Basic CCD Imaging by pengtt

VIEWS: 25 PAGES: 57

									Beginning CCD Imaging
           Major Topics
 CCD VS Other Cameras
 Choosing a CCD Camera
 Choosing a Guide Method
 Choosing a Telescope
 Choosing a Mount
 Choosing a Computer
    CCD VS. Film Cameras
A CCD Camera is better than film because….
CCD imaging chips are more sensitive than film
You can be successful with shorter exposures
You get instant feedback on your technique


BUT….
Most CCD chips are smaller, than a film negative. Larger CCD Chips
are available but expensive.
CCD Images are more difficult to print. Internet sites are available to
print large images, but you will need to experiment to get the brightness
levels right.
    CCD VS. Digital Cameras
A CCD Camera is better than a digital camera for
deep sky imaging because...
CCD Cameras have dramatically less noise because they are cooled.
This allows you to take long exposures.

BUT…
Digital Cameras take superb solar, lunar and planetary images,
especially through large, fast telescopes.
       CCD VS Video Cameras
A CCD Camera is better than video cameras
Because…
Unmodified video cameras are limited to extremely short exposures
– 1/60th of a second. CCD exposures may last an hour or more.
Modified video cameras that lack cooling produce noisy images.

BUT…
Software allows stacking of individual video frames to produce some
of the most stunning planetary images.
Video cameras are great for live shots of the moon, planets or sun
and for sharing the images with an audience in real time
Choosing a CCD Camera
Criteria for CCD Camera Selection
 The CCD Pixel size (should match the
  telescope).
 Sensitivity ( More sensitivity = less
  exposure time).
 Blooming or Anti-blooming.
 Micro-lens or not.
 CCD Sensor size.
     What is a pixel anyway?
 A pixel is a Picture Element
 May be thought of as a photon counter
 For color images, each pixel is
  represented by 3 numbers (1-red, 2-blue,
  3-green)
      Pixels arranged in an array
   Pixels are arranged in an array of rows and columns.
   Each pixel counts the number of photons striking it.
   More photons equal “brighter” pixels.
   All pixels displayed on the screen simultaneously
    produces an image.
    Matching the CCD imager to the
               telescope
   Match the pixel size to the telescopes resolution.

   Resolution can be defined as how much detail a
    particular telescope can see. It is dependent
    upon the size of the aperture and the quality of
    the optical surfaces

   Image Scale is defined as the arc seconds of
    sky “seen” by each camera pixel and is
    determined by the scope/camera combination
      Recommended Pixel Size
   Seeing conditions on any given night limit
    the image scale. For most locations, an
    image scale of 2.0 – 4.0 arc seconds per
    pixel is going to be useful on most nights.
    1 to 2 arc seconds per pixel will be
    possible on nights of exceptional seeing.
           Arc Seconds Per Pixel
   To calculate how much sky a pixel sees in arc
    seconds, divide the pixel size in microns by the
    telescope focal length in millimeters and multiply
    the result by 206.
             What is Binning?
   Binning combines groups of pixels
    together, in effect giving you different pixel
    sizes with the same camera. This allows
    you to match pixel size to telescope focal
    length more flexibly. For a given
    telescope, small pixels require longer
    exposures but provide higher resolution.
              Binning


Binning 1x1     Binning 2x2       Binning 3x3


Binning 1x1 - 1 image pixel = 1 camera pixel
Binning 2x2 - 1 image pixel = 4 camera pixels
Binning 3x3 - 1 image pixel = 9 camera pixels


  Binning reduces final image size because
  the image is made of fewer pixels.
             Example Image Scales
             Focal ratio/focal   arcseconds per     arcseconds per      arcminutes of sky
Telescope
             length              pixel unbinned     pixel binned 2x2    coverage
Stellarvue
SV80s        F4.8 / 384 mm       3.65               7.3                 89.5 X 132.8
(Reducer)

