Layout of a 4 1/4 inch combination f16 Cass - f4 Newtonian. This design uses an unperforated primary mirror and a standard Newtonian focuser mounted on the side of the tube with a single vane holder for the star diagonal to get the Cass focus to the left. When a similar focuser with single vane holder is added (on the right side) using a larger diagonal mirror the telescope is converted into a rich field Newtonian. Note the Cass secondary is kept permanently mounted on a standard spider. Removing the right hand focuser converts the telescope back to a Cassegrain without readjustment.
Cassegrain telescope are usually made with an f4 to f5 primary and a 3X to 5X secondary, while this easily provides medium and high eyepiece magnification the lower powered are difficult to get. Even with a recommended 40mm to 60mm eyepiece, used to locate objects after you have found them in the finder, the really low powers are not available. With any Cassegrain a 40mm to 60mm eyepiece is handy when observing conditions are not suitable for medium power eyepieces. A frequently considered idea is to substitute a Newtonian diagonal for the Cass Secondary thus providing a rich field. low power telescopes for all the objects not suitable for medium to high power. Such a combination or convertible Newt-Cass telescope would be flexible enough to give good performance for the wide range of observation many amateurs are interested in. The performance would be better than a single general purpose f7 or f8 telescope ultimately the idea as simple but making it work well often is not.
Both Newtonian and Cassegrain optical set-up use the same fixed primary mirror position, in most cases the same spider position, but with both secondary mirrors must be mounted close to the spider to minimize overhang. This is particularly important to rich field and Cass telescopes in keeping secondary vibration low. The layout of a Newt-Cass convertible telescope must be done carefully. If the back of the secondary holder is mounted in a position one inch or more different from the other secondary holder then the layout should be readjusted. Preferably, for the two secondary holders using the same spider, this difference in position should be a half inch or less. In some convertible systems such as the removable single vane holder and diagonal shown above and the interchangeable tube segment system have the diagonal mounted on a sperate spider or single vane holder to avoid this design problem.
It is often easier to design the Newtonian system first then the Cass system separation distance is matched to suit the position of the diagonal, with the back focus left as a variable. If the back focus is too long the difference can be taken up with focusing mount extensions or focusing tube extensions. If the back focus is on the short side a short Cass focuser can usually be used. In most cases you will find that the Newtonian diagonal is closer to the primary than the Cass secondary. This is solved by decreasing the separation and increasing the back focus. If you have Cass optic ready made and want to add a Newtonian focus, the problem is a bit more difficult. The Cass secondary position can not be changed isn't it will, for the most part, fix the position of the diagonal. This in turn fixes the position of the focuser. If the Newtonian focal plane extends too far from the focuser you can add focusing tube extension but if the focuser is too tall there is little selection in short Newtonian focusers,
This section is not the place to describe the technical aspects of various Cass optical systems. However it should be pointed out that only the Gregorian and the classical Cassegrain systems use a parabolic primary mirror as required for a rich field Newtonian telescope. So convertible Newt-Cass telescope are limited to these Cass optical systems. Even if you have a Cass with a parabolic primary there are a number of other problems in making it convertible to a Newtonian. The main problem stems from the accuracy needed in the adjustment of the Cass secondary. It must be optically centered to within +/-0.005" or less. (1/16th of the turn of a screw with 32 threads per inch). The single vane diagonal holder system shown in th illustration above avoids the problem of leaving the Cass secondary mounted to the spider at all times, even when the diagonal is used to get the Newtonian focus. However this system is limited to small telescopes.
