Jump to content
  • Choosing the Right Filter Size

    Reggie Jones

    If you plan to use a filter wheel for imaging with a monochrome camera, it is important that you obtain the correct size filter for your setup.  Filters usually come in 4 sizes; 1.25 inches, 31 mm, 36 mm and 2 inches.  As a general rule, 4/3” format sensors or smaller can use 1.25” or 31mm filter sizes.  APS-C sized filters use the 36mm filter size.  Full frame sensors require the 2” filter size.

    Like everything else in astro imaging, there is a formula to find the minimum acceptable filter size.  The formula is:



    S = sensor focal length

    d = the distance between the filter and the sensor

    f = telescope focal ratio

    One additional thought; for wide angle lenses with very short focal lengths, you may be able to use a slightly smaller filter than is calculated.

    Reference:  The Deep Sky Imaging Primer 3rd Edition, Charles Bracken

    User Feedback

    Recommended Comments

    There are no comments to display.

  • Create New...

Important Information

By using this site you agree to our Guidelines