Subsystems

The major subsystems, including the cover, the imager, its filter wheel, the cube mirror, and the heaters are discussed below.

Cover

The CFI optics are protected during launch by a one-time deployable cover. The cover is deployed by High Output Paraffin (HOP) actuators. There are two HOP actuators; each actuator has two heaters. Any one of the four heaters should be sufficient to release the cover. Each heater can be commanded on independently (CFI_COV_MODE and CFI_COV_DEPLOY). A mechanical switch provides a telltale that indicates that the cover mechanism has activated.

Imager

The imaging subsystem, from the CCD to the software, normally operates at a 1 Hz cadence. A simplified timing diagram of the CCD is shown below. The figure shows two seconds elapsing, i.e. two images being exposed. CCD operations are synchronized to a 1 Hz system clock. The internal CFI 1 Hz clock lags the system-wide clock by ~7 ms. Image exposures vary in length, but always end ~40 ms before the next CFI 1 Hz pulse. For science imaging, the exposure time will normally be under 1 second. However, additional seconds of exposure time can be commanded, for example, to do comet coma imaging. The figure shows a normal exposure followed by a minimum length "scrub" exposure. Scrub exposures are used to fill in time whenever no real image is needed. This ensures that the CCD is read out every second. The CCD is a frame-transfer device: at the end of an exposure, the image is rapidly transferred into a buffer within the CCD. Then the buffered image is read out of the CCD while the next image is being exposed. The DPU adds an image header provided by the DPU software. The read out of the entire image must be complete before the end of the exposure. The time between the end of exposure and the next CFI 1 Hz pulse is used by the software to configure the image read out (e.g., load the header data) and setup the next exposure.

Figure 2. CCD Timing

Each image collected requires three processing steps. Each step takes one second, but the process can be pipelined such that one image is taken every second. The resulting three-stage pipeline is shown below. For multisecond exposures, the expose stage is stretched out and subsequent processing stages are delayed.

Figure 3. Image Collection and Processing Pipeline

For purposes of command planning, only the first two image processing stages are relevant. During setup, commands are executed to configure image collection in the following stage. The following figure shows an example image taking sequence. The commands that control the sequence are being issued from macros; macros execute at the beginning of each second. A take image command (CFI_IMG_IMAGE) executed in t will cause the CCD to be exposed in t+1. Similarly, commands to set the exposure time (CFI_IMG_EXP) and select the filter wheel (CFI_FLT_MOVE) that execute in t will take effect in t+1. Filter wheel movements begin at the end of exposure; ensuring that the movement is complete before the next exposure is the responsibility of the macro and thus the macro author.

Figure 4. Example Image Sequence

The following figure relates the image on the CCD with the spacecraft's coordinate system. The numbers on the pixels in the figure indicate the order in which they are downlinked. Each pixel is 12 bits.

Figure 5. Visible Image Readout

CFI images come in six possible formats shown in the table below. The format is selected by command (CFI_IMG_FORMAT). The formats range from the full size of the CCD (1024x1024) down to smaller and smaller windows into the full image. A command (CFI_IMG_REGION) selects the origin of the window. Alternatively, the entire 1024x1024 image can be rebinned into 512x512 or 256x256 images. The rebinning is done directly in the CCD while the windowing is done by downstream logic. In all of the formats, each line has an additional four dark calibration pixels and a line of temperatures at the end of the image. Every data item in the image can be compressed from 12 bits to 8 bits if enabled by command (CFI_IMG_COMP_MODE). The compression is done via table lookup in hardware; one of eight tables can be selected by command (CFI_IMG_COMP_ALG). Any of these commands executed at time t affect the image exposed at time t+1.

Images are downlinked with a header describing the state of the instrument at the time the image was taken. The header starts with the time at which the image was exposed and a snapshot of the current DPU status. For multi-second images, the header corresponds to the last second of exposure. The header format is shown below:

The DPU software monitors the Focal Plane Unit's (FPU) latch-up indicator. If latch-up is detected, the DPU suspends FPU operations. The length of the suspension is set by an uploadable parameter.

Filter Wheel

The filter wheel is controlled by a stepping motor. The motor phases are controlled directly by the DPU software to move the filter wheel. The phase pattern can be adjusted by software to move the filter wheel forwards or backwards. A resolver provides an absolute position reading of the wheel.

The filter move command (CFI_FLT_MOVE) uses the commanded filter number to look up an absolute goal position from a table, reads the current position from the resolver, and computes the number of steps and travel direction needed to arrive at the goal in the least amount of time. The motor is then stepped to the desired goal. Alternatively, the filter step command (CFI_FLT_STEP) will move the filter wheel the commanded number of resolver counts forward or backward. The power to the resolver can be commanded on or off (CFI_FLT_PWR). If the power to the resolver is turned off, further move commands will be dead-reckoned from the last resolver reading. The filter wheel is characterized by several uploadable parameter including the motor power level, the scale factor between resolver counts and motor steps, the filter wheel positions, etc. The filters are listed in the following table.

Cube Mirror

The mirror move command (CFI_MIR_MOVE) uses the commanded mirror side to look up an absolute goal position from a table, reads the current position from the resolver, and computes the number of steps needed to arrive at the goal traveling in a preferred direction. The motor is then stepped to the desired goal. Alternatively, the mirror step command (CFI_MIR_STEP) will move the mirror the commanded number of resolver counts forward or backward. The power to the resolver can be commanded on or off (CFI_MIR_PWR). If the power to the resolver is turned off, further move commands will be dead-reckoned from the last resolver reading. The mirror is characterized by several uploadable parameter including the motor power level, the scale factor between resolver counts and motor steps, the mirror side positions, and the preferred move direction.

Heaters

The CFI DPU controls the temperature of the radiator used to cool the CCD. The heater may be commanded off, on, or controlled by software (CFI_HTR_MODE). If the heater is being controlled by software, it uses the setpoint and hysteresis commanded (CFI_HTR_TMP). The software uses one of four temperatures sensors to control the heater; the sensor used is commandable (CFI_HTR_SENSOR). Every second the selected temperature is compared against the limits. If the temperature is too low, the heater will be turned on. If the temperature is too high, the heater will be turned off.

CONTOUR Hardware Experimental Acquisition Package

The CFI DPU monitors the CONTOUR Hardware Experimental Acquisition Package (CHEAP). CHEAP contains three experiments: digital solar attitude detectors, a dust shield monitor, and a dosimeter. These are described below. Because of CHEAP's experimental nature, commands are provided for doing raw reads and writes to the hardware (CFI_CHE_PEEK and CFI_CHE_POKE). CHEAP is present as a flight test and is not an operational part of CONTOUR.

Digital Solar Attitude Detector

The CFI DPU controls two Digital Solar Attitude Detector (DSAD) imagers. Each device has a 200 x 200 pixel array; each pixel is ten bits. The only difference between the DSADs is that one views space through a pinhole and the other views space through a lens. Image collection is controlled via command (CFI_SAD_IMAGE). Another command (CFI_SAD_EXP) sets the exposure time for each device.

Dust Shield Monitor

The CFI DPU monitors degradation of the spacecraft's dust shield. Sixteen measurements are taken over different regions of the dust shield; each measurement is ten bits. Data collection is controlled via command (CFI_DUS_DATA).

Dosimeter

The CFI DPU monitors the total radiation dose that the spacecraft receives. A single ten-bit dosimeter measurement is taken on command (CFI_DOS_DATA).

Return to CFI Software User's Guide. Report problems to John Hayes.