Perhaps the most frequently tried idea is simply interchanging secondary holders in the spider. It can be done but it requires readjustment of the optics which is practical a few times a year but not on a daily or weekly basis. In practical use a convertible telescope should be easily changed to suit the objects being observed and the conversion should take only a few minutes. Interchanging secondary holders in the spider that:
Shown above is one system for indexing secondary spiders to fit one spider. It can be made using commercial secondary holders and spiders but one accurately machined parts must be added. The index arm is adjusted to the stem to fix the spacing between the secondary holder and the spider. The end of the index arm has a narrow slot that fits on one vane to keep the holder unit from rotating. The stem that passes thru the spider center body must be accurately round or tapered to fit a carefully reamed hole in the center body. Only the ends are threaded, one for mounting the collimation plate and the other for a wing nut to hold the secondary on the spider. Even if a machinist makes the required parts and modifies commercial parts it does require some skill to set it up on a telescope. The set screw on the index arm is a bit difficult to reach between the spider and secondary holder. A stack of washers between the index arm and the collimation plate could be used to adjust the distance between the two so the set screw only locks the rotation in place. Note that the vane that the index arm should be marked with a bit of white paint to indicate which of the four vanes to use. The major disadvantage of the index arm system is the added overhang because of the arm position between the spider and secondary holder. It is important to keep the index arm as thin as possible, perhaps using two small set screws rather than one large one.
The removable Newtonian focuser with single vane holder system shown in Figure 5 is suitable only for small Cass systems. It works well in a 4 1/4" aperture. The Newtonian focuser is attached to the tube with thumbscrews into threaded holes in the tube wall. The two screws can be removed in only a few seconds. Many Newtonian focusers can be mounted over a 1 3/4" focuser hole. Since the diagonal must also pass thru the same hole easily without damage or losing adjustment, the maximum size diagonal is about 1.52" minor axis size. Adapting the design to a 6" aperture is possible but additional problems are encountered. If you want to use a larger diagonal a large base focuser must be used. The single vane holder is prone to be the source of diagonal vibration in rich field telescopes. Many amateurs do not want to accept the vibration level normally found with a single vane holder used with a common sized diagonal for a 6" rich field.
The index arm system shown in Figure 5 can be used with almost any size Newt-Cass telescope but does require some special machining, the next two systems for changing secondary holders in Newt-Cass telescopes have been used on many professional telescopes and are sometimes used on amateur telescopes. Both are limited to at least 8" aperture usually and typically to 12" apertures or more. If they are properly built they work well but to build them properly they should be machined and fitted to the tube.
First is the flip spider-secondary holder. A metal ring is pivot mounted to the inside of the tube with the axis of rotation thru the center of the ring. The spider is mounted to the ring with the two secondary holders on opposite sides. Since collimation and the conversion from Newtonian to Cass or visa versa is accomplished by flipping or rotating the ring.
The difficulties are that there can be no side play of the ring and it must be locked in place accurately after rotating. This is often done with a tapered pin passing thru the tube into the flip ring. However if the space between the tube and the ring changes due to thermal expansion or other reason the tapered pin will seat in the tube or the ring but not both as required for accurate alignment. Since the spider is mounted to the ring inside the tube, the spider usually can be reached only thru holes in the tube. Often a specially made heavy duty spider is needed to carry the weight of the two secondary holders. Taper pins and taper pin reamers are available from machinists suppliers and their use is described in "Machinery's Handbook" published by Industrial Press, Inc. The rotating ring can be made from a flat aluminum bar rolled into a ring and welded or spliced together. If a wide bar is used it must be considerably smaller in diameter than the tube inside diameter to allow rotations. A narrow bar size can be used but must be thick enough not to deform under spider tension. The flip ring reduces the effective inside diameter of the tube, often by several inches, so the tube should be large enough to hold the flip ring without reducing the effective field. Otherwise the rich field Newtonian will not be so rich. The pivot shaft can be made simply by using a shoulder screw threaded into the flip ring. The shoulder screw is accurately machined and hardened. The two pivot shaft must be parallel to each other and divide the ring in half exactly. Otherwise the ring could be locked in one position but not flipped over in the other position.
The second method is the interchangeable tube segment system. In this system the upper end of the tube holding the spider is made removable. Thus the tube segment with spider and Cass secondary can be exchanged with the other segment carrying the spider diagonal holder and Newtonian focusing mount. The tube segments do not have to be the same length so each secondary holder can be closely mounted to the spider and one spider does not have to carry the weight of both secondary holders and mirrors. Alignment of the tube segment to the main tube is best done with tapered pins although dowel pins have been used. Since it is difficult to index the main tube to both segments using the same pin locations this can be a major source of problems and should be avoided. Three or more clamping screws to attach the segments to the main tube can be interchangeable but a sperate set of three alignment pins should be used for each segment which in turn mate with a sperate set of holes in the main tube.
