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Weird Dark Gradient - Any Ideas?


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I've noticed that some of my subs and darks in my last few imaging sessions have contained a weird gradient on the bottom edge. This may have been happening for a long time, but I've only just noticed it.

I originally thought that it was due to some kind of intermittent light pollution, but then I did a run of 3 minute darks at ISO800 and found that out of 18 darks, about half of them had this weird gradient and the other half were perfectly fine.

I've loaded one normal dark and one dark with the weird gradient into Nebulosity and let it do an "Auto" screen stretch. This makes the gradient very obvious. I've then zoomed out so the image fits the screen and told Neb to save a JPG as displayed in each case.

Here are the images, weird gradient first.

This dark run was produced by some automated control software I wrote for my Canon 450D. It works by having the camera in Bulb mode. It then switches the camera to Live View mode, waits 3 secs for the mirror to settle, then exposes for 3 minutes, exits Live View, and waits 10 seconds before starting the same thing for the next image. During the capture run, the LCD screen is switched off to conserve power and not to generate heat etc.

What I can't understand is why it only happens on some darks? I thought maybe it was a light leak through the viewfinder, so in this particular run, I lay the camera on its back with the viewfinder on the floor (in the dark). The camera just had the bayonet cover on.

So I'm pretty certain, given its a dark, that it isn't light pollution from outside the camera. I wondered if it was Amp Glow or something from inside, although people tell me the 450D doesn't suffer from that. But if it isn't amp glow, then what is it? Could it be the battery pack heating up, or some other heat source?

Is my camera developing a fault?

By the way, the 450D is an unmodified camera, with warranty intact, and is about a year old maybe. Bought new.

Any ideas what this can be? I can provide the RAW dark files if anyone has a clue and wants a closer look.




Edited by OzDave
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I'm sure I read on here, fairly recently, that when the camera is in Live View it generates quite a lot of heat. So it may indeed be amp glow. Don't know why it's only on some frames though.

On the other hand, maybe I dreamt it.


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Hmmm, well the camera is in live view only for a few seconds until the exposure starts. And the screen is turned off for the whole period. However, I guess that essentially the image sensor is active for most of the time and never really gets a rest, except for the 10 second delay between shots.

I wonder if I gave it an extended rest between images whether things would improve.

Have any of you other 450D owners seen anything like this? How do you capture your darks? Do you do anything very different to what I described?


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Aha, ok...I think my software is similar to APT (except it runs on my Mac) in the way it uses Live View to do mirror lock and long exposure without a special adapter.

What I'm wondering is if the use of Live View in this way is the problem? I mean, supposing I used a long exposure adapter instead, would this eliminate the problem? I guess there is only one way to find out.

However, does anyone have experience of using long exposure adapters?


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