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Photometrica was developed by Geir Klingenberg, Norway. It is now owned and operated by the AAVSO in collaboration with Geir Klingenberg. Contact Information and SupportFor general questions please send an mail to geir with extension @photometrica.org.For technical support, please go to the forum, join the group and post your question. We will try to answer you within one business day. Many practical questions can be answered by reviewing the video tutorials, and by referring to the "help" links available on many of the screens within the program. Theses help links are specific to each different screen. About the softwareCurrently Photometrica is mainly an aperture photometry software program that analyses remote CCD images online. The main steps involved in the aperture photometry analysis process are:
Centroid determination: The centroiding algorithm is based on the DAOPHOT FIND algorithm in IRAF (1987PASP...99..191S). In short, it goes something like this:
Sky fitting Photometrica uses the following outlier rejection algorithm to remove high valued pixels in the sky annulus (from stars, hot pixels etc.):
mode = 3 * median - 2 * mean This is the value subtracted from each pixel in the aperture. Aperture integration The star signal (instrumental magnitude) is estimated as -2.5 * LOG( SUM( ADU - Sky ) / exptime ) where the sum runs over all pixels in the aperture. That is, for each pixel that is fully contained inside the aperture, the software sums the pixels ADU, subtracted the estimated sky background (from step 2). The sum is divided by exposure time to get total intensity pr second before it is converted to the magnitude scale. Along the rim of the aperture there are pixels that are only partially inside the aperture. If the distance from a given pixel to the centroid is less than the aperture radius - 0.5, it is included in the sum as described above. If the distance is greater than the aperture radius + 0.5 the pixel is excluded. If the distance is in between, a fraction of (ADU - Sky) is included, proportional to the amount of the pixel inside the aperture. This is an approximate algorithm, but works fairly well. It is similar to the aperture integration algorithm in PHOT IRAF module, see http://iraf.noao.edu/docs/photom.html. Final Magnitude Estimate: Calculating the instrumental magnitude of a target, It, and a comp star Ic, as described in the sections above, the magnitude estimate of the target is given as V = It - Ic + C where C is the known magnitude of the comp star. If we use more than one comp star, we get instrumental magnitudes I1, I2, ..., In. And hence n estimates of the targets magnitude, V1, V2, ..., Vn. Photometrica calculates the final magnitude as the average of these n estimates. Error Estimation In an ensemble solution with more than two comp stars, the magnitude is estimated as the average of the individual comp stars estimate, and the error is taken as the standard deviation of this sample. This error estimate will cover all error sources. If one or two comp stars are used, the error estimate is based on the SNR of each measurement (the target measurement and the comp stars measurements). The standard error of a measurement is defined as 2.5 * LOG(1 + 1 / SNR) where LOG is the 10 based logarithm, and SNR is defined as S / Sqrt(S / (G + Ns * Std^2 * (1 + 1 / Nr))) S: Total ADU in aperture Ns: Number of pixels in aperture Nr: Number of pixels is sky annulus Std: ADU standard deviation in sky annulus G: Gain of the CCD detector For more information see the AAVSO CCD Observing Manual and 'Handbook of CCD Astronomy' by Steve B. Howell, 2000. Finally, the standard error of each measurement is squared and summed, and the error estimation is the square root of this number. Limitations Since Photometrica is based on aperture photometry, it has the limitations of this method. In particular this means that one has to be careful when working in crowded fields, and when there are large variations in the sky level, such as for a star close to a galaxy center. AknowledgementThis software uses source code created at the Centre de Données astronomiques de Strasbourg, France.Plate solving is done by
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