October Imaging Challenge (NB) - M16 (Eagle Nebula)

[PhotonCollector (Paul)]

Originally Posted by tornado33

Howdy Paul
I have not made a master dark but I guess I should try averaging several darks together and see how that goes
Scott

Yes that's right. Remember the CCD camera days, we used to take lots of darks and average them to make a master dark for that imaging session. I always did the imaging processing so you may have not been aware of that.

In some instances I would do just one dark frame immediately after the image and subtract that, but experience and recommendations by others, certainly showed that images were generally smoother with averaged dark frames.

Another reason for averaging them was that the temperature will potentially change (get cooler) over the period of your imaging session and this means so too will the amount of dark noise generated by the camera. So, if you were to subtract a dark which was taken early in the night from an image taken late in the night, then chances are the amount of dark noise your subtracting is too much - since the camera only generated that amount of noise when it was warmer.

Paul M

[Itchy]

Originally Posted by PhotonCollector

In some instances I would do just one dark frame immediately after the image and subtract that, but experience and recommendations by others, certainly showed that images were generally smoother with averaged dark frames.

Hi Paul,

I've been doing a bit of research into all this lately. It seems that if you subract a single dark, you may be reducing the dark noise, but you will actually increase the random noise by as much as 41%. At 5 darks the additional noise is around 10%. At 10 it is below 5%. To get to only 1% aditional noise you need to average 50 darkframes.

Another reason for averaging them was that the temperature will potentially change (get cooler) over the period of your imaging session and this means so too will the amount of dark noise generated by the camera. So, if you were to subtract a dark which was taken early in the night from an image taken late in the night, then chances are the amount of dark noise your subtracting is too much - since the camera only generated that amount of noise when it was warmer.

The latest ImagesPlus Beta has a automatic dark frame matching routine that is specifically designed to take account of the temperature variation in a session. It measures the light frame and scales the master dark before subtracting it. It seems to work very well and in fact many imagers (including myself) are starting to use it with a single master dark made from many darks (>50). The concept is that this "super master" dark can be used for a range of different exposures taken at different temperatures and even at different ISO's. It should be possible to make a few of these master darks that would eliminate the need for ever taking another dark frame.

Works for me (if indeed I can make it work at all ).

Cheers

[PhotonCollector (Paul)]

Originally Posted by cventer

you can but the result is not as good as an averaged master dark. In built camera noise reduction will subtract the noise it sees in that particular exposure. If you get a cosmic ray strike during that partcular dark you end up with a messed up exposure.Taking several and avergaing gives a better and msoother view of the cameras dark noise.

Using in built darks subtraction also mean you waist valuable imaging time for each exposure while camera does noise reduction.

Regards
Chris

Hi Chris,

Do Cosmic Rays actually make landfall ? I thought they collided with particles in our atmosphere and created other sub-atomic particles, like Muons, which then make it to the ground and allow us to detect the cosmic ray event ?

Paul M.

[cventer]

They do make landfall. And produce artifacts on CCD Detectors. Although sometimes the same affect is causes by terrestrial background radiation but is still called a cosmic ray hit.

try google "Cosmic Ray CCD" and you will get a truckload of reading on the subject.

regards
Chris