscillate around (maybe because of periodic error).

It would be wonderful to automate dithering, but how? Maybe I could just start guiding and acquisition sequence on the camera, and set a timer somehow to stop autoguiding every, say, 6 minutes, during the pauses of the acquisition (commands GUIDE -> STOP and GUIDE -> RESUME).

Note; I do dithering without guiding on a EQ6 for short focal length, the most simple and reliable solution I found is to drive mount and camera with a small Arduino. But for dithering and auto-guiding I use a laptop (EEEPC), it's the most simple (+ cheap) way.

Interesting. Can you give me more details about it, or a link to read?

regards,
gerlos

Just downloaded latest nightly and put it on my sdcard.

Unfortunately, there not seems to be any scripting menu Is this feature been removed? Or I need to load some module (first time I see them on my camera).

Looked also at dmilligan's post about adv_int.mo module, but didn't understand how could I add this module to magic lantern (I don't see it in the modules menu). Anyway, the lack of bulb mode makes it less useful to me.

But, good news: latest nighly's intervalometer lets me set any number I want in the "Stop after" option! Not a perfect solution, but better than before.

Also: kudos for the latest nightly, its look is a lot more polished than my old stable!

gerlos, you can write a script with the PicoC scripting API to do this.
-> http://www.magiclantern.fm/forum/index.php?topic=4362.0

But I'm sure it's the most simple way to do what you want.

Thanks both for the fast answer! I'm looking at PicoC, and it seems to fit perfectly my needs.
Just to be sure: to use it, I can just pick latest nightly, is it right?

The 30s between shoots because of dithering?
And what about darks in this sequence?

Right. During the pauses, I'll move slightly the mount to do dithering. I'm also looking for a way to do it automatically with my SkyWatcher Synguider, with no luck at the moment, but I don't give up hope (also sent a mail to SkyWatcher asking for this feature, low probability of success, but it worth a try).
Pauses help also with sensor temperature, since during the idle time it cools a little.

PS. 300s looks to much for the core IMHO, face to 600s. Make a try with 180s and I'm sure you find it better.

Actually, I'm going to do some experiments, since the 250 lens I'll use is not very good. I'm still unsure if I could use it at f/4, f/5.6 or f/8, that seems my best option for sharpness, but it's really too dark.
I'm planning also several sequences at different exposures, to span 3 or 4 stops of exposure (maybe 60s, 150s, 300s, 600s) so I can pick the best settings for my setup.

Now I'm going to do some experiments, and wait for a clear new moon night!

thanks again
gerlos

Hello everyone!
Going to do astrophotography in the next weeks, using my Canon 550D. Last time Magic Lantern helped me a lot: I used time lapse feature to setup an automatic shot sequence and leave it go while I was doing something else (like sleeping).

This time I'm planning to shoot M31, the Andromeda Galaxy, and need several different exposures to get it right.

I remember that months ago someone here talked about a scripting feature, not present in the current stable, that could help me in this project. I actually need to do something like this: 10 shots at 600 sec with 30 sec of pause between each photo, 10 shots at 300 sec, and so on. Perfect for a script!

Is this possible with recent nightly builds? Is there any reference for the scripting language used?

If scripting isn't possible at the moment, is it possible to change the minimum value for the "stop after" option of the time lapse tool? 100 frames are really too many for my needs, I actually would be happy with 10, 15 or 20 frames.

thanks in advance
gerlos