G15, Strange Light in Pictures

granite4brains

8

G15, Strange Light in Pictures

« on: 03 / November / 2013, 23:56:48 »

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Hello,

I just set up CHDK on my Canon G15

CHDK 1.2.0, Revision 3186, 100e

Anyhow, I was using to take some night sky photos. Using exposure times of 30 secs via CHDK.

Worked fine the first night. On the second night, at least half the time it would take a picture, it would barely process the image afterwards - you know when you get the busy message for a while after a long exposure. Was really quick. Every time that would happen, the image would look okay, except for a what looked like an overexposure on the right side of the screen, covering maybe 20% of the total picture.

Other times, I would get the busy message for a while, as expected, and the image would look fine.

Don't see how it could be ambient light. Same setup, same position, same everything. Only thing different was the absence of the busy message sitting there for a while in proportion to the exposure length.

Any help would be much appreciated. Couldn't do my timelapses with this happening

Thanks!

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reyalp

14118

Re: G15, Strange Light in Pictures

« Reply #1 on: 04 / November / 2013, 00:13:46 »

Quote from: granite4brains on 03 / November / 2013, 23:56:48

On the second night, at least half the time it would take a picture, it would barely process the image afterwards - you know when you get the busy message for a while after a long exposure.

The "busy" is dark frame subtraction.

Normally, when the camera takes an exposure longer than some value (usually something like 3 seconds), it will take a second exposure with the shutter closed (a dark frame) and subtract that from the actual image. This reduces some defects caused by long exposures.

Quote

Was really quick. Every time that would happen, the image would look okay, except for a what looked like an overexposure on the right side of the screen, covering maybe 20% of the total picture.

It would be easier to tell if you posted a sample image, but this sounds like amp glow. Most of these cameras suffer badly from amp glow with a long exposure like that, and dark frame subtraction greatly reduces id.

Which CHDK, you should be able to control the firmwares dark frame subtraction behavior user the "Dark frame subtraction" option in the "enhanced photo operations" menu. You can either force it on, force it off, or let the Canon firmware decide.

The default is auto, which should always use dark frame subtraction for a long exposure like 30 sec, but it's possible that the canon firmware is confused by the override.

If you don't want to take the time for a dark frame after every shot in your timelapse, you can take dark frames yourself and subtract them after the fact. It is better to do this with raw if possible.

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granite4brains

8

Re: G15, Strange Light in Pictures

« Reply #2 on: 04 / November / 2013, 00:37:55 »

Thanks for the response - super informative and helpful! My problem seemed so bizarre I didn't exoect to hear anything.

I will try hard setting that to on next time i am out and see if that doesn't solve the problem. (Or possibly to off and do the dark frame after the fact.)

Here is a sample image of the problem.

http://granite4brains.smugmug.com/Astrophotography/Wagon-Wheel-G15-November-2nd/i-N7GqWQZ/0/X3/IMG_2915-X3.jpg

Thanks again!

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granite4brains

8

Re: G15, Strange Light in Pictures

« Reply #3 on: 04 / November / 2013, 00:49:00 »

I think you pin pointed the problem. Also, if you take a look at my image, you can notice funny little tails on some of the stars. Don't know what that is, but it's not in the images w/o the light, that apparently also took a dark frame.

I'd be curious what you think though still based on the image.

I was bummed this was happening, so I'll be very happy for such an easy work around

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reyalp

14118

Re: G15, Strange Light in Pictures

« Reply #4 on: 04 / November / 2013, 01:31:30 »

Quote from: granite4brains on 04 / November / 2013, 00:49:00

I think you pin pointed the problem. Also, if you take a look at my image, you can notice funny little tails on some of the stars. Don't know what that is, but it's not in the images w/o the light, that apparently also took a dark frame.

The purple at the top looks like typical amp glow. It's normally more intense in the corners, but I guess that's a crop.

I'm not sure about the streaks, I've had something like that from the camera moving when I started the exposure but it doesn't look quite the same.

This script http://chdk.wikia.com/wiki/Meteor_Intervalometer_with_Dark_Frame_Management can be used to create dark frames.

Programs like raw therapee let your specify a dark frame to use when processing your raw file.

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blackhole

942
A590IS 101b

Re: G15, Strange Light in Pictures

« Reply #5 on: 04 / November / 2013, 05:39:19 »

Quote

I'm not sure about the streaks, I've had something like that from the camera moving when I started the exposure but it doesn't look quite the same.

The camera is moved on the beginning of the exposure. The tails are visible only on the brightest stars because the sensor has not yet collected enough light from stars that are not so bright.

