[Markovo]

Lets say I want to shoot some low light scenery with my K-1 (that has the White Dot problem) and the only solution that removes the "white dots" is the build in LENR. It works, OK, but for 10min exposure I have to wait for 20 minutes to capture a single image. That's long...
My questions are:
1. Can I do a Dark Frame Subtraction, using pre-captured image (with cap on) with 2 minutes of exposure and use it for the real 10 min exposure shot?
2. Or does the Dark Frame has to be with the same exposure as the first shot?
3. Can I shoot one Dark Frame with, lets say, 5 min of exposure, and use it with all my long exposure shots after that for subtraction?
4. Does this technique works with the ASTROTRACER function?

Greetings!

[UncleVanya]

My understanding after reading about this on my k-3 is that:

1)You can create your own dark frames to use outside the in body method.

2)Accuracy of the background noise of sensor is best if performed at the time of the exposure and with the same exposure parameters. However in practice it is typically possible to use one dark frame when using a bunch of shots with the same exposure in the same conditions.

3) sensor heat can play a role; which depending on exposure and ambient conditions could vary over a session.

4) hot pixels may be worse at the end of many long exposure shots due to sensor heating.

[jbinpg]

Yes, the dark frame exposure should be done with the same shooting conditions as the light exposure: same ISO, same exposure length, same ambient temperature. The standard method for astrophotography post-processing dark frame subtraction is to take 15-20 dark frames after the shoot is over. Seasoned astro togs make what is called a "masterdark" frame which is essentially an average of all your dark frames. Since this should be done to encompass all possible shooting conditions for a location, they end up with what is called a library of masterdarks saved by ISO, exposure length and ambient temperature (technically, it should be sensor temperature but the sensor temperature reported in the exif data is always the temperature of a circuit board component, not the actual sensor. So ambient temperature is the most convenient proxy). Golden Rule is that the warmer the ambient temperature, the higher your sensor noise level will be.

Having said all this, some of the current astrophotography programs out there can do "scaling" of the masterdark such that they do not have to be so stringent with the dark frames used. This is possible since the amount of sensor noise per ISO is roughly linear with respect to exposure time per sensor. But, since shot noise is also random across the sensor, each masterdark should be made from a longer series of dark frames to be useful. It also helps that Sony Exmor sensors used by recent Pentax camera models produce comparatively low amounts of noise.

As for hot and cold pixels (white dots), most post-processing programs are equipped with routines to remove these easily.

- Jack

[beholder3]

Lets say I want to shoot some low light scenery with my K-1 (that has the White Dot problem) and the only solution that removes the "white dots" is the build in LENR. It works, OK, but for 10min exposure I have to wait for 20 minutes to capture a single image. That's long...

LENR works the way it does because that is the best way. For maximum efficiency you want to subtract exactly those pixels which glow at the time when you shoot the real image.

If you only take one DF in advance the sensor is not heated up exactly like it is after the shot and certainly not the same way after taking say 20 shots.
It is all about heat effects.

If you want top quality results there is no alternative to LENR / DFS which is faster and more accurate.
If you go for multiple exposures and then create an median image that takes even much longer.

[Markovo]

All my questions have been answered. Thank you for the fast response! Best of luck to you guys and may the focus be with you!

[mikeSF]

I was getting good results on the 645D by manually shooting one dark frame at the end of my entire astro series, ie as I was hiking back to the car, allowing the 2 to 4 minutes to expose while I am multitasking. It was incredibly disrupting to engage the compulsory dark frame subtraction for every exposure.

Then, in post, I set up a reasonable workflow to subtract the hot pixels using the single dark frame for multiple exposures.
good luck.

[DeadJohn]

A common astrophotography technique is to build a "dark library" that can be reused for many photography sessions. Dark frames should have the same exposure length and ISO as your light frames, and be taken at the same temperature. Cooled CCD astro cameras have the benefit of constant temperatures so the dark library only has to account for variable exposure length.

Noise patterns can change with sensor age, so dark libraries may need to be redone.

The approach mikeSF used "I was getting good results on the 645D by manually shooting one dark frame at the end of my entire astro series" is great. He's redoing his dark library each photo session to take care of temperature and sensor age.