Part III: Modifications and Operating Tips for the Schmidt Camera

Homemade Roll Film Holder

The use of roll film in the Schmidt camera is perhaps the single greatest convenience that can be added to the camera beyond the use of an autoguider. A homemade roll film holder can be made from easily obtained hardware items and an hour's worth of do-it-yourself work.

The mechanics of a homemade roll film holder are simple. The film leader is passed through the Schmidt's film holder just as when loading single shot film chips. Instead of cutting the film, the supply cassette is left hanging from the film holder. The leader extending through the film holder is then spooled into a reusable 35 mm cassette. Now there are two cassettes hanging from the film holder. The problem is to keep the cassettes from drooping into the light cone reflecting from the Schmidt's mirror back onto the film holder. The solution is to fabricate a stiff wire cradle to support the cassettes.

To fabricate the cradle, remove the pressure plate and its locking screw to avoid damaging them. Next, drill two holes into the side of the film holder using a #29 drill bit. Tap these with a #8x32 machine screw tap and carefully de-burr the inside of the film channel. Next, using two #8x32x1/4-inch oval head machine screws and flat washers, secure six inches of stiff wire to each side of the film holder. Two straightened thick paper clips will work quite nicely. With needle nose pliers, fashion the wire to form the necessary cradle and loop the ends of the wire so they have no sharp ends. The cradle should hold the cassettes level with the film plane.

The wire used for the cradle needs to be at least 3/64-inch thick because it also acts as a spacer between the film holder and flat washers holding the wire. This 3/64-inch space is needed so the film holder's dust cover or filter holder will still snap into place.

A simple wooden jig will measure the proper film advance between exposures. This device is used in photo changing bag because the film is open during the procedure. The jig is an L-shaped fixture with an indexing hole drilled into its base. The film holder's pressure plate locking screw fits into this hole and the supply cassette is drawn out 2.5 inches until it bottoms against the vertical leg of the L-shaped jig. This distance provides enough film to prevent overlapped exposures and damage to the previous exposure by the pressure plate clamping against the film holder frame.

Once the supply film is pulled out 2.5 inches, the film holder dust cover is removed, the pressure plate is loosened, the fresh film is drawn through, the pressure plate is locked again and the exposed film wound into the take-up cassette. At this point, make sure the felt lip of the supply cassette isn't caught between the film holder frame and pressure plate. The dust cover is then snapped into place and the film holder is ready for reinstallation in the camera.

To tell which cassette is the supply or take-up while in the changing bag, one of them should be marked with tape.

In an 8-inch Schmidt camera, the two cassettes on either side of the film holder block about 10 percent of the incoming light. This means a 30-minute exposure will have to be extended to approximately 33 minutes to achieve a similar exposure.

Keith Shank used an alternative approach. He made a roll film holder out of 1/16-inch aluminum plate. He drilled a matching hole in the plate that allows a simple compression fit to the Schmidt film holder. The film spools are held to the plate with plastic tie wraps. The aluminum plate is painted black to prevent reflections inside the camera. To wind the film, the whole assembly is placed in a dark bag. The film holder is popped out of the aluminum plate with a push from the back. Slipping the tie wraps off the outer edges of the cassettes loosens the film containers. Once the film is advanced, the film holder is pushed back into the aluminum plate, the tie wraps resecure the cassettes, and it's ready to go back into the camera in less than a minute.

Gas Purging

In order to use hypered TP-2415 in the high humidity of south Texas, I had to modify my Schmidt camera with a dry nitrogen purge system. The need for this comes from the fact that water vapor will cause severe film fogging with hypered TP-2415. Over the course of a long exposure, the film in its holder is exposed to the large volume of open air inside the camera. During this time, high humidity will substantially degrade the image contrast. High humidity also degrades the Wratten gel filters used with the filter film holder. Additionally, some unhypered films swell and creep as they absorb water vapor. At the least, this creep will elongate star images at the edge of the field of view. But since the film is firmly clamped in the Schmidt film holder excessive creep will lead to film buckling that will cause focus problems.

