Celestron CGEM II 800 EdgeHD Telescopes Whitepaper EdgeHD Optics - Page 11

Autocollimation Testing Telescope being checked Precision optical flat Beamslitter Eyepiece and Ronchi grating Green Laser (532 nm) FIGURE 9. In autocollimation testing, light goes through an optical system, reflects from a plane mirror, and passes through again. This super-sensitive test method doubles the apparent size of all errors. After we remove each set of optics from the autocollimator, we send the components to our in-house coating chamber. Here, the primary and secondary mirrors receive their high-reflectance aluminum coatings, and the corrector lens is anti-reflectance coated. Each set of optics is then installed into an optical tube assembly (OTA). Completed OTAs undergo the Visual Acceptance Test. In a temperature-stabilized optical test tunnel, green laser light at 532nm wavelength is reflected from a precision paraboloidal mirror to act as an artificial star. With a high-power ocular, a QA Inspector views the artificial star critically. To pass the Visual Acceptance Test, an optical tube assembly must meetthe following requirements: • T he in-focus Airy disk must be round, free of scattered light around the disk, and display only one bright ring. • Inside and outside focus, the diffraction patterns must be round, uniform, and appear similar on both sides of focus. • O bserved with a 150 line-pairs-per-inch Ronchi grating, the bands must be straight, uniformly spaced, and high in contrast. Because its optics have been tested and tuned in error-revealing double-pass mode, and because each assembled OTA has been tested again and qualified visually, the telescope's images should be flawless when observing and imaging the night sky. FIGURE 10. We test all of our primary mirrors on an optical bench by means of laser interferometry. In the picture, stacks of polished primary mirrors await testing. FIGURE 11. To correct any remaining optical errors, the figure of the secondary mirror is fine-tuned against the entire optical system in double-pass autocollimation setup. This delicate match process ensures that every telescope performs to the diffraction limit. 7. FINAL ACCEPTANCE TESTING AND CERTIFICATION Before it can leave Celestron's facilities, every EdgeHD must pass its Final Acceptance Test, or FAT. We conduct the FAT on an optical test bench in a specially-constructed temperaturecontrolled room (Figure 12). Rather than use laser light for this test, we use white light so that the FAT reproduces the same conditions an observer would experience while viewing or photographing the night sky. To avoid placing any heat sources in the optical path, the light for our artificial star is carried to the focus of a precision parabolic mirror through a fiber-optic cable. After striking the parabolic mirror, the parallel rays of light travel down the optical bench to the EdgeHD under test, through the telescope, to a full-frame format digital SLR camera placed at its focus. Using a set of kinematic test cradles, there is no need to change the test configuration between different EdgeHD telescopes. We simply place the telescope in its test cradle on the bench, and it's ready for testing. The Final Acceptance Test verifies an EdgeHD's ability to form sharp star images in the center and on the edges of a full-frame (24×36mm format, with a 42mm diagonal measurement) digital SLR camera. The QA Inspector attaches the camera to the telescope, focuses carefully, and takes an on-axis image. The telescope is then pointed so the artificial star image falls in the corner of the frame, and without refocusing, the inspector takes another image. The process is repeated for each corner of the camera frame, and another picture is taken at the center of the frame. To pass the test, the telescope must form a sharp image at the center of the field, at each corner of the camera frame, and again at the center. The images are examined critically. To pass, every one of the test images must be tight, round, and in perfect focus. Any EdgeHD that does not pass the FAT is automatically returned to the assembly room to recheck the collimation and centering of its corrector plate. No EdgeHD can leave the factory until it has passed its FAT. The Celestron EdgeHD 11