- Slides: 21
Webcam Astro-imaging Workshop Dave Dockery & Steve Barkes
Goal – To help everyone learn to achieve better images with their equipment. Session 1 Overview • • Why Webcam Astrophotography? How Do Digital Cameras Work? Camera Settings Drift Imaging vs. Tracking Focusing Basic Operation of K 3 CCDTools Exercise – Lunar Imaging
Why Web Camera Astrophotography? • Inexpensive alternative to astronomical CCDs • Quick. Cam and Unconventional Imaging Astronomy Group (QCUIAG) • Digital video format • Video Captures (AVI files) • Single frame capture • Some support long exposure modification • Imaging advantages • Full color • Immediate results • Can take many images to get a few at moments of good seeing • Can combine many images to enhance image features and reduce noise
How Do Digital Cameras Work? l l l CCDs invented as a type of analog computer memory that can store any value in it’s range, not just 0 or 1 These capacitive cells are arranged in an array and accumulate a charge when struck by a photon The charges are transferred out of the array and digitized into picture elements (pixels) The number of charge cells in the array determines the maximum image resolution in pixels (typically 640 x 480) The resolution of the digitizer (8 bit, 12 bit, 16 bit, etc. ) determines the number of shades each pixel can represent (typically 8 bit) Advantages – Sensitivity and digital format l Disadvantages – Noise is caused when a cell charges due to energy from sources other than incoming photons (thermal, electrical, cosmic rays, etc. ) – Charge cells can leak into adjoining cells when fully saturated causing “blooming”
Camera Settings • Frame resolution • Nominally want to use max to get the most pixel information across the target. For smaller targets, you can sometimes use a sub-window to increase frame rate. • Frame rate and video compression • Lower frame rates mean less compression • Cameras advertise 30 fps but this is at the cost of image quality • Typically must image at 5 -10 fps at highest resolution for best quality (limitation of USB bandwidth) • AGC and manual gain settings • Can use Automatic Gain Control (AGC) for lunar or solar imaging. The object must mostly fill the frame to get good levels.
Camera Settings (2) • AGC and manual gain settings (cont. ) • • • Shutter speed vs. seeing conditions • • • White balance is normally left in auto-mode or in the outdoor setting. Gamma • • Short exposures will capture more brief moments of good seeing. The higher the gain, the faster the shutter speed that can be used for a given light level. This is limited by the acceptable level of noise. (Higher gain means higher noise and it varies by camera) White balance • • Use manual gain to set proper level for small objects (e. g. planets) Also use manual gain control to minimize shutter time during poor seeing conditions Intensity linearity scaling factor that is normally left in the default position. Saturation • Color intensity – normally set midrange
What factors effect image quality? l l l l Observing conditions Magnification Aperture and optical quality of telescope CCD resolution, sensitivity, and noise Tracking (for longer exposures) Ambient temperature Image Processing
Drift Imaging vs. Tracking • Drift imaging • Does not require a telescope mount that tracks the apparent motion of the stars. • Use a fixed mount pointed ahead of the object (moon) and record an AVI file as the object drifts through the field of view. • Creates the effect of flying over the terrain. • Disadvantage – can’t stack images • Tracking • Alt-az vs. equatorial mounts (how many? ) • Field rotation • Alignment accuracy vs. magnification • Polar drift alignment
The Subtle Art of Focusing l Problem: Air turbulence and diffraction can make an image inherently blurry no matter how well you focus. (Covington) Focusing Procedure using a Hartman mask: 1. Point to a bright star (overhead will minimize turbulence) 2. Install Hartman mask 3. Optimize camera sensitivity so that star is visible but not saturated. 4. Collapse star pattern to a tight point by adjusting telescope focus 5. When using a SCT, lock focus if possible 6. Remove Hartman mask and point to the target Alternate Method: l Use a high contrast area of the Moon or a set of sunspots and focus for max sharpness in place of steps 2 & 4
Setup and Capture (So, how do I make this stuff work? ) l l l l Setup telescope (tracking is preferred but not required for bright objects) Install camera and Barlow lens (if applicable) Set desired camera resolution Set frame rate for minimum compression Set camera exposure and gain levels Focus using the focus procedure Frame the shot Collect Data (Lots)
Image Processing Introduction l Image processing software for digital enhancements – Levels histogram – fill dynamic range – Curves/Colors – Detail enhancement – spatial frequency algorithms Smoothing l Sharpening – Combining images l Stacking l Summing l l Save during each processing step – Use compressed format to share (50 Kb rule of thumb)
K 3 CCDTools Basic Functions - Steve Overview and history • Basic features and settings • • Camera controls • How to use the meter to set levels manually • How to capture single images and video • How to open and save files • How to save BMP images from AVI files
Session One Exercise – Lunar Imaging 1. 2. 3. 4. 5. 6. 7. Set up telescope, camera, and laptop outside (You can use a Barlow lens, if needed) Turn on AGC, set max resolution, and 5 fps frame rate Locate the Moon and then focus on a high contrast feature using the focusing procedure Turn off tracking and position the telescope so that the moon drifts into the FOV. Capture an AVI video called “Moon. Drift” and save it in the My Videos folder Turn on tracking and center the moon or an interesting feature in the FOV. Turn off AGC and manually adjust the gain to 50% then adjust the shutter speed to achieve 75% full scale on the level meter.
