INNAME Input name INCLASS Input class INSEQ 0.0 9999.0 Input sequence, 0 -> high INDISK 0.0 9.0 Input disk, 0 -> any OUTNAME Output name OUTCLASS Output class OUTSEQ -1.0 9999.0 Output sequence 0 -> highest unique OUTDISK 0.0 9.0 Output disk 0 -> highest with room IMSIZE 0.0 16384.0 Output image size BPARM Output map parameters 1) coordinate system 1: equatorial (default) 2: galactic 3: ecliptic 2) epoch of mean coordinates (default: 2000.0) 3) epoch prefix (default: J) 1: Julian (eg J2000.0) 2: Besselian (eg B1950.0) 3: Besselian without E-terms (eg b1950.0) 4) projection (default: NCP) 1: SIN 7: AIT 2: TAN 8: STG 3: ARC 9: CAR 4: NCP 10: MOL 5: GLS 11: PAR 6: MER 5) blanking control 0: "magic blanking" 1: zeros CPARM Output axis specification See HELP for important information concerning the usage of CPARM. 1-5): first axis 1: hour (or degree) 2: minute (or arcmin) 3: second (or arcsec) 4: reference pixel 5: coord increment (arcsec) 6-10): second axis similarly

SKYVE Type: Task Use: SKYVE will regrid a Digitized Sky Survey (DSS) image to a coordinate frame and projection recognized by AIPS. The DSS is based on photographic material obtained using the UK Schmidt Telescope operated by the Royal Observatory Edinburgh (RGO), with funding from the UK Science and Engineering Research Council (SERC) until 1988 June, and thereafter by the Anglo-Australian Observatory (AAO). The DSS was produced by the Space Telescope Science Institute (STScI) under U.S. Government grant NAG W-2166. SKYVE USAGE Digitized Sky Survey images may be extracted from the CD set as FITS files by a program called 'getimage'. This is supplied with the CD set (on CD #61). The FITS file created by 'getimage' may be read into AIPS using IMLOD. The DSS image coordinate system is encoded as a set of plate solution coefficients in FITS header cards which are not generally recognized by AIPS. IMLOD stores these unrecognized header cards in the history file associated with the AIPS image. These header cards are listed in the EXPLAIN file. SKYVE retrieves the plate solution parameters from the history file and regrids the image into a coordinate system recognized by AIPS. Why would you want to do this? a) So you can use AIPS to measure positions from the optical image. Verification tests done on a selection of about 70 quasars with VLBI positions by Martin Anderson (ATNF/UWS) using MAXFIT show that an rms accuracy of better than 0.5 arcsec is achievable. b) So that you can overlay radio images on top of the optical image. If you want to overlay images then be sure to extract an optical image slightly larger than the radio image to avoid edge effects when the optical image is regridded. You must also exercise caution in changing the pixel spacing in the optical image to be much greater than that of the original DSS image (approximately 1.7 arcsec). If you make the spacing too great then stars and other small scale objects may be skipped over. If you already have the radio image then: a) If the pixel spacing in the radio image is much less than 1.7 arcsec then you should regrid the optical image to the same spacing as the radio image. You should be able to do this directly with SKYVE using the IMSIZE, BPARM, and CPARM adverbs to match the coordinate system and projection of the radio image. b) If the pixel spacing in the radio image is much greater than 1.7 arcsec then you should regrid the radio image to the same spacing as the optical image. This may be done with REGRD or HGEOM. If you don't already have the radio image then you should synthesize it with a cell spacing to match the optical image. COORDINATE SYSTEMS Coordinate transformations between the IAU1976 and Bessel-Newcomb systems are done with full precision assuming zero proper motion, parallax, and recessional velocity at J2000.0 Specifying a Julian epoch 'J' to SKYVE implies that the output coordinates are referenced to the new IAU1976/FK5 system. Specifying a Besselian epoch to SKYVE implies that the coordinates are referenced to the old Bessel-Newcomb/FK4 system. An epoch prefix of 'B' indicates the convention that the coordinates include the effect of the E-terms, whereas 'b' indicates that they have already been removed. FK4 catalogue coordinates were not corrected for the elliptic terms of aberration (E-terms) except for positions within 10 degrees of the pole. Most earlier catalogues did not correct for them. The default behaviour here is to assume that the E-terms are included in all Besselian coordinates (including near the pole). This can be defeated if it is known that the input coordinates have already been corrected, or if it is required that the output coordinates not contain them. See the EXPLAIN section for a brief description of the algorithm. STATISTICS SKYVE reports statistics of the pixel displacements (output map pixel coordinate minus input map pixel coordinate) for the regridding operation. The mean and rms for pixel displacements in X and Y are reported, and also the correlation coefficient. If all SKYVE defaults are adopted - same image size, J2000.0 equatorial coordinates, same centre coordinates and pixel increment - then the mean shift should be approximately zero, the rms should be a few pixels, and the correlation coefficient much less than unity. However, these statistics do not directly account for a net rotation of the image, and this is usually the main systematic difference between the output and input maps. Adverbs: INNAME......Input image name, standard defaults. INCLASS.....Input image class, standard defaults. INSEQ.......Input image sequence number, 0 -> highest. INDISK......Input disk drive number, 0 -> any. OUTNAME.....Output image name, standard defaults. OUTCLASS....Output image class, standard defaults. OUTSEQ......Output image sequence number, 0 -> highest unique OUTDISK.....Output disk drive number, 0 -> highest with space. IMSIZE......Output image size (pixels), maximum 16384. Defaults to the input image size if negative or zero. BPARM.......Coordinate frame and projection of the output map 1) Coordinate frame 0: -> 1 1: equatorial (mean of epoch) 2: galactic 3: ecliptic (mean of epoch) Anything else produces an error. 