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Home , Astrophotography

ASTROPHOTOGRAPHY (What is all the noise about?)

Chris Woodhouse ARPS FRAS Havering Astronomical Society

• a bit about me • living on the edge • what is noise? • break • noise combat strategies • and sensors • questions A bit about me living on the edge counting photons: portrait

• EOS CMOS sensor • 1/2,000 second , ISO 250 • face has value of 40,000 • about 80,000 photons • 1 photon every 6 nano seconds living on the edge counting photons: deep sky • KAF8300 8 MP CCD sensor • 20-min exposure • has pixel value of 65 • about 30 electrons, or 50 photons • 1 photon every 24 seconds ~4000 million times dimmer SNR - portrait vs deep sky

SNR = Signal to Noise Ratio portrait 1/1000s SNR = 200 (almost 100% shot noise) narrowband 1200s SNR = 3 (20% shot noise, 80% sensor noise) M63 Sunflower what is noise ?

Noise is everything in an image we don't want. • Light has Noise • (unwanted background) + shot noise (random noise) • deep sky (wanted signal) + shot noise (random noise)

• Sensors have Noise • pixel variations (pattern) • read noise (pattern and random) • thermal noise (average and random) light has noise

Photons are like raindrops; random in nature. If you measure and compare the accumulated amount over equal areas () you will find:

The randomness (noise) between pixels increases with the average amount, defined by:

shot noise = √mean pixel value

Compare a bright scene with a dim one: √40,000 = 200,1/200th of mean value √400 = 20, 1/20th of mean value light pollution and noise

Individual RGB filters have a noise advantage over RGB sensors; they reduce shot noise by reducing yellow light pollution. (As do LP filters)

the red and green exclude yellow (@ 590 nm) noise combat strategies

• take up fishing • add more exposure • calibrate the image files • find a darker site • cool the sensor • image processing • add even more exposure ripples in the space time continuum add more exposure

Random noise becomes less significant with more exposure. There are two similar strategies:

1. lengthen each exposure (but do not clip) 2. take more exposures and average them

In essence, choose an exposure that just does not clip and take multiple exposures. Each time you double the exposure count, the averaged image has 40% less noise. more exposure = better images

M31 :

4 hours luminance 4 hours through separate R, G & B filters

8 hours total over several weeks M31 Andromeda Galaxy even more exposure

M13 Globular Cluster:

130 x 5-minute exposures through separate R, G & B filters.

~11 hours over 3 weeks M13 Globular Cluster more is not enough

IC1396 Nebula

Shot with SII, Hα and OIII filters

120 x 20 minute exposures SHO 60 x 5 minute exposures RGB

45 hours over two months Elephant Trunk Nebula- enhanced sensor calibration

Why? If you average thousands of image files, you will still have sensor pattern noise, hot pixels, dust shadows and in each image. This 'noise' is constant but still annoying.

Calibration uses dark frames, zero-exposure frames and images of featureless T-Shirts to create an image of pattern noise to subtract from each image and normalize with another, which makes all pixels behave the same. sensors - require calibration

calibration typically requires:

• 50 averaged zero-length exposures of nothing • 50 averaged exposures of nothing at image exposure time and temperature • 50 averaged exposures of a flatly lit uniform subject • 50+ exposures of the image itself

calibrated image = (image - dark) x (normalized flat) Break Astrophotography welcome back Rosetta Nebula deep exposure and dynamic range

increasing the exposure count increases the dynamic range

example: - particularly tricky 10 hours Hα in 30, 120, 300 second exposures 8 hours SII in 120, 300 second exposures 8 hours OIII in 120, 300 second exposures 4 hours RGB for acquired over 10 nights (due to low altitude) Orion Nebula dynamic range > 25 stops

Sensor noise also reduces the sensors dynamic range but averaging multiple exposures improves it:

e.g. CCD “full-well capacity” = 25,000 electrons, read noise = 5 electrons >> effective dynamic range i O s: 25,000/5 = 5,000 levels or just 12 bits but averaging 50 exposures increases that by 50x to 18 bit and then can be processed as a 32-bit image image integration (an aside)

A simple average of the calibrated images may show traces of aircraft, satellites, cosmic ray hits and meteors. While not noise, they are still annoying.

