Current Product Plans
This note covers both short-term storage of AP products to disk, and long-term storage to tape. Per discussion with Robert Gruendl I assume that we will not have different compression policies for disk and tape storage (in particular, if a product is compressed for disk storage, then the compressed copy is what will be transferred to tape).
Currently, the AP pipeline is following the DPDD. It will store raw images, PVIs, and difference images for both short- and long-term use. There are no plans to dynamically generate, e.g., difference images on the fly.
We are working on a system for creating DCR-corrected templates, which will use a set of basis images to create a modeled template for each exposure. It is not yet decided whether we will be storing the template associated with each exposure, or whether we will only store the basis images and recompute the model on the fly when somebody requests "the template used for such-and-such an image". Depending on the answer, either basis images or templates may be candidates for compression.
Data Re-Use
AP will rely on DRP-produced coadded templates that may be up to two years old (LDM-151).
AP will also perform forced photometry ("precovery") on difference images. Precovery is usually done over a 30-day interval but may revisit all previous exposures in selected fields. Note that AP will also be performing forced photometry on newly acquired difference images; there may be a consistency problem if this photometry is done on exact images while precovery is done on compressed ones.
Data Quality Requirements
Compressed images are most likely to affect Alert Production by increasing the false positive or false negative rates, particularly if quantization introduces "noise" into subtracted stellar images.