iraf.net

Hello,everyone!
I met a problem to ask for help.When using rd2xy,the RA= 193.86719,DEC=2.470158(deg),the output x=59.10,y=-4411.95.why y
is a negtive number?
Do anyone can help me please?

The results might in fact be correct, it's impossible to tell without seeing the image header and/or WCS keywords, it all depend on where the CRPIX reference pixel is defined and the CRVAL values at that point (plus the CD matrix defining the scale/rotation). An alternative way to do this is with the WCSCTRAN task where the 'inwcs' is "world" and the 'outwcs' param is 'logical'. The 'physical' system will tell you where the point is in the image independent of whether it was copied out of another image section. Post the image header if you still have problems.Cheers,
-Mike

Hi,
The image header is bellow:
SIMPLE = T / Fits standard
BITPIX = -32 / Bits per pixel
NAXIS = 2 / Number of axes
NAXIS1 = 2048 / Axis length
NAXIS2 = 1489 / Axis length
EXTEND = F / File may contain extensions
ORIGIN = 'NOAO-IRAF FITS Image Kernel July 2003' / FITS file originator
DATE = '2007-09-28T12:36:30' / Date FITS file was generated
IRAF-TLM= '20:36:52 (28/09/2007)' / Time of last modification
OBJECT = '11 S ' / Name of the object observed
TAI = 4464229733.94 / 1st row - Number of seconds since Nov 17 1858
RA = 193.86719 / 1st row - Right ascension of telescope boresigh
DEC = 2.470158 / 1st row - Declination of telescope boresight (d
SPA = 90.385 / 1st row - Camera col position angle wrt north (
IPA = 43.476 / 1st row - Instrument rotator position angle (de
IPARATE = 0.0002 / 1st row - Instrument rotator angular velocity (
AZ = 299.73242 / 1st row - Azimuth (encoder) of tele (0=N?) (de
ALT = 41.118844 / 1st row - Altitude (encoder) of tele (de
FOCUS = -999999999999.999999 / 1st row - Focus piston (microns?)
DATE-OBS= '2000-05-05' / 1st row - TAI date
TAIHMS = '07:48:53.93' / 1st row - TAI time (HH:MM:SS.SS) (TAI-UT = appr
COMMENT TAI,RA,DEC,SPA,IPA,IPARATE,AZ,ALT,FOCUS at reading of col 0, row 0
BUNITS = 'ADUs '
TELESCOP= '2.5m '
TIMESYS = 'TAI '
RUN = 1462 / Run number
FRAME = 461 / Frame sequence number within the run
CCDLOC = 14 / Survey location of CCD (e.g., rowCol)
STRIPE = 11 / Stripe index number (23 <--> eta=0)
STRIP = 'S ' / Strip in the stripe being tracked.
FLAVOR = 'science ' / Flavor of this run
SYS_SCN = 'mean ' / System of the scan great circle (e.g., mean)
EQNX_SCN= 2000.00 / Equinox of the scan great circle. (years)
NODE = 95.00000 / RA of the great circle's ascending node (deg)
INCL = 2.50000 / Great circle's inclination wrt cel. eq. (deg)
XBORE = 22.74 / Boresight x offset from the array center (mm)
YBORE = 0.00 / Boresight x offset from the array center (mm)
EXPTIME = '53.907456' / Exposure time (seconds)
SYSTEM = 'FK5 ' / System of the TCC coordinates (e.g., mean)
CCDMODE = 'DRIFT ' / 'STARING' or 'DRIFT'
C_OBS = 26322 / CCD row clock rate (usec/row)
COLBIN = 1 / Binning factor perpendicular to the columns
ROWBIN = 1 / Binning factor perpendicular to the rows
DAVERS = 'v14_24 ' / Version of DA software
COMMENT CCD-specific parameters
CAMROW = 1 / Row in the imaging camera
BADLINES= 0 / Number of bad lines in frame
EQUINOX = 2.00000000000000E+03
SOFTBIAS= 1000 / software "bias" added to all DN
FLUX20 = 1.88043000000000E+03 / Number of DN in 20th mag object
FLUX0 = -9.9990000000000E+03 / Number of DN in 0th mag object
FCALI_ID= 'None '
SKY = 1.59980000000000E+02 / Approx. sky level (DN)
BUNIT = 'DNs '
FILTER = 'r ' / filter used
CAMCOL = 4 / column in the imaging camera
VERSION = 'v5_4_24 '
DERV_VER= 'v8_16 '
ASTR_VER= 'v5_16 '
ASTRO_ID= '2003-07-09T16:53:02 06073'
BIAS_ID = 'PS '
FRAME_ID= '2003-07-14T23:42:00 28595'
KO_VER = 'devel '
PS_ID = '2003-07-09T11:23:48 21176 camCol 4'
ATVSN = 'v5_16 ' / ASTROTOOLS version tag
RADECSYS= 'ICRS ' / International Celestial Ref. System
CTYPE1 = 'RA---TAN'
CTYPE2 = 'DEC--TAN'
CUNIT1 = 'deg '
CUNIT2 = 'deg '
CRPIX1 = 1.02450000000000E+03 / Column Pixel Coordinate of Ref. Pixel
CRPIX2 = 7.44500000000000E+02 / Row Pixel Coordinate of Ref. Pixel
CRVAL1 = 1.94435716340000E+02 / RA at Reference Pixel
CRVAL2 = 2.57240785000000E+00 / DEC at Reference Pixel
CD1_1 = 8.30431860597186E-07 / RA degrees per column pixel
CD1_2 = 1.10001297896051E-04 / RA degrees per row pixel
CD2_1 = 1.09997822265625E-04 / DEC degrees per column pixel
CD2_2 = -7.8793010752684E-07 / DEC degrees per row pixel

