DM-9769 - Getting issue details... STATUS

Requirements on LSST Image Cutout Service (DRAFT!!)

A. Requirements

LSST has two kinds of images, single exposure images and deep co-adds.  After the image search, one or more single exposure images or deep co-adds images are returned. The cutout service shall return the cutout image made from those images , based on user's inputs on the cutout, like the orientation, the center location, and the size of the cutout.

For cutouts from the single exposure images:

• • Since the focal plane array is made of 189 separate CCDs each CCS has 4x4 cm² so the cutout very likely needs to stitch multiple single exposure images together. We require all the single exposure images in one cutout must be from the same full field of view (FOV), in other words, they are all taken at the same time. 
  • When stitch single exposure images together, the cutout service needs to be able to cross image edges and to handle gaps in the coverage (e.g., gaps between CCDs) and set those pixels to some value (NULL?) that can be read by the visualizers and set to 'black'.
  • User could request the orientation of the cutout be in the camera array coordinate system (equivalent to the pixel space coordinate system) or any world coordinate system. 
  • If the orientation of the cutout is in the camera array coordinate system, the cutout service should respect that and do not do re-projection. The flux value in the output should be the original measured. 
  • If the orientation of the cutout is in any world coordinate system, the image re-projection will be needed to generate the cutout.

For cutouts from the deep co-add images:

• • The deep co-adds images will be stored in some form of tiling across the sky. There are no gaps between co-adds images. The cutout image can be a sub image from one co-adds image or when the cutout is large the cutout service might need to stitch several co-adds images together. 
  • User could request the orientation of the cutout, either in the pixel space coordinate of the co-adds image where the cutout center locates or in any of the world coordinate systems.
  • If the cutout is a sub image from one single co-adds image and the cutout orientation is the pixel coordinate, the cutout service should respect that and do not do re-projection. The flux value in the output should be the originals in the co-adds image.
  • If the orientation of the cutout is in any world coordinate system, the image re-projection might be needed to generate the cutout and the flux value in the output should be interpreted from the co-adds images.
  • If the cutout is from several co-adds images, and the cutout size reaches a boundary of a co-adds image,  the cutout service must be able to cross that boundary and build the remainder of the image from the next image over. Re-projection might be needed.

The cutout service should be able to handle many calls without performance degrading.

B. Inputs to the cutout service

The users will specify the orientation, the center position, and the size of the cutout. 

(1)Orientation (string):

       This input defines the orientation of the cutout, or the cutout would be done along which coordinate system. 

        The orientation choices are the pixel space coordinate or any world coordinate:

• • Pixel Coordinate 
  • Equatorial J2000
  • Equatorial B1950
  • Ecliptic J2000
  • Ecliptic B1950
  • Galactic
  • Super Galactic

Note. If the cutout contains more than one input images, the Pixel Coordinate is the one of the image where the cutout center locates. 

(2)Center (decimal, decimal (string)):

x, y (units)

• • The center parameter consists of a comma separated coordinate pair followed by an optional units specification.
  • If units is set to px, pix or pixels, then x,y are interpreted as the pixel space coordinates of the desired cutout center. 
  • If units are angular (rad, deg, etc...), then x,y are interpreted as the right ascension/longitude (x) and declination/latitude (y) of the desired cutout center, depending on the world coordinate system selected in Orientation.
  • If no units are specified, degrees are assumed.
  • Declinations outside of [-90, 90] degrees are illegal; right ascensions are range-reduced, so any value will do.
    
    

Examples:

Question: If the image search result has single exposure images from different times or FOVs, how to get which FOV to make the cutout? How to get which CCD to use pixel coordinate?

  (3)Size (decimal (, decimal) (string)):

x(, y) (units)

• The size parameter consists of one or two (comma separated) values followed by an optional units specification. 
• Units can be pixels (px, pix, pixels) or angular (arcsec, arcmin, deg, rad); the default is degrees.
• The first size value (x) is taken to be the full-width of the desired cutout along the first image axis (NAXIS1), and the second (y) is taken to be the full-height along the second axis (NAXIS2). If only one size value is specified, it is used as both the full-width and full-height. 
• Negative sizes are illegal.

        Examples:

C. Notes

We require that all the cutout images shall be North Up. In other words, if the orientation is one of the world coordinates, the line of the equatorial RA or of the ecliptic/galactic longitude through the center of the cutout must be upright and at the middle of the image. (Detail wording TBD. ) See attached images which are from https://skyview.gsfc.nasa.gov.