Unistellar Help Center

Taking a Dark-Frame: How and Why

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Key Takeaways

  • Dark-frame subtraction 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 and share with your friends;
  • On the eVscope, it is done by simply tapping on a button within the "User" 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);
    • taking a dark-frame at least twice a year: during the winter and during the summer. If you observe at a high altitude, you might have to do it more often.

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

If there is no dark frame in your eVscope, 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" 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 eVscope 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 eVscope and do not remove the top dust cap cover. Leave it as-is for 1 or 2 minutes;
  3. Make sure your eVscope is not exposed to direct streetlight. The darker the eVscope surroundings, the better;
  4. Go to the "User" tab of the app and tap on the "TAKE DARK FRAME" button;

    Note: this process takes a few seconds

  5. If successful, the message "New Dark Frame Created" 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.

Tip: dark-frame subtraction 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, are also steps that should be followed for better image quality and ultimately a better experience.

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