Darks Library

[Fay]

I want to start a DARKS LIBRARY.
Can I just put the camera & laptop in a dark place & do them? The camera does not have to be on the scope etc etc?

Thanks,

Fay

[JohnP]

Fay - ALL you have to do is cap the end of the camera (make sure no light gets in). You don't need to put laptop in dark place...

If the darks you take are at a different temp to the actual images you acquire then you will need to do a ksigma stack..

John

[Fay]

John, is that a ksigma stack with all the subs? L, flats & darks etc 

[JohnP]

No - just the L's when you subtract the darks etc.

[Daniel]

I was reading an article Jerry Lodriguss wrote about what he does, on cloudy nights he simply sets up his camera in his garage, gauges the temperature and takes darks for specific ISO's at that temperature. I think i might start doing this too, leaving the camera out after a long night of imaging is beginning to kill me

[Rick]

...so you end up with a library of dark frams at known temperatures, presumably. Then all you have to remember is how cold it was on the night you went imaging.

I can see another use for Mike's temperature logger already.

[Fay]

I thought it did not matter about DARK temperature. It is obviously more involved than I thought.

[MarkS]

I thought the laptop itself had to be in a dark place else light leaks in via the gaps in the keyboard.  I suppose I could always put flocking under the keys 

[JohnP]

Fay

I thought it did not matter about DARK temperature.

YES - it does - that's why you do a Ksigma stack instead of the normal average stack.....

[Mike]

A dark frame is mainly a capture of the Thermal Noise for the same length of exposure time for your main image. Therefore, temperature is critical in making the darks work properly. hence, the best time to take your darks is after your main images so they are at roughly the same temperature.

[Daniel]

One thing that always worries me with my dark frames is that the temperature at which I start imaging is usualy higher than when I take the frames at the end of the day, I guess the only way around this would be to use the camera's automatic dark subtraction or use a library for variation

[MarkS]

Daniel,

You are absolutely right about the temperature changing during an imaging session.

Roughly speaking, dark current doubles for each 6C temperature rise.  This means that the thermal noise resulting from the dark current increases by square_root(2) for each 6C temperature rise [technically, bacause it is a random Poisson process].  So it is possible to mathematically adjust the master dark frame to compensate for temperature difference before applying it to the light frame.  The adjustment would be different for each - depending on temperature.

A corollary is that you could produce a master dark frames for a number of different temperatures - these would need to be created in the usual way.  Then dark frames for all intermediate temperatures could be mathematically synthesised (by square root interpolation).  Then you have a complete library of dark frames for not too much work.

For myself, I am already working towards this goal - you will notice that I have recently been publishing ambient temperature along with my images for precisely that reason.

Of course, this all assumes that the physical properties of the CCD don't change too much over the weeks, months, years.

Mark

[Rocket Pooch]

I want to start a DARKS LIBRARY.

Fay, I have to complain about that one, its not very PC!

[Rocket Pooch]

If you take a set of BIAS at the same time as the DARK and then use both and do constant dark scaling during processing it makes no difference at all, that what I do.

Sorry Fay read the title wrong, shucks!