The PanSTARRS images are interpolated onto a regular grid of images that cover the sky. The grid consists of large 4°x4° projection cells that are centered on lines of constant declination spaced 4 degrees apart. For convenience of access, each projection cell is divided into a 10x10 grid of skycells, each 0.4°x0.4°. The skycells for a given projection cell share a seamless world coordinate system with the same tangent projection, so they can be easily pasted together to produce larger images. The pixel size is 0.25 arcsec.
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The RINGS.V3 Tessellation
The PS1 image layout for the 3PI survey is called the RINGS.V3 tessellation. It was defined by Tamas Budavari to meet the goals of defining a regular image pattern with nearly square cells that covers the sky while minimizing redundant image overlap. The sizes of the large projection cells are approximately constant over the sky.
The projection cell centers are located on lines of constant declination spaced 4° apart. At a given declination, the pointing centers are equally spaced in right ascension around the sky, with the number of RA points changing to account for the convergence of RA lines in the spherical sky. The pattern is defined to cover the entire sky from δ = −90° to +90°. There are 46 declination zones numbered starting at 0 for an image at δ = −90°, 1 for a row of 9 images centered at δ = −86°, 2 at δ = −82°, and so on; the images in zones 22 and 23 are at δ = −2° and +2°, and zones 44 and 45 are at δ = +86° and +90° respectively.
Within a given declination zone, the projection cells are centered at RA(n) = n Δα = n 360°/M where M is the number of RA cells in the zone. Finally, the projection cells themselves are numbered consecutively (ordered by increasing RA) starting at 0 at the south pole, 1–9 at δ = −86°, etc.
Since the PS1 survey covers the sky only for δ > −30°, not all projection cells are used for PS1 images. The first PS1 projection cell is number 635 at RA=0°, δ = −30°, and the last one is 2643 at δ = +90°. The table below gives the details on the number of cells in each row, the exact sizes of the images, etc.
Skycells
Image showing all projection cells
Image showing region near pole
Image showing skycells with overlap
Discuss skycell overlap and projection cell overlap
Note that this tessellation applies to the PS1 3PI survey. For the Medium Deep Surveys (which do not yet have image products
Projection Cell Table
The table below gives all the details on the PS1 projection cells. Zone is the declination row number, which starts at 0 at δ = −90°. ProjCell is the projection cell number for the first cell on that row. M is the number of cells in the row (so the projection cell number runs from ProjCell to ProjCell+M−1). Dec is the declination of the image center. Xsize and Ysize give the size of the full projection cell image in 0.25 arcsec pixels. Xsub and Ysub give the size of the skycell images into which the projection cell is divided for storage in FITS files. Note that Xsub = (Xsize-480)/10+240 pixels. The final two columns, MinDec and MaxDec, give the declination range over which this row is the best choice. These columns are used to select a declination zone in the region where adjacent zones overlap.
Zone | ProjCell | M | Dec | Xsize | Ysize | Xsub | Ysub | MinDec | MaxDec |
---|---|---|---|---|---|---|---|---|---|
15 | 635 | 79 | -30.0 | 63220 | 63170 | 6514 | 6509 | -31.989 | -27.991 |
16 | 714 | 82 | -26.0 | 63030 | 63130 | 6495 | 6505 | -27.991 | -23.992 |
17 | 796 | 84 | -22.0 | 63270 | 63090 | 6519 | 6501 | -23.992 | -19.993 |
18 | 880 | 86 | -18.0 | 63220 | 63060 | 6514 | 6498 | -19.993 | -15.994 |
19 | 966 | 88 | -14.0 | 62900 | 63020 | 6482 | 6494 | -15.994 | -11.996 |
20 | 1054 | 89 | -10.0 | 62960 | 62980 | 6488 | 6490 | -11.996 | -7.997 |
21 | 1143 | 89 | -6.0 | 63390 | 62950 | 6531 | 6487 | -7.997 | -3.998 |
22 | 1232 | 90 | -2.0 | 62880 | 62910 | 6480 | 6483 | -3.998 | 0.000 |
23 | 1322 | 90 | 2.0 | 62880 | 62910 | 6480 | 6483 | 0.000 | 3.998 |
24 | 1412 | 89 | 6.0 | 63390 | 62950 | 6531 | 6487 | 3.998 | 7.997 |
25 | 1501 | 89 | 10.0 | 62960 | 62980 | 6488 | 6490 | 7.997 | 11.996 |
26 | 1590 | 88 | 14.0 | 62900 | 63020 | 6482 | 6494 | 11.996 | 15.994 |
27 | 1678 | 86 | 18.0 | 63220 | 63060 | 6514 | 6498 | 15.994 | 19.993 |
28 | 1764 | 84 | 22.0 | 63270 | 63090 | 6519 | 6501 | 19.993 | 23.992 |
29 | 1848 | 82 | 26.0 | 63030 | 63130 | 6495 | 6505 | 23.992 | 27.991 |
30 | 1930 | 79 | 30.0 | 63220 | 63170 | 6514 | 6509 | 27.991 | 31.989 |
31 | 2009 | 76 | 34.0 | 63130 | 63220 | 6505 | 6514 | 31.989 | 35.987 |
32 | 2085 | 72 | 38.0 | 63530 | 63270 | 6545 | 6519 | 35.987 | 39.986 |
33 | 2157 | 68 | 42.0 | 63680 | 63320 | 6560 | 6524 | 39.986 | 43.984 |
34 | 2225 | 64 | 46.0 | 63550 | 63370 | 6547 | 6529 | 43.984 | 47.982 |
35 | 2289 | 60 | 50.0 | 63090 | 63430 | 6501 | 6535 | 47.982 | 51.980 |
36 | 2349 | 55 | 54.0 | 63310 | 63500 | 6523 | 6542 | 51.980 | 55.977 |
37 | 2404 | 50 | 58.0 | 63260 | 63590 | 6518 | 6551 | 55.977 | 59.974 |
38 | 2454 | 45 | 62.0 | 62880 | 63670 | 6480 | 6559 | 59.974 | 63.970 |
39 | 2499 | 39 | 66.0 | 63550 | 63810 | 6547 | 6573 | 63.970 | 67.965 |
40 | 2538 | 33 | 70.0 | 64130 | 63980 | 6605 | 6590 | 67.965 | 71.958 |
41 | 2571 | 27 | 74.0 | 64610 | 64190 | 6653 | 6611 | 71.958 | 75.950 |
42 | 2598 | 21 | 78.0 | 65000 | 64460 | 6692 | 6638 | 75.950 | 79.941 |
43 | 2619 | 15 | 82.0 | 65290 | 64770 | 6721 | 6669 | 79.941 | 83.938 |
44 | 2634 | 9 | 86.0 | 65490 | 64670 | 6741 | 6659 | 83.938 | 87.975 |
45 | 2643 | 1 | 90.0 | 62880 | 62880 | 6480 | 6480 | 87.975 | 90.000 |