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The Minor Planet Observer
and
Palmer Divide Observatory

2007 Shoemaker Grant Recipient

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Canopus Overview

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MPO Canopus/PhotoRed is a complete package for astrometry and photometry. With it you can easily measure the positions of asteroids or other targets and perform photometry on just about any variable object. Combined with the supplemental program, PhotoRed (Photometric Reductions), you can transform raw instrumental magnitudes taken through one or more filters to standard magnitudes. This makes combing your data from night to night with that of other observers much easier.

The screen shot above shows the main screen of Canopus after the program has automatically measured an image. The numbers on the chart (left) indicate reference stars while the concentric circles on the image indicate the measuring apertures for the same stars and asteroid (771 Libera in this case). Depending on your computer, the process can take five seconds or less. This is only the beginning of how Canopus can make working with your images for astrometry and photometry fast and easy.

Canopus is designed to make astrometric measurements as a stand-alone program. When it comes to photometry, things are a bit different. A good number of asteroid lightcurve researchers have used Canopus "as-is" for years and produced high quality work. The one missing step was putting the magnitudes on a standard magnitude band or at least being able to link all observations of a given target to an internal standard.

For the most part, reducing to a standard band is not critical if one is working alone and trying to find only the period and amplitude of the asteroid’s lightcurve. In this case, differential photometry is used and so standardized magnitudes are not essential in most cases. Also, Canopus provides several features that allow the user to combine data from several nights by setting an arbitrary zero point for each night’s run so that all runs appeared to be based on the same comparison star. This can be accomplished even if using observations from several observers.

However, the work of combining data from different nights and observers can be difficult if the observations span a considerable amount of time. If all the observations are reduced to a standard magnitude band or at least to an internal standard that is highly repeatable, setting the zero point for the observations becomes very easy. Some minor adjustments may be required but not to the degree of when each observer is reporting his own instrumental magnitudes.

Amateurs can no longer claim that standardized photometry is beyond their means or capabilities. The demand for more critical work on the part of amateurs is justified and is just plain good science.

This is where PhotoRed comes in. The current version has a history that goes back to DOS programs written in the mid-1980s. The purpose of PhotoRed is to measure images taken for the specific purpose of determining the "transforms" that convert raw instrumental magnitudes to a standard magnitude band. However, PhotoRed does more than just that. PhotoRed can import observations made in Canopus, use the derived transform values to convert the instrumental magnitudes, and then export the converted data back to files Canopus can read. At that point, true, or at least near, standard magnitudes can be used for period analysis and reporting.

PhotoRed provides many checks and balances. The data is presented in graphs and you can compare derived standard magnitudes for known stars against the catalog values. A typical example of the latter is to image a field with well-known photometric stars, e.g., M67. You would measure the images in PhotoRed to determine the transforms and nightly extinction and zero point values and then apply those to the raw instrumental values for the same stars to determine the computed standard magnitude for the stars. PhotoRed can then compare the computed values against those in the catalog, giving you an exact idea of the quality of the transformation solution and/or the raw data itself.

Field tests using this approach often had average errors (computed – catalog) on the order of 0.01-0.02m with the standard deviation of 0.01m.

 

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This page was last updated on 01/19/11 05:14 -0700.
All contents copyright (c) 2005-2011, Brian D. Warner
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