PinPoint Astrometry

The PinPoint Astrometric Engine from DC-3 Dreams (http://pinpoint.dc3.com) is a tool for precise position measurement or astrometry. When manually or automatically controlling a telescope, it can also be used as an incredibly accurate ”digital finderscope.” PinPoint works by matching stars from a catalog with those in the image.

Availability of this feature depends on Product Level.

MaxIm DL includes PinPoint LE (light edition). The full version of PinPoint, available from DC-3 Dreams, adds a number of capabilities, including additional information on detected and catalog stars, asteroid and supernova detection, generating output in MPC format, and more.

PinPoint requires an accurate star catalog, such as the Guide Star Catalog. For information on downloading catalogs, please see http://gsc.dc3.com/

 

Although PinPoint is fully automatic, there are various setup parameters that are required for it to work. Once these are set up, they should suffice for any image taken with the same equipment.

The Detection Setup is used to recognize stars and differentiate them from non-stars, and to limit the number of image stars that will be used for solving. Any object less than Brightness will be ignored. The Standard Deviation multiplier is multiplied by the standard deviation of the background noise, and any object less than the resulting value is ignored. If there are more than the Maximum Number of Stars Used to Solve, only the brightest down to this number are taken (This control is disabled if you are using an older version of PinPoint which does not support the Plate.MaxSolveStars property.). There are also advanced parameters that are accessed using the Advanced… button. These parameters are described below.

The Image Parameters give PinPoint a starting point for its analysis. It requires an approximate image scale and position on the sky. PinPoint needs to be given a starting point on the sky. The Right Ascension (RA) and Declination (Dec) can be entered manually, but are more often obtained from the FITS header OBJCTRA and OBJCTDEC keys.  MaxIm DL inserts these keys automatically when an image is acquired if the Observatory Control window is linked to a telescope.

The Pixel Scale (arcsec/pixel) gives the scale of the image in each axis. If FITS scale is checked, these will be extracted from the FITS header keys CDELT1 and CDELT2 if they are available, or in their absence, calculated from the camera pixel size XPIXSZ and YPIXSZ and the telescope focal length FOCALLEN. Most cameras supported by MaxIm DL provide the pixel dimensions automatically, and the instrument focal length can be configured on the Site and Optics tab prior to imaging.   If an image has no pixel scale information available in its FITS header, you can specify it manually using the Pixel Scale Editor, described below.

Finally, we have the Catalog Setup. You can select the catalog to use from the list. The choice of catalog depends on which catalogs are available and the scale of the image. One common choice is GSC-1.1 (corrected), which uses the Guide Star Catalog with adjustments to improve its accuracy. You need to select the location of the catalog using the Path… button.

To make sure the analysis is performed quickly, you can instruct PinPoint to ignore catalog stars outside of a particular Magnitude Range. Enter appropriate minimum and maximum magnitudes into the two fields labelled Use stars from magnitude.  Stars outside this range will be ignored. This is useful if the catalog goes much deeper than the image you are analyzing. The Search Area is usually set to include more area than the image itself. This allows for additional stars to be brought in from the catalog, which is important if the image is offset from the expected position.

If you think the image might be substantially off-target, turn on the Search outwards spiral check box. Then, if a solution fails because the image stars cannot be matched against those in the catalog at that location, MaxIm DL will try to solve it using a sequence central image positions that spirals outward from the original, assumed position.

Once everything is set up, click the Process button to start the analysis. The progress will be shown as the search proceeds. If the analysis is successful, this will be indicated, and you can close the dialog box. Otherwise you may wish to adjust some settings and try again. You can abort an analysis with the Stop button.

Once you have analyzed an image, World Coordinate System (WCS) information is added to the FITS header. You can now inspect the image using the Information window’s Astrometric mode. For users with the full retail PinPoint version, additional catalog and image star information is available in the Information window display. This extra information is retained only as long as the image is open.

The results of the analysis are also shown in the window. You will see the average residual, which gives an indication of how accurately the star positions have been measured. The computed image center RA and Dec are also shown, as well as the computed position angle (measured counter-clockwise from North), the calculated focal length of the telescope (if the pixel size in microns is available), and the image scale in arc-seconds per pixel (based on X axis).

When using the Observatory Control window, the solved image can also be used to synchronize the telescope position. A typical use is to resynchronize the telescope after a long slew; if the polar alignment is not sufficiently good the image may be completely off-target. To fix this quickly and easily, run an astrometric analysis with the Search outward spiral option enabled. When the analysis is complete, click the Sync button on the Observatory Control window Telescope tab, select the Solved Position and click OK. Now click the Go To button again to position the telescope precisely on the intended coordinates.

For information on individual dialog controls, close this help window and use the button in the dialog’s title bar.

Advanced PinPoint Parameters

PinPoint has a number of advanced parameters which do not normally require adjustment by the user except in special cases. Access to these is through the  Advanced PinPoint Parameters dialog. Each numerical parameter has an associated Auto checkbox; when set, PinPoint determines the optimum value for this parameter itself. This allows you to override only one or two parameters without having to worry about values for all of them.

Projection Type specifies the mathematical model used to map the Cartesian coordinates of image pixel locations to the angles inherent in spherical coordinates used to describe object positions in the sky. This is usually set to Gnomonic (tangent plane)(TAN). Please see the PinPoint documentation for more information on projections.

The Maximum Residual (arc sec) is checked against the actual residual in the image, which is a measurement of how accurate the analysis results are. The residual is calculated by measuring how far off predicted catalog star positions are from their actual positions in the image. If the maximum residual limit is enabled, and the residual exceeds the entered value, the astrometric reduction fails.

Minimum Number of Stars to Identify sets a lower limit on how many stars must be used for the reduction. If too few stars are used, the analysis may not be accurate. This ensures that the astrometric reduction will fail if the results are inaccurate due to too few matched stars.

The Fit order specifies the minimum order of the fitting polynomial used.

The Border Width (pixels) is used to discard edge artifacts, including objects cut off by the edge of the image.

Any object less than Size (pixels) is ignored.

Please see the PinPoint documentation for more information on any of these parameters.

Pixel Scale Editor

If the pixel scale is already in the FITS header, simply select Pixel Scale and click Load from FITS. Alternatively, you can enter values manually in the X and Y fields in this mode. The scale is in arc-seconds per pixel.

Another method is to use the Calculator mode. Enter the Pixel Size (microns) and Focal Length (mm), or click the Load from FITS button within the Calculator frame to extract the values directly from the FITS header. Most cameras supported by the Camera Control window provide pixel dimensions automatically when an image is taken. The Focal length (mm) field will contain the telescope focal length if it was specified on the Site and Optics tab of the Settings dialog. This required value can often be found in the manual for your telescope, or it can be calculated by multiplying the aperture by the f/ratio. Click Calculate scale to compute and display the pixel scale. Then click OK to close the window and return the values to the Information Window.

 

PinPoint Troubleshooting

PinPoint should solve quickly and very accurately with default settings and appropriate choice of catalog.  If you have problems solving then here are a few simple things to check: