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My astronomy project:
Implement of a stepper motor focus


  1. Introduction to stepper motor focus
  2. Making brackets to motor focus
  3. Test of focus system out in the field
  4. Temperature compensation 1
  5. Temperature compensation 2
  6. Collect temperature compensation data
  7. Find best focus point, polynomial fit

I take no responsibility or liability for what are written here, you use the information on your own risk!

6: Collect temperature compensation data

Now I have collect more data points on how temperature affects the focus setting. The values I have entered into an excel sheet to obtain the value that the focus should be used when compensate for the temperature changes.

temperature compensation

New updated estimate:
The orange curve shows how many micrometers relative to a start position the focus needs to be moved to different temperatures. The blue correspond to how many steps of motor. A trend calculation gives a kind of average that shows how many steps that the stepper motor should move for each degree change in temperature. This calculation gives 55 steps / degree Celsius, or set in the unit of length 95.6 my / degree C. It is usually the value corresponding to 55 you specify in your stepper motor control applications.

95.6 / 55.0 = 1.738 gives the change in micrometers per step.

The three columns in the table at right shows telescope tube's relative length. Apparently increases the tube's length at increased temperature. But the strange thing is that I have to compensate and make the tube even longer to maintain focus. If the tube is increased 0.138 mm in length, I should of course have to compensate with -0.138 mm, instead it will be 10 times the wrong direction, + 1.460 mm. The image is sharp then, probably some logical error or miscalculation even if I can not find it right now. Another option is that the temperature compensation cell that holds the triplet lens doing something crazy. It shall normally only hold the lens elements in the triplet relative each other independent of temperature variations

I talked to the famous Swedish mirror maker Joel von Knorring, he told me that he noticed that the mirrors change focal length with temperature. Maybe I more temperature compensate for the triple lens than the telescope tube variation with temperature, lets see what I find out in future.

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