red tint on dark frame

[funkysandman]

to understand dark frame subtraction, I took a light frame and then a dark frame of m33.  I shot both in camera raw and turned off all color correction/noise reduction, however if I examine the dark frame it seems to have much more red..when I subtract it from my light frame in photoshop, it tints it green...  did I miss something?

[ronin]

You had better add what the cameara is and if a DSLR is it modified.

If a modified DSLR then red will show up more as without the filter more gets through and whatever the software does will boost that bit of the spectrum more then the G or B section.

As to light getting in whilst taking a dark then again if a DSLR did you blank off the viewfinder?

Light can get in from that and then gets to the sensor, has happened to a few people.

A combination of modded DSLR+"open" viewfinder could be a reason.

If however you are running an Atik ccd or similar then all the above falls down.

As said need to have camera details.

[funkysandman]

-camera is a Canon 70D with no mods other than firmware (magic lantern)

-shot one dark with lens cap on after shooting light frames with same iso...hmmm, did not blank the view finder, but it was dark outside

subtracting this raw shot from  a light frame doesn't seem to improve things much, and seems to turn things a little green

[D4N]

It is a bad idea to use a single dark frame, you will add a lot of noise, you need quite a lot of them to average the noise out.

Light leak sounds plausible, especially if you happened to be using a red torch at the time.

/Dan

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