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The Minor Planet Observer
and
Palmer Divide Observatory

2007 Shoemaker Grant Recipient

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MPO Canopus - Ensemble Photometry

One of the big "buzz words" these days in photometry software is "ensemble photometry." There can be many definitions but, in general, this can be taken to mean that two or more comparison stars are averaged to find a single value. This value is then used to find the difference between the target and the value - thus "differential photometry." How many stars are used and how they are averaged is where the big differences derive.

Honneycut produced a sophisticated method that does not require all stars in the original comparison star set to be in every image and gives weights to each star. A simpler approach, one used in MPO Canopus, is to use up to five stars and find the average of the instrumental or derived magnitudes. In either case, the error in the value decreases with the sqrt(N), so that doubling the number of stars reduces the error to about 0.7. Going from one star to four stars cuts the error due to random noise to about half. By having a more stable reference set, the differential values have less random noise, thus improving the overall photometry.

Another significant benefit to using more than one star is that you have one or more "backups" should a selected star be variable. One of the important features that makes MPO Canopus stand out for photometry is that not only does it use multiple comparisons but it plots the comparison star data so that you can check on the quality of the comparisons.

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The screen shot above shows the Sessions form's Comparison Plots page. Here, the raw data of the #1 comparison has been plotted. The curve shows low noise and the expected brightening of the star as the field rose higher in the sky. However, just the raw plots alone may not be enough. Canopus goes a step further by doing differential photometry on the comparisons, a result of which is show in the screen shot below.

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In this case, the average of the other comparisons used in the given session was found and then subtracted from the value for Comparison #1. Again, the scatter is low, about 0.02m and, more important, the general trend of the plot is a flat line, meaning that this comparison was not variable (and - most likely - neither were any of the others).

Should you ever see an average plot like this:

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You should be very suspicious! At the least, remove this comparison from the set and, better yet, measure it as a target after you're done with the current target.

Using this feature of MPO Canopus has resulted in a number of variable stars being discovered and then studied, again using MPO Canopus, to determine the type and period of the variable.

 

 

 

 

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This page was last updated on 01/19/11 05:14 -0700.
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