Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

The typical reduction procedure

If you read a paper in the technical astronomy journals, you'll often see the phrase "we applied the standard reduction procedure to our images." What does that mean? It means:

  1. subtract a (master) dark frame
  2. divide by a (master) flatfield frame

Here's the overview of the work required to convert a night's worth of raw CCD images into "clean" images which you can then analyze.


Cleaned images

The result of your work should be a directory full of "clean" images: CCD frames from which the thermal contribution and variations in sensitivity have been removed. You should be ready to perform the next step in your analysis, whether it is photometry, astrometry, or just plain looking.

Before you can move on to measuring the light of the stars in your images, you need to figure out some properties of the data. For example, consider the following "clean" image of the region around the star "V585 Lyr" (click on the image below for a larger version of the picture):

You can download a copy of this clean image by executing the command


    cp $dd/v585_clean.fit .

  1. make a finding chart of the field using the Aladin tool from SIMBAD
  2. find the orientation of the images. Which way is North? East?
  3. what is the pixel scale? That is, how many arcseconds does each pixel represent?
  4. which star is V585 Lyr?
  5. what are the catalog magnitudes -- in the standard R passband -- of the stars at (108, 146) and (161, 265)? Which catalog is the source for these values?
  6. just what sort of star is V585 Lyr, anyway?

One way to look for information on a particular star is to use SIMBAD's list of stellar properties and references. Another way is to go to the Astrophysics Data Service Abstract Search site. Choose either the "Object name/position" or "Abstract Words/Keywords" box, and type the name of your object. Then click on the "Send Query" button.

Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.