Autoguided versus Unguided Long Exposure Astrophotography

I wanted to demonstrate, visually, the difference between an unguided versus a guided exposure.  When taking photographs of deep sky objects, stars can move as fast as 500 kilometers per second or as slowly as a few kilometers per second.  This makes taking nice round images of stars a bit difficult.  To counter the movement of the stars, an Autoguider is needed, which is an additional camera sensor (either on the camera that is taking the photo or an independent camera strictly used for guiding).  In short, an Autoguider is a sensor that constantly makes short exposures to take a picture of a star and put it in memory.  A software program calculates the stars position, monitors its movement, and calculates the necessary correction to bring the star back to its original location.  The software then issues commands to the telescopes drive system to make the correction.  Since the sky is constantly moving, the telescope mount is able to move with the sky and object you are photographing looks like it is frozen in time.

Below, you will find an Autoguided, 4 minute exposure of the Andromeda Galaxy.

Below, you will find an Unguided, 90 second exposure of the Andromeda Galaxy

If you notice the difference in the stars from guided and the unguided exposures, you will see that the stars look smudged in the unguided exposure and the stars look like round balls of light in the guided exposure.  If you can’t see the difference above, here is a side by side view of a section from both images





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