Celestron 11007 Betriebsanweisung PDF herunterladen (Seite 51)

2 I The Celestron EdgeHD

The Celestron EdgeHD A Flexible

Imaging Platform at an Affordable Price

By the Celestron Engineering Team

ABSTRACT:

The Celestron EdgeHD is an advanced, ﬂat-ﬁeld, aplanatic

series of telescopes designed for visual observation and imaging

with astronomical CCD cameras and full-frame digital SLR

cameras. This paper describes the development goals and

design decisions behind EdgeHD technology and their practical

realization in 8-, 9.25-, 11-, and 14-inch apertures. We include

cross-sections of the EdgeHD series, a table with visual and

imaging speciﬁcations, and comparative spot diagrams for

the EdgeHD and competing “coma-free” Schmidt-Cassegrain

designs. We also outline the construction and testing process for

EdgeHD telescopes and provide instructions for placing sensors

at the optimum back-focus distance for astroimaging.

1. INTRODUCTION

The classic Schmidt-Cassegrain telescope (SCT) manufactured

by Celestron served an entire generation of observers and astro-

photographers. With the advent of wide-ﬁeld and ultra-wide-ﬁeld

eyepieces, large format CCD cameras, and full frame digital SLR

cameras, the inherent drawbacks of the classic SCT called for a

new design. The EdgeHD is that new design. The EdgeHD of-

fers clean, diffraction-limited images for high power observation

of the planets and the Moon. As an aplanatic, ﬂat-ﬁeld astro-

graph, the EdgeHD’s optics provide tight, round, edge-to-edge

star images over a wide, 42mm diameter ﬂat ﬁeld of view for

stunning color, monochrome, and narrow-band imaging of deep

sky objects.

2. SETTING GOALS FOR THE EDGEHD TELESCOPE

The story of the EdgeHD began with our setting performance

goals, quality goals, and price goals. Like the classic SCT, the

new Celestron optic would need to be light and compact.

Optically, we set twin goals. First, the new telescope had to

be capable of extraordinary wide-ﬁeld viewing with advanced

eyepiece designs. Second, the optic had to produce sharp-to-

the-edge astrophotography with both digital SLR cameras and

astronomical CCD cameras. Finally, we wanted to leverage

Celestron’s proven ability to manufacture high-performance

telescopes at a consumer-friendly price point. In short, we sought

to create a ﬂexible imaging platform at a very affordable price.

Given an unlimited budget, engineering high-performance optics

is not difﬁcult. The challenge Celestron accepted was to

control the price, complexity, and cost of manufacture without

compromising optical performance. We began with a compre-

hensive review of the classic SCT and possible alternatives.

Our classic SCT has three optical components: a spherical

primary mirror, a spherical secondary mirror, and a corrector plate

with a polynomial curve. As every amateur telescope maker and

professional optician knows, a sphere is the most desirable

optical ﬁgure. In polishing a lens or mirror, the work piece moves

over a lap made of optical pitch that slowly conforms to the glass

surface. Geometrically, the only surfaces that can slide freely

against one another are spheres. Any spot that is low relative to

the common spherical surface receives no wear; any spot that is

higher is worn off. Spherical surfaces result almost automatically.

A skilled optician in a well-equipped optical shop can reliably

produce near-perfect spherical surfaces. Furthermore, by

comparing an optical surface against a matchplate—a precision

reference surface—departures in both the radius and sphericity

can be quickly assessed.

In forty years of manufacturing its classic Schmidt-Cassegrain

telescope, Celestron had fully mastered the art of making

large numbers of essentially perfect spherical primary and

secondary mirrors.

In addition, Celestron’s strengths included the production of

Schmidt corrector plates. In the early 1970s, Tom Johnson,

Celestron’s founder, perfected the necessary techniques.

Before Johnson, corrector plates like that on the 48-inch

Schmidt camera on Palomar Mountain required many long

hours of skilled work by master opticians. Johnson’s innovative

production methods made possible the volume production of a

complex and formerly expensive optical component, triggering

the SCT revolution of the 1970s.

For more than forty years, the SCT satisﬁed the needs of

visual observers and astrophotographers. Its performance

resulted from a blend of smooth spherical surfaces and

Johnson’s unique method of producing the complex curve

on the corrector with the same ease as producing spherical

surfaces. As the 21st century began, two emerging technologies

—wide-ﬁeld eyepieces and CCD cameras—demanded high-

quality images over a much wider ﬁeld of view than the clas-

sic SCT could provide. Why? The classic SCT is well-corrected

optically for aberrations on the optical axis, that is, in the exact

center of the ﬁeld of view. Away from the optical axis, however,

its images suffer from two aberrations: coma and ﬁeld curvature.

Coma causes off-axis star images to ﬂare outward; ﬁeld curvature

causes images to become progressively out of focus away from

the optical axis. As wide-ﬁeld eyepieces grew in popularity, and

as observers equipped themselves with advanced CCD cameras,

the classic SCT proved inadequate. To meet the requirements of

observers, we wanted the new Celestron optic to be both free of

coma and to have virtually zero ﬁeld curvature.

1 2 ... 46 47 48 49 50 51 52 53 54 55 56 ... 74 75

Kommentare zu diesen Handbüchern

Keine Kommentare

Celestron 11007 Betriebsanweisung Seite 51

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Teleskope Celestron 11007