My solution to the humidity problem was to drill two small holes into the Schmidt camera tube, one just above the mirror, and the other just below the corrector plate. I installed Weatherhead brand 1/8-inch pipe brass fittings through these holes and plumbed them with 1/4-inch rubber hose connected to a nitrogen cylinder and pressure regulator. The nitrogen is regulated to several pounds pressure and piped into the camera for about half a minute after the film is loaded into the camera to flush out the humidity. If the exposure is lengthy, the purge is repeated half way through the exposure. There is no concern about pressurizing the interior of the camera, possibly flexing or breaking the corrector plate, because the low-pressure nitrogen bleeds out the loose-fitting film loading door. This humidity purging system is the key to my being able to produce high-contrast long-exposure TP-2415 negatives with the Schmidt in spite of the image-busting local humidity.

Using a Glass H-Alpha Filter

Richard Payne reports that he has good success using a glass Lumicon Hydrogen-Alpha filter with his 5.5-inch F/1.65 Schmidt camera. He made a metal adapter ring which slip fits over the film holder on one side while the other side of the adapter has 48 mm female threads to accept a 48 mm Lumicon filter. To achieve focus, Richard slowly machined the back of the film holder over several days while taking Tech Pan exposures to verify the focus during the machining process.

A Simple Shutter for a Schmidt Camera

Many folks have rigged mechanical shutters or devised caps that go over the front of the corrector plate to act as a shutter for the Schmidt camera. One of the simplest shutters is a plain black T-shirt. It is flopped over the front of the camera before the film is loaded, then gently pulled off as the exposure begins. A lighter weight black scarf will also work well, but may be so light that it will blow off prematurely in a light breeze.

Dew Cap and Shutter Assembly

Chris Nisbit also sent me a note explaining how he created a combination dew cap and shutter for both his 5.5-inch and 8-inch Schmidt cameras. He wrote:

"I live in Minnesota so dewing of the corrector is always a problem. After seeing a picture of the Hubble Telescope, that gave me an idea for a dew cap/shutter assembly. On the 5.5", a standard coffee can (painted flat black of course) snugly fits over the end of the front cell and acts as the dew cap. I then fastened a piece of flat black painted cardboard to the dew cap. It is fastened in such a way that the cardboard shutter simply flips up when the exposure starts and is flipped down when the exposure ends. The 8" uses the same principle with the dew cap being the hard plastic Celestron Dew Star dewcap. Both work great!!"

Notice the classy way Chris has mounted his Schmidt in a Celestron Ultima fork mount with clamp rings holding the small Celestron refractor guide scope. It looks good!

Installing a Bigger Finder

The standard 6 X 30 finder, often called the "soda straw finder", that comes with the Schmidt camera is far from adequate to do anything other than align setting circles. For ease of locating targets, I consider an 8 X 50 as the minimum finder one should use, but other Schmidt users such as Kent Kirkley use a Telerad with great success. The mounting bracket used with larger aftermarket finders often will not fit the bolt pattern of the original finder. Simply plug the original mounting holes with short machine screws and drill new mounting holes for the larger finder a little farther up the tube.

Drilling holes into the Schmidt's tube may seem intimidating at first, but the finder will likely be mounted on the thin skin of the tube assembly further up from the mirror. Here, a drill bit penetrating the tube will not strike any internal components as long as it is located away from any of the Invar rods along the inside of the tube. Locate the area inside the tube where the drill will pop through the metal. On this location, use several pieces of masking tape, sticky side toward the tube, to build up an enclosed "tent" on the inside of the tube. This will capture drill shavings that fall into the tube. After the holes are drilled, collapse the tape against the tube wall so the shavings are stuck to the sticky side of the tape, and then peel it off, taking the shavings with it. Use a 3/8-inch deep socket as a drill stop so the drill bit will only penetrate a little way into the tube without puncturing the tape "tent" on the inside of the hole.

Once the holes are drilled in the tube, the new finder can be attached with machine screws and nuts. For installation, the nuts can be held through the film-loading door. Spray a little flat back paint on a cotton Q-tip to paint the nuts on the inside of the tube to reduce internal reflections.

Aligning the finder with the Schmidt's field of view may be a challenge when you consider that you can't actually see through the camera. But in a way, you can see what the camera sees to adjust the aim of the finder. Draw a crosshair on a piece of white paper and insert it into the film holder. Load the film holder into the camera and aim the Schmidt at a bright target such as the Moon or a bright planet. In twilight you can view the paper in the film holder and see the target you are aimed at. Center the target on the crosshairs drawn on the paper, then align the finder. If no Moon or planet is visible, a first magnitude star is bright enough to be seen on the paper in post-twilight darkness. Indirectly illuminate the film holder with a dim red light in order to see the crosshair.

