Large Telescope Optics
Quality
Lomo is our standard supplier of the optic set. We have already delivered many large sized optics (1.1m, some 1.2m, several 0.8m and 0.9m and many 0.6m). Lomo according to our opinion is one of the best manufacturers of large Cassegrain optics and is unbeatably regarding the price performance ratio. Both the main mirror and the complete Cassegrain-Sysystem are interferometrically tested. We think this is a superior method compared to the testing of the main mirror and secondary mirror separately. The encircled energy will be better than 80% in 0.5 arc seconds.
The used material is Sitall, which is the Russian equivalent to Zerodur. There is a slight disadvantage with Sitall, which we do not want to conceal. There are nearly always some bubbles in the blank and sometimes it happens, that some small bubbles are open in the optical surface. But the total area of the bubbles is so small, that it does not affect the optical performance.
Design
The optical design regularly consists - besides the primary and secondary mirros - of a powerfull 3-lens corrector, Filter and CCD cover slip. It will be pixel- and seeinglimited in most conditions. The corrector consists of a 3-element in 3 groups. The maximum useful format is D=70mm (even larger diameter can be used with some vignetting), which is 1.15 degree in diameter. The corrector has a very good performance through a wavelength interval of 400 to 1000nm. The design gives 117mm back focus, which is enough for the filter wheel and typical CCD camera.
The drawing shows the lens corrector for the prime focus together with the holder for the secondary (example 80cm). To use the prime focus all you have to do is remove the secondary mirror. This may be easy in case of 80cm, but it is to heavy to do it by hands in case of 1.2m telescope. In this case, we have to find a solution together with the customer to use a small crane (for example attached to the dome) to exchange the secondary mirror.
We measured the image resolution which was down to 1.2 arc seconds for the best exposures which is an exceptional value for European conditions. For the secondary focus a seeing limited 2-lens corrector is offered and sufficient.