When averaging, statistics sample equivalent image pixels in every image and reject individual pixels that are very different from the average value. M51 find a darker site

A darker site has several benefits:

• less light pollution • less shot noise contribution from light pollution • less sky gradients • same image quality with less exposure • benefits visual too • still has mosquitos • no Internet? cool the sensor

Thermally generated electrons accumulate randomly during each exposure ... • average noise level is proportional to duration • rate is proportional to temperature • randomness increases with average level, like shot noise. • the rate halves with ever 5–6°C reduction. ... equivalent noise reduction requires averaging double number of exposure frames. Horsehead Nebula, Natural Color image processing

image process is not the cure, but it helps; noise reduction tools (PixInsight) • MURE • MMT / MLT • TGVDenoise • SCNR - specific • Cosmetic Correction MURE Denoise (script)

• easy to use and remarkable: • requires gain, read noise and # of images in the stack • with no loss in resolution

Before After Heart Nebula SH2-190 CCD vs. CMOS sensors

• CCDs were more linear, less noisy but more expensive than CMOS sensors. • security and scientific users still use CCDs • 8 mpixel KAF8300 CCD is 15 years old • cameras have mostly abandoned CCDs and spent their resources improving CMOS sensors over the last 15 years. CCD vs CMOS sensors

• CMOS sensor architecture has faster readout speed and frame rate • high frame rate encourages use of "lucky " • specifications are alluring but you need to be careful • gain-setting changes alter dynamic range and noise • caution! rapid CMOS development for astro is creating hardware and software reliability issues M45 - Nebula 35 hours exposure sensor parameters datasheet values: • read noise (e-) • well depth (e-) • pixel size microns • sensor size (mm x mm) • ADC 12-, 14-, or 16-bit • gain (e- / ADU) • dark noise rate (e- / sec) • quantum efficiency (%) • linearity inferred Values - more meaningful: • dynamic range (well depth / read noise) • full well / area (FWD / pixel area) • noise / area √(read noise2 / pixel area) M27 - Dumbbell Planetary Nebula - 35 hours of exposure sensor comparisons

Dynamic full well read noise dark noise QE Ha Sensor ADC Range e-/area e-/area @-10C % KAF8300 16 3125 857 1.48 0.02 48 ICX814 16 5806 1322 0.84 0.002 65 ICX834 16 5625 937 0.52 0.002 70 KAF16200 16 4444 1111 1.5 0.06

Atik 12 5764 1385 0.91** 0.009 ~35 Horizon EOS 60Da 14 2595 663 1.1 ??? < 30 ISO400

IMX183 12 5172 2604 1.2** 0.008 54

ASI1600 12 5714 1385 0.92** 0.024 typical CMOS architecture

1 e- read noise 10,000 4096 65536

Gain (2) Output

x16 Note: • Well depth > ADC resolution • Read noise < ADC resolution (at low gain) • Read noise > ADC resolution (at gains higher than 3)

pixel 12-bit 16-bit well ADC output M33 - Triangulum Galaxy traits of CMOS sensors

**12-bit ADC limit dynamic range and increase read noise at low gain settings due to quantization noise. as gain (or ISO) is increased • dynamic range reduces • e- / ADU decreases • noise / area reduces • full well / area reduces • less quantization noise contribution

Amp Glow and associated shot noise Random Telegraph Noise - Cosmetic Correction gain settings - Atik Horizon has very different characteristics at different gains; leading to composite images: deep sky (long exposure of dim nebula) gain = 30, 512 effective dynamic range, 0.08 e/area colorful stars (short exposure) gain = 1, 5330 effective dynamic range, 0.94 e/area

>> process both and combine to give non-saturated colorful stars Eastern Veil Nebula QUESTIONS?