Insert WCS/AutoAstrometry command (Research menu)

Figure 4 shows the Insert WCS/AutoAstrometry dialog box. If you took the image with CCDSoft version 5 with TheSky also present, the image coordinates are already stored in the image header, and will appear in the dialog in the Image equatorial coordinates box. You can also click Get Previously Entered Coordinates to recall the last set of coordinates you used. If you took the image with an earlier version of CCDSoft or another camera control program, you can manually enter the image coordinates using the format shown in figure 4.

Note: You can also use the FITS Header Editor (click View | File Information | Edit Header tab) or the FITS Header Modifier (click Research | Analyze Folder of Images | Pre-Analyze and then click the FITS Modifier tab) to add or change equatorial coordinates in an image or set of images.  

Figure 4: Adding astrometry and World Coordinate System (WCS) information to an image.

You must specify the image scale of the image in arcseconds per pixel. You can calculate the approximate image scale for your camera/telescope combination using the following formula:

( pixel size / pixel length ) / 206

where pixel_size is expressed in microns, and focal_length is expressed in millimeters. For example, to find the arcseconds per pixel for a 10-inch f/10 SCT using an f/5 focal reducer and an ST-8E CCD camera:

1.46

The focal length is calculated using 25.4 (the number of mm in an inch), 10 (the aperture in inches), and 5 (the focal ratio with the reducer in place). If the ST-8E is binned 2 x 2, the calculation then uses 18 microns as the pixel size, with the result:

2.92

You can find the pixel size of your CCD camera in microns in your camera documentation, at the manufacturer’s web site, or by contacting the manufacturer.

Note: The image scale you get from this calculation is only approximate, but it should be close enough to allow the AutoAstrometry routines to perform an Image Link with TheSky. You can get the exact image scale calculated from your images by clicking on the image in TheSky, and choosing Image Link Information from the Object list. See figure 5, where the image scale is shown as 3.53 arcseconds per pixel.

Figure 5: Viewing information about the linked image in TheSky.

If you have installed the USNO (U.S. Naval Observatory) star database, you can click the Use faint stars checkbox. When this is checked, the SExtractor tool is used to find objects in the image. You must have the USNO database available and turned on in TheSky to use this feature. To turn on the USNO database, from TheSky, click Data | File Locations to open the dialog shown in figure 6. Make sure that Plot USNO Stars is checked. If your field of view is under 10-15 arcminutes on a side, you may find it necessary to use the USNO database to get astrometric solutions, as the catalogs used in TheSky may not be sufficient for very small fields of view even though the catalogs contain millions of stars.

Figure 6: Turning on the USNO database in TheSky.

Note: If astrometry is successful, CCDSoft creates a .SRC file with the same name as the image file. The .SRC file allows for the storage of many more stars in the astrometric solution. The FITS format allows room for a limited number of stars, while the SRC file has room for hundreds of stars. Some of the advanced operations on the Research menu require the best possible astrometric precision, and such operations typically require the presence of .SRC files.

If AutoAstrometry is successful, you will see a list of the stars found in the image (see figure 7). For each star, the following information is shown:

• Whether the star is used in the astrometric solution. If the star’s position doesn’t match the catalog closely, it will not be used. The default value is 1.5 arcseconds.

• The star’s catalog ID. For USNO stars, the right ascension and declination server as identification.

• The error factor for the star catalog used, in arcseconds.

• The star’s coordinates.

• The X and Y coordinate of the star in the image, in pixels.

• The residual error for the star, in arcseconds. Residual error is the amount by which the position of the star in the image varies from the catalog position.

Figure 7: A sample astrometric solution.

The numbers at the top of the Astrometry dialog (figure 7) display information about the astrometric solution. The calculated image center is shown at top left. It may vary from the image center originally stored in the image header if the mount was not perfectly polar aligned and positioned at the time the image was taken. Also shown are:

Image scale in arcseconds per pixel

Position angle (the amount in degrees by which the vertical axis of the image varies from celestial north)

Overall RMS (root mean square) error of the solution. Values under 0.50 indicate a very good solution.

RMS for the X- and Y-axis. Values under 0.35 indicate a very good solution.

The number of stars used in the solution. The leftmost column Used in Solution, tells you which specific stars were used in the solution. By default, stars with a residual error of 1.5 arcseconds or less are used in the solution.

If the RMS error of the solution is too high for your needs, you can click Setup to set a different residual filter. The solution will be recomputed, and stars that have too large of a residual error will be dropped from the new solution. By default, stars with a residual error of 1.5 or less are included in the solution.

Figure 8 shows the Astrometry Setup dialog. Specify the maximum residual error to allow, and then click OK. When 0.7 is used instead of 1.5 for the sample image, the RMS drops to a 0.36 in both axes, and 0.25 in the X- and Y-axis. The number of stars used in the solution drops to 89, which is still more than enough stars to provide a useful solution. The minimum number of stars in a useful solution is typically about 10-15. If you have too few stars, it will usually drive up your RMS error to unacceptable levels. If you have checked Use faint stars, the USNO stars in the solution have a higher level of accuracy and you may get by with fewer stars than when you use stars from just the GSC catalog.

The USNO catalog has an error factor of 0.1 arcseconds, while the GSC catalog has error factors in the range of 0.2 to 0.4 arcseconds. The larger error factor of the GSC catalog leads to larger residual error on individual stars, and a higher RMS error. As long as your solution provides 0.50 arcseconds of RMS error or better, your results should be acceptable to the Minor Planet Center and other institutions.

Figure 8: Adjusting the residual filter.

You can also manually add or remove stars to the astrometric solution. In star-rich areas of the sky, you may be able to eliminate some stars from the solution to obtain a better RMS error. Click on the Use in Solution box for the star you want to change. Then type “Y” or “N” depending on whether you want to use the star in the solution. Click on any other star to save the change. You can then review the statistics at the top of the Astrometry dialog to see if the change has given you the results you want. For example, star number 3 in figure 7 has a residual error of 0.94. You could remove this star from the solution, which would change the RMS error to 0.51 instead of 0.52. You might also remove a star if you notice that it is over-saturated, bloomed, or otherwise problematic. To see which star corresponds to a line the astrometry solution, click on the line to highlight. A yellow circle appears around the star associated with that line.

The buttons across the bottom left of the Astrometry dialog allow you to open a previously saved solution; save the current solution to a file; and to print the solution. The buttons at bottom right allow you to access the Astrometry Setup dialog (figure 8), and to close the Astrometry dialog.

Note: The stars used in the astrometric solution are added to the image header automatically, primarily as reference data for magnitude and star catalog information. The astrometric solution is not dependent on the objects stored in the FITS header, but rather as WCS (World Coordinate System) parameters. You do not need to take any action to save the solution, unless you want to save the solution in text format to disk for later use. The changes are not saved to disk unless you explicitly save them by clicking the File | Save or File | Save As to save the image. If you try to close the file without saving, CCDSoft will ask if you wish to save the changes.

If CCDSoft cannot find an astrometric solution, you will usually receive this error: “Image link failed, no pattern matching found. Check image coordinates and image scale. Error code = 653.”

As the error suggestions, verify that the image coordinates (right ascension and declination) are correct, and that the arcseconds per pixel value is within +/-0.25 arcseconds of the actual value. You can test coordinates and image scale by manually pasting the image into TheSky and perform an image link using the Link Wizard menu option. The Link Wizard can tolerate a large error in right ascension and declination, and can work without an image scale. When the link is successful, you can obtain right ascension, declination, and image scale from the Object Information window.