Cat-Star Atmospheric Extinction Coefficient Calculator (CAT-X)
In the Image Processing section of the SOCO website, I describe the procedures for calculating the values of atmospheric extinction coefficients (k) from imagery acquired of stars. Manually, this is a simple but rather tedious procedure. To make the effort much easier, I've written a new computer application that will do all the calculations for you. This application is the Cat-Star Atmospheric Extinction Coefficient Calculator, or "CAT-X". All you need to run it is a set of monochrome FITS images (16-bit Integer or 32-bit Floating Point data format) of the target star acquired with red, green and blue filters and some additional information (latitude and longitude of the observing site, and the RA and declination of the target star). As a bonus, CAT-X will also calculate the values of the white-balance weight factors (W) if the target star is a spectral class G2V (sun-like) star. It will analyze up to 1000 images at a time and runs on any 32-bit or 64-bit Windows PC. You can download this application for free in the "DOWNLOAD" section of this webpage. CAT-X is a "stand-alone" program that you run by simply double-clicking on the EXE file— you don't need to install it in Windows.
Image Requirements:
(1.) When you run CAT-X, one of the first things it will do is display one of your images and ask you to move the cursor to the target star and click on it. This tells CAT-X where the target star is in the image. Since CAT-X will use this information in analyzing all your images, all your images should be registered. When you run your image registering routine, be sure that the resulting registered images all have the same image size (rows and columns). Some registering routines may want to change the size of each registered image based on the results of the registering process in an effort to "optimize" their size. In this case, be sure you specify that you want all the registered images to have the same numbers of rows and columns.
(2.) CAT-X needs to distinguish between the red, green and blue images in your image set when it does its analysis. Some camera control utilities write the filter color in the FITS file header where it can be retrieved when the file is read. Unfortunately, some don't, so this method is unreliable. To distinguish between files, CAT-X looks at the last character in the file name (right before the ".fit" extension). Here, it expects to see either "R", "G" or "B", indicating the filter color for the image. For example, "" could be the name of a red image file.
Some camera control utilities can be set to automatically append an R, G or B to the end of the file name to indicate the filter color. If yours can't do this, I've developed a utility program ("FIX-FILENAME") that will take your files and modify their names so that they appropriately end in R, G or B. You can download this utility program for free in the "DOWNLOAD" section of this webpage. To use it, place your red, green and blue image files in separate folders. Also put a copy of FIX-FILENAME in each folder and run it (i.e., double-click on it). It will ask you what color the images are in the folder. It will then append the appropriate letter to each filename. It won't change anything else about the files or their contents.
When you've got your files registered and appropriately named, put them all in a single folder along with a copy of CAT-X. When you run the application, it will first ask you to key in the latitude (degrees, minutes and seconds) and longitude (degrees, minutes and seconds) of your observing site and the right ascension (hours, minutes and seconds) and declination (degrees, minutes and seconds) of the target star. The latitude should be negative for the Southern Hemisphere and West longitude should also be negative.
CAT-X will next display one of your images on the screen and ask you to indicate the target star as described earlier. When you click on the star, CAT-X will draw a red circle around it and ask you if the star is inside the circle. If it isn't, you can try again. Once this is done, CAT-X will ask you to place the cursor in a part of the image that is mostly background. When you click on the image, CAT-X will draw a red rectangle and ask you if it contains mostly background. It's okay if it contains some stars and/or parts of nebulas or galaxies— as long as it contains a fair amount of background. If you're not satified with your choice, you can try again. When you're finished, the image should look something like the example below.
When you accept your choice of rectangle, the image will disappear and CAT-X will start processing your images. CAT-X is pretty fast— on my old Dell XPS 8900 (6th Generation Intel Core i5 Processor) it takes about a second to analyze each image. When it's done, CAT-X will display a graph of the data extracted from your images, with optical thickness on the x-axis and stellar magnitude on the y-axis. Trend lines will be fit to the three sets of data points (red, green and blue). It is the slopes of these trend lines that represent the values of the extinction coefficient (k) for the red, green and blue spectral bands. An example is shown below.
This graph allows you to assess how good the analysis is. As with all statistical analyses, there are sometimes points that obviously don't belong with the rest of the data— "outliers"— that can be removed to improve the analysis. CAT-X allows you to use the cursor to remove outliers one-at-a-time and will re-calculate the appropriate trend line each time a point is removed. When you're satisfied with the analysis, CAT-X will allow you to save the graph as a BMP image file. At the same time, the CAT-X application window will show the values of the extinction coefficients along with the ratios of the trend line intercepts, which represent the values of the white-balance weight factors (W) if the target star happens to be a spectral class G2V (sun-like) star.
CAT-X uses a photometric approach to analyze your images, but you don't have to specify the sizes of the rings making up the photometer aperture. Instead, CAT-X uses an iterative method to determine the optimum aperture size for each star it analyzes. This makes it much easier for the user.
While it analyzes your data, CAT-X writes intermediate results to a text file called CAT-X_LOG. While this file is primarily used to de-bug an analysis if it terminates prematurely, it also presents a summary of the final results at the end of the file for later inspection. This summary includes regression parameters, extinction coefficient values, and weight factor values for both an optical thickness of 1 and an optical thickness of 0 ("exoatmospheric" values). An example is shown below.
One of the great things about using CAT-X is that, because it is so easy to use, you can evaluate extinction coefficients for many stars in a set of images. By averaging the results, you can get very robust values for the extinction coefficients to use in color-calibrating your imagery without having to rely on "typical" or default values provided by many image processing software packages. And by analyzing imagery obtained for G2V stars, you can determine white-balance weight factors that are truly representative of your imaging equipment (camera and filters).

Click on the link immediately below to download the executable file for CAT-X.

CAT-X Cat-Star Atmospheric Extinction Coefficient Calculator (ver. 1.0)

Click on the link immediately below to download the executable file for FIX-FILENAME.

FIX-FILENAME Image File Renaming Utility (ver. 1.0)


CAT-X Version 1.0 April 2022 Initial release.

FIX-FILENAME Version 1.0 April 2022 Initial release.

I'm indebted to Keith Burnett ( for his outline of the computational procedure for calculating astronomical elevation angles (Converting RA and DEC to ALT and AZ), which is used in CAT-X.

Many of the procedures used in developing CAT-X are described in The Handbook of Astronomical Image Processing (Second Edition) by Richard Berry and James Burnell, Willmann-Bell, Inc. (pub), July 2009.

Contact me by email at with questions or comments.

Computer applications available from this website are provided "as is" without any warranty for compatibility or accuracy.



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