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The Minor Planet Observer
Palmer Divide Observatory

2007 Shoemaker Grant Recipient

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MPO Connections:
Working with MPO Canopus

If you're also using MPO Canopus for your measuring images, you should be aware of some interrelations between Connections and Canopus. This allows you to make the most of Connections for astrometry and photometry.

When Canopus loads a FITS image, the format used by Connections, it attempts to use the values saved under the OBJECT keyword and/or the OBJCTRA and OBJCTDEC keyword pair. If one or both sets are found, then when Canopus generates a chart, it first tries to determine the position of the object (if an asteroid). If it can’t, it uses the RA and Declination values to center the chart.

By having the RA and Declination values available, Canopus can automatically generate and match a chart to the image for measuring reference stars and the target.

Canopus and the OBJECT Field

Canopus attempts to interpret the OBJECT field in the following ways.

Example: A1417

If the value starts with ‘A’ and is followed by a number, the ‘A’ is removed and Canopus attempts to convert the rest of the value to a number.

If that succeeds, the assumption is that the object is a numbered asteroid and chart center is based on the computed position using the date and time in the FITS header, adjusted as necessary by the configuration settings to Universal Date/Time.

If the conversion to a number does not succeed but the OBJCTRA/DEC keywords have valid entries, then Canopus uses those fields to center the chart.

Example: 1417

If the first and second characters are a number and there is no space in the value, then Canopus attempts to convert the value to a number. It then proceeds as outlined in Example 1.

Example: 1999 CZ1

If the first and second characters are numbers and there is a space in the value, the assumption is that the value is an unpacked MPC designation. Canopus converts the value to the packed form and then back to unpacked. If the original value and the returned unpacked string are the same, then Canopus tries to find the asteroid in the MPC database and, if found, proceeds as in the case of a numbered asteroid. If the packed-to-unpacked test fails or the asteroid is not in the MPC database but the OBJCTRA/DEC fields have valid values, the chart is centered on the position in those fields.

Example: Ceres

Canopus attempts to do a name lookup in the MPC data. If found, then Canopus proceeds as in the case of a numbered asteroid. If not found but there are valid entries for the OBJCTRA/DEC keywords, the chart is centered on the position indicated in those fields.

How does this all fit into Connections? When you set the Default Base Name in the Configuration settings or Arg2 of a TakeImage command in a script, the OBJECT field is set with one or the other value. Thus, if you properly set one and/or both, Canopus can automatically determine which asteroid you were working, numbered or unnumbered, and generate the chart based on the asteroid’s position at the time that the image was taken.

This has the particular advantage that when using the AutoMeasure feature of Canopus, where it matches a chart to the image (as the AutoMeasure method does in Connections), then Canopus can try to measure the position of the asteroid for you. If the chart is based on position only, no such attempt is made.

Canopus and the OBJCTRA/OBJCTDEC Fields

If the OBJECT field does not contain a valid entry as defined above, then Canopus uses the settings in these two fields. Connections sets these fields for you.

Whenever you take an image with the telescope linked, the RA and Declination reported by the scope are recorded into the FITS header under these two keywords. This is particularly important if you are working a target other than an asteroid since, most likely, the OBJECT field will not be interpreted as an asteroid.

If there is no telescope linked, you can still have Connections insert the position into the FITS header by using the "RA/DEC from GoTo" checkbox in the Configuration settings. This means that the RA and Declination are taken from the two fields used to manually slew the scope to a given position. Naturally, you need to be sure these two fields are set correctly before taking each image.

If all else Fails

If for some reason your image(s) don’t have entries that can be properly interpreted by Canopus, you can use the Image Header editor in either Connections or Canopus to enter values into the appropriate fields. This could be rather tedious if you have a lot of images so your best bet is to follow the guidelines above to the maximum benefit of using Canopus and Connections together.

Valid Formats for OBJCTRA and OBJCTDEC

The entries for these two fields in the FITS header must be in the following format so that Canopus can convert them correctly.

   OBJCTRA HHxMM[xSS[.ss]]
   OBJCTDEC ħDDxMM[xSS[.ss]]

Where ‘x’ represents either a space or colon ( : ) and the entries within the square brackets are optional. For declination, you must enter the sign ( + or – ) as appropriate, i.e., the sign symbol is not optional.


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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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