Homemade Removable Finder Bracket

Stefan Beck from Germany has made a homemade removable finder bracket that allows the finder to be removed from his 5.5-inch Schmidt during transport, but does not loose its alignment. His dovetail-type finder bracket is fabricated from aluminum.

Mounting Rings Allow Rotating the Schmidt Camera

A good method for mounting a Schmidt on various popular German equatorial mounts is to use Parallax brand rings sized to fit the diameter of the Schmidt tube. These in turn are mounted on a Losmandy DC14 dovetail plate originally made to fit the Celestron C-14. The rings are placed at the extremes of the Schmidt tube, the bottom ring against the mirror cell, the top ring against the corrector cell. This places the rings at the most rigid part of the tube, preventing deformation of the Schmidt tube, which would cause focus problems. It also keeps the Schmidt from moving up or down and acts as a 'bearing surface' when the rings are loosened to allow rotating the tube for composing images with the rectangular imaging surface of the 35mm holder or for easier access to the loading door.

It would be a good idea to contact Joe Nastassi of Parallax before ordering rings for a Schmidt camera. He can put thicker felt in them to prevent unwanted movement. Jim Bandy replaced the felt in his rings with cork, which is very positive about holding the Schmidt still, but makes turning the tube difficult unless the rings are almost opened.

Interchangeable Tricolor Filters

John Mirtle has come up with an easy way to swap tri-color filters between exposures. The factory-made filter holder is really designed to have a single filter glued into place. Mirtle's system uses 2-inch superslide holders for the filters, and a small aluminum jig that is siliconed onto the original filter holder to hold the superslides. This is not a permanent change because the silicone can be peeled off, restoring the filter holder to its original state. The superslide holder is U-channels built from aluminum scraps using silicone, JB Weld, a hacksaw and file. The basic idea was to make no permanent change to the Schmidt (and therefore do no permanent harm). The superslide-mounted filters simply slide in and out of the channels. John has used this system for tri-color work, and for regular Wratten #92 red filter work for two years.

The filter holders were made from Gepe brand two mm thick superslide glass slide mounts. The glass mounts were chosen because they are thicker and fit the U-channel better, even though the glass is not used. The tricolor filters are labeled on the superslide holder frame with black felt pen, as you can't tell them apart with a red flashlight.

The U-channel forming the superslide holder started off as an aluminum extrusion for a weatherstrip designed to be screwed to the bottom of a door. The rubber weatherstrip skirt was removed and only the aluminum channel was used. It was cut into three strips approximately 2-inches long and filed down so that the slide mount made a snug fit in the U. They were then JB Welded onto the 2.5" square piece of flat aluminum that had a hole for the filter holder. It now only takes a few seconds to slide out one color, and slide in another. The assembly does add a bit of diffraction, but less than the roll film adapter.

Homemade Wratten Filter Holders

Stefan Beck addapted Wratten filters to his 5.5-inch camera by fabricating filter holders from heavy construction paper and tape. The red and blue filters slip-fit onto the film holder with a push fit. Stefan notes that on his camera the filters do not change the focus. The shorter focal length 5.5-inch camera is less sensitive to filter focus shift than its 8-inch cousin.

Another Homemade Wratten Filter Holder

Chris Nisbet also shares and idea on how to mount Wratten filters in the Schmidt camera. He wrote:

"When I bought the 5.5-inch a few years back, it came with a filter film holder and gel filters but no filter holder assembly. At a local camera store, I scrounged around in their used filter bin for a 48mm metal filter ring. Since I was only interested in the ring, the glass didn't matter. I found a decent one, paid $5 for it and went to work making it fit on the film holder assembly. Since the 48mm ring was just slightly larger than the film holder, I simply put several pieces of electrical tape on the side of the film holder until the ring fit snugly over the film holder. Now I had a way to securely fasten filters to the film holder".

