CCD Imaging
Processing Your Images
Once the image(s) have been taken, processing begins. We recommend that you
take dark and flat frames during the CCD imaging session. While it is
possible to keep a "library" of these files, we have found that you're better
off taking them at the same time as the images. Remember: these two important
files are your best defense against noise and other imperfections in your
images. You want the best possible; darks and flats from other CCD imaging
sessions will just not be as good as the ones you take right then!
Color images produced using monochrome CCD cameras involves taking
black-and-white images through color filters (red, green and blue; we recommend
a fourth: luminescence, or clear7#41;. Once these black-and-white images have
been processed, then they are combined to make a color picture. The processing
steps for each of the three or four images (taken through each of the color
filters) are the same as that applied to monochrome images. And, most of the
time, you will make each of the color image components from multiple images
taken through each of the color filters and combined the same way as
black-and-white images.
This also needs to be said: At every step in the CCD imaging process the
battle against noise is your biggest job. Noise avoided during the image-taking
steps will be noise you won't have to deal with later. We need to talk about the
sources of noise and how they can either be avoided or reduced in the CCD
imaging process.
Noise produced during CCD imaging is affected by the following:
- Exposure time
- CCD QE (Quantum Efficiency)
- CCD Dark Current
- CCD Temperature
- CCD Read Noise
- The focal ratio of the telescope
- Light pollution and atmospheric conditions
- The brightness of the object imaged
Once you've taken the image,
processing steps affect noise:
- Applying dark and flat frames
- Image stretching
- Selective blurring or sharpening and deblooming
- Rescaling
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