Santa Barbara Instrument Group ST-5C, Руководство по эксплуатации

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Section 3 - Advanced Topics
Page 21
3. Advanced Imaging Techniques
The following sections describe some of the advanced uses of CCD cameras. While you may
not use these features the first night, they are available and a brief description of them is in
order for your future reference.
3.1. Taking a Good Flat Field
If you find that flat field corrections are desired due to vignetting effects or for more accurate
measurements of star magnitudes, try either taking an image of the sky devoid of stars or take
an image of a blank wall or neutral grey card that is evenly illuminated.
Finding areas of the sky devoid of stars is very difficult after twilight. Therefore, you
should take flat field images of the night sky after sunset, but long before you can see any stars.
If this is not possible, take an image of a featureless wall or card held in front of the telescope.
However, if using this second method, be sure that the wall or card is evenly illuminated. Do
not use a flashlight to directly illuminate a card, but rather allow the light to bounce off of
another card or the wall of your observatory opposite the one you are imaging. Use a diffuse
light source.
If you plan on flat fielding Track and Accumulate images, you should also refer to
section 3.6. Since the same flat field is added to itself a number of times, be sure that you do not
saturate the flat field image by starting with pixel values too high. Typically try to keep the
pixel values between 10% to 20% of saturation for this purpose. For single flat field images, try
to keep the values to approximately 50% of saturation. That is a value of roughly 8,000 counts
for the TC-255 in Hi-Res and 32,000 counts binned.
3.2. Track and Accumulate
An automatic Track and Accumulate mode is available in CCDOPS which simplifies image
acquisition for the typical amateur with a good telescope drive. Drives employing PEC or PPEC
technology and accurate gears, only need adjustment every 30 to 120 seconds. With Track and
Accumulate the software takes multiple exposures and automatically co-registers and co-adds
them. The individual exposures are short enough such that drive errors don't show up and the
accumulated image has enough integrated exposure to yield a good signal to noise ratio.
Procedurally, the camera will take an exposure, determine the position of a preselected
star, co-register and co-add the image to the previous image in the CPU, and then start the cycle
over again. Up to 64 images can be co-added. The resulting exposure is almost as good as a
single long exposure, depending on the exposure used and sky conditions. The great sensitivity
of the CCD virtually guarantees that there will be a usable reference star within the field of
view. This new patented feature provides dramatic performance for the amateur, enabling
unattended hour long exposures!
3.3. Color Imaging
The field of CCD color imaging is relatively new but expanding rapidly. Since your CCD
camera is equipped with a monochromatic CCD, discriminating only light intensity, not color,
some provision must be made in order to acquire color images. Your camera has been designed
with color imaging in mind, with the ability to easily upgrade the camera with an internal color
filter wheel in place of the standard shutter wheel.
The color filter wheel allows conveniently placing colored interference filters in front of
the CCD in order to take multiple images in different color bands. These narrow band images
are then combined by software to form a color image. The Red, Green and Blue pass filters are
used to acquire three separate color images of the object. The resulting images are combined to
form a tri-color image using the CCDCOLOR software.