Part II: Collimating and Focusing the Celestron/Epoch Schmidt Camera
The Schmidt camera is a rugged instrument being simple in design with no moving parts to wear out or
break. The camera's focus is preset at the factory and without interference of some catastrophic event,
the Schmidt camera will retain its precise infinity focus virtually forever. However, in the real world, the
camera is transported, handled, and subjected to variations in temperature and humidity which all
conspire to eventually alter the instrument's focus and collimation.
The term collimation means placing the mirror, corrector plate, and film holder on axis and not tilted
relative to each other. Focus means proper placement of the mirror and film holder relative to each
other.
If the camera is out of collimation, star images will be comatic, or flared and comet-like in one direction
over the entire field. If the camera is out of focus, star images will be fuzzy or swollen, sometimes
showing the shadow of the film holder at the center of the bloated star image. The image may not
necessarily be out of focus over the entire field. In an extreme case, such as a tilted mirror, the camera
can be out of collimation and out of focus at the same time.
A quarter century of use eventually softened the focus on my Schmidt and forced me to research the
techniques of servicing the Celestron/Epoch Schmidt camera. Since all other owners of these
instruments are also now without factory support, I decided to chronicle my efforts to repair my 8-inch
Schmidt as to guide others in the repair of their own Schmidt cameras.
I should say that my experiences are with the original Celestron 8-inch Schmidt camera, not the 5.5-inch
or the modified version that has been upgraded by Epoch Instruments with a different mirror cell and
spider assembly. References to design or assembly differences in the Epoch Schmidt cameras as
opposed to the Celestron models are gathered from anecdotal information provided by owners of these
instruments.
There are a number of critical measurements in a Schmidt camera. The curve of the film holder must be
actly concentric with the curve of the mirror. The distance from the mirror to the film holder must be equal
between the mirror and the center of the film holder and the mirror and the edges of the film holder. The
mirror, film holder, and corrector plate must be on-axis and collimated. If any of these measurements are
incorrect, the camera will be out of focus. In fact, an f/1.5 Schmidt camera is sensitive to focus
variations of only .001 inch.
Go to the previous page ---- Pros and Cons of a Schmidt Camera
Go to the next page -------- Initial Inspection
Back to -------------------- Table of Contents
