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 cooadds. The cutout service shall take user's inputs and return the image cutouts in FITS for both kinds of images.
The cutout service must be able to cross image edges. The focal plane array is 189 separate CCDs and the co-adds will be stored in some form of tiling across the sky. If the cutout size reaches a boundary, cutout service must be able to cross that boundary and build the remainder of the image from the next image over.
For the single exposure images
- all the images to be stitched together for the cutout must be from the same full field of view (FOV), in other words, they are all taken at the same time.
- the cutout service needs to be able 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'.
The cutout service (and stitching) needs to be able to handle the orientation of the cutout images on the sky at any angle with respect to the camera pixel orientation. The users will specify a position on the sky as the center of the cutout, the size of the cutout, and the orientation of the cutout which is in astronomical coordinates (equatorial, galactic, ecliptic).The image re-projection will be needed to generate the cutout FITS file.
For the single exposure image, user could request the orientation to be the camera pixel orientations which are oriented in XY pixel space. The cutout service should respect that and do not do re-projection.
The cutout service should be able to handle many calls without performance degrading.
B. Inputs to the cutout service
(1)Cutout Type (string):
Two options:
- Single exposure images
- Deep coadd images
(2)Cutout Orientation (string):
This input defines the orientation of the cutout, or the cutout would be done along which coordinate system.
The orientation choices are:
- Cameral Orientation (Only for Cutout Type = "Single exposure images")
- Equatorial J2000
- Equatorial B1950
- Ecliptic J2000
- Ecliptic B1950
- Galactic
- Super Galactic
(3)Cutout Center (decimal, decimal):
The Cutout Center parameter defines the center of the cutout, in the coordinate system defined in Cutout Orientation. If the Cutout Orientation is Cameral Orientation, the Cutout Center is (ra, dec) in J2000. The Cutout Center must consist of a comma separated coordinate pair followed by an optional units specification.
Examples:
| Cutout Center | Cutout Orientation | interpretation |
|---|---|---|
| 10,10 deg | Cameral Orientation | (RA = 10 deg, Dec = 10 deg) in Equatorial J2000 |
| 10,20 arcmin | Equatorial B1950 | (RA = 10 deg, Dec = 20 deg) in Equatorial B1950 |
| 45, 89 rad | Ecliptic J2000 | (Long = 45 rad, Lat = 89 rad) in Ecliptic J2000 |
| 30, 60 deg | Galactic | (Long = 30 deg, Lat = 60 deg) in Galactic |
(4)Cutout Size (decimal, (decimal)):
The Cutout Size parameter defines the width and the height of the output cutout. The Cutout Size 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 input is the full-width 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:
| Cutout Size | Cutout Orientation | Interpretation |
|---|---|---|
| 100, 200 pix | Cameral Orientation | The width is 100 pixels and the height is 200 pixel along the input images' NAXIS1 and NAXIS2 respectively. |
| 10, 20 deg | Cameral Orientation | The width is 10 degrees and the height is 20 degrees along the input images' NAXIS1 and NAXIS2 respectively. |
| 200, 150 arcmin | Equatorial J2000 | The length of the bottom side of the cutout is 200 arcmin along Equatorial J2000 RA direction. The length of the height through the cutout center is 150 arcmin along Equatorial J2000 Dec direction. |
| 45, 60 deg | Ecliptic J2000 | The length of the bottom side of the cutout is 45 degrees along Ecliptic J2000 longitude direction. The length of the height through the cutout center is 60 degrees along the Ecliptic J2000 latitude direction. |
| 0.1 | Cameral Orientation | The length of the bottom side of the cutout is 0.1 degrees along the input images' NAXIS1 and the height through the cutout center is also 0.1 degrees along the input images' NAXIS2. |
Note. If Cutout Orientation is "Cameral Orientation", no re-projection will be needed. The flux value in the output FITS should be the original measured. The pixels in the gaps between CCDs should have NULL.
Note 1. We require that all the cutout images shall be North Up. In other words, the line of the equatorial RA or 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.
Note 2. Re-projection from the camera pixel orientation to the user selected coordinate system will be necessary to produce the output FITS. The flux value in the output FITS should be interpreted from the original measurement.
C. Emphasize the differences between the single exposure image cutout and the deep coadd cutout:
The single exposure images are taken along the camera coordinates. Users might want the cutout oriented along either the camera coordinates which should be the original FITS NAXIS1 and NAXIS2 or any astronomical coordinates. If along any astronomical coordinate, the image reprojection will be needed. There are 189 separated CCDs in one FOV taken at the same time. A cutout very likeyly needs more than one CCD image. So when stiching multiple CCD images together, the cutout service needs to handle the gaps between CCDs (NULL/black) and the image edges.
The deep coadd images are produced by many signle exposure images and could be along any orientation. The image reprojection would be very likely needed to meet users' required cutout orientation. There are no gaps between coadds. When the cutout is large, the cutout service might need to stitch several coadds together. The image edges need to be handled properly.