"Gel filters, as you know, do not come mounted in a filter ring. I went back to the camera store and bought a supply of the $5 used filters. What is critical in buying these rings is the way the glass is mounted in the ring. I needed the type that the glass is secured in with a retaining ring, NOT a snap ring. The next task was to mount the gel filter in the 48mm filter ring. The basic principle that I came up with is to have the gel material in-between a "sandwich" of two thin rings that will fit in place of the original glass. To start with, I removed the glass from the filter ring and made a tracing of it on a THIN sheet of wood I picked up at a craft store. This will serve as the outer dimension for the ring that will fit into the filter ring. I then used a prescription bottle to trace the inside circle for the gel filter (the inner dimension). Using a new razor blade, I cut out the wood ring. (If it doesn't quite fit, use sandpaper to trim it down). The rings were then painted flat black. To assemble, put one ring in the bottom, put in the gel material, put in the top ring and secure it with the original retaining ring. You now have a mounted 48mm gel filter that will screw into the filter ring that is on the film holder. And, you can get a 48mm camera lens cap that can be fastened onto the gel filter assembly when the exposure is finished. The picture of the gel filter holder I sent is slightly different than the description above. I found out that not all camera filters have the same depth. The "sandwich" was thick enough that while I could get the retaining ring on, I couldn't get the 48mm lens cap on. Not enough thread showing. Back to the camera store for another $5 filter. I removed the glass from this one and the filter ring became the retaining ring for the "sandwich". Now I had plenty of thread for the lens cap to grab."

Controlling Vignetting With an Internal Aperture Stop

Another Schmidt modification by German astrophotographer Stefan Beck is the addition of an internal aperture stop to control vignetting. Instead of placing the stop on the corrector plate on the outside of the camera as is often done, Stefan permanently mounted a 120 mm aperture stop inside the camera at the focus of the mirror. He performed this modification on the advice of Austrian astronomer Gerald Rhemann, who in turn relied on tests made by fellow Austrian Michael Jaeger. The paper aperture is attached to the lower side of the spider cage around the inside of the tube.

Sealed Film Holder

Michael Treacy has designed a sealed and desiccated film holder for his Schmidt. Two decades ago, the precedent for this was set by the Rosemary Hill Observatory in Florida as a way to combat the ill effects of humidity on photographic plates exposed to the open air inside the telescope tube. Michael has designed the device on a CAD system and will soon start construction using a computer-controlled milling machine.

Homemade Dew Cap and Shutter Assembly

The 5.5-inch Schmidt camera is small enough that an effective dew cap and an aperture cover (aka: a "shutter assembly") can be fabricated from back construction paper, staples, and tape like in this example from Stefan Beck.

Focusing a Vehrenberg 10-degree Film Holder

Kent Kirkley notes that once his camera was focused by Epoch Instruments for the 35 mm film holder, he only uses the push-pull screws on the back of his Vehrenberg wide-field film holder to adjust its focus. He notes that the Vehrenberg changes focus slightly from cold weather to hot weather. He also states that the three sets of push-pull screws on the back of the Vehrenberg can address the tilt issue with the holder, but it is also a problem when attempting to make 'straight-line' focus movement without introducing any tilt.

Kent has also noticed another problem with the Vehrenberg holder. With 120-format roll film, the star images are focused evenly all over the plate. But with thicker 7 mil 4 x 5 sheet film, the center of the plate has slightly enlarged images. This is, no doubt, is because the thicker base of the sheet film does not bend as easily or evenly as the thinner 120 films. However, the 120 roll film suffers from curling and is difficult to handle. For this reason, Kent continues to use the sheet film.

Schmidt Camera Carrying Case

Kirkley also has an unusual recommendation for a carrying case for the Schmidt camera. If you have a large ice chest just over two feet wide, it makes a perfect transporter for the 8-inch Schmidt. For the 8-inch Schmidt, an Igloo brand 90-quart cooler works perfectly. Currently, this size cooler retails for $49.95 at at the Academy Sports retail chain. For a telescope carrying case, this is a real bardgain! A sheet of 2-inch thick foam rubber is inserted into the ice chest to cradle the Schmidt. Ice chests naturally resist heat, seal out dust fairly well, their insulation acts as shock padding for the optics, and the built-in handles make carrying the Schmidt an easy task.

I had previously mentioned that if you had a large ice chest that leaked, and otherwise could not be used, it might make a good Schmidt case. I now reverse myself on that after realizing that if the cooler had been leaking, it is very likely that the insulation between the cooler's walls is water soaked. At the least, this water might seep back into the case with the Schmidt, at worst it might start a nasty case of mold on the Schmidt. So, I now suggest that you do what we have always done, toss out the leaky cooler and buy a dedicated cooler only for the Schmidt.

Go to the previous page ---- Removing the Mirror
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