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A description of persistence in WFCAM data

Typical values in counts

Filter Sky Level Sky Noise Persistence1
Z 150-250 6-8 80
Y 370-400 8-12 80
J 850-2500 15-30 40
H 6500-8500 40-60 40
K 3000-6500 25-40 25
1 Value at delta-t=25s
Persistence has been observed in WFCAM data and is typical of infrared detectors. This is when a saturated image in a previous exposure leaves a persistent or remnant image in following exposures. This is also known as remnance. This phenomenon is not clearly understood and varies considerably between detectors.

Persistent images are very approximately top-hat shaped and cover the area that was saturated in the contaminating frame. The height decayed exponentially with an e-folding time of ~40s. This is approximately the same for each filter. The size of persistence is approximately the same for chips 1, 2 and 4. Chip 3 has slightly higher values.


The above shows an example of the persistence caused by a H=6 magnitude star after a delay of about 25s. This time delay is typical for the exposures taken for UKIDSS.

It is not clear that an accurate correction for persistence can be made since it doesn't seem to be repeatable. The height of a persistent image can vary considerably even though the observing parameters (filter, chip, magnitude, sky level, delta-time, exposure time, detector temperature) are the same. The error of a correction with the current understanding would be of order the size of the correction.

For JHK frames, persistence does not seem to be a large and significant problem, even for non-interleaved data. If data is interleaved and stacked then the effect is further reduced. For Z and Y frames, where the level of persistence is high, the sky noise level is low and no interleaving is carried out, the number of detected images caused by persistence will be higher.

No correction for persistence is carried out in the current version of the pipeline. A future possibility might be to flag catalogue entries that might be caused by persistence (cf. 2MASS) if it is established that there is a need for it.