Session One Exercise (continued) 8. 9. 10. Capture an AVI and save it as “Moon. Tracking” Capture several single images and save them as BMP files in the My Pictures folder. Homework: 1. 2. Load the Moon. Fixed AVI into K 3 CCD Tools and look through the individual frames. Find several of the sharpest and save them as BMP images. Use your favorite image processing software to adjust the brightness, contrast, and sharpness of your best lunar BMP file and make sure to save it under a new name.
Resources l Internet Links – http: //www. qcuiag. co. uk/ - QCUIAG Site – http: //www. pk 3. org/Astro/ - Peter Katreniak K 3 CCDTools – http: //webcaddy. com. au/astro/adapter. htm - Mogg Adapters – http: //www. zianet. com/dave. dockery/Astro. Photos. htm - Dave’s site – http: //www. barkosoftware. com/index. html - Steve’s site l Egroups – [email protected] com – [email protected] com l Books – Astrophotography for the Amateur - Covington – Splendors of the Universe - Dickinson
Webcam Astro-imaging Workshop Session Two Dave Dockery & Steve Barkes
Goal – To help everyone learn to achieve better images with their equipment. Session 2 Overview • • • Camera Settings Effects Focusing Review K 3 CCDTools Planetary Wizard Exercise Registax Demonstration – Jupiter Imaging
Camera Settings • Frame resolution • • Frame rate and video compression • • Typically must image at 5 -10 fps at highest resolution for best quality (limitation of USB bandwidth) AGC and manual gain settings • • • Nominally want to use max to get the most pixel information across the target. For smaller targets, you can sometimes use a subwindow to increase frame rate. Use manual gain to set proper level for small objects (e. g. planets) Also use manual gain control to minimize shutter time during poor seeing conditions Shutter speed vs. seeing conditions • • Short exposures will capture more brief moments of good seeing. The higher the gain, the faster the shutter speed that can be used for a given light level. This is limited by the acceptable level of noise. (Higher gain means higher noise and it varies by camera)
Effects • Sky conditions - probably the biggest factor in our ability to image fine detail. • Take advantage of good conditions – watch the Clear Sky Clock • Equipment – aperture, telescope type, condition of optics, dirt on sensor, collimation, and thermal stabilization. • Tracking - mount alignment accuracy vs. magnification • • • Stacking - why stack images? • • Polar drift alignment Two Star Alignment (Alt-Az) Improved SNR Planetary rotation limit on stacking • • • Jupiter – 90 seconds max (. 41 day period) Saturn – 120 seconds or so, less critical (. 44 day period) Mars – 180 seconds (1. 03 day period)
The Subtle Art of Focusing Procedure using a Hartman mask: 1. Point to a bright star (overhead will minimize turbulence) 2. Install Hartman mask 3. Optimize camera sensitivity so that star is visible but not saturated. 4. Collapse star pattern to a tight point by adjusting telescope focus 5. When using a SCT, lock focus if possible 6. Remove Hartman mask and point to the target
K 3 CCDTools Planetary Wizard Exercise - Steve