Aperture Radius: The radius of the synthetic aperture around the brightest pixel (the approximate center of each detection). All pixels within the aperture are used by the software to determine the centroid and flux of the object. Choose a size large enough that the peak of the PSF of the stars will be within the aperture, but small enough to resolve close pairs of stars. Generally, an aperture radius of two to three times the FWHM of the PSF is a good value. As a start, choose the radius by visually fitting the colored circles that appear after the data reduction.
Detection Limit: The minimum signal to noise ratio (SNR) required for the central pixel. The software will try to fit a stellar profile any time it finds a pixel with a SNR above the limit set here. A value of 4.0 to 5.0 is recommended. For noisy images, or images without proper calibration, you may want to increase the value to avoid spurious detections. You can set the value as low as 3.0 if you want to detect very faint objects if you use clean, well calibrated images.
Minimum FWHM: The minimum FWHM for real objects. And detections with a FWHM lower than the value specified here will be rejected. This will prevent the software from accepting hot pixels, random background noise peaks or cosmic ray strikes as real objects. A value of 1.0 will effectively reject hot pixels and most cosmic ray strikes from being detected. Depending on the scale of your images and the FWHM of real objects in your images, you may want to raise that limit to exclude further noise sources, such as clusters of hot pixels or bumps in the background. For example, if real objects in your images typically have a FWHM of 4", and the scale of your CCD images 1.5"/pixel, you may want to set the minimum FWHM to 2.0 pixels (corresponding to 3"). The maximum FWHM allowed, by the way, is assumed by the software to be the diameter of the aperture, as specified by the aperture radius described above.
PSF-Fit RMS: The maximum RMS for the difference between the model PSF and the actually fitted profile. In other words, this value sets a limit on how much a real detection may differ from the assumed PSF. A value of 0.25 is usually a good starting point. Brighter stars will generally have a much lower RMS, but fainter objects will show larger values.
Search Radius: The search radius, in pixels, used by the software when it tries to match detections on the individual images. Sources for which the position does not change by more than the value specified here will be considered as stars by the software, whereas minor planets or comets must move by more than this value from one image to the next to be recognized as moving objects by the moving object detection routine. The positions determined for brighter objects will generally not vary by more than a small fraction of a pixel from one image to the next, but you may want to set a larger value to accept more variation for fainter sources. In most cases, a value corresponding to about 1"-3" will be useful.
Tip:After you have successfully completed data reduction on several images, you can click on various (bright and faint) sources in the image. The software will display the SNR, FWHM and RMS values found. You may want to use this information to fine-tune the settings specified here.
Note: If you select a quadratic or cubic fit, the software will automatically switch to a linear fit if less than 12 reference stars are available. If you select a cubic fit, the software will switch to a quadratic fit if less than 20 reference starshave been included in the solution.
Astrometric Limit: The maximum residual in the place of a reference star, specified in arc seconds. Any reference star that if found to show larger residual in either Right Ascension or Declination will be rejected from the astrometric solution.
Photometric Limit: The maximum residual in the magnitude of a reference star. Any reference star that if found to show larger residual in the magiutude will be rejected from the photometric solution.
Note: The UCAC 2 covers the sky from the south celestial pole up to a declination of about 45°. If the software notes that no UCAC 2 reference stars are available because the image to process is from a region not currently covered by the UCAC, it will ask the user to access the USNO-B1.0 instead.
Upper Limit: The upper limit for the magnitude of reference stars. If the field is rich in bright stars, the reference star match can fail because the brightest stars from the catalog (which are used to match the catalog to the image) are saturated in the image and are therefore not detected by the software. To avoid this problem, you can set the upper magnitude limit to approximate the magnitude where the stars in the CCD frame are saturated.
Lower Limit: The lower limit for the magnitude of reference stars. When the field is rich in stars, you might want to exclude the faint (potentially less precise) reference stars by specifying a lower limit. (But note that the settings in the 'Residuals' group, described above, will automatically reject reference stars with large residuals.) You can also avoid reading large numbers of faint reference stars from the star catalogue which are not detected in your images by specifying the approximate limiting magnitude of your images here.
Note: The lower magnitude limit will also be set as a constraint for the USNO-B1.0 online query. It is recommended to set a reasonable magnitude limit (for example, 18mag or so) to limit the size of the data to download. Also note that the USNO-B1.0 query is limited to 9999 stars by default, and if a faint magnitude limit is set, the query will possibly abort due to that limitation, resulting in reference star data covering only part of the field. (Note that data for 9999 stars correspond to a data volume of about 1.7 MB.)
Number of Stars: The number of stars from the image and from the catalogue, respectively, that will be used for the pattern matching. The software will use the brightest stars in the image and in the star catalogue for the matching routine. When using a star catalogue that is complete down to a certain limiting magnitude (i.e., the USNO-A2.0 or the UCAC ), a small number of stars (10 to 30) is usually sufficient. When using the USNO-SA2.0, which is only a subset of mainly faint stars, a higher number of stars is required, as many of the brighter stars in the image will be missing in the catalogue. As a crude rule of thumb, the number of stars used for the match usually is between 10% and 50% of the stars detected in the image. As a starting value, you may try to set this to 50. Note, however, that the matching algorithm is of quadratic complexity, which means that it will take about four times longer when the number of stars used for the match is doubled. If you can set the number of stars to 0, the software will not attempt to match the reference stars automatically, but rather asks the operator to match the stars manually.
Search Radius: The search radius, in pixels, used by the software when it tries to find the stars listed in the reference star catalogue in the image(s). If a star is located within the search radius from the predicted position, the software assumes that this star is identical with the star listed in the catalogue. It is recommended that you use a search radius is set to a value corresponding to a few arc seconds. If larger values are used, the software will more frequently find stars within the search radius which are not identical to the reference stars it is searching for, causing the reference star match to fail completely in some cases.
Number of Stars: The number of stars from each image that will be used for the pattern matching. The software will use the brightest stars in the images for the alignment routine. As the inidividual images are probably very similar, the number of stars required to align the images is usually small, and a number of 10 or 30 is probably sufficient.
Alignment Area: The size of the area, in pixels, used by the software to align the images for blinking and for finding fixes objects (stars) during data reduction. If you have very large images, you can speed up blinking and data processing by specifying a smaller area. For example, wehn using 1024 x 1024 images, you may want set this value to 512, and the software will only scan the central 512 x 512 pixles (i.e., one fourth of the whole image) to align the images.
Include Magnitude: When this option is checked, the MPCReport file will include magnitudes. If this is not checked, no magnitudes will be listed.
Include extra Digit: When this option is checked, the software will write an extra for the coordinates (i.e., the Right Ascension will be given to 0.001 seconds, and the Declination will be given to 0.01") and the magnitude (0.01mag) to the MPC Report file. Please note that the absolute positions or magnitudes derived by the software would, in almost any case, not justify to include these extra digits! However, if you are interested in relative positions (for example, during last minute astrometry for stellar occultations) or magnitudes, this information might be of some use.
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