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Plate Solving Notes

Plate solving is a technique that measures precisely where the telescope is pointing by taking a CCD image and then using various pattern matching techniques, matches the stars in the image to a given star catalog.  Knowing approximately where the telescope is pointing and the system image scale, plate solving algorithms can calculate the center of the image to sub-arc-second accuracy.  Successful plate solving is essential for automated imaging.

In order to be successful, three things are required: telescope coordinates, known image scale and sufficient catalog stars in the image.
  • Telescope coordinates: CCDAutoPilot gets this information from the mount via the chosen telescope control program.  Generally the coordinates must be within one or two FOV's and is easy to do.  Problems with plate solving are generally not related to telescope coordinates.  The telescope coordinates, RA (OBJCTRA) and Dec. (OBJCTDEC), are written into the image's FITS header for any images acquired by CCDAutoPilot. (see note below.)
  • Image scale:  CCDAutoPilot gets this information generally from the FITS header.  Image scale is calculated from the pixel size (XPIXSZ), binning (XBINNING) and imager focal length (FOCALLEN).  These keyword values are written into the image's FITS header for any images acquired by CCDAutoPilot.  A successful initialization will cause these values to be properly written, based on the Imager Settings information.  See note below.
  • Star Catalog: When using TheSky6, catalog selection is automatic.  In the recommended default setup, GSC, Tycho and UCAC are used.  TheSky6 starts with the GSC and proceeds to more dense catalogs as needed.  When using PinPoint, the chosen star catalog must be defined by the user and only one at a time is available.  Fortunately for most uses, the GSC is sufficient.  Very narrow FOV's may need to use USNO A2.0.  This has more faint stars but may fail on wider FOV's.

    Note: If you are using image data that was not acquired by CCDAutoPilot, as for example using the "From FIT" button on the targets page, these parameters must be correct in the fits header.  If CCDAutoPilot is not initialized, then the FITS header information may not be correct.  Camera control programs typically provide for entering most of the data based on the camera driver.  However, the user must enter the correct focal length for the imaging system in the appropriate setup section.  Failure to do this will cause plate solves to fail.

    Assuming all of the above, then the next issue is the plate solve exposure parameters.  The goal is here is to get a sufficient number of stars in the image.  For most users, a binning of 2x2 or 3x3 is more than sufficient and will both speed plate solving time and increase sensitivity.  Of course, use the most transmissive filter, usually the clear or luminance filter.  Sub-framing is appropriate for wide FOV's both to speed plate solving, and in the case of CCDSoft/TheSky plate solving, to insure success.  CCDSoft/TheSky has a maximum FOV limit of 1 square degree.  Set a suitable sub-frame to get below this limit.  While PinPoint doesn't have such a limit, sub-framing may make it a bit speedier.  Lastly, the exposure must allow sufficient stars for reliable plate solving.   


    Determining plate solve exposure parameters

    A good starting point is 5 sec., binned 3x3, through a clear filter. Take some images manually first and use your desired plate solve program.  For CCDSoft, use Tools/Insert WCS AutoAstrometry.  You will need to manually enter your image scale.  For PinPoint with Maxim, use Analyze/PinPoint Astrometry.  For PinPoint with CCDSoft, use Visual PinPoint to solve saved CCDSoft exposures.

    Experiment with different exposures and note the number of stars used in the solution.  You will need at least 10 and more is always better.  Adjust the exposure to achieve a good number of stars.  Move to a star-poor region of the sky and repeat the test a couple of times to insure good results.  Adjust exposure times as needed.  If your FOV is small and you are using PinPoint, you may need to use the USNO A2.0 catalog.

    During a CCDAutoPilot session, the number of stars used in the solution is shown in the log for every plate solve.  Examine your log to make sure you have a sufficient number of stars for reliable plate solving and adjust your plate solve parameters appropriately.