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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.

Contents

Projection Cells in 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 at the bottom gives the details on the number of cells in each row, the exact sizes of the images, etc.

Note that this tessellation applies to the PS1 3PI survey.  For the Medium Deep Surveys (which do not yet have image products), special projection cells are used that are centered on each medium deep field.

The figures below display the projection cells for the entire PS1 sky and for the region around the north celestial pole.

Figure 1: Aitoff plot of all 2,009 PS1 projection cells for the 3PI survey. The coverage extends from declination −30° to the north celestial pole.

 

Figure 2: PS1 projection cells near the north celestial pole, where the image overlap is greatest due to convergence of the RA grid.  The projection cells are 4°x4° in size and are on rings spaced by 4° in declination.

Skycells

The projection cell images would be large and inconvenient to download – they are typically about 63000x63000 pixels and so would be about 8 GB each.  To make them easier to work with, each projection cell is divided into a grid of 10x10 skycells.  The skycells are typically 0.4°x0.4° in size. These images are simple rectangular FITS images that share the same tangent world coordinate system with the original projection cell image (i.e., they have the same CRVAL1, CRVAL2, and pixel size).  They can be pasted together to make a larger image. 

Adjacent skycells within a projection cell overlap by 240 pixels (60 arcsec) at each edge with the neighboring skycells.  Consequently, for objects and structures smaller than 2 arcmin it is generally not necessary to combine data from adjacent skycells.

Skycell images have names like skycell.nnnn.0yx where nnnn is the projection cell number (which ranges from 635 to 2643) and 0yx gives the skycell location in the image, with y and x ranging from 0 to 9 indicating the respective y and x section of the projection cell.   The 000 skycell is in the bottom left corner of the projection cell, 010 is just above it, and 099 is in the upper right corner.

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.

This table is also available as a FITS binary table.

 

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

PS1 Image Grid Layout
Projection cell size

~ 4° ×
63k × 63k pixels

Pixel size0.25 arcsec
No. of projection cells2,009
Declination spacing
Skycell size

~ 0.4° × 0.4°
6.3k × 6.3k pixels

No. of skycells~ 200,000
Skycell edge overlap240 pixels = 60"
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