A major design goal of PinPoint was to minimize the number of adjustments needed. Compared to other packages, PinPoint has the fewest adjustments, yet it it very reliable. Unlike other packages, PinPoint does not require precise input of image rotation or plate scale. Its solver is independent of rotation, and the image scales are only used to estimate the field in which catalog stars are retrieved. Wide variations in background noise and shading are handled by PinPoint's zonal background estimator and synthetic aperture locator.
First, here are some facts that you need to know:
If you still have problems, look at the plate solve logs. Ask yourself:
There are a few things that you might want to tweak in order to get the most out of PinPoint. Plate solving, the most common task, requires that the object detector find most of the stars on the image. It also requires "enough" catalog stars in the image field for a reliable match and solution. This document concentrates on tuning for plate solving, but the principles may be applied to other tasks.
Leave the advanced settings alone except for special applications. Contact DC-3 Dreams for assistance before playing with those settings. Virtually no one needs to play with them for plate solving. They may be removed from Visual PinPoint's user interface in a future version.
The most important factor in success of plate solving is the selection of the reference catalog. PinPoint requires a certain number of stars to match, depending on the number of image and catalog stars available. The absolute minimum is 7 stars. As the number of stars detected in the image increases, and/or the number of catalog stars in the field increases, that minimum number is dynamically adjusted upward to 200. If there are many catalog stars in the area, yet few image stars, the chances of a false match are increased. Thus, you need to expose down to the faintest stars in the catalog.
The Guide Star Catalog 1.1 (GSC) (with PinPoint's internal "Gray" corrections) is the most commonly used, and is suitable for most images. The GSC is supplied with PinPoint in the CD-ROM. It contains stars down to about 17th magnitude, so any fainter stars in the image cannot be used for matching and solution. The density of stars over the celestial sphere is modest. The primary factor that may prevent plate solutions with the GSC is a narrow field of view. If your images cover less than 10 arc minutes on the shortest side, the GSC doesn't contain a high enough star density for reliable solving.
If you do have a narrow-field imager, or if you want the ultimate in reliability, then your best bet for plate solving for pointing applications only is to use the USNO A2.0 catalog. This catalog is large (6 gigabytes), but today's disk drives can easily hold its contents. Acquiring a copy of this catalog is a significant downloading project, even if you have broadand connection. But it's worth the work. See the support knowledge base article Downloading and Using Reference Catalogs. Copy the readxxx.xxx files and the zonexxxx.acc and zonexxxx.cat files in this directory to a directory on your system. All A2.0 files must be accessible, there is no CD-shuffling logic in PinPoint. Set your catalog type to USNO A2.0. Set your catalog path to the directory into which you copied the tree from USNO.
For precision astrometry and some light photometry, the USNO UCAC4 catalog (8 gigabytes) is the one to use. It does not have stars below mag 16, so it will not be as sueful for demanding narrow field pointing applications. It is, however, the recommended catalog for astrometric reductions. See the support knowledge base article Downloading and Using Reference Catalogs.
If you have an internet connection and are willing to accept longer plate-solving times, you can use the USNO NOMAD catalog via internet access. This is an outstanding catalog for plate solving. On a typical broadband connection, a moderately crowded half-degree field takes about 10-20 seconds to download. The great thing about this catalog is that you can use it immediately with no downloading of the catalog (if you have an internet connection). On the flip side, it slews your plate solves.
PinPoint supports other catalogs (Tycho-2, UCAC4, B1.0 on local hard drive) but these are for special purposes such as precise astrometry.
The primary control over detection sensitivity is the Sigma value. Typically this will be set to 2.0, and this should serve for the vast majority of images. If your images are calibrated, you may be able to decrease sigma to 2.5 or even 2.0. As you decrease sigma, more stars will be detected, but at the lower values, some false star detections will also be produced. To a point these are harmless, but if there are "too many" false detections, plate solving will be slowed.
Using the Catalog Maximum Magnitude setting, you can exclude faint stars from catalog lookups. This can be useful if your images are not nearly as deep as the catalog. Suppose you have a wide-field camera and images that only go down to 14th magnitude. Using the GSC, you'll get catalog stars down to 16th magnude. All of the catalog stars below 14th magnitude are useless, and serve only to slow down plate solving. Too many useless catalog stars will also cause PinPoint to raise it's minimum number of matched stars too high for the number of image stars. By lowering catalog maximum magnitude to the limiting magnitude of your images, you provide the best balance of image and catalog stars to the solver.
By default, PinPoint retrieves catalog stars to cover the image area plus a 30% expansion along each border of the image. Thus, with 30% expansion, the area over which the catalog stars are retrieved is (1 + 0.3 + 0.3) squared or 2.56 times the image area. As the catalog expansion is increased, the number of stars retrieved from the catalog increases exponentially.
The effect of increasing the number of catalog stars is to slow the plate solution process. Thus, you want to reduce the catalog expansion as much as you can. On the other hand, if the approximate centerpoint coordinates of your images contain significant errors (e.g., from poor telescope pointing), decreasing the catalog expansion will prevent PinPoint from solving reliably. In the extreme, if none of the retrieved catalog stars overlap the image field, plate solution is impossible. If too few catalog stars overlap the image field, solving reliability will be compromised. So you need to adjust the catalog expansion depending on how well your telescope points.
If your telescope doesn't point well, then consider using All-Sky solving (a checkbox option in Visual PinPoint). For significant pointing errors it will usually be much faster than a local solve with a large catalog expansion! The maximum expansion is 80%. FOr desperation situations (like no internet connection for All-Sky) If your images contain pointing errors too large for this expansion, Visual PinPoint's solve Plates feature has a "spiral search" feature where the catalog stars are retrieved in multiple areas and a plate solve is attepmted for each. This when spiral searching, the expansion is typically set to 50%, and the areas overlap. This technique increases the coverage for misalignment beyond the internal limit of the PinPoint engine. This can be incredibly slow if you have large pointing errors. Please consider working it out so you can use All-Sky solving.
PinPoint provides three centroiding methods (for object detection and measurement) Unless you have a very good reason to do otherwise, use the Gaussian PSF method (the default in Visual PinPoint starting with V6). The others (synthetic aperture and thresholded box) will yield similar results but with lower performance and robustness in most cases. If you are familiar with SExtractor, you can use that instead of PinPoint's internal detector. It is, however, slower and not quite as sensitive. It will not make much difference in plate solving accuracy or reliability. It is included for special applications only.
Unless your images are severely undersampled (1-pixel stars), you can usually benefit from applying PinPoint's hot pixel removal filter before the Gaussian Filter (see below) and before scanning for stars in the image. It works well with the Gaussian PSF detector to maximize the number of usable matchable stars in the image. This is a checkbox option (default on) in Visual PinPoint starting with V6.