The Celestron/Epoch Schmidt Camera....
Beyond the Owner's Manual
By Robert Reeves
(These pages are published for anybody interested in Schmidtcameras on
this page with permission by Robert Reeves)
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
Pros and Cons of as Schmidt Camera
Collimating and Focusing the Celestron/Epoch Schmidt
Adjusting Corrector Plate Position
Removing the Corrector Plate
Checking Mirror Collimation
Adjusting Mirror Collimation
Special Equipment for Focusing the Celestron/Epoch
Checking Camera Focus
Adjusting Camera Focus
Removing and Installing the Mirror
Modifications to the Schmidt Camera
Common Schmidt Camera Errors
Miscellaneous Schmidt Notes
A Look at a 14-inch Schmidt Camera
Schmidt Camera Survives House Fire
Lessons Learned While Adjusting Camera Focus
Part One: An Introduction to 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.
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
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.
|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.
(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.
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.
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.
|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.
Go to the next page ----The Pros and
Cons of a Schmidt Camera
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
Contributors who have shared their Schmidt camera experiences with me
for this article:
Andreas and Sabine Philipp
References used in preparing this article:
| Astrophotography With a Schmidt Telescope
Atlas of Deep Sky Splendors
ATM Book 2
ATM Book 3
Practical Computer-Aided Lens Design
The Celestron Schmidt Camera
| Marx & Pfau
Albert Ingals, Editor
Albert Ingals, Editor
Gregory Hallock Smith
Rutten & van Venrooij