Because of the size of UVI images, 4 major frames of the Polar telemetry stream are required to transmit each image. This, in turn, sets the nominal integration period of each UVI image to 36.8 seconds (4 frames times 9.2 seconds/frame.)
UVI collects photons for 4 telemetry frames, then buffers the image internally for an additional 4 telemetry before it is written to the Polar telemetry stream. There is thus a nominal delay of 8 frames (73.6 seconds) between the start of the UVI integration and the time it appears in the Polar telemetry stream.
The time the image data appears in the Polar telemetry stream is referred
to as ATC time and is given in UT.
The beginning of the UVI integration is referred
colloquially as UVI time. The relative time is illustrated in the figure
below.
The situation is more complicated when the filter wheel is rotated. Since
the integration-buffer-telemetry cycle operates continuously, whenever the
wheel is moved its motion is seen in the current image.
To avoid seeing blurred images,
the CCD is cleared after two frames and a fresh integration
begins. Two frames is selected as the delay since all possible wheel
rotations are completed in less than 18 seconds.
The integration is stopped after only two frames to keep sync with the 4
frame requirements of telemetry download. In this case there is a nominal
delay of 6 frames from the beginning of the integration and the time the
data appears in the Polar telemetry stream.
For the rarely used 9 second integration, only frame 4 is used for integration and there is a nominal 5 frame delay between UVI and ATC time.
The results are summarized in the table below.
| Nominal Exposure | Nominal Frame Delay | Nominal Time Delay |
|---|---|---|
| 36 sec | 8 frames | 73.6 sec |
| 18 sec | 6 frames | 55.2 sec |
| 09 sec | 5 frames | 46.0 sec |