Thank you very much!I use the WCSCTRAN task,and the result is same with rd2xy.I can't understand why it is a negtive number,and where's the object's position in the image.Because i want to trim the image with the object in the center.Best
yanqin

Yanqin,I think the numbers you're getting from the task are correct, even if they aren't what you expect. Your query is for the values in the RA/DEC keyword, however the WCS for image indicates that the image is rotated 90 degrees (i.e. North to right, East is up) where the RA range is 194.353004 to 194.518554 and the Dec runs 2.46040833 to 2.6844I'll append the output of a script I have that prints some information about the WCS in human-readable form. Because of the orientation the X value actually refers to the Dec in the image, so it is the RA that is off the plate. If your target is obviously in the image then the WCS may be wrong, or else are you sure your target position is also in ICRS??Cheers,
-Mike[quote:ee191677ac]
ecl> wcsinfo test.fits
CRPIX1 = 1024.5
CRPIX2 = 744.5
CTYPE1 = RA---TAN
CTYPE2 = DEC--TAN
CRVAL1 = 194.4357163400003 / X wcs = 12:57:44.572
CRVAL2 = 2.572407850000004 / Y wcs = 2:34:20.668
CD1_1 = 8.3043186059719E-7
CD1_2 = 1.1000129789605E-4
CD2_1 = 1.0999782226563E-4
CD2_2 = -7.8793010752684E-7
CDELT1 = 0.
CDELT2 = 0.
CROTA1 = 0.
CROTA2 = 90.
EPOCH = 2000.
EQUINOX = 2000.
PLTSCALE= 0.39600913810941 / arc/pix
PLT_ROT = 89.988924937053 / degrees
PLT_WID = 13.5171119 / Field width (arcmin)
PLT_HGT = 9.82762678 / Field height (arcmin)
PLT_LLX = 194.353004 / LL X wcs = 12:57:24.721
PLT_URX = 194.518554 / UR X wcs = 12:58:04.453
PLT_MX = 194.435662 / Mid X wcs = 12:57:44.559
PLT_LLY = 2.46040833 / LL Y wcs = 2:27:37.470
PLT_URY = 2.6844 / UR Y wcs = 2:41:03.840
PLT_MY = 2.57235278 / Mid Y wcs = 2:34:20.470
AXMAP = 1 2 / Axis mapping
[/quote:ee191677ac]

Hi,fitz!
Thank you very much.I have known where the problem is.I make a mistake for the position of my object.The DEC and RA are for the telescope boresight.So it is not in the image.
Thanks for your kind!I am sorry to reply later for my illness these days.
Best,
yanqin