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granite4brains

8

Re: G15, Strange Light in Pictures

« Reply #6 on: 04 / November / 2013, 23:53:09 »

Quote from: reyalp on 04 / November / 2013, 01:31:30

The purple at the top looks like typical amp glow. It's normally more intense in the corners, but I guess that's a crop.

I was noticing the corner thing on most of the photos I found the web showing amp glow. My photo was not actually a crop. Each time it was showing up fairly uniformly on that same side. I'll probably try it out again this weekend with the dark frame take care of. I'll report back on whether it fixes the problem. But, I'm feeling lucky ;-)

Quote from: blackhole on 04 / November / 2013, 05:39:19

The camera is moved on the beginning of the exposure. The tails are visible only on the brightest stars because the sensor has not yet collected enough light from stars that are not so bright.

Quote from: reyalp on 04 / November / 2013, 01:31:30

I'm not sure about the streaks, I've had something like that from the camera moving when I started the exposure but it doesn't look quite the same.

You guys are probably both right. I didn't use my timer remote on a few shots, so maybe I shook the camera/tripod a bit when taking the shot

Quote from: reyalp on 04 / November / 2013, 01:31:30

This script http://chdk.wikia.com/wiki/Meteor_Intervalometer_with_Dark_Frame_Management can be used to create dark frames.

Programs like raw therapee let your specify a dark frame to use when processing your raw file.

Thanks again for all the great info. I'll check this out,

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reyalp

14118

Re: G15, Strange Light in Pictures

« Reply #7 on: 05 / November / 2013, 18:15:20 »

Quote from: granite4brains on 04 / November / 2013, 23:53:09

I was noticing the corner thing on most of the photos I found the web showing amp glow. My photo was not actually a crop.

The shot you posted is not the sensors native 4:3 aspect ratio, so it's effectively a crop even though you didn't crop it. If you shoot 4:3, I bet you will see more in the corners.

Quote

You guys are probably both right. I didn't use my timer remote on a few shots, so maybe I shook the camera/tripod a bit when taking the shot

In the script I use to take ISS shots I put in a start delay to let things settle a bit.

edit:
Example of jpeg and dark frame subtracted image in http://chdk.setepontos.com/index.php?topic=10159.msg101969#msg101969

« Last Edit: 05 / November / 2013, 18:28:01 by reyalp »

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granite4brains

8

Re: G15, Strange Light in Pictures

« Reply #8 on: 12 / November / 2013, 11:52:38 »

Quote from: reyalp on 05 / November / 2013, 18:15:20

The shot you posted is not the sensors native 4:3 aspect ratio, so it's effectively a crop even though you didn't crop it. If you shoot 4:3, I bet you will see more in the corners.

Hmmm, I almost always shoot that camera at 4:3. So, not sure what is going on there. The ones w/o the amp glow may have been cropped by me after the fact, if you're looking at those?

Quick follow up on another night out. I set dark frame to "on" instead of "auto". This time every image had that glow on the right hand side, although this time it was more green in color. Last time it was more blue in color

Seems like it got worse with a higher ISO setting.

I wonder if the camera just can't handle the higher ISO. I found a web site that tells you what your max quality ISO setting will be and the G15 was around 200. I am shooting between 1600-3200.

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reyalp

14118

Re: G15, Strange Light in Pictures

« Reply #9 on: 12 / November / 2013, 13:38:08 »

Quote from: granite4brains on 12 / November / 2013, 11:52:38

Hmmm, I almost always shoot that camera at 4:3. So, not sure what is going on there. The ones w/o the amp glow may have been cropped by me after the fact, if you're looking at those?

Oops, my mistake. Looking at http://granite4brains.smugmug.com/Astrophotography/Wagon-Wheel-G15-November-2nd/i-N7GqWQZ/0/X3/IMG_2915-X3.jpg I thought it was at one of the wider aspect ratios but I guess the orientation threw me off, it appears to be 4:3. Sorry for the confusion.

Quote

Quick follow up on another night out. I set dark frame to "on" instead of "auto". This time every image had that glow on the right hand side, although this time it was more green in color. Last time it was more blue in color

Is the camera taking a dark frame or not? This should be very easy to tell, if it is, then it will be "busy" for exactly the same amount of time as the original exposure.

It would not be at all surprising if the CHDK dark frame control was broken on this port. It might also only be broken at certain settings.

It would be helpful if you can do a controlled test, you don't need to wait until there is a scene you actually want to shoot, just set the exposure settings and check if the dark frame is actually taken or not.

All else being equal, amp glow will be worse at high ISO.

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