Telescope Reviews: Hypo/Dialyte testing discovery

In the case of a stopped-down system, as occurs when the aperture of the afocally employed system (camera or eye) had a pupil smaller than the telescope's exit pupil...

Assuming the camera's iris is correctly located in the plane of the exit pupil (as it should be for evenness of illumination, and especially when the iris and exit pupil are close to the same size), an image of the iris is formed at the object glass. And so all image forming light contributing to the image passes through this reduced central aperture at the OG.

And even if it were such that off-axis image-forming light passed through the OG offset from the latter's center (as though an off-axis mask were installed), the same reduced fraction of its aperture would be employed.

The point is that no one image point anywhere in the field is being formed by the full aperture of the OG, and so aberrations are reduced. Make up a 'telescope' using a single-element objective, which with a suitable eyepiece produces an exit pupil of about 2mm for daytime testing, or better yet 5mm for nighttime. The chromatic and spherical aberrations will be awful. Now place a mask up front which reduces the exit pupil to 1mm, or a bit less (starting from the larger, low-light exit pupil will make for a more dramatic change). The image, while dimmer, will be of rather better quality.

This is why I'm stressing the importance of knowing whether and by how much the afocal imager is reducing the effective aperture by virtue of its small iris. Any aperture reduction artificially improves performance, especially when the reduction is significant. And for the Hypo (in its simpler configuration), which is limited to larger exit pupils, images are only properly representative when the camera's iris at least comes close to equaling the exit pupil diameter.

One way to obtain an idea of the actual working aperture is to find the diameter of the circular obstruction which just blacks out the image. With the camera's iris located at the exit pupil plane, the eyepiece will form an image of the iris on the OG, the diameter of which is the iris's diameter times the telescope magnification. An obstructor having the same size as this projected iris will, when positioned on top of it, will just fully block all image forming light. Experimentally try successively smaller obstructors until one is found which just blacks out the image when positioned just so; this is, to reasonable accuracy, the effective aperture of the system.