Although the following document is 12,000 words long, it is far from complete. I ask all Schmidt camera owners to share their experiences with me so I can add to this Schmidt Camera guide. Please share your insights about adjusting, repairing, modifying, and user tips for the Celestron/Epoch Schmidt camera.
The very name Schmidt camera seems to invoke some sort of mystique in astrophotography circles. A Schmidt camera is basically an ultra-fast telescope with a film plate holder instead of eyepiece at its focus. The device is an exotic, yet deceptively simple high precision instrument that is also practically bulletproof. Since f/ratio determines how dim an extended object (nebulae) we can photograph, and aperture determines how dim a star we can photograph, the fast optics and wide aperture of the 8-inch f/1.5 Schmidt camera give us the best of both worlds in one instrument.
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| The author's 8-inch Schmidt camera mounted atop a C-11 fork assembly with an 8-inch Celestron guide scope. | The C-11 fork assembly tracks well enough to secure good photos, but to poorly allow the use of an autoguider. |
Three decades ago, astrophotographers who wanted a high-speed, high-resolution, fixed-focus wide-field Schmidt camera were rewarded when Celestron began manufacturing affordable 5.5-and 8-inch Schmidt cameras. Limited quantities of 14-inch Schmidt cameras were also manufactured, but they were difficult for the average skyshooter to work with and were discontinued. The author has owned an 8-inch Schmidt camera for the past 25 years.
The design of the Schmidt camera is unusual in that it has no moving parts: no shutter, no focusing mechanism, and no film transport, none of the usual items you expect in a camera. The Schmidt was designed for the singular purpose of quickly recording large areas of the sky without any aberrations in its optical image.
Although it represents the ultimate in high-tech optics for wide-field celestial photography, the principle of the Schmidt camera is far from new. The basic design has been in existence since 1930 when a previously unknown European optician named Bernard Voldemar Schmidt completed a revolutionary 14-inch aperture camera with a fantastic f/1.7 focal ratio.
Prior to Bernard Schmidt's optical invention, astrophotography was a lengthy, tedious affair, with exposures often extending over several successive nights in order to record celestial scenes with the slow focal ratio lens-based astrographic cameras of the day. Thanks to Schmidt's new design, the photographic exploration of the sky was catapulted into the 20th Century. Exposure times were radically cut while celestial detail recorded on film skyrocketed.
It has been said that without Schmidt's invention, our current understanding of the universe would have been considerably delayed. The instrument's fast f/ratio, wide field and unparalleled sharp optics uncovered cosmological relationships that may have taken much longer to find with the narrow field of regular optics.
The key to both the Schmidt's simplicity and its precision is the optical design. An extremely fast spherical mirror gathers light and focuses the image on the film plane. The short focal length mirror possesses a very shallow depth of focus. The zone of focus is so narrow that rods made of a special zero-expansion metal alloy called Invar are used to position the film holder at a precise distance from the mirror. If steel or aluminum were used instead of Invar, expansion from minor temperature changes would move the film out of the mirror's focus.
The spherical aberration produced with very short focus spherical mirrors will not allow a good focus across the entire field of a flat photographic plate. In a regular Newtonian telescope, spherical aberration is eliminated by aspherizing the primary mirror. Unfortunately, this also introduces coma, another aberration that increases with greater distances from the center of the field of view. Thus, the Schmidt design retains a spherical mirror because its aberration is constant across the field of view, and therefore can be corrected.
The Schmidt design has two features that combat the primary mirror's spherical aberration. First, a corrector plate, a thin zero-power aspheric lens, is placed at the radius of curvature of the primary mirror. This lens introduces an inverse spherical aberration to the incoming light rays. To do this, it is slightly convex at the center and slightly concave at the edge, and warps the incoming light rays. Light rays at the center of the aperture are converged, those about halfway out from the center are unaffected, while the light rays at the edge of the aperture are diverged. This bends the incoming light rays in such a way that when they are all reflected to the focal plane by the spherical mirror, they are focused across the entire field of view.
(A) loading port, (B) mirror, (C) mirror cell, (D) Invar (spacer) bar, (E) spacer nuts, (F) spider vane, (G) magnetic film holder saddle, (H) film holder, (I) film, (J) corrector plate
Diagram courtesy Celestron
A unique feature of the Schmidt design is that while the corrector and mirror optics of the Schmidt focuses e incoming light rays perfectly, the resulting focal plane is curved. This introduces the need for the Schmidt's second innovation; the film plane is deliberately curved to match the curved focal plane of the mirror.
