Data Reduction

Reducing CCD images taken with telescopes like those in the GTN is a somewhat involved process. There are several steps that must be completed to convert your raw images into scientifically meaningful data. Even if your primary goal is to take “pretty pictures” several of these steps are important. This page is designed to give you a rough guide for reducing your images and to point to resources that you will need or want to have for that purpose. For a more thorough discussion of the topics on this page see the AAVSO CCD Observing Manual. Another excellent resource for all aspects of astronomical image processing is The Handbook of Astronomical Image Processing by Richard Berry and James Burnell. To learn how to actually perform the steps discussed below you will have to consult the documentation for whatever software you are using for the reductions.

Image calibration refers to corrections that must be made to CCD data before you can extract scientific measurements from them. If you do not know what flat fields, darks and biases are, see the page on Image Calibrations before you begin this overview. For the rest of this page we assume that you have a set of calibration images and science images. We will describe how you should apply the calibrations to your science images. In certain instances, the calibration images might have been made for you. In any case, you will have to get a set of calibration images in order to reduce your data.

A Note About Image Calibration

During each of the steps outlined in our description of image calibration you should check the resulting images and make sure that the output is reasonable. A bad calibration correction will give you bad science images at the end, so you want to be sure that at every step you are not introducing any errors. Visually inspecting your images is one way to check that things are not going horribly wrong. You can also do hand-calculations with a few images to check pixel values. You definitely want to find any discrepancies now, before you havve spent hours working on science images that will have to be redone because the calibration was done incorrectly. Keeping these things in mind, click on Image Calibration to read a general overview of the steps required to calibrate your CCD data. Note that this gives only an overview of what is to be done; though the steps will be identical, the particulars of how to execute them will vary depending on the software package you use.

STEP 2 – Image Stacking

After you have calibrated your science images you must stack them. This operation is much like combining your flats, darks and biases, but there is one important difference: Your science images contain objects, and before you combine them you have to align the objects so that they add together. Aligning your images is called registering them. If your images are not registered before you combine them you will end up with multiple instances of each object, one from each of your raw images, in your final image. Of course, if your tracking is very good or if you have an auto guider then your images might already be well enough aligned (you should check to make sure). In any event, other than this very important difference, you merge science images for the same reason you merge calibration images: it helps remove cosmic rays and it improves the statistics in the final image.

The details of how to align and merge images will vary for different software packages. Some make it very easy by analyzing the field and calculating the necessary offsets for each image automatically. They then register the images and add them together. Other packages require you to determine the positions of a subset of stars from which you calculate offsets and apply shifts by hand. You should look at the documentation for your own software to find the details on how to register and combine images.

STEP 3 – Doing the Reductions

Once you have calibrated and stacked your images you are ready to begin the real reductions… and about time! If you wish to determine the brightness of objects in your field, look at the photometry section. If you want to do astrometry, go to that section – you actually might not have to bother about calibration if all you want is positions of things, though for the most accurate positions possible calibration will still be required.


Photometry is the measurement of the brightness of objects in your images. There are two types of photometry, relative and absolute. For most GTN projects you will probably want to do relative photometry, in which you compare your program objects to other objects in the image. This is also often called differential photometry. If you want to do absolute photometry instead, then you must do additional observations of photometric standards in order to put your comparison objects on an absolute brightness scale. For a description of how to do either kind of photometry you should consult a reference like the AAVSO CCD Observing Manual. To gain a basic understanding of how photometry works, have a look at our Basic photometry and Astrometry page. Again, the particular steps you must complete to do your photometry will depend on the software package you are using. Consult its manual for details.


Astrometry is the measurement of positions of objects in your images. This is always done relative to some set of reference objects within an image. In the event that you have absolute positions (RA, DEC) of these reference objects, you will be able to put all the objects in your field on this same absolute system. To do this you need a reference catalog of stars to determine the absolute position of your field. Read our astrometry page to learn how to do this.

MaximDL Photometry Exercise

We have put together a step by step tutorial that takes you through the process of reducing a set of images using MaximDL. This is one of the popular packages used on Windows. A list of other packages can be found in the table below.

Start the Photometry Exercise