Author: Glenn Gombert Date: 970110 Revision: #0 970110 Key Words: FITS, astrometry
In TASS Technical Note #22 the subject of adding World Coordinate System parameters was addressed and an example was shown of the WCS.exe program was used to generate WCS FITS Header parameters. In this technical note the focus will be on demonstrating how to use the WCS parameters in the header file of a FITS format image to supply the Astrometric solution to TASS star lists that are generated by several different methods that are used in the analysis of TASS image data.
To summarize from Technical Note #22 the following items need to be present in the header of a TASS image before the WCS library of programs can be used to compute an astromertic solution:
In the example below an image of the three stars in the belt of ORION that is a well known field and should be easy to verify the astrometric solution to the star list. The WCS parameters were added to this image using the ATFTOOL Kit's WCS.exe program (see previous TASS technical note #22). When this program is run it produced the following output:
31t0433.fts: CTYPE1 = RA---TAN CRVAL1 = 83.8282 CDELT1 = -0.003832 CRPIX1 = 400 CROTA1 = 0 CTYPE2 = DEC--TAN CRVAL2 = -0.461914 CDELT2 = -0.003832 CRPIX2 = 448 CROTA2 = -3.07828 RA= 5:35:24.0 DEC= 0:06:00 W=800 H=896 ArcSecs/Pixel=13.7952 found 10370 GSC stars found 746 image stars using only 25 brightest GSC stars using only the 25 brightest image stars 10 bin hits Delta RA: 0:00:05 = 5.67701 pixels Delta Dec: 0:33:43 = 146.637 pixels Rotation: 3.07828 degrees CTYPE1 = RA---TAN CRVAL1 = 83.8282 CDELT1 = -0.003832 CRPIX1 = 400 CROTA1 = 0 CTYPE2 = DEC--TAN CRVAL2 = -0.461914 CDELT2 = -0.003832 CRPIX2 = 448 CROTA2 = -3.07828 31t0433.fts: rewritten successfully.
After these values were added to the FITS header a "dump" of the data in the FITS header produced the following listing:
SIMPLE = T BITPIX = 16 NAXIS = 2 NAXIS1 = 800 NAXIS2 = 896 TYPE1 = 'RA---TAN' CRPIX1 = 400 CRVAL1 = 83.82824565 CROTA1 = 0 CTYPE2 = 'DEC--TAN' CRPIX2 = 448 CRVAL2 = -0.4619144279 CROTA2 = -3.078280257 BSCALE = 1.0 BZERO = 32768.0 DATE-OBS= '14-12-1996' TIME = '17:35:12' TELESCOP= 'TASS Mark III #0' INSTRUME= 'CCD1' LATITUDE= 39.4800 LONGITUD= 83.5200 EXPOSURE= 469.0000 SHIFTIME= 0.9160 XFACTOR = 13.8 YFACTOR = 13.8 CDELT1 = -0.003832 CDELT2 = -0.003832 EPOCH = 2000 EQUINOX = 2000 RA = '+05.59.00' DEC = '+0.10.00' END
The above indicates that the WCS parameters have been successfully added to the header of 31t0433.fts image and at this point a star list can be generated by any of the several programs that can be downloaded from the TASS Home Page. One nice feature of Sextractor is that a star list of just X & Y locations can easy be generated and saved away to a file this is important for the next step which is to convert the X & Y locations into RA & DEC coordinates. The first 50 stars of the star list X & Y positions from Sextractor for the above image look like the following:
393.31 1.70 587.82 2.55 477.73 3.04 699.80 3.83 17.51 3.09 745.78 4.85 81.84 8.57 697.09 18.49 148.74 25.59 750.64 25.53 611.90 26.84 771.35 27.61 579.64 28.03 348.14 30.81 624.58 31.04 383.80 31.83 430.34 31.05 505.37 31.48 85.52 32.66 441.71 37.47 746.99 38.38 113.21 39.76 280.00 40.84 727.89 41.14 289.40 42.98 82.61 43.84 234.65 43.88 337.98 40.71 446.45 43.56 226.11 43.30 378.88 50.16 259.44 50.03 387.63 51.22 231.41 51.61 498.53 51.92 283.24 53.59 323.12 55.07 339.36 54.78 450.62 56.10 175.21 57.10 399.20 56.09
These points were saved to a file and will be used in the next step to calculate the RA & DEC for each X & Y pair.
After the X & Y locations have been saved away in a file the final step in calibration is to compute the RA & DEC for each star found in the image. To accomplish this the WCSTools program xy2sky.exe is used to perform this operation. The above X & Y locations have to be saved away in a file and then run through "xy2sky.exe" . The command is: xy2sky 31t0433.fts @starlist (where star list is a file containing the X & Y positions).
