Dark Frames; Flat Field Images; Pixels Vs. Film Grains - Santa Barbara Instrument Group STL-1001E Operating Manual

possible readout noise. At 10e
performance.

2.4.3. Dark Frames

No matter how much care is taken to reduce all sources of unwanted noise, some will remain.
Fortunately, however, due to the nature of electronic imaging and the use of computers for
storing and manipulating data, this remaining noise can be drastically reduced by the
subtraction of a dark frame from the raw light image. A dark frame is simply an image taken at
the same temperature and for the same duration as the light frame with the source of light to
the CCD blocked so that you get a "picture" of the dark. This dark frame will contain an image
of the noise caused by dark current (thermal noise) and other fixed pattern noise such as read
out noise. When the dark frame is subtracted from the light frame, this pattern noise is
removed from the resulting image. The improvement is dramatic for exposures of more than a
minute, eliminating the many "hot" pixels one often sees across the image, which are simply
pixels with higher dark current than average.

2.4.4. Flat Field Images

Another way to compensate for certain unwanted optical effects is to take a "flat field image"
and use it to correct for variations in pixel response uniformity across the area of your dark-
subtracted image. You take a flat field image of a spatially uniform source and use the
measured variations in the flat field image to correct for the same unwanted variations in your
images. The Flat Field command allows you to correct for the effects of vignetting and
nonuniform pixel responsivity across the CCD array.
The Flat Field command is very useful for removing the effects of vignetting that may
occur when using a field compression lens and the fixed pattern responsivity variations present
in all CCDs. It is often difficult to visually tell the difference between a corrected and
uncorrected image if there is little vignetting, so you must decide whether to take the time to
correct any or all of your dark-subtracted images. It is always recommended for images that
are intended for accurate photometric measurements.
Appendix D describes how to take a good flat field. It's not that easy, but we have
found a technique that works well for us.

2.4.5. Pixels vs. Film Grains

Resolution of detail is determined, to a certain degree, by the size of the pixel in the detector
used to gather the image, much like the grain size in film. The size of the pixels found in the
Research cameras ranges from 7.4 to 24 microns square. One must match the size of the pixel
in a particular camera to the appropriate focal length to achieve the maximum resolution
allowed by the user's seeing conditions. The effects of seeing are usually the limiting factor in
any good photograph or electronic image. On a perfect night with excellent optics an observer
might hope to achieve sub-arcsecond seeing in short exposures, where wind vibration and
tracking error are minimal or 2-3 arcsecond seeing on long exposures. With the average night
sky and good optics, you will be doing well to achieve stellar images in a long exposure of 3 to
6 arcseconds halfwidth. This will still result in an attractive image, though.
Section 2 - Introduction to CCD Cameras
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to 15e rms per read these cameras are unsurpassed in
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