Helix 250 - vignetting & distortion

Vignetting leads to light drop-off near the corners of the field. Every astrophotographer knows this effect. In the Helix 250 the primary mirror is larger than the entrance pupil of 250 mm. This means that vignetting only starts at 12.5 mm off-axis. At a distance of 21 mm (corner of small film) vignetting is only 4%.

The Helix 250 has almost no distortion. At 10 mm off-axis the distortion is only 0,004% which corresponds to 0,4 micron. At 15 mm the distortion is 0,01 mm which corresponds to 1,5 micron. At the corners of a small film negative distortion is 0,02% and 4 micron. This means that even in the corners distortion is only visible at a sub-pixel scale. This makes the Helix 250 very useful as an astrograph to determine the position of super novae, planetoids etc.