Astrophotography Primer
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This document provides an overview of astrophotography, including the types of targets photographed (planets, nebulae, galaxies), necessary equipment (telescopes, mounts, cameras), and techniques (tracking mounts, stacking images). It discusses factors that impact image resolution like diffraction limit and sensor resolution. It also covers how to reduce noise through techniques like calibration frames, increasing exposure time, and stacking multiple images. The document lists common astrophotography software and provides tips for processing images, including emphasizing desired details and artistic composition.
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Astrophotography Primer
- 1. Astrophotography Primer Mark Cheverton Instagram: @photonfog
- 3. Equipment • Lenses, Telescope – Newtonian, refractor • Tracking mount, Tripod • DSLR, Webcam, Astroimaging CCD • Guide scope and camera • Intervalometer, computer capture
- 4. Resolution • Resolution = SQRT of sum of the squares of • Diffraction limit ~ wavelength/aperture • Sensor resolution ~ pixel size/focal length • Seeing ~ 3 arcsecs in suburbs (image at nearer zenith means less atmosphere) • Usually CCD and seeing are similar and outweigh the diffraction limit
- 5. Widefield • Targets • Milky way • Meteor showers • Aurora • Timelapse
- 6. Planetary • Targets • Planets • Moon • Comets • Lucky Imaging
- 7. Deep sky nebula • Targets • Emission nebula • Reflection nebula • Dark nebula
- 8. Small deep sky targets • Targets • Galaxies • Globular/Open Clusters • Planetary nebula
- 10. Sources of noise
- 11. Calibration frames • Used to reduce fixed noise. Calibration frames have their own noise so need lots • Dark frames • Reduces fixed pattern thermal noise in camera, e.g. hot pixels, sensor glow • Need to match exposure and temperature • Also helped by camera cooling • Flat frames • Reduces fixed pattern noise in optical train assembly, e.g. vignetting, dust motes • Aim is equal pixel values for equal exposure • Bias frames • Reduce read noise gradients in camera sensor • Per pixel offset and gain adjustment • Zero exposure frames • Read noise is independent of exposure (reading of the sensor) • Exposure for long enough to swamp the read noise
- 12. Exposure stacking • Even a perfect sensor evenly lit will have a distribution of values around mean due to quantum mechanics. Noise = SQRT(Signal) • Shot noise is reduced by increasing exposure • Shot noise means long exposures are needed for faint subjects but… • Wash out from light pollution reducing dynamic range • Over exposure of stars • Errors in long exposure tracking • Satellites, clouds and airplanes • Stack multiple images instead to improve signal to noise ratio and avoid saturation • Signal is reinforced • Shot noise is averaged out • Need minimum exposure time to separate from read noise which is independent of exposure
- 14. Increasing signal to noise • Filters • Light pollution • Monochrome & Bayer • Narrowband • Larger aperture • Longer exposure – polar alignment, guiding • Dithering • Brighter targets
- 15. Processing
- 16. My software stack • Apps • Photopills • Clear outside • Sky safari • Polar finder • ASIcap • Processing • Pixinsight • Astropixelprocessor • Observatory control • Kstars • Video capture • FireCapture • Winjupos • Widefield • StarStax • Starrylandscapestacker • Photoshop • Lucky imaging • Registax • Autostakkert
- 17. Artistic interpretation • Processing to show the details you care about • Composition • Colour
Editor's Notes
- Lucky Imaging is a remarkably effective technique for delivering near-diffraction-limited imaging on ground-based telescopes. The basic principle is that the atmospheric turbulence that normally limits the resolution of ground-based observations is a statistical process. If images are taken fast enough to freeze the motion caused by the turbulence, we find that a significant number of frames are very sharp indeed where the statistical fluctuations are minimal. By combining these sharp images we can produce a much better one than is normally possible from the ground.
- Light from subject and light pollution altered by dust and vignetting Quantum efficiency determines photon to electron conversion Random thermal electrons dark current ~ double for each 7deg rise Error in amplifier pixel gain and noise Quantisation error of electrons to output value
- Noise increases with exposure
- A single two minute exposure (left) vs. 60 two-minute exposures stacked (right).
- A single two minute exposure (left) vs. 60 two-minute exposures stacked (right).