How-To: Find a Suitable Star For Autofocus

by Hamza Touhami

How To Choose The Right Star To Focus


Avid astrophotographer

 How to choose a good star for focus?


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.

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:


  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 image
    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
    4. Keep moving the focuser and or out until the star is sharp.
    5. Now it is time to identify is 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 as such:

9. Repeat the process until you find a suitable star for your system.

Video Tutorials

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

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