3.5" Prime Focus Field Flattener for 180 TEC Refractors (180F735FF)

Brand :


October 2023 - This field flattener was introduced to optimize performance of the 180mm f7 refractors from Telescope Engineering Company (TEC).  Limited quantities available, so if you would like one, don't wait. If you have lost the original field flattener provided with your AP 180f7 or 175f8 StarFires, we recommend this one as a replacement. 

Field flatteners are designed to accurately compensate for the inherent field curvature of a refractor lens. For each lens design there is an optimal spacing that works best for sharp focus over the specified field size. As camera designs have advanced, the performance requirements have become more stringent. This is now especially important when using the newest CMOS cameras with pixel sizes of 3.76 microns and smaller. This field flattener has been optimized for these 180mm refractors.

Backfocus Spacing from the Rear Flange

The backfocus spacing for these flatteners (as measured from the rear flange) is 3.2597" +/- 1/10" (82.8 mm +/- 2.5 mm). Spacer rings (1mm) are provided to achieve the 80.80mm spacing on which our various adapters are designed.  


35mm SLR or DSR Camera - For full-frame Canon and Nikon DSLRs, we offer our large DSLR adapters with their large 1.875" (47.6mm) inside diameter bayonets: ADATCCEOS and ADATCCNIK.

CCD Camera - Choose the appropriate adapter to space the field flattener properly with your CCD camera. Consult the Diagrams and Instruction Sheets below for a wide variety of options.

180F735FF Includes

  • Field Flattener housing with optics optimized for this version
  • 3.27" Spacer Rings , Set of 3 Rings (SRSET-327)
  • Two beautifully machined and black anodized covers
  • 18035FF Label to differentiate from similar assemblies

Required Adapters 

TEC180  scope: You will also need to order a Drawtube Endcap (S35RFAA), shown below, for the focuser. This only applies to the TEC focusers. This part is integral to the Astro-Physics 3.5" focusers and is not needed as a separate purchase. The S35RFAA screws onto the drawtube's 4.100" x 24 thread (same as Starlight Instrument's 3.5" focuser endcap thread).

Diagrams and Instructions

If you own items not shown in the diagrams above, please refer to the Discontinued Products section of this website and Technical Support for Legacy Products that are No Longer Available

Custom Adapters

If you need an adapter that is not listed above, we recommend that you contact PreciseParts. Refer to the link for the spacing requirements document above.