250mm f/8 Ritchey-Chrétien Telescope
After a very interesting and productive period of time that I had spent working at the Public Astronomical Observatory of Belgrade, I had to pause my astronomical activities as I had no instrument to work with. Together with my colleagues and friends from the Observatory, I concluded that I need a flexible high quality instrument that can fulfill my narrow field of view imaging demands. Since I live in urban environment, this instrument and complete imaging setup needed to be transportable. There are a lot of good commercial products that qualify for this, but building your own presents a unique engineering challenge and rewards you with precious development experience that is so much more exciting compared to a typical end user.
This is how I got an idea of building a Ritchey-Chrétien telescope. In it's design I wanted to incorporate all experiences that I collected while working with various instruments from the Observatory. I am an experienced electrical engineer, but I always liked venturing into the world of mechanical design as well. Precise mechanics are my favorite, and this telescope was a chance not to be missed. Therefore I invested considerable effort to design and make a high quality device, trying to avoid as much compromises as possible. On the other hand, each part has been carefully designed with simplicity in mind for easier production on general purpose machines available.
250mm aperture opening is in my opinion close to an optimum between light gathering ability and excessive weight for a small photographic instrument that needs to be easily transportable. 250mm f/3 primary mirror together with secondary mirror that produces f/8 at the focal plane is a standard configuration for this type of telescope. Back working distance of 250mm provides enough room for filter wheels and other accessories, and large 100mm clear optical diameter secondary provides for wide field of view that is very important for optional focal reduction. As soon as I decided that optical set is going to be produced by famous craftsman Germano Marcon from Italy, I rounded up all design ideas, and fixed the specification:
| Optical System: | Ritchey-Chrétien |
| Main optical parameters: | D=250mm, f=2000mm, f/D=8 |
| Primary mirror parameters: | D=250mm, R=-1450mm, f/D=2.9, K=-1.1455 |
| Secondary mirror parameters: | D=100mm, R=-813.8mm, m=2.76, K=-6.1401 |
| Primary - secondary mirror separation: | s=-465.59mm |
| Back working distance: | BWD=250mm |
| Fully illuminated / maximum usable field diameter: | 50mm / 57mm |
Optical system with these parameters has been simulated in the ray tracing CAD software. Aside from assessing optical performance, simulation is needed to determine the size and positioning of some mechanical parts close to the mirrors. It is also very helpful for some trivial things like verification that Cassegrain baffles have been correctly designed for desired field of view.
Structural design was also done using CAD software, complete telescope and each part individually. Truss tube design was chosen because of its stiffness, precision, ease of adjustment and convenient disassembling for telescope maintenance. From the outside most of these designs look similar, but it's the details of mechanical construction that yield final performance, so most of my attention was directed to these. With regard to limited resources available for this project in amateur environment, I am very satisfied how it turned out in the end.
These parts were manufactured first. Most of them, notably the octagonal rings were cut out from aluminium alloy plates on a CNC waterjet cutting machine. As an abrasive process, waterjet leaves rough surface after the cut, but is much faster and less expensive compared to precision milling. For this reason, waterjet cutting process was adjusted in a way that roughness can be smoothed out later on a milling machine using the same CNC program. Other smaller parts were independently made from aluminium alloy bars on a milling machine.
Small holes and high precision holes (eg. for linear bearings) should not be cut using waterjet. These were drilled on a CNC milling machine using precision tools. This work was entrusted to a small but capable workshop ran by former employees of the Military Technical Institute of Belgrade (VTI). These experienced guys really helped me out, so that we could succeed in manufacturing many different parts for a complicated device in only one attempt per each, reducing cost and saving time.
Technician Steva is verifying alignment of the piece he is working on against the coordinate system of his CNC machine. Having a lot of experience, he had the final word on all decisions related to machining of the parts. He was satisfied that my design was well adapted to capabilities of his not so specialized machines, but still there was a lot of work to be done. He never wanted (nor did he have time) to get involved in my design solutions, he just asked me where and what to cut/drill. I don't know whether he was worried if my design made any sense, but he was very happy at the end seeing that all of the effort resulted in a nice looking and functional instrument.
Since all of the plates were previously cut using waterjet, running the same CNC program on a milling machine took considerably less time. Rough surfaces of the cut were smoothed out, as waterjet cutting process was adjusted to leave some excess material behind. This process also increased the precision of manufactured parts, as it corrected some of the bending that occurs each time shapes are cut from any type of rolled metal plates. At the end, small holes for screws were drilled.