2) Epoch of mean equatorial or ecliptic coordinates, e.g. 1950, 2000. Defaults to 2000.0 if negative or zero. 3) Epoch prefix 1: "J" - Julian (as J2000.0) 2: "B" - Besselian (as B1950.0) 3: "b" - Besselian without E-terms (eg b1950.0) Anything else defaults to "J". 4) Spherical projection (geometry) 0: -> 1 1: SIN, sine (orthographic) 2: TAN, tangent (gnomonic) 3: ARC, arc (zenithal equidistant) 4: NCP, north celestial pole tangent 5: STG, stereographic ----- all sky types ---------------- 6: GLS, global sinusoid (Sanson-Flamsteed) 7: MER, Mercator 8: AIT, Hammer-Aitov 9: CAR, Plate Carree ("cartesion") 10: MOL, Molweide's 11: PAR, Parabolic (Craster) These should have ref latitude = 0 or they will be "oblique" which you probably do not want. Anything else produces an error. 5) output blanking control 0: "magic" blanking 1: zero (less than or equal to 0.5 -> 0; greather than 0.5 -> 1) CPARM.......Output axis specification 1-5) Apply to the first axis 1-3) Specify the coordinate reference pixel. IF THE VALUE SPECIFIED IS OUTSIDE THE RANGE -24HR TO +24HR, OR -360 TO +360 DEGREES, THE COORDINATES OF THE CENTRE OF THE INPUT MAP WILL BE USED (transformed to the coordinate system of the output map if necessary). 1: hour for equatorial, degree for the others 2: minute for equatorial, arcmin for the others 3: second for equatorial, arcsec for the others 4: Coordinate reference pixel. If zero, the centre of the output map is assumed. 5: Coordinate increment (arcsec per pixel, should be negative). If zero, the DSS plate scale is assumed. 6-10) Apply to the second axis as for the first except the range is -90 to +90 degrees, and the coordinate increment (if non-zero) should normally be positive. THE ABSOLUTE VALUES OF CPARM(6:8) ARE USED TO COMPUTE THE DECLINATION (LATITUDE); IF ANY OR ALL OF CPARM(6:8) ARE NEGATIVE THE DECLINATION (LATITUDE) IS NEGATED.

SKYVE: Transforms DSS image coordinate system and projection Author and documenter: Mark Calabretta, ATNF Related tasks: REGRD, GEOM, HGEOM, LGEOM, PGEOM, COMB Algorithm ~~~~~~~~~ For each pixel in the output image: 1) Compute its sky coordinates. 2) Transform to sky coordinates on the input map - a) remove E-terms (only when the output map is in equatorial FK4 coordinates). b) perform the spherical coordinate rotation specified by three Euler angles. 3) Compute the pixel coordinates on the input map. This requires iterative inversion of the DSS plate solution equations. 4) Interpolate the pixel value from the nearest nine pixels - a) quadratic interpolation in X for each of the three rows. b) quadratic interpolation in Y of the result. Parameters used for the transformation (step 2) are recorded in the history file. E-terms ~~~~~~~ The E-terms are recomputed for every pixel (step 2a, computation of the E-terms is not done iteratively). 1) In RA: (E1*COS(RA) + E2*SIN(RA))/COS(DEC) 2) In DEC: (E2*COS(RA) - E1*SIN(RA))*SIN(DEC) + E3*COS(DEC) Euler angles ~~~~~~~~~~~~ For a spherical coordinate rotation from SYSTEM1 to SYSTEM2: 1) PHI0: Longitude of the ascending node in SYSTEM1. Of the two points of intersection of the equators of SYSTEM1 and SYSTEM2, the ascending node is the one where the equator of SYSTEM2 crosses from south to north as viewed in SYSTEM1. 2) THETA: The angle between the poles of the two systems. Positive for a positive rotation about the ascending node. 3) PHI: Longitude of the ascending node in SYSTEM2. Blanking ~~~~~~~~ Blank pixels are fully accounted for in the sense that one blank pixel in the input map produces only one blank pixel in the output map. The basic criteria is that the output pixel will be blank if and only if the pixel (P0) on the input map nearest the position computed at step 3 above is blank. If P0 is not blank and any of the eight pixels surrounding it are, then the quadratic interpolation reduces to a linear or, if necessary, a constant interpolation or extrapolation. In the worst case where all of the neighbouring pixels are blank, the "interpolated" value would be the value at P0. DSS FITS header cards ~~~~~~~~~~~~~~~~~~~~~ CNPIX1, The DSS pixel coordinates of the bottom and CNPIX2 left-hand corner of the bottom left-hand pixel of the image extracted by 'getimage'. PLTSCALE Approximate plate scale, arcsec/mm. XPIXELSZ, Plate pixel size in X and Y, micron. and YPIXELSZ PLTRAH, J2000.0 right ascension of the plate and PLTRAM, centre (hours, minutes, and seconds). and PLTRAS PLTDECSN, J2000.0 declination of the plate centre and PLTDECD, (sign, degrees, arcmin, and arcsec). and PLTDECM, and PLTDECS PPO3, Plate centre offsets, micron. and PP06 AMDX1, ... Plate solution coefficients for the xi to AMDX13 standard plate coordinate. AMDY1, ... Plate solution coefficients for the eta to AMDY13 standard plate coordinate.