Stellarvue
             F6 / 480 mm         2.92               5.84                71.6 X 106.2
SV80s

TSA102
             f6 / 610 mm         2.3                4.59                56.3 X 83.6
(Reducer)

TSA102       f8.0 / 816 mm       1.72               3.43                42.1 X 62.5

TMB 152
             F6.3 / 961 mm       1.46               2.92                35.8 X 53.1
(Reducer)

TMB 152      f8 / 1200 mm        1.17               2.33                28.6 X 42.5



                             Blue text indicates a useful image scale
To Bloom Or Not To Bloom
Blooms are artifacts created when the pixels imaging
bright objects (stars) become saturated.


Think of it as a bucket that is filled too full of water and
overflows.
       A Choice of CCD types
•A Camera with an anti-blooming gate is usually less
sensitive than one with no anti-blooming gate.
•A non anti-blooming camera is better if you plan to do
photometry. An anti-blooming camera pixel starts to bleed
off the electrical charge (becomes less sensitive) as the
pixel nears the point where blooming will occur. This fall
off in sensitivity is not linear.
•CCD imaging software makes removing blooms less
difficult.
The affects of the micro-lens
Micro-lens artifacts are artifacts that occur only on
certain CCD Chips with micro-lenses installed.
These chips typically have higher sensitivity. An
ST10XME (micro-lens) has up to 87% peak
quantum efficiency. The ST10XE (no micro-lens)
has up to 66% peak quantum efficiency.
Micro-lens artifacts can be removed with software,
but at this time no software package does this
automatically.
       Large CCD Chip Sizes
Recently large chips have become available. These
chips are the same size as a 35mm film negative or
larger.
These chips give a very wide field of view but….
The Large chips are usually less sensitive with peak
quantum efficiency less than 60%. More sensitive chips
are available but very expensive.
Make sure your imaging telescope can provide a flat field
large enough to cover the chip.
Make sure your imaging telescope has an unobstructed
field of view large enough to cover the chip.
        Your camera choice
 If you must set up and tear down for each
  imaging session, then sensitivity is king.
  Consider a non anti-blooming camera with
  micro-lenses.
 If you have a permanent setup with a great
  mount then a large anti-blooming camera
  is advantageous.
   Single Shot Color Cameras
Single shot cameras simplify color imaging
  but ….
 They are usually a bit less sensitive than
  monochrome cameras.
 They don’t do narrow band imaging as
  well as monochrome cameras.
 Some resolution is lost because each pixel
  records only one color.
    What Is Narrow Band (Emission
            Line) Imaging?
   Use of filters to isolate a particular wavelength of light emitted by a
    specific energy transition in a specific element.
   Most common emission line is Hydrogen-Alpha.
   When a hydrogen atom is struck by an energetic (ultraviolet) photon
    from a nearby hot star, it absorbs a photon. The atom’s single
    electron jumps to a higher energy level as a result. This is an
    unstable configuration, and the electron eventually drops back down
    to an intermediate energy level, giving off a photon in the process.
    The photon contains exactly the energy difference between the two
    levels. This energy corresponds exactly to light with 653.6
    nanometers wavelength.
    More on Narrow Band (Emission
            Line) Imaging.
   Other emission lines (i.e. Oxygen II, Sulfur II, etc.) may
    be combined by mapping each emission line to a
    particular color (red, green, blue) to form a “false color”
    or “mapped color” image.
   Because of the narrow band pass, long exposures are
    required and finding a bright guide star through the filter
    becomes very difficult.
Choosing a Guide Method
Guiding Using a Single Chip
 Camera with Auto-Guiding
     (Starlight Xpress)
Advantages:
-Less money than two chip cameras or separate guide camera
-The camera sees what the imager sees which reduces
problems with flexure and mirror flop
-You can pick any star in the field of view to guide

