Something wrong with my darks?

[poogle]

Since it's been cloudy for the last couple of weeks here in Sweden I've been focusing on taking darks that can be used when the clouds disappear

The problem however is that my darks get bright/purple in the corners (see attached, scaled, image). First I thought that is was due to light leaking into the camera, but the results were the same after placing the camera inside a closed briefcase.

The exposure is 120 secs and with ISO 1600.

Why am I getting these bright spots, and can the darks be used anyway?

/Patrik

[Uplooker]

This is or may be caused by amp glow. it gets greater the longer the exposure and as the temperature increases due to the number and length of exposures. This is part of the reason for taking darks as it enables you, during stacking, to subtract this noise and the normal hot pixels from the light frames and thereby gain a better image.

the fact that the temperature of the sensor increases due to the number and length of exposures is also why your dark frames should be taken at the same time as your lights to try and match the ambient temperature. You can take the darks at different time, and therefore different temperature, but they will not be as effective.

I am sure someone more informed and experienced than me will be able to give a better explanation.

Ian

[VigdisVZ]

Ja vi har verkligen inte haft något lysande väder.

That's the purpose of the darks, I think, since they subtract from the final image. They will wash away any purple corners on the real images. That's why the imaging train needs to be setup the same way when shooting darks, so you can wash away the noise that's generated at that setup, at that temp, at that iso etc

[poogle]

Thanks guys!

Then I guess I can use those darks (provided the temperature/humidity is approximately the same) when the sky clears. I'm going to try creating an archive of darks at different temperatures/ISOs/exposure length.

I've got to get me one of those remote timer cables, my current remote is the manual kind (my Nikon D200 can't be controlled from a computer when taking >30 sec exposures)

[RobH]

That's not quite correct Carl.

The camera doesn't even need to be on the scope to shoot darks, it's flats that need exactly the same imaging train that hasn't been moved, refocussed etc.

With darks you simply need the same exposure time as your lights, and the same temperature.

I got a free secondhand fridge which I used to create a library of darks for different temperatures in the days before I got a camera with regulated cooling. The last thing you need to be doing when the skies are clear is wasting time shooting darks!

Cheers

Rob

[VigdisVZ]

Thanks for clearing that up Rob.