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  • Gain and Offset Simplified

    Reggie Jones

    A major item to learn in improving your astro imaging has to do with understanding how to set your camera’s gain and offset values.  Gain and offset are not easily understood and I’ll try to clear up a couple of misunderstandings and provide some guidance on how to properly use gain and offset when capturing your image data.

    A primary misconception is that changing the gain on a CCD / CMOS camera (or ISO if you’re using a DSLR) has everything to do with changing the sensitivity of the camera.  The truth is that it doesn’t.  Camera sensitivity is governed only by a single characteristic of your camera - the quantum efficiency.  This is how well your camera’s sensor captures photons.  This characteristic is solely determined by the camera sensor’s manufacturer - there is nothing you can really do to change, adjust or improve it.

    So what is Gain? I think many people are familiar with the pixel bucket analogy.  This is the example view that your sensor pixel is simply a bucket that electrons are captured in.  The size of the bucket is determined by the chip sensor; the maximum amount of electrons the bucket can hold is called the Full Well Depth.  Also keep in mind that the bucket captures whole electrons; you can’t capture a fraction of an electron.  So the bucket will hold a finite maximum of whole electrons.

    When you adjust the gain or ISO of your sensor, you shorten the size of the pixel bucket.  This allows your bucket to fill faster; however, the trade off you need to come to terms with is that you will have fewer total levels to read from the pixel, effectively reducing your ability to record subtle differences in the brightness of your target.  It also means that your pixels will become saturated much sooner than if you did not adjust the gain.

    Offset is pretty simple.  When you set an offset value, you’re putting a small amount of value in the bucket before you fill it with electrons.  This is to prevent having zero value pixels in frame.  Zero values are effectively no information and it does not help the image.  You will not be able to create information from nothing when the pixel is read out.

    So, what are the best gain and offset settings are for their camera?  This is not an easy question to answer; the best answer I have found is that it depends on  your imaging setup (Telescope and camera combination) and you will need to experiment with these settings to find one that provides consistently good results.  Also, you may have heard the term Unity Gain.  Unity Gain means the gain you have at 1 electron per Analog Digital Unit (ADU).  It is NOT the best gain setting; it is only a baseline setting and a starting place for your camera to start imaging with.  From there, you’ll need to experiment and make changes to find a sweet spot for the imaging you’re doing.

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