Primary Telescope

 

General: The Alt-Az telescope will have two Couder focus ports. One port will be used primarily for visual observing or observing through a video camera. This port will serve most of the educational and observational needs of 4-H and the amateur astronomy organiazations. The telescope will be built in such a way as to minimize the height of the focuser port to make it accessible to observers. With the observing port just 48 inches above the floor, it is anticipated that a handicapped accessible observing platform will be built around the telescope as necessary. The second focuser port will serve primarily as an instrument port and will accommodate a research grade spectrograph and a large format, low noise CCD camera. The mounting interface at this port will be designed to accommodate additional instruments if necessary. This port will also include a removable wide field corrector lens set which will operate at F/10. Additionally, a focal reducer lens will be supplied which, when installed reduces the telescope's focus to F/4. A removable annular elliptical flat mirror can be fitted to the instrument port for auto-guiding. An automatic field de-rotator assembly will be installed at the instrument port to counteract the field rotation inherent in the Alt-Az design. To accommodate the research requirements of professional astronomers who might be allocated time on the telescope, the control system will be capable of functioning remotely and automatically. A priority based sorting algorithm will schedule observations. Astronomers will be able to submit observing requests and retrieve data via the Internet. The telescope control system will be able to interface to security and weather monitoring equipment. These features will be useful if unattended remote operation of the observatory becomes a priority. Mechanical: Drawings: The mechanical design of the telescope features an open truss tube assembly constructed primarily from anodized 6061-T6 aluminum. All fasteners are stainless steel. The primary mirror cell consists of a 36-point whiffletree flotation system with tangent-bar edge supports. Central support of the primary mirror is also provided. The secondary mounting/focusing mechanism is attached to a 4-vane spider in the telescope head ring. Focusing is accomplished by moving the secondary mirror via a stepper motor and precision lead screw assembly. The telescope's focus is directed to the Couder observing/instrument ports via an indexable turning flat mounted on the primary mirror central support post. The turning flat is indexable at 90 degree intervals. The main observing/instrument ports are located 180 degrees apart in the telescope's azimuth axles. Additional ports can be located in the telescope's waist section if desired, however these are not anticipated at this time and field de-rotation will not be provided at these ports. The telescope mount consists of an Alt-Az fork constructed from anodized 6061-T6 aluminum and assembled with stainless steel fasteners. All bearings in the mount are pre-loaded Timken tapered roller with dust seals. A Schematic drawing of the proposed telescope is provided. Telescope Drive Mechanisms: The telescope drive consists of zero lash friction rims on both axes. The main drive rim on each axis is a 30" diameter x 2" thick hard-coat anodized disk. This disk is driven by a 2" diameter hardened stainless steel roller. The drive roller is coupled to a high torque DC servo motor through a high precision, zero back-lash 60:1 Harmonic Drive Technologies gear reducer. The total reduction ratio between the servo motor shaft and the telescope shaft is 900:1. The telescope control system relies on Dynamics Research Corporation optical encoders for positioning and tracking. These reducers produce 80,000 counts per revolution of the encoder shaft. The encoder shaft is coupled to a ½" diameter idler roller which rolls against the 30" diameter main drive rim. This couples the encoder reading directly to the telescope shaft and results in encoder resolution of 0.27 arc second. Torus is currently operating a similar drive system on a 16" telescope with 0.57 arc second encoder resolution. This telescope consistently blind points to better than 2 arc seconds with repeatability of +/- 1 arc second. The proposed one-meter telescope is expected to blind point to better than 10 arc seconds with repeatability of +/- 2 arc seconds. Tracking is expected to be accurate to 2 arc seconds over 20 minutes duration. Telescope Control Software: The telescope control software was developed by Elwood Downey for the University of Iowa Automatic Telescope Facility. It is a Unix based package that will run on a PC or any other platform running Unix (or Linux). This software was designed specifically for remote, automatic telescope operation. Furthermore, the software was designed for multiple users of the same caliber and experience range as expected at the ATF Educational Observatory (amateur, student and professional). In it's current form the software's capabilities are an excellent match for the proposed ATF Educational Observatory's requirements. Elwood Downey will work with Torus and the staff at ATF to develop additional features specific to the requirements of ATF. Comprehensive user/reference manuals will be provided. Telescope Optics: The proposed optical system design was chosen to meet a wide range of observing and scientific imaging programs. The F/3 primary