A question about Flats

[Kirkus (Kirk)]

In the article "Digital Imaging - Flats and Darks Explained!" in the Projects and Articles section, relating to Flats, I don't understand this sentence:

The main aim is to get a flat field that has an average pixel value of about 30% of the maximum pixel value that your camera is capable of. For a 16-bit camera, this would be approximately 20,000. Most image processing software will give you the average pixel value in an image so this value is generally easily obtained.

I'm not sure what "pixel value" is. Is this speaking about resolution?

[iceman (Mike)]

No, it means the intensity of the pixel - how "bright" it is.

So when you take a flat, you're aiming for the histogram to spike at about 1/3 of the way along.

[Dennis]

Good question. Each “pixel” on the sensor is a photo-site that has a “well depth”. Think of each photo-site as a bucket and think of incoming photons as droplets of rain.

Each ccd/cmos chip has a specific well depth, how many photons it can register before a bucket (photo-site) fills up and overflows (saturates).

For my ST7E ccd camera, I get saturation or star blooming spikes at around 58,000 photons, so when I take my Flats, I aim for an exposure which gives me a reading of around 15,000 to 20,000 which in my case, is shown by the camera control software, CCDSoft.

From memory, I think that my ST7E is a 16 bit camera so in theory the “bit depth” is 65,536, although it seems my “well depth” is only around 58,000 before the photo site overflows.

Cheers

Dennis

[Kirkus (Kirk)]

Okey dokey. Thanks to you both. Now it makes a little more sense.

Would I adjust the exposure/iso to reach the optimum 30% value? Would changing the value in Photoshop after the fact be an acceptable way to reach that value?

[iceman (Mike)]

Use ISO100, and adjust the exposure to get the histogram to the right spot.

[Kirkus (Kirk)]

Originally Posted by iceman

Use ISO100, and adjust the exposure to get the histogram to the right spot.

Excellent! Thank you, Mike. Thank you, Dennis.

[higginsdj]

That Flat advice should probably read 'no less than 30%'. I take flats at 50-60% of well depth for my photometry work.

Cheers