Pros and Cons of a Schmidt Camera

Schmidt Camera Advantages:

There are a number of advantages in doing wide-field astrophotography with a Schmidt camera.

Very Fast F/ratio

The 8-inch Schmidt operates at F/1.5 while the 5.5-inch model is f/1.65.

Extremely High Resolution

The 8-inch Schmidt has a theoretical resolution of 1,333 lines per mm while the 5.5-inch resolves 1,212 lines per mm. The image remains sharp all the way across the field of view and there is no need to stop the aperture down to achieve maximum resolution. In practice, I doubt if the camera actually achieves such a high resolution, but the fact still remains that the Schmidt optics far outperform any lens system for resolving fine detail. The Schmidt will out resolve any film used, including Kodak's Technical Pan with its 320 lines per mm resolution.

The Schmidt camera's wide fast f/ratio makes it a natural for recording vast areas of faint Milkey Way nebulocity. The Schmidt camera's wide aperture, coupled with high-resolution optics, allow it to resolve detailed objects like the Omega Centauri star cluster.

Wide Field

The 8-inch Schmidt images a 4.5 X 6.5 degree field of view while the 5.5-inch covers 5.9 X 8.6 degrees on a 35 mm frame. The aftermarket "Vehrenberg" film holder for the 8-inch camera images a 10-degree circular field on cut film.

Fixed Infinity Focus

The Schmidt camera is permanently focused at infinity and needs no focus adjustment in the field. Anyone who has focused a prime focus camera on a telescope can appreciate what a convenience an accurate fixed focus is.

Efficient Use of Available Light

The Schmidt camera's fast focal ratio and wide aperture makes efficient use of the limited numbers of photons available from dim deep sky objects. Photographic emulsions are actually very insensitive detectors, having a quantum efficiency that can reach as low as one percent on extremely dim objects. This is because the light sensitive silver bromide crystals absorb only one in ten photons entering a photographic emulsion. The rest are reflected off the emulsion or absorbed by the gelatin suspending the silver bromide. Of the remaining photons, at least 10 have to strike a silver bromide crystal or it will not develop as black on the negative. If less than 10 photos strike the crystal, their effect is lost. The fast optics of a Schmidt camera helps gather a maximum number of photons on the film.

Schmidt Camera Disadvantages:

While these are attractive advantages for deep sky astrophotography, there are several inconveniences encountered when using a Schmidt camera. I mention these as a warning to know what to expect when operating a Schmidt, not as a condemnation of these instruments.

Low Image Contrast

This used to be a major problem with color films, though modern color saturated films like Kodak E200 have largely alleviated this problem. The Schmidt's super-fast optics reaches the film's sky fog limit so quickly that color images often cannot record faint objects. Low color contrast can be eliminated by using tri-color techniques, but while I greatly admire those who successfully do tri-color work, I figure life is too short for me to engage in that kind of aggravation when I can achieve reasonable success with the system I do use. Using a deep red filter such as a Wratten #92 with red sensitive black and white film like hypered TP-2415 can eliminate the low contrast problem. At my observatory's location, unfiltered exposures are limited to about five to 10 minutes, depending where I am aimed in the sky. With a #92 filter, I can expose in excess of an hour, but limit the maximum to 40 minutes out of practicality. To allow the use of filters, the Schmidt camera uses two different film holders. One is machined to focus the image without a filter; the other is machined to accommodate the slight focus shift that occurs when using a Wratten filter.

Small Image Scale

Make no mistake about it. The Schmidt camera is a wide-field instrument. While its optics are capable of extraordinary resolution, the camera itself still has a fairly short focal length. Even high-resolution TP-2415 film cannot resolve tiny objects while using a short focal length. Therefore, deep sky objects smaller than about 10 arc/minutes in diameter will begin to suffer poor resolution on the print. Singular small objects like the Ring Nebula are simply out of a small Schmidt camera's grasp.

No Viewfinder

The Schmidt camera is more like a telescope than a camera, but it has no eyepiece. There is no way to preview the field of view. A finder scope must be used to either point the Schmidt or to align the guide scope's setting circles in order to point at objects invisible to the eye. Aligning the Schmidt's finder scope is easy if you insert a piece of white paper in the film holder. In the dark, any celestial object 1st magnitude or brighter can be seen on the white paper when the film holder is installed on the magnetic spider. I drew a cross hair on the paper to insure the finder is set to the center of the field of view.

