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Friday, October 24 2014 @ 02:59 AM EDT

 

CCD Group

Please review the topics below for helpful tips and tricks.

An Introduction to CCD Astrophotography

Equipment Needed

  • Camera
  • Telescope
  • Mount
  • Computer
  • Software
  • Power supply
  • Willingness to climb a steep learning curve

CCD Explained

The CCD Camera Chip

  • CCD = “Charge Coupled Device”
  • Based on photoelectric effect (Heinrich Hertz)
  • Photons in – electrons out…Proportional
  • Primary unit is the pixel (Picture Element)
  • Chip can have 100K’s to millions of pixels
  • Circuit has an amp and A/D converter
  • Signal to noise ratio (SNR) is concern

News Print Analogy

Rough Analogy Half Tone Image
Half Tone news print picture made up of ink dots. Ink dot density is in proportion to image light and dark areas.
CCD image made up of pixels which have electron density proportional to image light and dark areas.

Schematic of CCD Chip

The Inside Story

High QE KAF-0402ME CCD

Sample Cameras

Atik 16C Celestron Neximage Meade DSI Pro
DSLRs SBIG Starlight Express

CCD Advantages vs. Film

  • CCD has greater light sensitivity
  • Avoids reciprocity problem of film
  • Almost Instant image examination
  • Image “Stacking” ~ long exposure
  • Can remove plane/satellite tracks

Telescopes For Imaging

  • Can use almost any type of scope, even a camera lens with an adapter
  • Shorter FL scopes = shorter exposure time and less demanding tracking
  • Typical scopes include 60-152mm refractors and 6-14” Newts & SCT
  • f/5 or less = “fast” focal ratio scope
  • Image scale is an important consideration

Image Scale and f/ratio

Image Scale versus Focal Length

10” SCT @ f/10 Atik 16C 10” SCT @ f/3.3 Atik 16C
Matching telescope to CCD chip size & object is critical

The Telescope Mount

  • The single most important component
  • Must be able to track & have low PE
  • Must be solid & vibration dampening
  • Must be capable of supporting wt. of gear
  • Should be capable of being guided
  • Needs to be polar aligned for long exposures

Mount Examples

Fork and Wedge Fork AltAz German Equitorial

Imaging Setup

Capturing Photons

Stark-Labs’ Nebulosity Software

Exposure & The Histogram

The histogram graphically depicts the exposure of the image. The “X” axis of the histogram shows from black on the far left to white on the far right. The “Y” axis illustrates the number of photons captured along that continuum.

Desired Histogram

Over-Exposed Image

Under-Exposed Image

After Capture Processing

  • Will need to manipulate digital image to bring out the image data
  • Post capture data manipulation divided into “Calibration”, preprocessing & “Post processing”
  • Calibration removes various types of “noise”
  • Post processing “stretches” (enhances) the image data
  • Involves another learning curve
Types of CCD Images
Type Nickname Purpose
Luminance lights Image
Bias Frame Bias Calibration
Dark Frames Darks
Flat Field Flats
Bias Frame
  • Represents bias signal already on the chip before the image is taken. (it’s always present)
  • Taken at min. exposure time & the scope covered
  • Taken at same temp as dark and light frame

  • Taken at lowest possible exposure time with scope covered to keep light out
Dark Frame
  • Records the thermal noise of the actual exposure
  • Noise increases with exposure time and ambient temp. (reason CCD’s are cooled)
  • A dark frame should be taken at same time & temp. as the light exposure

  • Electrical noise in system
  • affected by time & temp.
  • Taken with objective lens covered & same time & temp as light frame
  • Must subtract bias from dark to get a “clean ”dark frame
Flat Frame
  • Flats are another type of calibration image
  • They remove photosite to photosite variations and problems in the optical train (dust motes, gradients, vignetting, etc.)
  • Flats need their own calibration with bias & dark frames before being used
  • Must be taken with same optical set-up

  • Removes Optical Train Defects. Exposure is usually ½ to 1/3 ADU of Luminance
  • Must use “flat” uniform illumination
RAW Sub Frame

  • Image + noise, Hot pixels, amp glow & dust/gradient
Calibration Sub Frame

  • 30 sec. sub –(bias, dark frame)/ flat
Stacking and Aligning
  • Next step after image calibration
  • “Stacking” combines sub frames, increases S/N ratio, simulates a long exp.
  • Aligning reduces tracking errors of mount and some sky rotation in AltAz mounts
Before and After Alignment
Before After
Before and After Post Processing
Before After
B&W vs. Color Images
  • All CCD chips are B&W
  • Color requires separate RGB images (not unlike the photo pigments in the human eye’s cone cells)
EMS Spectrum Visible Light
Two Ways to Color
B&W (monochrome) with individual RGB filters
One Shot Color with filters on Chip
Color fom Monochrome
  • Must use separate images taken through individual RGB color filters (not visual filters)
  • Need Luminance image, usually with an IR block filter, to enhance image detail,
  • Result is higher resolution than 1-shot color but requires 4x the number of images

One Shot Color Image
  • Preprocess color image as if “raw” B&W with regard to “Bias, Darks & Flats”
  • Must “deBayer” into component colors
  • Software then magically combines RGB
  • Post process as Tiff or Bmp
Color Imaging with Filters
  • Requires significantly longer imaging sessions than One-shot color.
  • Time introduces variables (focus between filters, tracking, sky position, etc).
  • Most advanced imagers use LRGB imaging
Typical Imaging Session
  • Set-up and polar align mount & scope (drift align)
  • Find target to image & critically focus
  • Determine maximum exposure time
  • Set software to take a series of Lum exposures (25-50)
  • Take 10-15 bias frames at lowest exposure time
  • Take 10-15 dark frames at time & temp of light exposure
  • Take 10-15 flat frames at 1/3 to ½ ADU of light exposure
  • If color, repeat with individual RGB filters (not with one-shot)
  • Break-down equip. and head home (~4am)
  • Calibrate all sub frames
  • Align & combine all calibrated sub frames and save as separate files
  • Process to merge into an LRGB image
  • Post process in PhotoShop or similar to bring out detail
  • Swear you’ll do better next time

Advanced Subjects

  • Advanced post processing techniques
  • Mount tuning to improve tracking & minimize PE
  • Auto-guiding
  • Narrow band filters

The Questions to ask Before Getting Started

  1. Are you willing to make the commitment?
  2. Will you or your bank account go first?
  3. Will your marriage survive?

The Reward

You get to hang out with a great bunch of people!

Recommended Reading

  • Ron Wadoski’s “The New CCD Astronomy
  • R. Scott Ireland “Photoshop Astronomy
  • Michael A. Covington “Astrophotography for the Amateur” 2nd ed.
  • Plus many others

Video of this Course

Software and Drivers

Meade LPI and Vista

Many people have said they are having problems controlling the Meade LPI camera from Windows Vista. The drivers are not compatible. It appears (and Simon Porter has seen it work) that the following driver from TRUST WEBCAMS will work:

www.liutilities.com/device-driver/trust-webcam-14823-driver

Astro Tortilla

Presentation by May Astroimaging Group speaker Mark Casazza, on the AstroTortilla plate-solving software: astrotortilla-final.pptx

Links to useful tools:

htpp://clearskyalarmclock.com This link will alert you whenever the Clear Sky Clock is forecasting a clear night

http://tonightssky.com Will allow you to set your location and then find what objects, by magnitude and FOV are available to view or image.

 
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ccd_group.txt · Last modified: 2014/05/19 22:44 by David Moll
 
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