WFC3 Persistence Project

Persistence in the WFC3/IR Images

Like essentially al IR arrays, the WFC3/IR detector exhibits after-images, known as persistence, following exposure to light that approaches or exceeds the saturation level of individual pixels of the detector.   In extreme cases, this persistence can be seen for several hours in WFC3. As a result, persistence can affect science exposures not only within a single visit, but also in exposures of later visit.  A description of persistence in the WFC3/IR detector can be found here.

Normally, persistence due to images within a visit is not a problem, because most observers use small dithers and the after-glow images are faint.  As a result, afterglows within a visit are usually undetectable since they are found in the wings of the point spread function of the follow-on images. 

Persistence due to earlier visits, while less common, can be more of a problem scientifically, since the after-images from earlier visits can be anywhere in the science images of the current visit.  The numbers of programs which have been adversely affected are small, but observers need to be aware of the possibility.

In order to help observers assess the degree to which a their exposures are affected, STScI has developed a model for persistence as it exists in WFC3.  We have used this model and the time-history of earlier exposures to generate estimates of the amount of persistence in each "flt" file and to produce fits files that contain estimates for each pixel in each "flt" file.  We also produce a persistence-subtracted "flt" file. 

Since persistence due to images within a visit and those from previous visits reflect different issues, we distinguish between the two.   We call persistence within a visit "internal persistence", and persistence from earlier visits "external persistence." 

Access to Persistence Information for WFC3/IR

One can use this search form to locate persistence information about WFC3/IR observations. 

The output of the query contains the percentage of pixels estimated to have persistence > 0.1, 0.03, and 0.01 electrons/s.  It provides these for the external persistence (EXT1, EXT2, and EXT3, respectively) and for the total of internal + external persistence (TOT1, TOT2,TOT3).  Generally speaking, the primary columns to be concerned about are EXT1 and EXT2, the percentage of pixels with  external persistence in excess of 0.1 and 0.03 electrons/s, respectively.  For reference, the median dark current in WFC3/IR is about 0.015 electrons/s.

In any event, more details of the persistence estimates as well as evaluation images showing the regions of the detector where persistence is likely to be greatest can be seen by following the link associated with the dataset name. 

In addition, by clicking on the visit link, one can retrieve the fits files that are produced to as part of the persistence estimate for this visit.  When you uncompress the tar.gz file there you will find files of the following types:

• rootname_extper.fits - estimate of the external persistence
• rootname_persist.fits - estimate of the total (internal + external) persistence
  
• rootname_flt_cor.fits - persistence subtracted flt file
  

Datasets marked with a yellow background and followed by an "@" sign are still proprietary and are not available to the general user. You will be prompted to enter your archive user name and password and if that account is authorized to retrieve the data, then you will be able to see the web page and to download the tarfile.

These data are available through the browser at http://archive.stsci.edu/pub/wfc3_persist/ and are also available via ftp from archive.stsci.edu (logon with archive user name and password to obtain authorized proprietary data). cd /pub/wfc3_persist.

As with most standard MAST search interfaces, one may run the search interface has an HTTP GET request and embed the search in a script. For example: http://archdev.stsci.edu/prepds/persist/search.php?prop_id=11937&visit=01&outputformat=JSON&action=Search will find observations associated with Proposal ID 11937 Visit 01 and return the results as a JSoN format. For more information about options see http://archive.stsci.edu/vo/mast_services.html. See the field descriptions for the column names and data types.

Observers should be cautioned that the persistence model is only an approximation to the actual persistence. Therefore, one should not "blindly" replace the standard 'flt" files with the persistence-corrected flt files in downstream analysis.  A better strategy in some cases, may be to use the external persistence file to flag bad pixels for downstream analysis. In other cases scaling the models up or down by a small factor may result in a cleaner corrected image. 

Observers and archive users are encouraged to contact the help desk (help@stsci.edu) if the persistence subtraction substantially limits their ability to extract science from an image or set of images.

Users who have questions about the interface or encounter problems with the interface or downloading the data should contact the archive help desk archive@stsci.edu.