The Celestron EdgeHD
11
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.
Autocollimation Testing
Precision optical flat
Beamslitter
Green Laser (532 nm)
Eyepiece and Ronchi grating
Telescope being checked
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.
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:
• The 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.
• Observed 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.
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 temperature-
controlled 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 with-
out 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.
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.
