Set of three 1 mm thick spacer rings that can be used in special circumstances to modify the back focus in the following instances:

• Telecompressor Correctors: QUADTCC, 155TCC*, 160TCC*, or 175TCC* (*are no longer available, but you may already own them)
• Field Flatteners: 13035FF, 160FF*, or 175FF* (*are no longer available, but you may already own them)

Below are some recommendations from Astro-Physics to achieve proper spacing:

• Increase the spacing by adding one, two, or three of these spacer rings to your imaging train as shown in the diagram referenced below
• First, you may reduce the distance by switching to a shorter 3.27" back focus spacer extension. Then, add the appropriate number of these 3.27" spacer rings to achieve the optimum spacing, use the 2.7" spacer rings (SRSET-27) with the 2.7" extensions (EXT27-...), or a combination of both 2.7" and 3.27" rings. You can easily experiment with 1 mm increments

Please reference the documents below for usage of these 3.27" spacer rings

Field Flatteners, TCCs, and QUADTCC Imaging Train Options

Determining Ring Spacing Change

Optimizing your Field Flattener, Lens, Camera Combination