what causes this?

[Snakeguy101]

I took some long exposure shots and I was getting this nasty purple vignetting. What is it caused by? It is only appearing in the corners that would be the top corners of the sensor when the camera is held in the landscape orientation. Is this caused by the lens or something in the camera?

You can see it on the left corners of this picture:

Thanks for your help.

 

[Village Idiot]

I think it's heat? Is it a CCD sensor? Exposures that are too long will have that effect and I think I've read that CCD sensors are worse than CMOS at that.

Google it, lots and lots of pages on the issue

Purple corners

 

[Lodd]

I guess that is just a flare from the many lightsources in the scene. This is caused by the coating on the frontlens element.
And is when the light "bounces" between lens elements.

 

[Big Mike]

I think that either or both could be right.
It was shot with a D80, which does have a CCD, not a CMOS, sensor and it was a six minute exposure.
Also, there is a purple flare in the middle of the frame, so there might be another.

 

[Snakeguy101]

Is it bad to have exposures that long? The purple only showed up around the 5 minute mark. If I continue to do it will there be any lasting effects on the sensor?

 

[Big Mike]

I don't think that it will cause permanent damage...but I'm not an expert on sensor design.

The cause, is likely heat build up, which is a result of the electrical signal moving across the sensor...the longer the sensor is actively recording light...the more heat will build up. The specific way that CCD type sensors work, causes more heat build up than with CMOS sensors (which is what most other DSLR cameras use).

So now that you know this...you can be prepared for it. Maybe compose your long exposure shots so that you can trim off the affected corners. Or just avoid using exposures over 5 minutes. For example, you shot this one at F22. If you had shot it at F16, you would only have needed half the time (3 minutes).

 

[KmH]

It's called thermal noise.

The heat is caused by the disipation of power. CCD sensors use much more power than CMOS sensors, so they heat up quicker.

Astronomers have been using CCD sensors since they were invented, but use cooling systems (like liquid helium at 5.2 K) to control the heat and minimize thermal noise. Astronomers may make exposures as long as 8 hours.