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Techniques to taking Astronomical CCD pictures:

 

Recent software and hardware improvements have made it very straight forward to take advanced digital pictures with a telescope.  The actual science and methods to produce good pictures involves many steps that are vastly simplified with the help of the computer programs and hardware found at Blueberry Pond Observatory.

To get started at all, there are the steps of hooking up the camera, controlling the telescope, computer, finding your target, focusing, using filters, and other equipment tied into the camera.  The Observatory is set up to take care of all of this for you.  Controls can be operated while physically beside the telescope, or from nearby in a comfortable heated control room.  A variety of focusers, mechanized filter holders, and optics are used to speed up all steps to using the camera.

The first step to producing a good picture is to take an exposure of the object.  Generally, the longer the exposure, the better the results.  If an exposure is longer than a minute or so, the telescope and camera must first be set to track a guide star near the object being photographed.  This prevents the telescope from shifting away from the target, and making the stars in the picture appear as squiggles rather than round stars.  This first picture is called the "raw image".

Next, a "dark frame" is used to calibrate the picture by removing noise.  Strangely enough, the CCD camera is very sensitive to the temperature around it, and is cooled to -30 to -40 degrees Celsius when in use.  Even at these temperatures, the camera will actually take a picture of very faint static over time, even with the shutter closed.  This static is present in your raw picture, mixed in with the actual light from the stars you photographed.  This static in the "raw image" can be removed by subtracting a "dark frame" from the picture.   The "dark frame" consists of a picture taken for the same exposure length as your raw picture, with the camera shutter closed.

Additionally, the "raw image" contains some distortion from certain telescope optic configurations, dust that might be on the lenses, and even from irregularities built into the camera.  These are removed using a "flat field image".  This "flat field image" is created by taking a picture of an evenly lit white surface through the telescope.  The picture will look mostly white, with an occasional faint ring around a particle of dust.  The computer then adjusts your "raw image" using the "flat field image" to remove these small flaws.

The result is the "calibrated image" of your target.  This image will usually look distinctly better than your "raw image", and is ready to be further processed if desired.  The computer can use several mathematical routines to pull more detail out of the picture, and enhance it's contract immensely.  The difference between a processed picture and the raw image can often be stunning!

Planetary pictures are usually processed with "unsharp masking" which brings out details on the planet's surface that are very similar in brightness and contrast.

Moon pictures benefit from "sharpening" which helps remove some blurring of bright areas.

Galaxies and Nebulas often benefit from "digital development processing", which helps bring out details from the very faint parts of the object (such as the arms of a spiral galaxy) while toning down the overly bright parts of the image (such as the center of galaxies).

A host of other image taking tricks, and processing options can be used to take pictures of just about any astronomical object, and create just about any kind of picture you want.

 

  • First capture a "raw image" of the desired object.
  • Use a "dark frame" to remove background static from the picture.
  • Use a "flat field frame" to remove optical distortions and other small flaws from the picture.
  • The result is the "calibrated picture" ready for processing.
  • Use "unsharp masking" to enhance details that are similar in brightness to each other.
  • Use "sharpening" to removing blurring of bright areas on some images.
  • Use "digital development processing" to balance extremes between faint and bright parts of the image.
  • Many other creative combinations of processing options can be used when needed.

 

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