The Celestron/Epoch-type Schmidt cameras use single exposure film holders that are installed into the camera through a door in the side of the camera body. Thirty-five millimeter film holders for the Celestron/Epoch cameras all look alike but in fact, there are two separate types. One is designed for use in "white light", that is, without a Kodak Wratten gel filter. The other type is for use with a Wratten gel filter. The two are noninterchangable because the film holder designed for use with filters is machined to compensate for the focus shift when using filters. A dowel pin indexes the film holder's orientation while a magnetic retainer holds the film holder at the proper focus point on a spider assembly similar to that of a Newtonian secondary mirror holder. The indexing pin prevents lateral movement of the film holder by no more the .002 inches. But in practical terms, the film holder is untouched during the exposure so the magnetic mount keeps the film holder stationary. The retainer allows the film holder to be installed with the 35 mm film's long dimension oriented either north-south or east-west.
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| The white light filter holder exposes the film directly to starlight with no filter assembly attached. | The filter film holder is seemingly identical to the while light holder except it is machined to maintain focus with the Wratten filter attached in front of the holder. |
There are two types of film holder for a Schmidt camera, front referencing and rear referencing. What this means is the focus of the camera is measured relative to the film holder frame against the emulsion of the film, or it is measured from the pressure plate of the film holder behind the film. Both types of film holders employ a pressure plate that is bowed outward. When the pressure plate is clamped against the open frame of the film holder, it curves the photographic emulsion to conform to the Schmidt camera's curved focal plane. The advantage of the front referencing film holder is that any thickness of film, thin-based Technical Pan or thicker-based color film, can be used in the holder without having to refocus the camera. The focus point is the emulsion, which always stays pressed against the film holder frame, while the varying thickness of the film base merely spaces the pressure plate away from the holder's frame. On the other hand, changing the film thickness used with a rear referencing film holder will move the emulsion away from the camera's exact focus point. Fortunately, the 35 mm film holders used with the Celestron/Epoch cameras are front referencing so any kind of 35 mm film can be used in them.
A regular camera lens with a 50-degree field of view will vignette 30 percent at the edge of the field because the round lens aperture is projected as an ellipse on the edges of the film. A Schmidt camera on the other hand will be free of vignetting if the mirror size is equal to the diameter of the corrector plate plus twice the width of the film holder. In practicality to reduce size and cost, most Schmidt cameras have a mirror size equal to the corrector plus only one film holder diameter. This introduces a negligible amount of vignetting that is usually ignored. The 8-inch f/1.5 Schmidt camera has a nearly equal sized corrector and mirror. This system vignettes the field of view by 15 percent at the edges. This is tolerated in order to reduce costs by allowing the camera to be constructed with optical components common with an 8-inch Schmidt Cassegrain telescope.
Vignetting in the 8-inch Schmidt is mathematically significant, but in practicality it is barely detectable to the eye except on the highest contrast full-frame black and white prints. The light drop-off at the edge of the field is so slight that it is only of consequence to projects such as measuring magnitudes using densitometry. But then again, today such tasks fall to instruments such as CCD detectors.
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In the 1980's, Celestron ceased marketing the Schmidt camera entirely, but continued to manufacture the camera's optical components for Epoch Instruments. For another 20 years, Epoch continued to build and market the 5.5- and 8-inch Schmidt cameras, as well as repair and update cameras previously built by Celestron. Operationally, the instruments produced by Celestron and Epoch are virtually identical. There are variances in how the mirror is mounted and where assembly screws are drilled into the tube assembly, and the design of the spider assembly. Because of this commonality, the cameras are almost universally known as Celestron/Epoch Schmidt cameras, although the two manufactures were separate companies.
After 20 years of Schmidt camera business, the owner of Epoch instruments suffered serious health problems and was forced to close the company. Although he is now recovered, he never reentered the optics business, chosing instead to pursue other less stressfull astronomical pursuits for a living. Thus in 2001, Epoch Instruments ceased operation and owners of Celestron/Epoch Schmidt cameras found themselves orphaned with no factory support for repair or adjustment of existing cameras.
I do not clain originality for any of the ideas or techniques described in the document. I also aknowledge that some of this document is material quoted from the contributors listed below. I do, however, claim copyright to the manner in which this material is presented. I grant permission for the information in this article to be freely distributed only for personal or non-profit use.
Andreas and Sabine Philipp
Chris Nisbet
David Levy
Dean Ketelsen
Douglas Schmutz
Fernando Paniagua
Jim Bandy
John Mirtle
Kent Kirkley
Keith Shank
Michael Treacy
Richard Payne
Rick Dilsizian
Stefan Beck
| Astrophotography With a Schmidt Telescope Atlas of Deep Sky Splendors ATM Book 2 ATM Book 3 Practical Computer-Aided Lens Design Telescope Optics The Celestron Schmidt Camera Wide-Field Astrophotography | Marx & Pfau Hans Vehrenberg Albert Ingals, Editor Albert Ingals, Editor Gregory Hallock Smith Rutten & van Venrooij (owner's manual) Robert Reeves |
