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Sensor calibration / Dark-Frame: How and Why


Key Takeaways

  • The sensor calibration (or Dark-frame) is regularly used in digital photography, and in astrophotography in particular;
  • It improves your observations and the quality of the images you save on your smartphone or share with your friends;
  • On a UNISTELLAR Telescope, it is done by simply tapping on a button within the "My telescope" tab of the app;
  • Because it is linked to the telescope temperature, we recommend:
    • leaving the telescope outside for 15 minutes before taking a dark frame (thermalizing);
    • making a sensor calibration before each observation session.

In digital photography, sensor calibration or a dark frame is a process of recording a frame (or image) without a signal. It is then used to reduce the noise of images taken by subtracting this frame from the newly recorded ones. The noise can occur during a long exposure and when there are high-temperature variations. This process is called dark-frame subtraction, or dark subtraction

If there is no dark frame in your UNISTELLAR telescope, or if the dark frame is no longer accurate, you will be able to notice the existence of so-called "hot pixels" in the enhanced vision images displayed on the app or in the eyepiece. These "hot pixels" (some call them artifacts) appear as very small colored dots (red, green, or blue). This is not unexpected.

An example of blue "hot pixel"

To prevent this, simply follow the steps below:

  1. Place your UNISTELLAR telescope outside for at least 15 minutes before starting your observation. This will allow the tube to cool down and adjust to the external temperature;
  2. Turn on your UNISTELLAR telescope and do not remove the top dust cap cover. Leave it as-is for 1 or 2 minutes;
  3. Make sure your UNISTELLAR telescope is not exposed to direct streetlights. The darker the telescope surroundings, the better;
  4. Go to the "My telescope " tab of the app and tap on the "Sensor calibration" button;
    Note: this process takes a few seconds


You will be able to check the progression of the operation


If successful, the message "Calibration is successful" should appear briefly at the bottom of your smartphone screen.


Note: if you get an error message, please try again in darker surroundings and make sure your telescope was left outside for 15 minutes. Don't forget the cap and try to add a dark piece of fabric onto your eVScope.

Tip: sensor calibration is only one of the things you can do to improve the quality of the images you save during your observations. Making sure the collimation is correct, that the focus is optimal, (and that the eyepiece is adjusted to your sight if applicable), are also steps that should be followed for better image quality and ultimately a better experience.

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