Back Focus Troubles #Absolute_Encoders


M Hambrick
 

Hi David

In my sketch, item C is a custom adapter that I purchased from Precise Parts. On the Precise Parts web site I specified the A-P 92 Stowaway flattener (92FF) on the scope side, and on the camera side I specified the SBIG FW8S-STXL Filter Wheel with Dovetail noseplate. The dimension "C" on the sketch is the effective length of the adapter. The determination of this dimension is discussed below. 

If you look on the sketch you will see dimension "X". This distance (64.1 mm or 2.254") is the specified backfocus distance from the rear face of the 92FF to the CCD sensor surface. This dimension comes from the specifications for the 92FF from Astro-Physics.

This nominal backfocus distance is the sum of dimensions A + B + C + D where:
  • A = The OPTICAL backfocus distance from the face of the camera/filter wheel to the CCD surface. This dimension is fixed and comes from the specifications for the camera and filter wheel. Make sure to use the optical backfocus distance and not the mechanical backfocus. I made that mistake once and had to get the adapter modified.
  • B = The thickness of the dovetail nose piece on the filter wheel. This dimension also comes from the specifications for the camera.
  • D = The thickness of a 1 mm spacer ring (0.040"). It is recommended that the custom adapters should be designed so as to allow the actual backfocus distance to be tweaked to optimize the flatness of the images in the corners. The adapter length is therefore specified assuming that there will be one spacer ring to get the nominal backfocus. You can remove the spacer ring to get the nominal backfocus - 1 mm, and you can add an additional spacer ring to get the nominal backfocus + 1 mm.
  • C = This is the effective length of your adapter and is equal to X - (A + B + D)

I ordered the spacer rings from Precise Parts because Astro-Physics does not offer them - Yet. I believe I saw a post where they plan to do so soon. These spacer rings have to fit inside the custom adapter and rest against the shoulder. They were not cheap, and I would suggest asking Astro-Physics when they will be available before you go to Precise Parts. If you go the Precise Parts route, you will specify a washer/shim on the scope side, and on the camera side you must specify the A-P 92FF/92TCC Canon EF camera adapter. The effective length is 1.0 mm.

I hope this helps

Mike