Circular parts were machined on a lathe. In this photograph, secondary mirror cell is being made from a compact cylindrical block of aluminium alloy. A lot of wasted raw material, but this was the only way to produce a high precision cylinder and a matching cover. Central holder for the primary mirror was made in a similar way as well.
Telescope was assembled for a mechanical fitting test, and I was really happy that there were no mistakes (errors in part placing and dimensions). Since carbon fiber or other better materials were not easily available, truss bars were made of inox steel, which has lower coefficient of thermal expansion compared to aluminium alloys. Inox steel is also exceptionally strong and tough, thus being able to withstand common hazards of transportation. Holes for weight reduction in the octagonal rings are most important for the front pair, because large secondary mirror and complex spider shift the total weight balance forward.
Removable focuser plate is mounted on the back side. Under it, mounting space is reserved for future optional field correctors or focal reducers. For attaching cameras or eyepiece adapters, an adjustable stack of old Pentacon Six photo camera bayonet sockets and macro rings was used. These proved to be extremely convenient, in terms of required diameter, stiffness and ease of use. Traditional focuser is not needed, instead a servo motorized focuser is implemented on the secondary mirror, together with a high precision sliding mechanism for zero slop and backlash.
All aluminium parts received black anodized surface finish, while steel parts were powder coated (thermoplastic coating). After appropriate dust covers were made, mirrors were mounted in their cells and fixed by provided means of mechanical adjustment. Fans for speeding up thermal equalization were not mounted yet, instead only chrome masks were put to make the holes look nicer.
An almost complete telescope, with mirrors installed. Of course, it needs to be collimated before use, and initial rough adjustment has already been done by careful assembly of the truss structure. Effective length of each bar is adjustable, and this was used to rectify shift and tilt of the front rings with regard to the base where primary mirror is. Once tightened, truss structure remained a very solid platform. Next comes the precise shift of secondary mirror spider. For this purpose, supports for spider vanes are equipped with precision adjustment screws together with locking worm screws, visible on the side. These are set during secondary mirror collimation adjustment using a He-Ne laser.
Secondary tilt is adjusted in a traditional way, with six screws visible on the bottom of 3 pairs of holes standing in an inverted Y orientation. Step motor of the servo focuser is visible in the center of the spider, and has a back axle extension in case you wish to turn it by hand. Motor wire leads are connected to a thin flat cable which is glued onto one of the spider vanes. This is barely visible in this photograph (lower right corner), but can be nicely seen in the next.
One of the main features of this telescope is its precision servo motorized focuser for the secondary mirror. It has been designed with 6 small linear bearings and 3 shafts, in a configuration that ensures virtually zero slop and backlash. Step motor is operated in half-step mode yielding 400 steps per revolution, providing excellent precision coupled with 0.5mm per revolution pitch of the threaded axle that slides the mirror. Motor is driven by the microcontroller which was installed in a convenient hand control device that connects to the telescope by a cable.
Focuser hand controller, outside and inside. Black and white buttons rotate the motor in opposite directions allowing the user to reach full focusing range. Middle button (coupling button) operates together with potentiometer knob mounted on the side for precise focusing. When coupling button is pressed, motor axle will follow any subsequent rotation of the knob, giving you the exact same feel as a traditional focuser but without any vibration caused by handling. Coupling button is a preventive measure against accidental turning of the knob during imaging, and it also enables you to shift the range of focus with respect to available potentiometer range. Controller also has an RS232 port for computer connection, providing a convenient means of automated software control.
Precise alignment of the secondary mirror can be achieved using a set of collimating adapters and a He-Ne laser. One of these adapters is in front of the primary mirror hole, while the other similar one is near the focal plane on the back side of the telescope. They both have small central holes, enabling the laser beam to be precisely aligned with mechanical axis of the telescope. In such condition first the secondary mirror shift is adjusted so that laser is hitting the center of the mirror surface. Next, the tilt is adjusted so that beam is reflected back to the laser.
Collimation adventures performed on an optical bench in Institute of Physics, Belgrade. We were experimenting with the influence that adapter type and placement has on final precision of the adjustment. Photographed by Vladimir Nenezić.
First field use of the telescope was carried out on Andrevlje star party. We were not so lucky with the weather, as only a part of the second night was clear. Telescope was temporarily mounted on my friend's EQ6 mount and used only for short visual observing that passing clouds permitted. In near future I plan to purchase a good mount for astrophotography and complete the whole instrument. Photographed by Dušan Vučković.
Latest news (12.04.2011.): Telescope adjustment and tuning is still in progress, as I wish to experiment a bit more before finalizing it. After that I look forward for some preliminary imaging tests on the EQ6 mount, before my new mount arrives (current candidate is ASA DDM60).