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  • Determining the Image Resolution of Your Equipment

    Reggie Jones

    We strive to get as much detail as we can out of the images we take no matter what the field of view is of our equipment.  The resolution you can achieve is determined by the size of the pixel array of our image sensor (CCD or CMOS camera) and translates to the angular size of the smallest details the sensor will be able to see.  The base measure for this is usually given in arc seconds per pixel.  To maximize resolution, you’ll need the camera pixel sizes to be small enough to pick up the smallest details you can using 2 linked pixels or more.  High resolution also depends on how important the field of view is to you.  If you want a wide FOV, you may need to sacrifice some resolution to get the image you want which means less resolution.

    To calculate the angular size of a pixel, use the following equation:

             =  206265 x [Camera Pixel Size / Telescope Focal Length]

    Make sure you use the same units for the pixel size and the Focal length.  For example, my QHY 268C camera’s pixel size is 3.76 microns or .00376mm.  My Astro Tech AT102ED has a focal length of 709mm.  Do the math and the angular pixel size for this setup is 1.093 arc seconds per pixel.  In other words, for the given field of view of this setup, each pixel will cover about 1 arc second of detail in the image.

    What does this mean?  Mount tracking error and optical tube tremors of my imaging system can degrade the imaging to 2 arc seconds or more; it’s probably worse since I mostly image in a severely light polluted location.  If the seeing isn’t very good, let’s say it’s 2 or 3 arc seconds, then things will still be somewhat Ok for imaging.  However, if the seeing is very good at around 1 arc second, I’d benefit from using a longer focal length telescope assuming the FOV I’d like is still somewhat possible.  For deep sky imaging, the FOV you need will usually dictate the focal length of the optical system that you need to work with.

    The Handbook of Astronomical Image Processing by Richard Berry and James Burnell is a great reference work for all things related to taking and processing deep sky images.  It provides very detailed information and the theory on how imaging equipment works and offers techniques to use to get the most out of your equipment.  Section 4.1 on Sensors and Optics discusses how to determine the resolution of your equipment.  Get your own copy of The Handbook of Astronomical Image Processing, try the Sky and Telescope website; they bought the old Willamen-Bell website and resources.  You can also try Amazon or a used bookseller.

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