Technical Intro to Digital Astrophotography Part 1

How we capture photons CCD and CMOS sensors convert a percentage of captured photons (usually 30-75% = Quantum Efficiency) in electrons. Each pixel holds a max number of electrons (Full Well cap.). Electrons are converted to voltage and an amplification (Gain) is applied to them (also an offset to avoid zeros). The Analog to Digital Converter (ADC) takes voltage and converts it to ADUs (Analog/Digital Units), which are numbers representing intensities in our digital file. ADCs are usually 12-16 bit (12 bit = 4096 levels, 16 bit = 65536 levels)

Example 4000 photons arriving from M31 hit a pixel in 5 minutes, generating 2200 electrons. The electrons are converted to voltage and a gain of 2e-/ADU is applied, as well as an offset of 1000 ADU. If we have at least a 14bit ADC, it will convert the voltage to 5400 ADU (2200x2+1000), which is stored in our raw file. For a 12 bit ADC, max intensity level is 4096, so this pixel will be recorded as pure white at 4096, and probably so will its surrounding ones. We should have applied a lower gain, or less exposure. We can use gain less than 1 if needed – some people aim for “unity gain” (1e-/ADU), but it does not really have a technical advantage in multiple exposure astrophotography.

Noise & SNR Noise is the enemy of a good image. Maximizing the Signal to Noise Ratio is our main concern for getting the best possible images. Example simplified formula:

Types of Noise Shot/Photon Noise: Photons arrive at random intervals. The smaller the photon flux, the more noise. Solutions: Increase integration time, increase pixel size. Dark Current: Thermally generated electrons. Solutions: Cooling, Dark frames, Stacking, Dithering. Sky Background Noise: Light pollution, Sun/Moon. Solutions: Filters, Increase integration time. Read Noise: Noise generated by the sensor and ADC. Solutions: Dark Frames, Stacking, +Gain (ADC noise). Pixel Defects: Hot/Cold pixels, pixels with different QE. Solutions: Flat frames, Dithering. Amp Glow: Glow caused by electronics. Solutions: Dark Frames.

F/ratio, Pixel Size & Binning To increase the signal and reduce shot noise, we need to have more photons/pixel. Ways to do it: Increase f-ratio (larger aperture, or smaller focal length). Doubling the f-ratio means 4x signal (squared). Increase pixel size (needs a new sensor). Combine multiple pixels into one (Binning).

Color vs Monochrome All sensors are mono: color sensors just add color filters in a “bayer matrix”. Only 1/4th of the actual pixels on your color sensor will capture e.g. a red object (like an Ha nebula emission). The raw files of color sensors are still monochrome. A process called debayering or demosaicing tries to “guess” all 3 colors for a pixel by knowing only 1 color of the pixel, plus colors of neighbor pixels.

Color vs Mono QE Example of Quantum Efficiency for Color vs Mono version of the same Kodak sensor:

Sensor Size A larger sensor gives you a bigger field of view for the same focal length, or more light = better quality for the same FoV.

Object Size The size of an object (in pixels) depends only on your focal length and the pixel size of your sensor. Example: Assume Jupiter is 40” in diameter. With a Skywatcher 8” f/5, at 1000mm f.l. and a 4x PowerMate making it about 4000mm f.l., on a camera with a 3.75μm pixel (e.g. IMX224 chip) we get: Size = 4000 * 40 / (3.75 * 206.3) = 207 pixels

CCD vs CMOS vs DSLR CCD used to be lower noise and much higher QE than CMOS. CMOS has improved to the point that it has better noise/QE characteristics. Cooled CMOS cameras are being released, so far for the mid-range sector. They will replace CCDs in the near future. DSLRs use CMOS. Modern sensors are great in terms of QE and read noise, but they are not cooled and don’t have mono versions. Their advantage is sensor size and price. We will focus mostly on DSLRs for the next slides, although many things are the same for CCDs. Everything discussed, applies to shooting in RAW, which, unlike JPEG, contains the actual data that the sensor recorded.

ISO FALSE* “ISO is the level of sensitivity of your camera to light” *The sensitivity to light is called Quantum Efficiency and it is built-in a sensor, you cannot adjust it. ISO is the gain applied to the voltage from already detected photons. It amplifies both the signal and the existing read noise equally. It does not amplify the ADC noise, so a higher ISO can reduce noise by making the downstream read noise less in comparison to the signal. Increasing ISO reduces our dynamic range and can limit our exposure (we hit 100% intensity faster), so be aware that more exposure is ALWAYS better than higher ISO. Very high ISO (e.g. 6400+) involves digital amplification after the ADC and is not beneficial.

But why are my high ISO photos noisier? LESS LIGHT (signal) is the simple answer. Normally you either shoot in high ISO to “freeze” a frame (e.g. sports) by using a shorter exposure, or you are already in a low light situation and your only option is a high ISO, otherwise the image (along with the noise) will be dark. Less light = more noise. If you shot with the same exact exposure and f/ratio at ISO 200 and 3200, then increased the brightness of the dark ISO 200 image, you would get either exactly the same result, or a bit better with ISO 3200 if your camera had some ADC noise (seen next slide). Sensorgen.info has (rough) noise values per ISO for DSLRs.

ISO 200 & 3200 at the same exposure, aperture As shot Brightness matched

Astro-modified DSLRs DSLRs have filters over the sensors to block unwanted UV/IR and adjust the color balance to match human perception. Unfortunately, that involves blocking 75% of the Ha line:

Astro-modified DSLRs Why modify: Dramatic increase (4x) in sensitivity (QE) for red emission nebulae, some increase in overall sensitivity. Ways to modify a DSLR: Remove camera filter(s) to make it Full-Spectrum. You will need a UV/IR filter to shoot (esp. with lenses involved), but you can also shoot through UV-pass or IR-pass filters. Replace camera filter with Baader UV/IR or similar. For cameras with dual filter, remove only the IR block (hot filter). This leaves the anti-aliasing filter (less sharp). Example modification service: “Cheap Astrophotography”

What white balance should I use for astrophotography? It shouldn’t matter at all. In RAW, white balance is saved as a number, providing a guide to the image processing program of how to alter the raw colors. It does not actually alter data. Astrophoto processing programs normally do not apply a white balance, as it does not make sense for deep space photos - original linear data is preferred. Some people might find a workflow that they like that involves applying a specific white balance. Good for them I guess ;) DSS is a little “special” in that while you do not select to apply the image white balance in RAW settings, it does not actually leave the data linear. Hence, we select “white-balanced data” when opening in StarTools.