The first 50 lines of output from "x22sky" is shown below:
393.310 1.700 -> 5:35:46.958 +01:14:38.24 J2000 587.820 2.550 -> 5:32:48.331 +01:16:50.24 J2000 477.730 3.040 -> 5:34:29.381 +01:15:22.26 J2000 699.800 3.830 -> 5:31:05.476 +01:17:54.99 J2000 17.510 3.090 -> 5:41:31.848 +01:09:39.42 J2000 745.780 4.850 -> 5:30:23.226 +01:18:14.69 J2000 81.840 8.570 -> 5:40:32.549 +01:09:12.07 J2000 697.090 18.490 -> 5:31:07.238 +01:14:31.26 J2000 148.740 25.590 -> 5:39:30.311 +01:06:07.73 J2000 750.640 25.530 -> 5:30:17.742 +01:13:33.70 J2000 611.900 26.840 -> 5:32:25.024 +01:11:33.69 J2000 771.350 27.610 -> 5:29:58.634 +01:13:20.26 J2000 579.640 28.030 -> 5:32:54.581 +01:10:53.54 J2000 348.140 30.810 -> 5:36:26.995 +01:07:24.10 J2000 624.580 31.040 -> 5:32:13.176 +01:10:45.22 J2000 383.800 31.830 -> 5:35:54.203 +01:07:36.49 J2000 430.340 31.050 -> 5:35:11.509 +01:08:21.69 J2000 505.370 31.480 -> 5:34:02.598 +01:09:11.25 J2000 85.520 32.660 -> 5:40:27.987 +01:03:43.31 J2000 441.710 37.470 -> 5:35:00.753 +01:07:01.74 J2000 746.990 38.380 -> 5:30:20.456 +01:10:34.19 J2000 113.210 39.760 -> 5:40:02.224 +01:02:26.24 J2000 280.000 40.840 -> 5:37:29.062 +01:04:15.47 J2000 727.890 41.140 -> 5:30:37.848 +01:09:42.19 J2000 289.400 42.980 -> 5:37:20.326 +01:03:52.99 J2000 82.610 43.840 -> 5:40:30.107 +01:01:07.28 J2000 234.650 43.880 -> 5:38:10.547 +01:02:59.93 J2000 337.980 40.710 -> 5:36:35.835 +01:05:00.29 J2000 446.450 43.560 -> 5:34:56.100 +01:05:41.41 J2000 226.110 43.300 -> 5:38:18.416 +01:03:01.56 J2000 378.880 50.160 -> 5:35:57.816 +01:03:20.53 J2000 259.440 50.030 -> 5:37:47.485 +01:01:53.70 J2000 387.630 51.220 -> 5:35:49.729 +01:03:12.42 J2000 231.410 51.610 -> 5:38:13.141 +01:01:11.12 J2000 498.530 51.920 -> 5:34:07.869 +01:04:24.84 J2000 283.240 53.590 -> 5:37:25.459 +01:01:22.37 J2000 323.120 55.070 -> 5:36:48.770 +01:01:31.60 J2000 339.360 54.780 -> 5:36:33.874 +01:01:47.64 J2000 450.620 56.100 -> 5:34:51.652 +01:02:51.88 J2000 175.210 57.100 -> 5:39:04.462 +00:59:13.75 J2000 399.200 56.090 -> 5:35:38.865 +01:02:13.96 J2000 121.640 57.450 -> 5:39:53.617 +00:58:29.05 J2000 559.010 51.870 -> 5:33:12.342 +01:05:10.19 J2000 597.010 57.610 -> 5:32:37.172 +01:04:19.21 J2000 41.660 51.920 -> 5:41:07.289 +00:58:45.54 J2000 41.340 58.430 -> 5:41:07.262 +00:57:15.71 J2000 81.560 62.070 -> 5:40:30.174 +00:56:55.60 J2000 664.030 62.090 -> 5:31:35.426 +01:04:06.91 J2000 706.700 58.860 -> 5:30:56.419 +01:05:22.75 J2000 421.680 61.620 -> 5:35:17.951 +01:01:14.48 J2000
As can be seen from the above the X & Y star positions generated by Sextractor has been transformed into RA & DEC coordinates. This has been made possible by adding the WCS parameters to the FITS header of the 31t0433.fts image.
As we have shown from the example above WCS FITS Header parameters can be used to solve for RA and DEC positions from X & Y locations in TASS star lists. This is a relatively "crude" example where data from several different programs had to be "cut-and-pasted" from different program outputs.I am now working on integrating several of these programs with a GUI interface to make astrometric calibration of TASS star lists much easier to achieve once the appropriate WCS parameters have been added to the FITS image headers.