The following information is taken from Waters et al. and Magnier et al., which should be cited appropriately. archive image cutout interface. 

Image Added
Schematic of the images and analysis processing stage of the PS1 IPP Pipeline, described in Magnier et al.

Raw images

After an exposure is taken at the summittelescope, the raw image files of the 60 OTA CCDs (see PS1 GPC1 camera) are corrected for persistence issues and then handed over to the the PS1 IPP processing pipeline for further analysis. The STScI PS1 public archive Archive does not provide these images.

Camera

images

The camera images are created by by applying PS1 Exposure detrending to the raw images (i.e.g., masking, bias subtraction, flat fielding), and applying an determining the astrometric and photometric calibration. The STScI PS1 public archive Archive does not provide these images.

Image

The OTA CCDs have known artifacts and anomalies. A lot of work has gone into characterizing these artifacts, and removing them if possible. Pixels affected by these artifacts or anomalies are kept tracked of in the mask images with pixel flags.

Warps

Warps are the result of resampling and realigning the camera images onto regular areas (called skycells) on the sky (basically aligned N-S, E-W). How this is done depends on the particular tessellation. Different surveys may have different tessellations. A warp will generally consist of several different OTAs, therefore there are gaps between the OTAs, as well as the smaller gaps between the cells. Warps are astrometrically and photometrically calibrated. The single-epoch warp images are not part of the PS1 DR1 data release but will be included in PanSTARRS DR2 (along with the catalog of multi-epoch photometry).

Note that the skycell images are 0.4x0.4 sq deg in size, which is considerably smaller than the camera's field of view (~7 sq deg), so there are many more warp images than exposures.  There are more than 24 million warp images in the archive.  Warp images (and their ancillary data) make up the bulk of the PS1 data volume.

Stacks

Stacks are the 'optimal' combination of multiple warps on the same skycell. The stacks are on the same sky tessellation than the warps. For the 3pi there are typically 10 or so warps per filter, but for the Medium Deeps there can be several hundred. Stacks are also astrometrically and photometrically calibrated. These images can be accessed and downloaded through the PS1 archive image cutout interface. As well as the standard masks and weights, stacks come with two other auxiliary image files:

• 'mask' images indicate which pixels in the stack are good and which are bad
• 'wt' images are the stack variance images
• 'num' images contain the number of warps with valid data which contributed to each pixel
• 'exp' images contain the exposure time in seconds which contributed to each pixel
• 'expwt' images are weighted exposure time maps XXX

Stack images have filenames that include '.unconv' because the are constructed by combining warp images having observed (variable) PSFs.  The PS1 pipeline also generated stacked images with seeing convolved to a fixed value before stacking. The convolved image products are not included in the public archive because it was found that the unconvolved images are always preferred for scientific data analysis.

Difference Images

artifacts and anomalies

Warp images (available in DR2)

Stack images (DR1)

Difference images (planned to be available in future)

Difference Difference images are created by subtracting a warp from another image, in general the stack, after matching the PSFs and normalization using a spatially varying kernel. All static objects like such as galaxies and constant stars are subtracted out, and only the excess flux from the different epochs is left. Even though the differences images are not stored on disk, they can be created, and it is planned to make them accessible through the STScI PS1 interface in a later release.

Mask Image

All images, warps and stacks, have Pixel Flags set for each individual pixels. This information is saved in a mask image, which in general has the suffix .mask.fits(.fz) and mk.fits(.fz) for warp and stack images, respectively.

Weight Images

Weight images are variance maps. For single epoch warps, these variance contains the readnoise, Poisson noise, IfA: FILL with other contributions. For stacks, the noise is propagated from the individual input warps. These weight images can be used for estimates of the uncertainties in the photometry. We note that the deprojection of the chip images into the warps correlates the pixels, introducing covariance. Therefore the derived uncertainties might underestimate the true noise.

Weight Image