Camera Systems and Back Focus - Untangling the Gordian Knot!

Chris Hysinger

When developing an optical train for astrophotography, there is rarely an “off the shelf” solution that covers all telescopes and cameras.  This is partly a result of a lack of standardization among camera companies and telescope manufacturers in terms of thread types and metal back distances, but it also due to the wide variety of camera sensors and telescopes available to today’s astronomers.  At Takahashi, we recognize this challenge and have developed some tools to help make this task less painful.


The key to any telescope/camera combination is first determining the back focus required by the optical system, and then configuring adapters that enable the camera sensor to be placed at that focal point.  Before beginning this process, it is helpful (required) to have the following information:


  1. The back focus of the telescope, or the metal back distance required in the case of flatteners, reducers or extenders.
  2. The amount of back focus consumed by any necessary telescope accessories or adapters, such as camera rotators.
  3. The rear-most thread on the telescope in the desired configuration. This may be the thread on the back of the focuser drawtube, camera rotator, or flattener/reducer/extender.
  4. The amount of travel of the focuser. This is only necessary for configurations at “prime focus”, meaning without any flatteners, reducers or extenders.
  5. The metal back distance consumed by the camera system, including filter wheels, auto guiders, and any adapters required by the camera manufacturer to interface with common telescope threads (t-thread, SCT, etc.). Don’t forget to account for optical filters, which tend to decrease the amount of back focus consumed by the camera.


With this information in hand, determining what adapters are needed is a bit like solving a puzzle – a puzzle that may have more than one solution! The goal is to find a combination of adapters and spacers that take up the proper amount of spacing and that terminate in threads that can attach to the telescope and camera systems.


The basic formula for determining the length needed by these adapters is found using one of the following two formulas:


   A. For prime focus scenarios


  1. Telescope back focus,
  2. Minus ½ focuser travel,
  3. Minus necessary telescope accessories,
  4. Minus metal back of camera system,
  5. Equals the length of adapters needed.


   B. For situations where a flattener, reducer or extender is used


  1. Metal back of the flattener, reducer or extender,
  2. Minus necessary telescope accessories, if they are between the flattener, etc and the camera,
  3. Minus the metal back of the camera system,
  4. Equals the length of adapters needed.


To help with these calculations, Takahashi America has developed a series of resources.  These include:


A table of current Takahashi telescope specifications, including the back focus and metal back measurements using various flatteners, reducers and extender. (Telescope Backfocus Table)


A table of available the focuser travel for current Takahashi telescopes (Focuser Travel)


A table in Excel of available Takahashi adapters, with thread sizes and lengths, to help determine which combination can be used in a particular scenario (Available Adapters)


We hope you find this information useful.  If you have any questions, please email us at