No Shutter

A lightweight black cloth, in this case a black T-shirt, makes an effective shutter for a Schmidt camera.

The Schmidt camera is one of the most advanced optical systems used in astrophotography, but it is like a throwback to the Daguerreotype days as it is a camera without a shutter. The classical hat trick must be used to start the exposure once the film plate holder is loaded into the camera. Most users fabricate a blackened cardboard cap that nests over the front of the corrector plate. The best Schmidt camera "shutter" I have found is a large black silk scarf. It is draped over the front of the camera, then the film is loaded, and the guide star centered. Once the guide star is being tracked, a gentle tug on the scarf starts the exposure without disturbing the camera's aim. Unfortunately, any kind of breeze will blow the flimsy scarf off the Schmidt, so I now use a heavier black T-shirt. My mounting is a lot stiffer than the springy fork mount normally used with a Schmidt and pulling the heavier garment off the camera does not cause any perceptible guide star movement.

Unusual Film Handling

The commercial roll film holder relieves the inconvenience of shooting single frane film chips. An inexpensive roll film holder can be fashioned out of anything that will support the film cassettes so they do not fall into the light cone reflecting from the mirror to the film.

The Schmidt is designed as a single-shot camera. The film must be loaded one frame at a time into a plate holder that is then installed on a magnetic mount at the camera's focus. Handling and processing single exposure film chips can be a real hassle, especially when humidity fogs the individual chips of hypered TP-2415 while they are stored in a light-tight box. This is partially eliminated by a commercially available roll film holder, but this device is notorious for scratching the film. An alternative is a simpler homemade roll film holder which consists of a stiff wire cradle to support loose 35 mm film supply and take up cassettes which hang on the film clamped in the plate holder.

Film Easily Scratched

The film holder used in a Schmidt camera uses a curved pressure plate that is bowed outward to preform the emulsion to the curved focal plane. This device is notorious for scratching film when it is loaded or when the film is advanced using a roll film holder. By design, the film must be slid in through a slot in the side of the film holder, and then the pressure plate is tightened against the film with a screw to press the film between the curved back of the plate holder and the frame of the plate holder. Often tiny machining burrs on the edges of the film frame will damage the film as it is slides through. Initially both of my film holders were bad about this. I had to disassemble them and sand the sharp edges with the finest grade of polishing paper I could find.

Image Reversed on the Negative

The Schmidt camera's mirror optics reverses the image on the negative. This is a minor inconvenience, but it must be remembered to print negatives or project slides backwards.

Internal Reflections

The bright planet Venus provides an exagerated illustration of internal reflections in a Schmidt camera.

Light from bright objects can reflect off the film and filter, or the filter holder itself, or bounce from the mirror to the underside of the corrector plate and back to the mirror, where they will create ghost images on the film. With the 8-inch Schmidt, any object first magnitude or brighter will create ghost images. A pincushion-shaped ghost with some internal line structure will always be diagonally opposite the bright object on the image. A line drawn from the ghost back to its parent object will pass through the center of the field. A flare or streak originating at the edge of the field is usually caused by a bright object reflecting off the inside edge of the film holder. This is usually a bright star just outside the field of view. Schmidt camera images also display spikes radiating outward from a bright star just like images taken through a Newtonian telescope. This is caused by diffraction of starlight around the spider vanes supporting the film holder.

Limited Specialized Use

For a 300 mm focal length instrument, the Schmidt camera is pricey. The 8-inch model will command between $2500 to $3500 on the used astronomy equipment market. This is a lot of money for an instrument that cannot be used for terrestrial photography. Some premium f/2.8 sports telephoto lenses of similar focal length cost about as much, but they can be used for other applications beside astronomy. However, for long exposure wide-field astrophotography, nothing can beat the wide aperture and fast focal ratio of a Schmidt camera.

It would seem, on the surface, that the disadvantages of a Schmidt outweigh the advantages. But this is illusion. In spite of these disadvantages, the Schmidt camera is still one of the most popular instruments for wide-field, extreme resolution deep sky photography. The results achieved with these instruments are well worth the challenges of operating them. There are a number of short-focus refractors with photographic fields approaching that of the 8-inch Schmidt, but again it must be emphasized again that nothing matches the wide aperture and fast optics of a Schmidt camera. Any serious astrophotographer who has captured the deep, fluffy detail of the entire North America Nebula and its companion, the Pelican Nebula on the same frame knows that an investment in a Schmidt camera is worth every penny.

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