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The Minor Planet Observer
Palmer Divide Observatory

2007 Shoemaker Grant Recipient

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MPO Canopus - AutoMeasuring Images

In MPO Canopus, "AutoMeasure" means matching (or aligning) a chart based on catalog stars so that for any star on the chart and its counterpart on the image, the X/Y distance and angle from chart center is the same as the X/Y distance and angle from image center. In other words, the star has the same polar coordinates on the chart and image.

This is done in Canopus by using a two-star alignment method. Two stars in the image are selected which have counterparts in the initial chart. The stars should not be too close together in order to allow the best possible scaling. Also, a line joining the two stars should not be vertical or horizontal, i.e., not parallel to either the X or Y axis, otherwise it's difficult to determine the rotation of the image from true North/South.

In the manual process, you tell Canopus which two stars on the image matches two stars on the chart. The auto-matching method determines the two stars each on the chart and image automatically. Canopus then uses the X/Y positions of the four stars to determine how to scale, translate, and rotate the chart so that it matches the image.

Once the chart is matched, Canopus can then match any star in the chart to a counterpart on the image or vice versa, unless – of course – there is no counterpart. This can occur for any number of reasons, e.g., the chart catalog being incomplete to the limiting magnitude of the image or the star is a nova or asteroid. The screen shot below shows the result of an AutoMeasure and the asteroid being automatically measured as well (the asteroid is the one with yellow apertures about it).  The cross on the chart indicates the center of the image, not necessarily that of the target.

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The numbers on the chart correspond to entries in the data table on the Reductions page, which is shown below but for a different asteroid than in the image above.

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AutoMeasure Requirements

A bonus of the AutoMeasure feature is that if the image is of an asteroid, Canopus tries to measure the position of the asteroid automatically, saving you the time and effort of locating the asteroid on the image and manually indicating its position. Before Canopus can do this, it must be able to read the FITS or SBIG header "OBJECT" keyword and interpret it correctly as the number, name, or designation of an asteroid. Since there are so many variables, this may not always be possible. In which case, you can tell Canopus which object it's supposed to try to find before the final steps of the AutoMeasure routine are executed.

The pattern matching algorithm in Canopus (and MPO Connections as well) is fairly forgiving. However, if it's not given a good first estimate of the image scale and image orientation, the algorithm may make a match that provides incorrect astrometry. The configuration form allows you to specify several parameters such as focal length of the system, number and size of the pixels, and so forth.


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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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