Unless it is specifically designed for photography, a doublet or triplet refractor does not offer a flat and corrected field; the image circle at the focus is reduced and does not fully cover the sensors used in imaging. To solve this problem, opticians use a corrector whose design is, ideally, specific to the field curvature of the lens of the instrument on which it will be mounted.
For its refractors with triplets, TSA or TOA, Takahashi has designed the 35FL flattener. It offers good field correction over a diameter of 40 mm and will satisfy users of APS-C format sensors. Astrophotographers equipped with larger sensors, and TOA telescopes with a 3" focusing system, will have to turn to the giant 67FL flattener capable of covering up to medium format sensors.
The 35FL is mounted with a 2" (50.8 mm) barrel; an M48 thread is available for screwing in a filter. The outlet is also at the 2" barrel. The mechanical backfocus is very large (117.5 mm), which offers a high degree of freedom for mechanical installation behind the telescope.
Be careful! For astrophotographers wishing to work with the native focal lengths of TSA-120 and TOA-130NS, the only field corrector available is 35FL. It is therefore strongly recommended not to use a sensor that is too large on these instruments and these focal lengths. The TOA-35RD0.7X focal reducer and the EX-ED1.5X extender allow a better field coverage but at different focal lengths.
| Characteristic | Takahashi 35FL field corrector |
| Image circle diameter | 40 mm |
| Metal back distance | 117.5 mm |
| Input mechanical interface | 2" (50.8 mm) barrel, male |
| Output mechanical interface | 2" (50.8 mm) barrel, female |
| Backfocus length | 38 mm |
| Mechanical diameter | 60 mm |
| Total length | 74 mm |
| Weight | 200 g |
Diagram of threads and focal point position of the Takahashi 35FL field corrector