A question about Flats

[Kirkus]

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



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

[iceman]

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]

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]

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

[Kirkus]

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