Chip Characteristics
The ST-7 camera uses a Kodak KAF0400 chip. This chip is a full frame readout chip that shifts each row into a readout register before passing each charge packet to the onchip output amplifer.
As is customary, the displayed image has row-1 and column-1 at the upper left corner of the image.
The following table summarizes most of the key features of the KAF0400 chip.
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number of columns (x-direction) |
765 |
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number of rows (y-direction) |
510 |
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number of pixels |
390150 |
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pixel size (x-direction) |
9 microns |
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pixel size (y-direction) |
9 microns |
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size of chip (x-direction) |
6.9 mm |
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size of chip (y-direction) |
4.6 mm |
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read noise |
15 electrons per pixel |
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full well capacity |
50K electrons/pixel, antiblooming 100K electrons/pixel, non-antiblooming |
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saturation, high resolution |
~20,000 ADU, antiblooming ~40,000 ADU, non-antiblooming |
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saturation, med/low resolution |
~65,000 ADU |
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chip gain |
2.3 photons/ADU |
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The complete camera system consists of the camera head, a power supply, and a computer system.
The camera head is the upper box in this diagram. In addition to the CCD chip, the camera head also contains a mechanical shutter, the onchip amplifer (preamp), a 16 bit A/D converter that produces the data values for each pixel, and the timing circuits that control the readout of the chip.
The camera head has one cable that runs to the poser supply, and one cable that runs to the paralles interface port of the computer system.
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The camera head should be orientated in the following manner with regard to the right ascension and declination axies on the telescope.
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The Set Point temperature should be set approximately 30 degrees below the ambient temperature. Temperature regulation can hold the chip temperature to within +/- 0.1 degree C.
Exposure times can be set for exposures between 0.11 seconds and 3600 seconds (1 hour). Exposure times are controlled by a mechanical shutter.
For a properly focused unfiltered image an exposure time of 10 seconds will usually reveal most of the bright astronomical objects. For example, a 10 second exposure will show the ring nebula (M57) and structure in the whirlpool galaxy (M51). Exposure times between 10 seconds and 60 seconds should show at least the nucleus for nearly all of the NGC galaxies.
Images become saturated when, for whatever reason, pixels reach their maximum data value. Pixels in a CCD can only hold a specific maximum number of electrons. This number is termed the full well capacity for the chip. Once this number of electrons is reached, the pixel, and the image, are saturated. The image essentially becomes over exposed. If light continues to fall on the chip more electrons will be produced, but they will bleed off into adjacent pixels. This is termed blooming. The effects of blooming can be minimized by means of an antiblooming gate (ABG). This process involves reading out some of the charge before reading out the final image. ABG effectively diminishes the full well capacity for a chip and lowers the data values at which saturation occurs.
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full well capacity |
~50K electrons (ABG) ~100K electrons (non-ABG) |
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saturation at high resolution |
20,000 ADU (ABG) 40,000 ADU (non-ABG) |
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saturation at medium and low resolution |
65,000 ADU |
Note that the CCD chip will saturate before the 16-bit data maximum of 65536 is reached for high resolution images. This is a consequence of the (somewhat) limited full well capacity for the chip. Larger pixels have greater full well capacity. Saturation is determined by the full well capacity of the chip for high resolution images, but by 16-bit analogue-to-digital converter (A/D) for medium and low resolution images.
The camera can be configured to readout only a segment of the full frame image. Subframe readout (image size) may be specified by selecting Grab from the Camera menu and then selecting an appropriate image size. A Full frame image may be specified, but a Half or Quarter image may also be specified. The smaller images correspond to the central 1/2 or 1/4 of a full image. Specifying an image smaller than a full frame image decreases the time it takes to acquire an image and also saves disk space because the resulting images are smaller. That is, the resulting images contain fewer pixels. This can be useful if the area of interest only occupies a small region of the image. This is also especially useful for focusing. However, the Focus command has separate provisions for specifying subframe readout.
The ST-7 supports high, medium, and low resolution modes. Resolution may be specified by selecting Camera Setup from the Camera menu. High resolution uses the full resolution capability of the chip by reading out the individual physical pixels. The lower resolution modes use onchip binning to combine pixels before readout. Medium resolution combines 2x2=4 pixels and low resolution combines 3x3=9 pixels. The lower resolution modes take less time to readout and take less disk space when saved because there are fewer total pixels. The full physical size of the chip is utilized so that the angular field of view for the image is the same for all resolution modes.
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40000 (non-ABG) |
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The camera is more sensitive to light in the lower resolution modes. The larger pixels can record more photons coming from a source than a smaller pixel. The low resolution mode is effectively 9 times more sensitive (faster) than the high resolution mode. The low resolution mode is especially sensitive at recording faint extended regions of nebulosity.
Optimum detection for stars occurs when stellar images extend across no more than 2 or 3 pixels. This is termed Rayleigh's criterion or optimal sampling. If stellar images extend across more than a few pixels the stellar images are over sampled. The extra resolution is wasted and exposure times will be longer than necessary. If a stellar image occupies only a single pixel the images are under sampled. A faint star near the edge of a pixel will place photons in more than one pixel and may not produce sufficient electrone in either pixel to be detected. The faintest stars are recorded when the stellar images are optimally sampled. The extent of stellar images may be examined by using the crosshairs in the analysis image display mode.
In the Auto mode for resolution a full frame image is always read out in high resolution mode. However, when the camera is in Focus mode a full frame image is readout in low resolution mode to speed up the display of images used during focusing. When the subframe planetary mode is selected during focusing, the image is read out in high resolution mode to provide greater precision for focusing.
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