CCD Cameras – How They Work And How to Correct for the Errors They Make Along the Way CCD vs CMOS – in 25 words or less • CCD = “Charge Coupled Device” have a readout along the edge of the chip, and produce less noise, usually best in astronomical setting • CMOS= “Complementary Metal Oxide SemiConductor” • CMOS chips have a voltage converter for each separate pixel and read-out faster (but noisier) than CCD’s A. How Does the Chip Work? The photoelectric effect is fundamental to the operation of a CCD. • Atoms in a silicon crystal have electrons arranged in discrete energy bands. The lower energy band is called the Valence Band, the upper band is the Conduction Band. Most of the electrons occupy the Valence band but can be excited into the conduction band by heating or by the absorption of a photon. The energy required for this transition is 1.26 electron volts for silicon. • Once in this conduction band the electron is free to move about in the lattice of the silicon crystal. It leaves behind a "hole" in the valence band which acts like a positively charged carrier. • In the absence of an external electric field the hole and electron will quickly re-combine and be lost. In a CCD an electric field is introduced to sweep these charge carriers apart and prevent recombination. • Absorbing a Photon Raises electron to the Conduction Band, where it can be easily moved 1.26ev = mid IR Photon – CCD’s sensitive only to Photons of higher energy than this. • Thermally generated electrons are indistinguishable from photo-generated electrons. They constitute a noise source known as "Dark Current" and it is important that CCDs are kept cold to reduce their number. • 1.26eV corresponds to the energy of light with a wavelength of 1 micron. Beyond this wavelength silicon becomes transparent and CCDs constructed from silicon become insensitive. CCD Chip vs. old style Film • CCD’s record up to 90% of the photons that fall on them, vs. only a few percent for film emulsions • CCD’s are linear over nearly their entire range (double the light, double the counts), while film is very non-linear. Important for photometry • CCD’s have far higher dynamic range. In other words, they can record a far higher range of light intensities than film. The “Bucket Brigade” Analogy There are Several Sources of Noise in Silicon Detectors like CMOS and CCD’s • 2. Readout Noise. In the process of “bucket brigade”ing the pixel’s store of conduction electrons to the amplifier, they can gain or lose a few in the counting process. • 1. Thermal Noise. Jostling of atoms due to not being at Absolute Zero temperature can also knock electrons into the conduction band. 1. Read-Out Noise • Bucket-brigading those buckets full of electrons one column across at a time and then into the wires, you can gain or lose a few along the way. • How to deal? • A. If it’s a major noise source, don’t read out your chip any more often than necessary. • B. Spend more money on your detector chip. (CCD’s are better than CMOS chips in this regard; • CCD’s read-out noise typically about a dozen electrons per pixel. Tiny! • CMOS chips usually what is used in commercial nonastronomical digital cameras like in your iPhone and point/shoot pocket cameras, and even in expensive DSLR’s). • For daylight photography, usually you have PLENTY of photons and so readout noise doesn’t matter, so CMOS chips are fine for daylight photography. 2. Dark Frame Correction • Jiggling of atoms can knock loose electrons and they are collected just as if they were knocked lose by a photon of light from your subject • But, it’s just noise – thermal noise A proper dark frame is an exposure with the aperture closed (like, you forgot to take off the lens cap). • Thermal noise is being added to your real picture at (on average) the same rate as it is added to the chip with the lens cap on. • This is true as long as the chip is at the same temperature in both situations. • If the chip were at absolute 0 temperature, there would be no thermal noise. • Then a dark frame picture would look like this An Actual Dark Frame from Our SBIG ST2000 camera. 5 min at Temp=-21C 3. Pixel-to-Pixel Sensitivity Variance • Pixels are super tiny, like 7 microns typically (7 millionths of a meter) and not all identical • They can gain electrons at slightly different rates. There may even be obvious “hot pixels” which produce lots more signal than they should • The fix for this problem is easy, as we’ll see in a few minutes And there’s millions of pixels in that little gray square! 4. Dust on the Chip Window • In an astronomical camera, the chip sits in an airtight little chamber so dew won’t form on it, and the pixels stay perfectly clean. • But, the window above it which lets in the light can get dust on it which may not be easy to just blow off. • This dust will show up as “dust donuts” – out of focus “donuts” of dimness, because the chip window is not at the focal point of the optics 5. Optical Illumination of the Chip Can Be Uneven • Your square chip is at the back end of a large, long set of round lenses and mirrors, and the center, or “optical axis” will get the most light from the sky, while the edges and corners will get a little less • So you see, your pixels will send off the “wrong” amount of signal for two reasons – it may receive less proper light, and it may respond to it with different sensitivity The Fix for Pixel Sensitivity, Optical Illumination, and Dust is All in One - The Flat Field Correction • A flat field is an image of a perfectly uniformly lit field. Example: the twilight sky (over a small bit of that sky) • Dividing your image by a flat field corrects for all these error sources • Divide?? Yes – every pixel value is divided by it’s value in the corresponding flat field image Color Pictures? • So, how is color information gotten in this process? It seems that any photon more energetic than a 1-micron IR photon will get counted equally, regardless of color. • Yes, so you need to color filter what photons get to your pixel. • There are Two Ways to do this… 1. Use a Filter Wheel Advantages of a Filter Wheel • You use the entire chip for each color, so you have a more sensitive image • You can choose a different exposure time for each filter, if desired. • Color resolution is single pixel, just like the image itself. Lower resolution (~2 pixel) for the Bayer Mask method 2. A Bayer Filter Mask Advantages of a Bayer Mask • 1. Need take only one image. “Single Shot Color”. MUCH faster to accomplish. • 2. Moving objects such as comets and asteroids will yield better images, because a filter wheel tri-color image will have gaps in time between each color image due to time needed to save image and re-starting the next image. You get a dot-dash set of different color stars, which may not be appetizing. Steps in Creating a Final Digital CCD Image • Take your Image at the telescope • Subtract a dark frame taken for the same exposure length and at the same chip temperature with the same camera • Divide by a flat field • Convert to color. Now you’ve got a fully corrected, color image! Color for Cabrillo • Our Big Dome’s CCD camera uses the Bayer Mask approach – “single shot color” • Our Nikon D7000 camera does the same • No filter wheels needed! They can be a hassle anyway, which is why I don’t use them any more at Cabrillo How to Do All These Steps? • I’ve already taken dark frames for the various temperatures we may be able to get down to. We have a “dark frame library”! • We also have flat fields already taken • So for the user, it’s all done in software, and I’ll help you with the steps as we get our photos! • It takes much longer to talk about the above steps than it does to DO them – For you, just a minute or two to accomplish these steps • But, there’s more wizardry to come… After The Corrections… • There are two more software steps after you have a fully corrected color picture • 1. For the CCD images, you’ll take 2 back-to-back 5 minute pictures. We’ll stack these together to make the equivalent of a 10 minute picture, which will show you how shot noise is reduced. Free off-the-web software called Registax will do this for us • 2. Then we will import into Photoshop and use many of the options there to polish your picture into final form. It’s really in Photoshop that you take possession of your photo and give it your individual touch unlike anyone else’s! • So, if we have too much fog and not enough fresh CCD images, we’ve got plenty already in the bank we can give to you to work with