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6” f/8 Motorized Dob for Lunar/Planetary Viewing
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A 6” f/8 motorized dob for lunar/planetary viewing
IntroductionI have been an amateur astronomer since 1999 and currently own three telescopes, an 80 mm short tube refractor for wide-field views, a portable 8” Newtonian for dark sky sites, and the subject of this article, a home-made 6” f/8 dobsonian optimized for lunar and planetary viewing. My criteria for designing this telescope was that it should be capable of optical excellence and be easily convertible between manual and motorized tracking. This telescope is dedicated to visual observations on the moon and planets, undertaken from my balcony under suburban skies (limiting magnitude about 4.5). Light weight, portability or tracking accuracy compatible with astrophotography were not design requirements.
A fully baffled, long focal-length Newtonian seemed best suited to my needs, dobsonian mounted to minimize costs. An aperture of 6” was selected, since local seeing conditions here in the Pacific Northwest rarely support large aperture, high magnification viewing. To optimize image quality, the telescope would normally operate at f/16, using a apochromatic x2 Barlow lens with 15 mm and 9.5 mm Ploessl eyepieces (x160, x252). For motorized tracking, I built a variant of Gary Wolanski’s visually assisted drive system (VADS), consisting of independent tangent arm drives on the altitude and azimuth axes.
Building the OTA.The mirror box is based on a plywood shipping container, of 8” x 10” cross-section. The sides of the box carry the 5 3/4 “ diameter altitude bearings which are faced with Ebony Star laminate. A large iron bracket is mounted on the upper surface of the box. This serves both as a counter-weight and as a platform for the right-angle finder, cannibalized from a pair of 20 x 50 binoculars. A red-dot finder is also fitted, near the eyepiece.
The mirror cell simply consists of two perforated aluminum disks, held in compression by three spring-loaded bolts. The lower disk is bolted in place at the bottom of the mirror box. For collimation, three surplus micrometers tilt the upper disk, which carries the primary mirror. The mirror rests on three hard rubber pads at 70% of the primary radius, and no adhesive is used. To restrain lateral movement, three leather-covered tangs lightly contact the mirror sides, but do not obstruct the pupillary opening.
The upper tube assembly is supported by four, 1 1/4” square aluminum trusses, bolted securely to the mirror box inside corners. The upper tube is formed from rectangular panels of 3/16” mahogany plywood, held in place by self-tapping screws. The inner surface of the panels was roughened and painted flat black. Self-adhesive flocking material was applied opposite the focuser tube. A small viewing port at this location facilitates the use of a laser collimator for accurate mirror alignment. To suppress off-axis light, three baffles are fitted. A “moon baffle” made from weather-stripping was added near the top end of the tube. Circular baffles were added in the focuser tube and near the mirror box. A curved-vane spider made from a stainless steel ruler carries the 1” minor axis secondary mirror (17% central obstruction). This mirror is mounted on a universal ball-joint for easy adjustment. A thumb-screw mounted panel near the primary can be easily removed, speeding up thermal equilibration. This feature also allows the use of a mirror cover, slowing dust accumulation on the primary.
Presently, the 1 ¼ “ focuser operates on the simple draw-tube principle, which seems adequate when observing at f/16. A low-cost, helical focusing adapter will likely be incorporated into the design.
Building the mount.The rocker box and the ground-board are made from ¾ “ medium density fiber board, sealed with a white polyurethane finish. All exposed edges are trimmed with 3/16” thick strips of mahogany for protection against chipping. Fasteners are brass or stainless steel. This finish has proved very durable in four years of use. A “virtual counter-weight” consisting of a spring and pulley-blocks is incorporated into the rocker box base. This greatly reduces the mass needed to balance the heavy OTA. The ground-board features a Teflon on Ebony Star bearing, a bubble level, and four castors for mobility.
Building the drive system.This was the most challenging aspect of the project, involving subtle interplay between mechanical and electrical parameters. Each axis is equipped with a 3V DC, 1 rpm motor (Edmund Scientifics), coupled to a pulse-width modulation drive circuit (Jameco Electronics kit #67). Motors are mounted on rubber grommets to suppress vibration. The system is powered by a standard 12V DC wall transformer. A control box allows the selection of tracking speed, direction and rapid slew on each axis. The altitude axis employs a 5/16”, 24 tpi drive rod mechanism, salvaged from an old biomedical syringe pump. This mechanism incorporates a spring-loaded clutch pin, allowing rapid disengagement of the drive for manual tracking. As a side-benefit, residual friction in the drive guides suppresses backlash in the altitude bearing during manual guidance.
The scratch-built azimuth drive utilizes a 3/8”, 16 tpi ready rod, connected to the motor spindle via nested 3/8” bore and 1/8“ bore locking collars (Small Parts Inc.). A coupling nut rides along the ready rod and bears the drive tang, secured with a U bolt. The coupling nut threads were packed with epoxy glue to minimize free-play. Loosening a set screw on the large locking collar uncouples the drive rod from the motor, allowing the tang to be reset by manual cranking. The drive tang has a cut-away that permits free rotation of the telescope towards the celestial West, as required for manual tracking.
Observational experiences and problems.At the time of writing, the telescope uses an inexpensive Taiwanese plate glass primary. With this mirror, Jupiter shows dark blue festoons and barges, the Great Red Spot with central darkening , and moon shadow transits on nights of steady seeing. The south polar cap and Solis Lacus were noted on the 12” disk of Mars, early in the 2005 apparition. Saturn shows the Cassini division complete around the ball of the planet, the contrasting colors of the A and B rings, a gold C ring at the ansae, and dark green shading on the polar region. Fine rilles on the floor of the lunar crater Gassendi are readily seen. However, resolving Plato craterlets, Jovian white ovals, and Galilean moon transits has remained a challenge. I plan to replace this primary with a Strehl 0.95 mirror at some stage.
The telescope is used in the manual tracking mode whenever observing time is limited. Motion on the azimuth axis is very smooth with negligible backlash. Residual backlash on the altitude axis does not significantly impede high-power viewing. The use of castors unavoidably elevates the centre of gravity. This contributes to a vibration damping time of 3.5 seconds, a little longer than desirable. The use of lighter aluminum trusses might have improved this aspect of function, without compromising OTA rigidity.
Considerable practice and patience are needed to effectively operate the motorized tracking system, especially on planets well off the meridian. Motion is not entirely linear over the full rotation arc. Contributory factors here may be the use of relatively high-friction Teflon bearings, a heavy OTA, and unbalanced moments about the altitude axis. However, short-term (15 minute) tracking performance is adequate for visual use at up to x252. No vibration is evident in the eyepiece at this magnification, although motor noise would likely be intrusive at quiet observing sites. In my experience, motorized tracking does indeed make it easier to see fine lunar and planetary detail, and is a worthwhile addition despite these drawbacks.
Conclusions.In summary, I greatly enjoyed this project. When it comes to acquiring high-resolution optics at reasonable cost, the classic 6” f/8 Newtonian has few, if any, equals. Remarkably, however, the design seems destined for a lowly, “entry level” status in the market place.
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