anyone know ST120 flat field

[kimo]

Hi,

I am trying to know the orion St120 flat field circle size without flattener. anyone know what is the size of it? anyone have image sample without flattener?

thanks
Kimo

[orly_andico]

Refractors have a field curvature equal to their focal length. The ST120 is f5 I suppose so 600mm focal length.

It will need a flattener for any decent size chip. For a small chip like an ICX285 I think you can get away without one.

Rule of thumb, any refractor below 1000mm will need a flattener for 4/3 or DSLR chips. Unless it's a flat field refractor like a televue or Takahashi.