Ultraviolet Imager (UVI)

Some of the KP images seem brighter than others. Why?

First of all, make sure that your viewing software is displaying each image on a common intensity scale. The viewer supplied from the UVI utilities page (KPFvu) autoscales each image based on the range of values in the image. Noise in the image can confuse the autoscaling function, making some images appear brighter than others. An autoscaling feature is necessary since single pixel noise spikes can be quite large. If the viewer simply scales the color table from min to max of the image you may only be able to see the single pixel with the noise spike. In addition, check that all of the images are at the same filter setting. Auroral morphology and instrument sensitivity change with wavelength.

Also, be aware that UVI images may have different integration times. The default exposure time is 4 major frames or about 36.8 seconds. If the filter wheel was moved prior to the image exposure, however, the integration is shortened to 18.4 seconds. This is to prevent the image from being blurred as the filter wheel turns accross the detector field of view.

The UVI Key Parameter Images are calibrated in units of photon flux and the exposure time is included in the calibration process. Thus the peak intensity for a 36.8 second exposure should be the same as that for a 18.4 second exposure. The image noise statistics, however, will not be as good for the short exposure. For low intensity images this will appear as increased 'speckle' or 'graininess' in the image. This can cause some images to appear brighter (discounting changes in activity and viewing geometry, of course). This also applies to reduced image brightness when the door is closed.

How do I know the total exposure time for a KP image?

The relative starting and ending times for each image are given by the Key Parameters IMG_MINUS_MSEC and IMG_PLUS_MSEC. The image begins at Epoch - IMG_MINUS_MSEC and ends at Epoch + IMG_PLUS_MSEC. (Time_PB5 can be used instead of Epoch, if desired.) The total exposure, in milliseconds, is IMG_PLUS_MSEC - IMG_MINUS_MSEC. When using the UVI Key Parameter viewing software (KPFvu), the start & stop times are displayed in a text box, along with the other non-image Key Parameters.

Why am I having trouble matching KP images seen with those seen in the level zero data?

The raw (level zero) UVI images are tagged with the ATC telemetry time which is the time the UVI data was first placed into the spacecraft telemetry stream. This is NOT the time of the exposure. A nominal exposure time is 4 major frames or about 36.8 seconds. At the end of the exposure, the image is stored in an internal buffer for an additional 4 major frames. Only then is the image data placed into the telemetry stream and tagged with the ATC time code. Thus there is a nominal delay of 8 major frames (73.6 seconds) from the actual start of the exposure to the time given by the ATC time code.

There is an additional complication in that the nominal exposure time is shortened to only 2 major frames following a mechanism movement, such as a change in filter position. This is to prevent the image from being blurred or otherwise contaminated by the moving mechanism. In this case, there is a delay of 6 major frames between the start of the exposure and the ATC time code.

In the Key Parameters all these offsets have already been calculated and the times (absolute and relative) refer to the ACTUAL EXPOSURE TIME. The only tricky part is that the KPs are mandated to be tagged from the center of the exposure, not the beginning.

How can I figure out what the quality and gap flags mean?

There are software utilities on our utilities page to decode these values for you. If you are using KPFvu the 'Flags' option on the 'Display' menu will decode the flag settings for the current record.

Why are the quality flag bits for calibration and pointing always set?

These bits are set to indicate that you should use the intensity and pointing information in the UVI Key Parameters with caution. It takes time to fully check out all the calculations and this process is not yet complete. As a consequence, the smallest value for the quality flag is 1088.

Why are there no UVI Key Parameters for spacecraft distances below 6 Re?

A number of factors contribute to this. Below 6 Re the northern auroral oval is typically visible only from extreme viewing angles. Consequently the oval appears almost edge-on, often on the limb of the earth. It is difficult to extract spatial morphology from such images.

Pointing below 6 Re often focuses on nadir-viewing airglow, which is basically featureless and not very useful as a summary product. The southern oval is seen below 6 Re, but it is visible only during perigee passage and then only for a short time.

To make matters worse, the low altitude portion of the orbit passes through the Earth's radiation belts. This generates noise in the UVI detectors which is interesting to look at but, again, not of much use as a summary parameter.

For all these reasons we do not compute Key Parameters for spacecraft distances below 6 Re. We hope to eventually include southern oval passes in the KP calculations.

But even above 6 Re there are periods when no Key Parameters are available. Why?

Some operations, such as SEPS field-aligned pointing or star calibrations, require the despun platform to be pointed away from the earth causing the imagers to look into space. During these periods no auroral activity can be seen. During periods of sun danger UVI will be shuttered during these maneuvers, for safety. Therefore, no UVI Key Parameters are calculated if the despun platform is pointed more than 40 degrees away from nadir.

In addition, UVI Key Parameters are calculated only for normal UVI viewing mode and normal spacecraft telemetry mode. Any special operations that deviate from these modes will prevent the calculation of KPs.

Why does the filter change from time to time in the UVI Key Parameters?

The default filter setting for the UVI KPs is the LBH long filter. This is deemed to be the most useful single image since the intensity is directly proportional to the incident energy flux. If, however, there are no LBHL images available the KP generation software will use whatever image is available. This insures that some image will be available on a regular basis.

Sometimes the gap flag indicates that a gap occurred but it is not apparent. What defines a gap?

The UVI KP generation software tries to produce output records with a period given by NOMINAL_OUTPUT_PERIOD. This is typically 5 minutes so the goal is to have an image every 5 minutes. In practice, however, it may take longer than this to find the default image (LHBL) and an appropriate background image. (Consider, for example, a viewing sequence that produced a background only once every 20 minutes. Or one that used only 1356 filter settings.)

If the time since the last output record is greater than twice NOMINAL_OUTPUT_PERIOD then a gap is declared and the current image is placed in the output record.

How can I find out what the observation sequence number means?

We keep a table of observation sequence definitions. Look for it on our operations page. Note that the sequence definitions are not permanent and can change with time. The web page will be updated as necessary.

Some of the KP images are missing or seem to have no valid image data. Why?

In late July 1996, the high voltage power supply (HVPS) of the primary detector suffered an on-orbit failure. From this point until December 1996 when the secondary detector was activiated, there were periods in which the instrument could not return valid image data. Consequently there were no UVI Key Parameters computed. At other times the automated Key Parameter Generation Software computed KPs even though the HVPS failure meant that the camera didn't have enough gain to see any light. The KPs computed at these times are essentially featureless and have no scientific value.

A summary of the UVI image quality during this period is given on our Special Operations & Events Calendar.