Disadvantages:
-Half of the imaging exposure time is used to guide and half of
the exposure time is used to image ( longer exposures )
-May be difficult to find a guide star through a color or narrow-
band filter.
Guiding Using a Dual Chip Camera
        With Auto-Guiding
  Advantages:
  -Less money than a separate guide camera
  -The guide camera sees what the imager sees which reduces
  problems with flexure and mirror flop

  Disadvantages:
  -Sometimes difficult to find a guide star on the guide camera
  chip (narrow field of view )
  -Guide chip may have issues with a flat field because it is not
  centered on the optical axis. (small refractors)
  -Focal Length may be too long for accurate guiding.
  - May be difficult to find a guide star through a color or narrow-
  band filter.
Guiding Using a Separate Guide
Camera With and Off-Axis Guider
Advantages:
-The guide camera sees what the imager sees which reduces
problems with flexure and mirror flop
-Easy to find a guide star
-Guide camera does not guide through filters (narrow band
imaging)

Disadvantages:
-More expensive solution (requires second camera)
-Guiding a very long focal lengths can cause difficulty if the
guide camera is guiding too aggressively (chasing the seeing)
-Extra setup to get both cameras focused
Guiding Using a Separate Guide
 Camera With a Guide Scope
Advantages:
-Scope / camera combinations can be optimize for both imaging
and guiding
-Easy to find a guide star, guide scope can be aimed away from
subject
-Guide camera does not guide through filters (narrow band
imaging)

Disadvantages:
-Most expensive solution
-The guide camera does not see what the imager sees, which
may cause problems with flexure and mirror flop
Choosing a Telescope
Criteria for Telescope Selection
   Focal ratio - determines the exposure time, not
    the aperture! f-ratio = focal length / aperture
    Lower ratios are “faster” and allow shorter
    exposures Higher ratios are “slower” and require
    longer exposures.
   Aperture - Large aperture provides better
    resolution (smaller details are visible).
   Focal Length - Longer focal lengths generally
    provide narrow field of view. Shorter focal
    lengths generally provide wide fields of view.
    Focal Length Issues <400mm
 Camera lenses and extremely fast/short
  telescopes.
 Fields of view are extremely wide
  measured in degrees.
 Limited number of targets that require the
  very large field of view.
 Unguided images are possible
Example Image with a 80mm Refractor (384mm Focal Length)
Example Image with a 80mm Refractor (384mm Focal Length)
    Focal Length Issues 400-800mm
 Refractors or smaller Newtonian reflectors
 Great range for beginning CCD imagers
 Light to moderate demand on mount
 Fields of view are wide, perfectly suited to
  many nebulae
 Usually requires guided exposures
  especially as you near the 800mm range.
Example Image with 102mm Refractor (610 mm Focal Length)
Example Image With 102mm Refractor (816 mm Focal Length)
Focal Length Issues 800-1500mm
 5”-6” refractors, 8” SCT with a focal
  reducer
 Imaging becomes a little more technical
  requiring a mount suitable for imaging
 Excellent for imaging galaxies (near
  1500mm range)
 Unguided images are challenging at best.
 Medium field of view
Example Image With a 152mm Refractor (1200mm Focal Length)
Example Image With a 152mm Refractor (1200mm Focal Length)
Focal Length Issues 1500-2000mm
 Larger Newtonians and 8” SCT’s at f10
 Steeper learning curve
 A good mount is fundamental to success
 This focal range really opens up the
  galaxy imaging options
 Seeing is almost always a factor in
  imaging quality, use larger pixel sizes to
  reduce resolution
 Narrow field of view
Example Image with a 10” SCT (1575mm Focal Length)
Focal Length Issues >2000mm
   Zone of “serious imaging”
   Must have a superb mount to image in this
    range, with nearly perfect tracking, very low
    backlash, and the ability to carry the weight of
    larger scopes.
   List of potential imaging targets is nearly endless
   Seeing is a dominant factor in success, use
    larger pixels to reduce the resolution.
   Very narrow field of view
    Newtonian Reflector Telescopes

