LBC ASTROMETRYThe astrometricsolution is found by using a software suite developed by M. Radovich at Naples Observatory. The steps through which the images are astrometerized are:
1) match the image objects catalog with the astrometric reference catalog (usually USNOB1.0). In order to increase the number and reliability of matches, a standard presolution is applied before the initial matching. 2) find a preliminary solution chip by chip. 3) find a general solution for the whole mosaic at once. 4) write individual solutions for the four chips in the respective headers. The astrometric headers are then saved as separate files (with extension .head) that can be used later by a resampling program (Swarp, by E. Bertin), or used to computed sky coordinates of objects measured on the images. An astrometric header is shown below. From this file, the procedure to obtain RA,DEC from x,y is the usual one: first, the x,y must be transformed into the projected plane coordinates x',y' by applying the linear WCS (keywords: CRVALn, CRPIXn, CDi_j). Then, the coordinates are transformed into the native spherical coordinates xi,eta by applying the astrometric polinomial solution PVi_j. The meaning of the coefficients can be found in various documents over the internet, for example here: http://www.oacn.inaf.it/~radovich/WIFIX/astrom.ps The projected coordinates are then transformed into celestial ones by applying the deprojection as indicated in the keywords CTYPEn and CUNITn, relative to the tangent point on the celestial sphere CRVALn. Example of astrometric solution header: EXPTIME = 1.259 EPOCH = 2000.00000 EQUINOX = 2000.0 CRVAL1 = 102.98410 CRVAL2 = -0.34925 CROTA1 = -0.00177 CRPIX1 = -1179.12952 CRPIX2 = 2823.02173 CDELT1 = -6.389E-05 CDELT2 = 6.389E-05 CD1_1 = -6.325283e-05 CD1_2 = 4.880472e-07 CD2_1 = 5.258577e-07 CD2_2 = 6.325923e-05 IMAGEID = 1 INLIST = list.1.txt KTAB = Y LINXRMS = 0.210 LINYRMS = 0.246 LINCAT = USNO LINTOL = 5 ASTXRMS = 0.121 ASTYRMS = 0.136 NAST = 439 PRESOL = ./Presol/ps1 PHFLAG = 1 OLXRMS = 0.036 OLYRMS = 0.025 PSORD = 3 PV1_0 = 0.0001547168 PV1_1 = 1.0032149315 PV1_2 = 0.0009429324 PV1_4 = 0.0240833660 PV1_5 = 0.0166469825 PV1_6 = 0.0005309086 PV1_7 = -0.1984784774 PV1_8 = 0.0469841415 PV1_9 = -0.2529869539 PV1_10 = 0.0030073688 PV2_0 = -0.0000049588 PV2_1 = 1.0003792280 PV2_2 = 0.0012891438 PV2_4 = 0.0086788762 PV2_5 = 0.0053759130 PV2_6 = 0.0116861035 PV2_7 = -0.2621941903 PV2_8 = 0.0159960531 PV2_9 = -0.2434173057 PV2_10 = 0.0214992420 ZP = 0.250 FLXSCALE= 7.942812e-01 ZP0 = 0.000 END UCAC2 2230+35Here it is possible to download some exposure of the astrometric field UCAC2 2230+35 taken in B, U and V filters. Original MEF images have been pre-reduced (x-talk corrected, bias subtracted and flat-fielded) and splitted in single FITS images. Each tar file contains 40 pre-reduced frames with the corresponding weight image (.wht.fits file) and astrometric solution (.head file). The .head files can be immediately used with the SWarp program to obtain aligned images.
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