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The Minor Planet Observer
Palmer Divide Observatory

2007 Shoemaker Grant Recipient

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Transforms Errors
First Order - Hardie
First Order - Comps
Color Index
Comps Std. Mags
Target Std. Mag.
Quick Binzel Method


PhotoRed Reduction Routines

Most of the routines in PhotoRed are based directly on the algorithms used by Henden and Kaitchuck. The differential approach to standard magnitudes is based on a paper by Richard Miles in the Journal of the British Astronomical Association. Where possible, the data and results from those algorithms were used in Canopus to verify the correctness of the results.


Say you want to get one or more V and R images for a field and treat them as if they were taken at the same time, e.g., you want to derive a V-R measurement for that moment. With a single camera, that’s not possible. PhotoRed somehow needs to know that the images are to be treated as if they were taken simultaneously. This is accomplished by assigning a group number to a set of images. In the example, the V and R images taken at approximately same time would have the same group number. When PhotoRed runs through its routines, it can group the images as if you had a multi-channel camera.

Usually, you don’t take just one image through each filter. Instead you take two or three with the intent that the values be averaged. This reduces scatter in the data. Combined with the grouping just mentioned, PhotoRed can automatically do the averaging for you.
For example, say you take a quick succession of three images in each of three filters of a standard field. In PhotoRed, you would assign the same group number to all nine images so that PhotoRed knew that the data for the nine images was to be “lumped together.” Then, when running through the routines, PhotoRed sums the magnitudes for each star in the three images for a given filter and uses the average value as the magnitude for that star in that filter for that group. It is not required that you have the same number of images for each filter, though having at least two helps reduce data scatter. Getting more than three images in each filter is probably not warranted in most cases.

The concept of groups is an important one to understand when using PhotoRed. In the simplest terms, all observations of a given object in a given filter that have the same group number will be averaged to find a single magnitude for that filter and air mass value. Therefore, it makes little or no sense to put observations made many minutes or even hours apart into the same group.

The Color Index (CI) Value

For the V filter, the CI value is often B-V, i.e., the catalog B magnitude minus the catalog V value. However, in PhotoRed the solution can be based also on V-R and V-I. Some of the routines specifically ask which catalog color index to use when running the calculations. Later on, when you run the routines that find the standard magnitudes of comparisons and target, you must tell PhotoRed which CI was used in the previous methods.

What’s critical when you convert Canopus instrumental magnitudes to standard magnitudes is that you use the same filters that were used for determining the transforms. It just won’t do to find a slope for B-V magnitudes and apply them to images taken in V and R.

As mentioned before, the C filter is treated as its own band but uses V magnitudes from the catalogs (plus one other for transforms and CI determinations). In short, think of the C filter as a substitute V filter with a higher pass through but the reductions for with will not be as good in terms of matching the standard system as using a true V filter.


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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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