|
Hi
The common mass produced focal reducers are meant to be used on SCT's with a secondary F ratio of 10. The common size is refered to as F6.3. and reduces the effective focal length by about a third. However these devices do more than reduce the F ratio as they are designed to match the optical path of a SCT and perform field flattening as well.
A second common type is refered to as F3.3 will reduce the focal length by two thirds. However these also are designed for use with a F10 SCT for older digital imagers using small physical size chips to get an apparent wider field. These type are not suitable for use with the large CCD's that are now used extensively as distortion outside the designed area can be quite severe.
This means that they are not suitable for use on a refractor. Admittedly they will reduce the effective focal length by a third or two thirds but the other corrections will not match the optical path.
For a refractor you will need a focal reducer designed for the actual telescope and it will probably cost more than replacing the OTA with one of a shorter focal length.
The calculators can be used to give the approximate photo dimensions if you substitute the amended reduction in focal length.
Barry
|