Custom Mounts
ScopeCraft has been building custom telescope mounts since 1985. In the early days the mounts were designed and built following good engineering principles tempered with a heavy dose of astronomical practical experience. The designs unfolded from concept to hand drawings to the finished device without being able to test the concepts prior to construction.
 
Computer Aided Design
In 1995 we started to use SolidWorks, a Computer Aided Design software package, to develop our concepts and begin to test the designs prior to building them. In 1998 we added the capability to do Finite Element Analysis (FEA) on these designs. This capability allows the detail design to be tested under a virtual operating environment. Since large telescope mounts must maintain optical and mechanical alignment to very precise levels, the ability to move the mount on the computer screen and monitor the deflections of the components as the mount sweeps across the sky has proven to be a very effective tool. No longer do we design and build with our fingers crossed hoping that the mount will be strong enough to keep from deflecting too much.
The CAD and FEA software have also brought additional benefits to the custom mount arena. The first is the ability to show and discuss with the customer how the ultimate design will be built and function. With both the customer and ScopeCraft examining the design prior to building it, any miscommunications or assumptions are cleared up. The customer and ScopeCraft clearly understands the deliverable.
The second benefit is that when the mount is delivered a complete set of drawings is supplied as part of the documentation. The drawings are used to manufacture the each of components and provide backup for any future changes. This documentation package also includes the CAD electronic files.
The third benefit is cost saving. With all of the detail design work done up front the construction then becomes one of manufacturing, knowing that each component will function properly with the others. Designing on the shop floor is very susceptible to errors.
As an example of the CAD process, Figures 1-5 show the concept package that was sent to a customer with our bid.  This mount is a 36-inch LIDAR proposed to a company that is studying the upper atmosphere. The overall design features a full thickness 36-inch primary mirror, a 10-inch secondary mirror in a classical Cassegrain configuration and a 6-inch tertiary fold mirror that sends the optical beam out the side. Some interesting details of this design are that the truss members are made from carbon fiber wound tubes that have a zero coefficient of temperature expansion keeping the focus position constant over a wide temperature change. Additionally the structure had to withstand 10g's acceleration that it would encounter in its shipping container.
 
Figure 1, is the solid model of the device that shows the overall design concept. 
 
Figure 2, is a drawing of this structure giving general dimensions and locations. 
 
Figure 3, is a detail of how the primary mirror will be supported.
 
Figure 4, is a detail of how the secondary mirror support will be kinematically attached to the support structure. The secondary needed to be removable during shipment as it was deemed that the thin vanes could not support the secondary under the expected shipping loads. A kinematic attachment was required so that recollimation of the secondary would not be needed.
 
Figure 5, is a copy of the Mass properties of the mount.  This information was needed to construct the shipping container hold-downs.
These four drawings and the mass data sheet were the basis for the bid package and provided concept clarity between the customer and ScopeCraft.

 

Design Detail
The level of detail that is supplied with a final mount is shown in Figure 6, which is an exploded view of the RA roller drive of our SC-2438 telescope. This drawing has a complete parts call-out and clearly shows the relationship of each of the parts.
Figure 6, SC-2438 Right Ascension Roller Bearing Block Assembly

Finite Element Analysis
ScopeCraft began using FEA in 1998. FEA is a computer based analysis method which calculates the response of a system by solving a set of simultaneous equations that represent the behavior of a structure under applied loads. This process involves graphical generation of the model geometry, meshing it into finite elements, defining material properties and applying loads and boundary conditions.
An example of the FEA process is from the early concept phase of a fork mount for a 27-inch f/5.5 full thickness Newtonian telescope. The concern was how much deflection would occur from the estimated weight of the 400-pound OTA. These two views show the deflection due to gravity of the fork arms alone. 
The analysis in Figure 7 is what would be expected when the OTA was pointed to the eastern horizon.
Figure 8 is the analysis when the OTA is on the meridian.
Note the deflection scales for each of these analyses. The blue color represents minimal deflection while the red color is maximum deflection. The deflection seen in the fork graphic has been exaggerated some 10,000 times.

Custom Mounts  
26-Inch f/2.8 Telescope  
This 26-inch f/2.8 automated telescope is of unusual design. The primary is a parabola and it uses a flat 10-inch secondary that reflects the light into a CCD camera located between the primary and secondary. This unusual design was necessitated by the requirement that the observatory building not be enlarged from its original size when it housed a C-10. An additional design parameter was that the focus position remain stable over changes in temperature. This was accomplished by using temperature compensating polymer rods to hold the CCD camera which expanded at a rate exactly opposite to the expansion of the steel truss structure. The camera was also movable by a stepper motor to achieve initial focus. The drive used worm gears on each axis.

Figure 9, a 26-inch f/2.8 automated telescope.

20-inch f/8.1 Telescope  
This 20-inch f/8.1 Ritchey-Chretein automated telescope uses 16-inch worm gears on each axis and a large disk running on roller bearings for the north polar bearing. The truss structure is of the Serrurier type, which maintains parallelism of the optical components. The design utilizes a moving secondary for fine focus. This telescope is automated with the DriveScope drive system. In this case the optics, mount and drive system were supplied as a complete telescope.
 
Figure 10, is a 20-inch f/8.1 Ritchey-Chretein telescope.

 

14-inch Celestron OTA Mount  
Below is the fork assembly of a partially completed mount in our shop, which will support a 14-inch Celestron optical tube assembly. The customer had previously used a German Equatorial mount and was unhappy with the required "flip" when crossing the meridian as well as the flexure of the mount when he attached his CCD camera, filter wheel and focuser. The mount attachment to the optical tube involved two side plates that ran the length of the tube and increased the stiffness of the Declination axles. This mount uses a 13-inch worm gear made by Aeroquest Machining on the RA axis and a 10-inch worm gear on the DEC axis. This telescope is automated using a DriveScope drive system.

Figure 11, is the fork assembly of a partially completed mount in our shop.

This exposure was taken with the telescope in Figure 11 and shows the quality of the worm gear and it's mounting.

Figure 12 is a five minute unguided exposure with this telescope.

   
ScopeCraft prides itself on using the latest of design tools, which greatly aids the design, and manufacturing process as well as providing the customer with a mount that meets their needs. With our DriveScope drive systems and WinScope software we can provide complete mounts and computerized drive systems for virtually any type of large telescope.  We do not build optics but have worked with most of the major large optics fabrication firms to integrate the optics to the mount.
Should you desire a bid on a specific mount please contact us with your requirements.