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Long-Exposure Nighttime Images Washed Out in One Corner

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Offline SunMesa

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Long-Exposure Nighttime Images Washed Out in One Corner
« on: 21 / July / 2020, 17:27:55 »
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Hello all,

Still mostly a newbie, bear with me. I've been using CHDK v1.4.1.5167 successfully since I installed it in Jun 2019, but have recently run into a problem while trying to get images of comet NEOWISE. I'm using a Canon PowerShot SX120 IS.

Specifically, when I enable CHDK overrides on shutter speed, aperture, and ISO, it results in a purplish washed-out region in the upper-left corner of the image. I've tried it in various dark settings (including pitch black), so it's not a light pollution issue, at least not an external one. When I disable the CHDK overrides and take images with the equivalent settings in the normal Canon mode, the artifact does not appear.

It's always in the same upper-left corner position, most intense at the corner (virtually solid magenta), fading out as you go toward the center, with some vertical banding evident.

I've tried turning off the Dark Frame Subtraction, it makes no difference.

Thanks for any insight!
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Offline reyalp

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Re: Long-Exposure Nighttime Images Washed Out in One Corner
« Reply #1 on: 21 / July / 2020, 18:18:25 »
This is due to "amp glow". Searching the forum or google will give you additional information.

Enabling darkframe, or creating your own dark frames to subtract later can reduce the effect.

You can use this script https://chdk.fandom.com/wiki/Lua/Scripts:_Fixed_Exposure_Intervalometer to create a standalone dark frame. Programs designed for astrophotography generally have dark frame support. Raw Therapee also supports dark frame processing.
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Offline SunMesa

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Re: Long-Exposure Nighttime Images Washed Out in One Corner
« Reply #2 on: 21 / July / 2020, 22:15:18 »
reyalp,

Thanks so much for your reply. This is evidently a fairly deep topic, as I've gathered from your suggested searches.

What continues to puzzle me is that disabling the CHDK overrides seems to eliminate the problem, at least for the shutter speed/aperture/ISO settings that I can duplicate with the normal Canon options.
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Offline reyalp

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Re: Long-Exposure Nighttime Images Washed Out in One Corner
« Reply #3 on: 21 / July / 2020, 23:22:15 »
Quote from: SunMesa on 21 / July / 2020, 22:15:18
What continues to puzzle me is that disabling the CHDK overrides seems to eliminate the problem, at least for the shutter speed/aperture/ISO settings that I can duplicate with the normal Canon options.
Normally, the Canon firmware takes a dark frame if the exposure is longer than a certain value. The exact value varies by model and settings, but is often around 3 seconds. If a dark frame is taken, you should see "BUSY" after shooting, for the same length of time as the exposure. Amp glow should be minimal if the camera does a dark frame.

If you turn off dark frame in CHDK settings, then you won't see the BUSY and your image will have more amp glow and hot pixels.

It's possible that when you use CHDK overrides, the normal dark frame logic isn't triggered. For example, if you set shutter to 15s in the Canon UI, you get 15s of BUSY, but if you set 15s using CHDK overrides, you don't. In that case, you could try forcing dark frame on in CHDK settings. If you still don't get BUSY for the exposure time, it could be a bug in the port.

Note you may see BUSY briefly for other reasons, like noise reduction at high ISO, but it should if it's doing a dark frame for long exposures, because it will be equal to the exposure time.
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