Settings - Program

Object Detection

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 signal from 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 (point spread function) 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, but be aware that you may end up fitting cosmics or chip defects rather than your object.

Minimum FWHM The minimum FWHM for real objects. And detections with a FWHMlower 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 error (RMS) for the difference between the model fit to the 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 model profile. 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.

Plate Constants

Residuals

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.

Star Catalog

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. For the Hyades prject you will need to alter this parameter to exclude your objects.

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.

Reference Star Matching

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 UCAC4), a small number of stars (10 to 30) is usually sufficient. 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 user 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.

Image Alignment

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.

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jrl

2013-Dec-22 23:31:18 UTC