Two Mirror Catadioptric
Another large group shown in the telescope tree is the two-mirror catadioptric. It can be designed with a flat or
curved field, in a compact or non-compact configuration, and with a variety of types of cirrectors. In most cases,
the focal palane is outside the system, behind the primary mirror, for good accessibility.
In flat-field designs, the radii of curvature of the primary and secondary mirrors are approximately equal. For the image to lie behind the primary mirror, a relatively large secondary mirror (typically 50% to 60% of the diameter of the entrance pupil) is needed. Therefore, these flat –field telescopes are not well suited for visual applications. In designs that allow a curved field, the secondary mirror can be more strongly curved and smaller than in flat-field designs (typically 30% to 40% of the entrance pupil diameter). A field flattener must be used when these instruments are used for photography or CCD imaging.
In comparision with the compact design, the non-compact, two-mirror catadioptric design suffers from greater tube length, a secondary mirror that must be by a spider, and for the same degree of light drop-off at the edge of the field, larger primary and secondary mirrors. However, for flat-field designs, the baffle system required can be smaller or even omitted.
When the designer chooses between a non-compact or a compact system, he must take into account not only these points, but also the possibilities for correcting aberrations. With the exception of a few non-compact flat-field designs, all Cassegrain-derived telescopes require a baffle tube system. Depending on the width of the field, bafflinfg may cause a considerable light drop-off toward the edge of the field. This is a decided disadvantage compared to the one-mirror catadioptrics, which do not require such baffles.
In flat-field designs, the radii of curvature of the primary and secondary mirrors are approximately equal. For the image to lie behind the primary mirror, a relatively large secondary mirror (typically 50% to 60% of the diameter of the entrance pupil) is needed. Therefore, these flat –field telescopes are not well suited for visual applications. In designs that allow a curved field, the secondary mirror can be more strongly curved and smaller than in flat-field designs (typically 30% to 40% of the entrance pupil diameter). A field flattener must be used when these instruments are used for photography or CCD imaging.
In comparision with the compact design, the non-compact, two-mirror catadioptric design suffers from greater tube length, a secondary mirror that must be by a spider, and for the same degree of light drop-off at the edge of the field, larger primary and secondary mirrors. However, for flat-field designs, the baffle system required can be smaller or even omitted.
When the designer chooses between a non-compact or a compact system, he must take into account not only these points, but also the possibilities for correcting aberrations. With the exception of a few non-compact flat-field designs, all Cassegrain-derived telescopes require a baffle tube system. Depending on the width of the field, bafflinfg may cause a considerable light drop-off toward the edge of the field. This is a decided disadvantage compared to the one-mirror catadioptrics, which do not require such baffles.