## Eyepiece Extension??Posted By Vladimir Sacek |

If it's an 8" f/10, every inch of extending the focus will introduce ~1/16 wave of undercorrection, and increase the efective f# by ~5%. The final effect depends on what level and kind (overcorrected/undercorrected) of correction you have at the present focus position. If it is, for instance,

1/4 wave overcorrection, then a 2" focus extention would practically cut it in half. But if it is 1/4 wave undercorrection, you'd end up with 1/2.7 wave correction error.

For larger f/10 SCTs, the despace error diminishes proportionally (i.e. a 10" would have ~1/20 wave of undercorrection induced for every inch of focus extention).

1/4 wave overcorrection, then a 2" focus extention would practically cut it in half. But if it is 1/4 wave undercorrection, you'd end up with 1/2.7 wave correction error.

For larger f/10 SCTs, the despace error diminishes proportionally (i.e. a 10" would have ~1/20 wave of undercorrection induced for every inch of focus extention).