mirror focal ratio was chosen in order to produce a compact optical tube assembly. At F/3 the cost of fabricating the parabolic mirror to high precision is reasonable. The Cassegrain system focal ratio was chosen to be F/10. For visual observing, the F/10 Cassegrain telescope will provide good image quality over a reasonably wide field of view while keeping the central obstruction to a minimum. The F/10 beam will work well for many applications at the instrument port, however for applications requiring excellent off axis image quality, a wide field corrector lens should be installed. For applications requiring a faster beam, an F/4 focal reducer is installed. For high precision guiding during long exposure imaging and spectroscopy, a perforated elliptical turning flat is inserted in the beam. This perforated mirror will direct light to an auto-guiding CCD camera. Detailed optical design and performance data will be supplied as soon as all mechanical issues regarding telescope and instrument design are addressed. The primary mirror will be fabricated from a Hextek light weight Gas Fusion blank. This light weight material simplifies much of the engineering of the telescope mount. It also offers the advantage of a much lower thermal time constant compared to a solid blank of similar dimensions. The Cassegrain secondary mirror will be fabricated from fine annealed Pyrex. Optical coatings for the primary and secondary mirrors will be Miro-Bright coatings, supplied by P.A. Clausing, Inc. This is a 91% reflectivity enhanced aluminum with SiO protective overcoating. This coating provides very good reflectivity with excellent durability. All lens surfaces in the corrector and focal reducer will be anti-reflection coated. The optical quality of the Cassegrain system will be such that 80% of the energy of a point source is concentrated within a 0.5 arc second diameter. All optical surfaces will be 60/40 scratch dig. The primary mirror will be tested in autocollimation against a 40" diameter precision optical flat. The Cassegrain system will be tested as a set in autocollimation against the same flat. In each case, officials from ATF (4-H, EO, ISU) will be invited to Torus' facility to witness final certification tests. Scope of project: Torus will fabricate the telescope, corrector lens, focal reducer lens and perforated guiding flat as described above. In addition, Torus will supply a Pentium based telescope control computer and software. Finally, Torus will deliver and install the completed telescope system at the ATF Educational Observatory. If a crane is required at the installation site, The One-Meter ATF Educational Observatory will be responsible for arranging and paying for the crane service. As a service to the amateur community, Torus will fabricate a Truss Tube Dobsonian mount for the existing 24" F/5 Newtonian primary mirror fabricated by Dean Ketelsen. Maintenance: The mirrors should be cleaned frequently with a CO2 snow gun. The set-up for this costs about $1600 but is well worth it in the long run. A coating that is cleaned once a year will last about 5 years in our climate. A mirror that is cleaned weekly with a CO2 snow set-up will last much longer. Obviously, there is no need to remove the optics to do this. The optics will be supplied with re-usable shipping containers. Removing the primary is accomplished with a set of Jack-screws that raise the mirror up out of the cell. These are supplied with the scope. (In fact, we will use them to install the primary on site.) Then, nylon strap slings are used to lift the mirror out of the cell. I would suggest that you install a gantry or swinging boom crane in the observatory for handling the primary mirror. We will give you complete instructions on removing the mirror and re-installing it safely. Actually, most of the details have yet to be worked, but we do have experience handling large mirrors and I can assure you that it is relatively straight forward if you follow a routine. Most freight companies will take the mirror but I recommend Emery World Wide for their excellent service, and for their reasonable cost. I always ship optics over night or second day air. This minimizes the exposure and an airplane ride is generally much softer that a truck ride. Shipping the mirror to Chicago for coating will cost around $400 each way. Coating the primary will cost around $4,000. Four healthy people can actually lift the crate onto the Tommy-lift of an Emery truck but I wouldn't recommend it. Actually, Emery has a palette jack on every truck so they can safely roll the mirror crate onto the lift. All bearings are lubricated with Krytox grease. This is the stuff NASA uses on satellites. It is impervious to temperature variations and does not absorb water. Therefore the bearings are essentially maintenance free. The drive control mechanics are relatively maintenance free because there are no real wear parts. Also, the friction rim, drive roller and encoder components are covered to seal out dust. All electronic components are off the shelf, Mill-spec components. If something fails, repair is simply a matter of swapping out replacement parts. It might be a good idea to keep a spares of critical parts to eliminate down time. The guys at the U of I did an excellent job of selecting reliable and robust electronic components for the drive control system. We will select components for the meter class scopes from the same companies or companies of similar repute.