Subsections

• 5.8.1 LW5: quarter- versus half-second processing
• 5.8.2 Summary of processing for various strength sources

5.8 Quarter-Second Processing

The strong source correction (see Section 5.7) was made with Saturn quarter-second ramps. Applying this correction directly to observations of strong sources, where the ramp lengths are one half-second, does not remove the non-linearities as the data are de-biased more than corresponding quarter-second ramps. This is seen in Figure 5.13 where a non-linear half-second ramp is shown by the + symbols. The dotted vertical lines mark out the first quarter-second of the ramp and whole half-second ramp. It can be seen that the ramp curves over more as time increases. This de-biasing effect is even more apparent when one considers the value of the first quarter-second and the whole half-second ramp (indicated by the slope of the long and short dashes respectively). This shows that there is a difference in calibration for quarter and half-second ramps, due to the de-biasing. Hence half-second data which suffer from `saggyness' should have their ramps reprocessed as if they are quarter-second ramps. This is done by discarding the 22 samples from the end of a ramp, which make up the extra quarter-second. Also, as in the normal processing, a further sample from the end is discarded because of the end of the format point and the usual amount is discarded from the start. This makes the ramps equivalent to the data used in real quarter-second ramps, by using exactly the same part of ramp. If any of the remaining points are unusable, such as being at the voltage limit of the ADC (see Figure 5.14), they are also discarded. [The illuminators however are processed as normal half-second ramps, as described in Section 4.3.6. This is done as their calibration is based on half-second length ramps and also their ramps should not be affected by non-linearity.] In doing this the non-linearity suffered will be to the same degree as those ramps of the correction source, Saturn, and the correction can be applied successfully to the photocurrents found from these `quarter-second ramps'.

Figure 5.13: A half-second ramp of SagittariusB2 from LW3. The gradient of the long dashed line shows the value of from the first quarter-second of the ramp, marked out by dotted lines. The short dashed line gives for the half-second ramp, also marked by dotted lines.

Figure 5.14: Half-second ramps of SagittariusB2 from LW3 hitting the ADC rail.

5.8.1 LW5: quarter- versus half-second processing

For LW5, with quarter-second ramps, the strong source correction is not needed as these ramps are linear. However, when strong sources are observed with half-second integrations the ramps may become non-linear. Hence for this detector the quarter-second and normal processed data (Figure 5.15 shows an example of these data) are compared with the corrected scaled LW4 data. The data which better match those of LW4 are used.

Figure 5.15: LW5 SagittariusB2 with quarter-second processing (upper) and half-second processing (lower).

5.8.2 Summary of processing for various strength sources

• Weak sources have linear ramps and therefore quarter- and half-second ramps have the same calibration and do not need a correction.

• Strong sources with quarter-second ramps need to have the strong source correction applied to LW1-LW4. The other detectors do not need a correction.

• For strong sources with half-second ramps SW1-SW5 produce linear ramps and so they are processed using the usual amount of ramp. LW1-LW4 should be processed using only the first quarter-second of the ramps and have the strong source correction applied to them. LW5 should also be processed using both half- and quarter-seconds of the ramp and then be compared with the LW4 data (but they do not require a strong source correction).