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The Minor Planet Observer
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Palmer Divide Observatory

2007 Shoemaker Grant Recipient

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PhotoRed Reductions Routines - Color Index

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In order for the differential reduction formulae to work in PhotoRed, you must know the standard, not instrumental, color index of the comparisons and target. The color differences between each comparison and the target factors into converting the raw instrumental magnitude in a given filter to the standard band.

What’s handy about the PhotoRed approach is that you need only one to three images in each filter of the target field. If you have more than one image, PhotoRed averages the values of the set to generate a single value. You need a minimum of three to compute a true standard deviation.

When you get those images depends on how you did the transforms, more specifically, if you used first order extinction terms in the reduction process. If you set the FOE values to zero, then the assumption was that the target field and the reference field were 1) close to one another and 2) that the fields were not too low to the horizon such that extinction differences were exaggerated by high air mass values. If the two fields are within 5 of one another and you shoot them when they are at least 45 high, the assumptions will be valid.

If you used good assumed or actual FOE values, then the only requirement is that you not shoot the fields too low to the horizon, though you can go lower than 45.

Another handy feature within PhotoRed is that you do not have to take and measure images in Canopus to find the color index values, as was required in early versions of the software, which would leave a number of very small sessions used for a limited purpose. In PhotoRed, you can run one of two photometry wizards that produces data in the same style as if you had imported data from Canopus.

If you did or will do a time series session in Canopus with the intent of importing the data for reduction to standard magnitudes, then you must be certain when measuring the images in PhotoRed for color index values that you use the exact set of comparisons. This means not only the same stars but in the same order, i.e., Comparison1 in Canopus must be Comparison1 in PhotoRed.

Those doing limited work, e.g., keeping track of variables where precise photometry has not been done, can use this feature to find the color index of the comparisons and target so that they can used PhotoRed-only data to find the standard magnitudes for their targets. Again, it is no longer necessary to measure images in Canopus if your only purpose is to get just enough images to find the standard magnitude of a target and not worry about a prolonged time-series.

 

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