Part 3J: Understanding the Data
To easily access the table on the SAS Graphics Accelerator (it will be referred to as SAS from now on) and the instructions, either have two adjacent tabs open, or arrange the windows side by side, whichever is appropriate for your vision.
You should now be looking at your table in the SAS laboratory.
The first thing you should notice is how many columns there are. You should be able to easily scroll across all the columns, which includes the calculated column you added before importing the data to SAS.
Let’s take a brief tour of the meaning of some of this data. The 4:03 video below has Chris explaining the meaning of the data in the columns. Or you can skip to below the video.
Column Information (you can skip this if you watched the video)
There are 3 columns which start with the letters pm. ‘pm’ stands for proper motion. All stars have motion with respect to each other. This is a VERY small motion and is always reported as standard output data. Asteroid astronomers typically ignore the data since the objects of interest are all located within our solar system.
- If you have ever experienced a simulation of a fly through of space or a galaxy, the proper motion of the stars is shown.
Two columns contain the Declination and Right Ascension. The star’s position should not change, but why is the asteroid’s position not changing? Do you recall we marked the position of the asteroid in several different images and then used a tool to merge those positions? So we can ignore the RA and Dec of the asteroid.
- The x and y coordinates are used instead. These are what changed when you scrolled over the image (they are seen on the bottom of the image).
The x and y numbers are found in two other columns and are what is used to do photometry calculations.
The columns with telescope, filter, file id, id, and exposure length is information contained in the image header (info which the CCD camera records on the image) and is used as needed by astronomers.
- This information can also be found on Afterglow Access for each image when you cllick on the 2nd tool in the column called “File Info”.
flux, mag, and mag error are also listed.
- flux is found using an algorithm on the counts recorded by the CCD camera. It is used to find the magnitude.
- mag and mag error are calculated with algorithms similar to other equations we have used.
There are also several columns containing the words zero_point. These all have to do with catalogs of accepted magnitudes. AgA does not have the catalogs availabe yet, so we can ignore these columns.
Errors are found using statistics.
The time is also contained in the image information. It usually has the date along with the Greenwich Mean Time. This time has been converted into a Julian date (jd is the column title), and then a modified Julian date (mjd is the name of that column), which is easier to do calculations with. Read below for a brief explanation.
- The Julian date is simply a continuous count of days since the beginning of the Julian Period and is used primarily by astronomers, and in software, for easily calculating elapsed days between two events. Julian day number 0 is assigned to the day starting at noon on Monday, January 1, 4713 BC. The Julian date includes the fraction of the day, so it is a decimal. The modified Julian date starts at midnight on November 17, 1858.
- This date corresponds to 2400000.5 days after day 0 of the Julian calendar, so it is a little easier to deal with.
We will be using the mjd after we analyze the asteroid light curve.
- Once we find the time for one rotation, we will convert it to hours by multiplying the mjd by 24.
The next part will show you how to use the data in preparing the table to make a graph.