Short lengths of angles or blocks can be attached to the tube to provide a mounting face for the pins and screws. However a continuous ring rolled from metal angle is easier to work with, is more accurate and has a better appearance. Metal fabricating shops can roll aluminum or steel angle and channels "leg out" for rings outside the tube or roll "leg in" for rings inside the tube. The rings are then cut off at the ends until a butt fit is achieved when the rings fit tightly to the tube wall. The butt joint is then welded if desired and the rings screwed or riveted to the tube and segment. Pilot holes should be drilled in the rings of the segments. Since these holes are close to the tube wall, long aircraft drills will probably be necessary in extending the pilot holes into the main tube ring. After the pilot holes are drilled the segments can be removed for drilling to the finish size hole. Clamping screw holes are drilled out first to securely hod the segment to the main tube while the pin holes are extended into the main tube ring. After each pin hole is drilled it should be reamed with the proper reamer, then pinned. The taper reamer may require a tap extension to reach the hole yet allow the reamer handle to be turned. Keep the taper reamed hole on the small side. As wear occurs causing poor pin alignment the hole can be reamed again slightly.
The segments should be marked on the outside of the tube to give an index to prevent rotating the segment to the wrong position. Outside mounting rings will not reduce the tube inside diameter to the tube segment system with outside rings can more easily be used with standard tubing. The clamping screws are also easy to reach. One variation of the tube segment system to use flat bars of metal firmly bolted to the upper segments with part of the bars extending into the main tube. These bars are bolted directly to the tube wall so they are compact. The extended end of the bar forms mounting tangs. The main tube is drilled thru into the tang. Countersinking the main tube allows ordinary flat head screws to be used as tapered pins. The disadvantages are that the tangs are too easily damaged and that the tube should be metal to withstand wear. Using continuous rings helps greatly to reinforce non-metallic tubing. This particularly helpful since the tubing loses stiffness when cut into segments.
A variation of both the flip ring and the tube segment system is being called the flip-segment system here. A rigid ring that carries the spider with the secondary holders is used without the pivot shafts. Instead the ring is indexed to a ring attached to the inside of the tube. It is indexed in the same way the tube segments are but usually the rings are inside the tube. The flip-segment is simpler that the tube segment system and is easier to make accurate than the flip ring system with pivot shafts. It is also possible to place the flip-segment system on the end of the tube. This leaves part of the spider and one of the secondary holders to outside of the tube. But it also allows an angle ring to be used on the outside of the tube to make the mounting bolts easy to reach on the outside of the tube and minimize obstruction inside the tube. a section of tubing can be attached to the end ring to serve as a dew cap if desired.
For smaller Cass telescopes it would be well to seriously consider two telescopes rather than trying a convertible Newt-Cass. A telescope tube with Cass optics can be easily interchanged with a telescope tube with Newtonian optics on a dingle telescope mount. Because of the difficulty and/or expense involved in most reliable convertible systems it may be less expensive and more convenient to interchange telescope tube units. It does require an extra primary mirror and tube. On small telescopes the extra cost of these parts can be less expensive than a convertible system unless the system is easy to make and use. For larger apertures the extra cost of the additional mirror and tube can be quite large and interchanging telescope tube units is no longer simple or easy.
Bob's Note: Another way to do the conversion setup from a flat for the Newtonian to the convex hyperboloid for the Cass is to do it with magnets. Apply three magnets in the part of the secondary holder that is attached to the spider and provide changable holders for the two secondaries. Each of the secondaries needs to be adjustable by itself and you can do that when you remove the secondary from the holder if desired or put holes in the spider half of the holder to access the three screws. You may also wish to apply a set of
adjustment screws on the spider part of the secondary holder so that you can make preliminary adjustments. You can do either a push-pull adjustments (6 screws
and holes in the mount) or rocking over a ball (three holes in the mount) as you desire. The only requirement is that the secondary carrier have a steel back on it and that there is an indexing pin(s) to locate the secondary so that it wil always return to the same location each time you put it onto the telescope. End Bob's Note.