Digital SLR Astrophotography

Transcription

1 Digital SLR Astrophotography by Michael A. Covington Cambridge University Press 2007 The following pages are a DRAFT TABLE OF from an unfinished version of the book manuscript. The page numbers DO NOT APPLY to the finished, printed book.

2 Contents I Basics 16 1 The DSLR Revolution What is a DSLR? Choosing a DSLR Major manufacturers Shopping strategy Choosing software Photo editing Astronomical image processing Freeware Judging software quality Is a DSLR right for you? Is film dead yet? Main technical issues Image files File size Raw vs. compressed files Digital film and camera software Focusing

3 2.3 Image quality Combining images Overcoming sky fog Dark-frame subtraction The Nikon star eater Grain Sensor size and multiplier (zoom factor) Dust on the sensor ISO speed settings No reciprocity failure How color is recorded The Bayer matrix Low-pass filtering The Foveon Nebulae are blue or pink, not red Basic camera operation Taking a picture manually Shutter speed and aperture Manual focusing ISO speed White balance Do you want an automatic dark frame? Tripping the shutter without shaking the telescope Mirror vibration Vibration-reducing lenses The camera as your logbook Limiting light emission from the camera

4 3.4 Menu settings Things to set once and leave alone Settings for an astrophotography session Using Nikon Mode Determining exposures Four simple projects Telephoto Moon Afocal Moon Stars from a fixed tripod Piggybacking Going further II Cameras, Lenses, and Telescopes 71 5 Coupling cameras to telescopes Optical configurations Types of telescopes Types of coupling Fitting it all together Optical parameters Focal length Aperture f-ratio and image brightness Field of view Image scale in pixels What is the magnification of this picture? Vignetting and edge-of-field quality

5 6 More about focal reducers Key concepts Optical calculations Commercially available focal reducers Lens types Meade and Celestron f/ Meade f/ Others Lenses for piggybacking Why you need another lens Big lens or small telescope? Field of view f-ratio Zoom or non-zoom? Lens quality Sharpness, vignetting, distortion, and bokeh Reading MTF curves Telecentricity Construction quality Which lenses fit which cameras? Testing a lens Diffraction spikes around the stars Lens mount adapters Adapter quality The classic M42 lens mount Understanding lens design How lens designs evolve

6 7.6.2 The triplet and its descendants The double Gauss Telephoto and retrofocus lenses Macro lenses Focusing Viewfinder focusing The viewfinder eyepiece The Canon Angle Finder C Viewfinder magnification Modified cameras LCD focusing Confirmation by magnified playback LCD magnification Computer focusing Other focusing aids Diffraction focusing Scheiner disk (Hartmann mask) Parfocal eyepiece Knife-edge and Ronchi focusing Focusing telescopes with moving mirrors Tracking the stars Two ways to track the stars The rules have changed Setting up an equatorial mount Using a wedge Finding the pole

7 9.3.3 The drift method Guiding Why telescopes don t track perfectly Must we make corrections? Guidescope or off-axis guider? Autoguiders A piggyback autoguider How well can you do with an altazimuth mount? The rate of field rotation Success in altazimuth mode What field rotation is not Power and camera control in the field Portable electric power The telescope The computer and camera Care of Li-ion batteries Ground loop problems Safety Camera control Where to get special camera cables Tripping the shutter remotely Controlling a camera by laptop Networking everything together Operating at very low temperatures Sensors and sensor performance CCD and CMOS sensors

8 11.2 Sensor specifications What we don t know Factors affecting performance Image flaws Binning Nebulae, red response, and filter modification DSLR spectral response Filter modification Is filter modification necessary? Filters to cut light pollution Didymium glass Interference filters Imaging with deep red light alone Reflections III Digital Image Processing Overview of image processing How to avoid all this work Processing from camera raw Detailed procedure with MaxDSLR Screen stretch Subtracting dark frames Converting to color (de-bayerization, demosaicing) Combining images Stretching and gamma correction Saving the result Processing from linear TIFFs

9 Making linear TIFFs Processing procedure Processing from JPEG files or other camera output Digital imaging principles What is a digital image? Bit depth Color encoding Files and formats TIFF JPEG FITS Image size and resizing Dots per inch Resampling The Drizzle algorithm Histograms, brightness, and contrast Histograms Histogram equalization Curve shape Gamma correction Sharpening Edge enhancement Unsharp masking Digital development Spatial frequency and wavelet transforms Deconvolution Color control

10 Gamut Color space Color management Techniques specific to astronomy Combining images How images are combined Stacking images in Photoshop Who moved? Comparing two images Calibration frames Dark-frame subtraction Bias frames and scaling the dark frame Flat-fielding Removing gradients and vignetting Removing grain and low-level noise The extreme brightness range of nebulae Simple techniques Layer masking (Lodriguss method) Other Photoshop techniques Where to learn more IV Appendices 264 A Astrophotography with non-slr digital cameras 265 B Webcam and video planetary imaging 269 B.1 The video astronomy revolution B.2 Using a webcam or video imager

11 B.3 Using RegiStax C Digital processing of film images