Image artefact query

alan72

alan72

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Alan
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Hope this is ok to post here, it's not a critique but a technical question specific to Astro. In a number of recent shoots using an astro modified camera (I purchased a second hand modified canon 6D mk ii last year) I have started to see a strange purple flare artefact on the images. I initially thought this was lens flare from an external light source, however shooting last night and the location was absolutely dark with no external lights and it is visible in my frames. I tried covering the eye piece to keep out any stray light but it still seems to be there in shorter frames when stacking.

The artefact isn't visible in all images and I think it starts to appear over time which makes be wonder if this is a sensor problem over heating. It's impossible to remove by post processing.

It seems to disappear by switching off the camera for a bief period and then shooting again. Last night i was shooting 2 minutes tracked exposures, but it also seems to be visible in 1 minute frames and

I've cleaned the senor using a dry cleaning brush i have but it seems the same.

Has anybody seen anything similar or had issues wih modified cameras? It's getting very frustrating and to the point of unusable.

Thanks


IMG_1082.jpg
 
Do you blank off the viewfinder?
Are there any lights hitting the camra body?
 
Is it in the same place on all frames? And has it always been there with this camera or just crept in, maybe getting worse? It's always possible the sensor mod wasn't done properly. If the image you've posted was the only time it's happened I'd be tempted to say some sort of distant light reflecting off cloud because it doesn't look totally clear low down (or the aurora - are you currently in the UK or elsewhere?), but if it's happened a number of times and it's the same each time it's obviously not. Who did the mod?
I have an ancient 350D that was modded some years ago by a friend of mine (he did his own camera first and he's very, very good at things like this) and have never had any issues at all with it, and neither has he with his own.
I have never blanked off the viewfinder
Software with background extraction might remove it, but might not
 
To eliminate light pollution, it might be worth shooting a couple of long exposures with the lens cap on.

I have managed to get light in through the viewfinder but it did take a long exposure (with the lens cap on) in bright sunlight with the viewfinder pointing at the sun. It didn't look anything like the purple area in your shot.
 
I've done some more digging this afternoon in my image archive, it looks to be in the same place consistently, it's been present throughout since early last year but perhaps more intermittent and less pronounced in the past now I look and compare. I wondered if it was caused by a certain lens and some odd effect at certain focal lengths or appertures but there seems to be no pattern, it happens wide open from f2.4 to f4 and from 14mm to 60mm on 2 different lenses,

I think I dismissed this initially last year as flare from external light sources but after some recent shoots in good dark locations when i'm confident there were no external lights I'm sure that is not the cause.

When I look at some dark frames I have taken and boost exposure and contrast you can see an area of hot pixels(?) on the sensor that seems to line up with the area in question in the images. (below).

I got the camera from Andy Ellis at Astronomiser which is a reputable source.

I shoot with either the modified 6d or a conventional 5d mkiv and the same lenses get used for both. I have not seen this aertefact with the 5D which makes me think this is linked specifically to the 6D.

IMG_0552.jpgIMG_0937.jpg
 
Last edited:
Nod said:
To eliminate light pollution, it might be worth shooting a couple of long exposures with the lens cap on.

I have managed to get light in through the viewfinder but it did take a long exposure (with the lens cap on) in bright sunlight with the viewfinder pointing at the sun. It didn't look anything like the purple area in your shot.
Click to expand...


What I had with a 7D and an 800D was mainly wrong exposure, but also had vague areas of low contrast
 
I emailed Andy and it looks like thermal noise on the sensor from running the live view. I shot on Friday night and made a conscious effort to turn off live view after focussing and framing before starting my shots. That seems to have done the trick.

I think there must be some damaged pixels in that area that is susceptible but keeping live view off for shooting seems to work.
 
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