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How to find a suitable star to successfully autofocus Your telescope

Astronomy is All About Sharing


Choosing the right magnitude star to focus on is a critical part of a successful autofocus run. Automatic observation relies heavily on acquiring sharp images all night long to accumulate as much data as possible. This increases signal-to-noise ratio which in turn lead to less noise and more beautiful astrophotos.

Things to consider:

Hardware requirements:

  • A focuser
  • A motor to move the focuser in and out
  • A good driver( preferably ASCOM compliant)
  • No Mirror flop: Some moving mirrors offer some great challenges to a successful autofocus run.

Software requirements:

  • PRISM v10
  • Any astronomy software that allows the user to highlight saturated stars.


  1. Find a suitable star: To do this, perform the following:
    1. Center a magnitude 5 or higher star
    2. Make sure it is centered: Some fields are quite curved and the best place is to have the star in the center of the field.
    3. If the star is defocused, use the manual focus tool to render the image as sharp as possible by using this function: Observatory > Focusing > Manual Focus. We have a great video at the end of this tutorial.
    4. Keep moving the focuser in or out until the star is sharp.
    5. Now it is time to identify if the chosen star saturates at the exposure time and binning you chose. Go to Image Display > Show Saturated Pixels.
    6. This window pop up:

7. Choose the right bit depth for your camera, here are some examples:

QSI583 = 16bit

ZWO ASI1600 = 12bit

SBIG STL11000 = 16bit

8.If the star is saturated, PRISM will display all the saturated pixels in RED as such:

9. Rinse & Repeat: redo the process until you find a suitable star for your system that does not saturate at the optimum focus position you found earlier.

Video Tutorials

Here are some video tutorials we have of different types of focus in PRISM.

  1. Manual Focus: Start with this

2. Automatic focus: Once successfully focused, use automatic focus

3. Direct Forucs: Once automatic focus is successful, go to boss mode: direct focus !

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