beginner astrophotography - page 2 - Hello, I'm a NEWBIE - HELP!! (Newbies assistance, User Guides and thank you notes) - CHDK Forum
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beginner astrophotography

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Offline whatalesyou1

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Re: beginner astrophotography
« Reply #10 on: 03 / October / 2012, 10:40:28 »
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Wow!  You guys have all given me a lot to think about.  I am going to try to get some pics soon.  The moon is too bright right now (correct me if I am wrong).

My plan is to use the native Canon settings for almost everything.  I need to turn of the dark frame subtraction using CHDK.  I actually received an external intervalometer when I bought my camera.  I am going to try that first.  I will set the camera for a 15 sec exposure and set the intervalometer for a pic every 20 sec.  Then I wll try startrails to stack my images and fill in any gaps.

I can also load a few dark frames into startrails (http://www.startrails.de/html/software.htm).  Any idea on the best way to acquire these?  My camera (G12) doesn't have a lens cap!  ::)

Please feel free to point out any flaws in my methodoloy!
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Offline whatalesyou1

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Re: beginner astrophotography
« Reply #11 on: 04 / October / 2012, 18:15:34 »
Thanks for the help everyone!

My first pic is here:

http://chdk.setepontos.com/index.php?topic=6919.30

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Offline whatalesyou1

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Re: beginner astrophotography
« Reply #12 on: 05 / October / 2012, 09:28:51 »
Quote from: lapser on 01 / October / 2012, 20:23:46

Here's a couple of videos I made using the time lapse script I'm writing.

http://www.youtube.com/watch?v=zkLBPlY6DRE#ws

http://www.youtube.com/watch?v=Nr-Q5aztka4#ws

These are great!!!

What exposure techniques did you use?  I am just guessing you left your camera on auto to handle the lighting transition?
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Offline lapser

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Re: beginner astrophotography
« Reply #13 on: 18 / October / 2012, 19:16:50 »
Quote from: whatalesyou1 on 05 / October / 2012, 09:28:51
What exposure techniques did you use?  I am just guessing you left your camera on auto to handle the lighting transition?
I was on a 2 week trip. Sorry to take so long to reply.

I'm developing a Lua script that sets the exposure parameters over time, using the measured brightness value from the camera. I'll post it when I get it figured out a little better. Leaving the camera on auto doesn't work very well, because the camera adjusts exposure in 1/3 ev steps or larger. This causes "flashes" in a time lapse.

With chdk, you can adjust exposure in 1/96 ev steps, eliminating the flash problem. But the brightness measurement at low light levels, when the stars come out, loses resolution and jumps by around 1/2 ev or so, leading to flash problems if you use it for exposure setting. I'm working on ways to smooth out the brightness changes.

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Offline waterwingz

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Re: beginner astrophotography
« Reply #14 on: 18 / October / 2012, 20:10:27 »
Quote from: lapser on 18 / October / 2012, 19:16:50
I'm developing a Lua script that sets the exposure parameters over time, using the measured brightness value from the camera. I'll post it when I get it figured out a little better. Leaving the camera on auto doesn't work very well, because the camera adjusts exposure in 1/3 ev steps or larger. This causes "flashes" in a time lapse.

With chdk, you can adjust exposure in 1/96 ev steps, eliminating the flash problem. But the brightness measurement at low light levels, when the stars come out, loses resolution and jumps by around 1/2 ev or so, leading to flash problems if you use it for exposure setting. I'm working on ways to smooth out the brightness changes.
I've been working on turning this script into a continuous shooter for astrophotography.   

http://chdk.setepontos.com/index.php?topic=8833.msg92152#msg92152

Sounds like it would be nice to merge your version.
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Offline reyalp

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Re: beginner astrophotography
« Reply #15 on: 18 / October / 2012, 22:56:56 »
Quote from: lapser on 18 / October / 2012, 19:16:50
But the brightness measurement at low light levels, when the stars come out, loses resolution and jumps by around 1/2 ev or so, leading to flash problems if you use it for exposure setting. I'm working on ways to smooth out the brightness changes.
One option is to use shot_histogram to get the actual raw pixel values from the previous shot. See http://chdk.setepontos.com/index.php/topic,2156.msg20057.html for an example. The earlier development thread http://chdk.setepontos.com/index.php?topic=1145.0 also has a lot of measurements.

There's another variant of the script here http://chdk.setepontos.com/index.php?topic=3079.0
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Offline lapser

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Re: beginner astrophotography
« Reply #16 on: 19 / October / 2012, 01:33:56 »
Quote from: reyalp on 18 / October / 2012, 22:56:56
One option is to use shot_histogram to get the actual raw pixel values from the previous shot.
Thanks. I've experimented a little with get_histo_range, but haven't figured out how to use it to set the exposure value. I did notice that the output is percent, so there's some truncating and loss of precision with 1,024 possible values. If you add up the 1024 individual values, they should be 100% but the result is around 65% due to truncation.

I think I can smooth out the brightness with an optional maximum for rate of change of brightness slope, i.e. 2nd derivative. I collected a lot of sunset data on my trip to analyze.

get_live_histo() sounds interesting too:
https://trac.assembla.com/chdk/changeset/2060/trunk/core/luascript.c
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