 Back focus issues. Will you be able to
  reach focus with the filter wheel, motorized
  focuser, focal reducer and camera?
 Diffraction Spikes
 Typically wide field / short focal length
  requires small pixels
 Issues with flat field (coma)
                   SCT
 Moving mirror issues (required secondary
  focuser, mirror flop)
 Large apertures
 Narrow field / long focal lengths require
  larger pixels
 Slow f-ratios (few f6s, mostly f10s)
 Relatively flat field
               Refractors
 Unobstructed aperture
 Wide field / short to mid focal lengths
  require small pixels
 Fast focal ratios
 May have flat field issues
 May have back focus issues
 Non-Apo refractors may have spurious
  color issues or bloated stars.
    Ritchey-Chretien Telescopes
 Typically have a long focal length requiring
  a very good mount.
 Typically are optimized for imaging with a
  large flat field.
 Nearly endless number of imaging targets.
  Great scope for imaging galaxies.
 Typically are very expensive even for used
  telescopes. Prices start at $7000.00 for a
  10” scope (excluding Meade RCX series)
Choosing a Mount
     Start with a Solid Mount
 You can have the best imaging camera
  and telescope and still get poor results
  with a shaky mount.
 If you have a great mount, you can take
  good images with an inexpensive imaging
  camera and telescope.
              Fork Mount
 Typically found on commercial Schmidt-
  Cassegrain telescopes
 Typically have some periodic error and
  some random error to deal with.
 Typically have some backlash issues in
  declination axis to deal with.
 Requires the use of a wedge to image in
  equatorial mode
    German Equatorial Mounts
 Most Amateur astro images, CCD and
  film, are taken using some form of German
  Equatorial mount (source: Ron Wodaski)
 Relatively light, and flexible as to what
  kind of telescope you put on it.
 Can be made to track very accurately
 Can be awkward to point near the
  meridian.
Choosing a Computer
         Imaging Computer
 Pentium Laptop
 Windows 98 or newer
 500 MB or more disk space (more
  required for large imaging chips)
 64 MB or more Ram (more is better)
 USB Port is desirable
 Serial Port is desirable/required for some
  equipment.
    Image Processing Computer
 As much RAM as possible
 As much CPU power as possible
 Monitor types (CRT VS Flat Screen)
                 My Imaging Setup

   TMB 152 F8 APO Refractor (1200mm or 960mm with reducer)
   Takahashi TSA102 F8 APO Refractor (816mm or 610mm with
    reducer)
   Camera Angle Adjuster for Takahashi Refractors
   Stellarvue SV80s F6 APO Refractor (480mm or 384mm with
    reducer)
   Mountain Instruments MI-250 Mount
   SBIG ST10XME (main imaging camera)
   SBIG Remote Guide Head (guide camera)
   2 Ghz laptop computer with 2GB ram and 160gb hard drive.
   4 port USB2 card for the laptop.
   Starizona Microtouch focuser system for Feathertouch focusers
   Dew Buster Dew heater controller and Dew Not heater strips
My Current Imaging Setup
                   Helpful Hints
   Take multiple long exposures
   Remember, this is fun! Don’t get too frustrated and don’t
    give up if things don’t work the first time.
   Don’t try to image more than one object in a single night.
   Try a relatively bright object first.
   Don’t change more than one piece of equipment at a
    time.
   Take the time to get good polar alignment. Guiding will
    be much more accurate. Polar alignment can begin
    before it is dark enough for imaging.
   Learn to use a good image processing software
    package. Don’t over-sharpen!
   There is no substitute for exposure time. The more data
    you collect, the easier image processing will be.
Where to get this presentation
If you would like to have a copy of this presentation, it
is available in PowerPoint format at the following URL:


http://www.dougsastro.net/files/BasicCCDImaging.ppt

								
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