This page summarizes installation of encoder mounting hardware and subsequent use of an Argo Navis computer on an 18 inch Starmaster Dobsonian telescope.

I purchased a used 1997-98 Starmaster telescope with an 18 inch f/4.5 Pegasus mirror in 2005. I have owned two other Starmaster telescopes with Sky Commander digital setting circles/computer (Sky Engineering, Inc.) and have been satisfied with their price and performance. However, after seeing the newer Argo Navis Computer (Wildcard Innovations, Pty. Ltd.) in operation by many observers at the 2005 Queensland Astrofest, I wanted to install this larger, more expensive computer with its high resolution encoders and increased capability. My used Starmaster had never had an encoder set installed, so my first step in installation was to find an encoder mounting hardware kit.

18" Starmaster Measurements and Encoder Kit Installation

I started to take measurements on my telescope to find out the best JMI encoder mounting kit or combination that might be work on my Starmaster. Initial surveys and questions in September, 2005 revealed that neither JMI representatives nor Gary Kopff at Wildcard Innovations had a stock recommendation for encoder hardware kits for Starmaster telescopes. They told me that the JMI encoder kit for the 18" Obsession telescope would not mount on my Starmaster. I took initial measurements to have Gary Kopff recommend JMI encoders that might work with my system.

Azimuth bushing and bolt installed by Rick Singmaster during original contruction of my Starmaster appears to be an Astro-Systems pivot kit with drilled pivot bolt and encoder shaft lock screw. This original superb construction by Starmaster greatly simplifies encoder installation.

Altitude Bearing (3/4 inch thick). The center of the altitude bearing diameter is about 12.5 inches. I ordered a 15 inch long encoder mounting arm to be able to place the tangent arm anchor pin on the rocker box (see below)

• Other measurements:
  • Clearance between rocker box and optical tube at closest point: 2 3/8 inches
  • Clearance between top of azimuth bolt and optical tube at closest point: 1 3/4 inches
  • Ground board and rocker box are each 3/4 inch thick with 5/16 inch between.

After looking at the drawings for the JMI encoder kits, I was struck by what appeared to be their relative complexity of design. I went back and looked at photos I had from my other two Sky Commander installations. The encoder mounting kit that Sky Engineering sells is the epitome of simple and elegant functional design. I abandoned my plans to use JMI encoder mouting hardware (Price: $170 USD) and ordered Sky Engineering's ALT-AZ-1000 General Purpose Encoder Mounting Kit ($40). Because the stock kit elevation tangent arm is too short for the altitude bearing on the 18" Starmaster, I also ordered the 15" encoder mounting arm (ARM-EN-1500, $12). I had to order the encoder kit through the mail from South Florida since Sky Engineering doesn't have a web commerce site or accept credit card purchases on line. On the same day, I ordered my Argo Navis computer and long encoder cable on line from South Australia. About 10 days later, I received items from Florida and Australia on the same afternoon.

Sky Engineering Encoder Mounting Kit

The general purpose encoder mounting kit (ALT-AZ-1000) made by Sky Engineering is well engineered and easy to install.

Sky Engineering Encoder Mounting Kit (ALT-AZ-1000) A = Elevation Tangent Arm ( I ordered the 15" arm for my 18" Starmaster); B = Azimuth Tangent Arm; C = Elevation coupler and wood screws; D = Azimuth and elevation anchor pins and bolts; E = Azimuth pivot bolt and bushing with retainer clip; F = Encoder cable clips and screws.

Bottom of rocker box showing azimuth pivot bolt.	Mounted encoder on azimuth encoder arm. Encoder shaft was locked in pivot bolt. The tangent arm faced towards the front corner of the rocker box. Rick Signmaster usually mounts tangent arms towards the rear (better interface with GoTo system?)	The hole site for anchor pin was marked with nail, and the hole was drilled with a 1/16" drill bit. (Note: These encoders are 4000 tick encoders that were replaced the same day with the 10000 tick encoders for the Argo Navis.)
The anchor pin was fixed in place with a 6 x 1 1/2 wood screw.	The one minor problem with the Sky Engineering encoder mounting kit is that the holes for the elevation coupler just barely allow the wood screws to purchase the altitude bearing on the Starmaster. The Starmaster name plate has been removed to allow the coupler to sit flat and orient the encoder shaft hole as close to perpendicular on the bearing surface as possible (paint chipped out where the name plate had been attached). Rick Singmaster avoids this hole alignment probem with a custom machined aluminum coupler that mounts over the name plate and has holes drilled farther away from the edge. See photo.	The anchor pin hole site for the elevation tangent arm is located on the rocker box using a similar process to that used for the locating and drilling the azimuth pin hole.
Elevation tangent arm and encoder mounted in place.	I used the battery compartment screw to affix an encoder cable clip to the Argo Navis. The Argo Navis is well built and has a high quality screw that treads into metal receiving threads. I used the encoder cable clip to hold a split ring key for attachment. See the next photo legend for details. Note: The Argo Navis comes with a mounting holster and screws/velcro strips to mount the holster to the telescope. If you are mounting the computer on your upper cage, however, the suggested method isn't secure as you raise and lower the telescope.	A key clip with strap is used to attach the Argo Navis to the upper cage via the knobbed assembly bolt.
This mounting method is very stable and secure. The computer repositions roughly perpendicular to the ground as the telescope is raised and lowered through normal observing motions.	Argo Navis on 18 inch Starmaster. With the computer mounted from the upper cage, you always have the computer accessible at hand near your focuser, eyepiece and Telrad. This is particularly handy when you are touring an area of the sky while standing on a ladder.

Comments on Argo Navis Use:

Initial Setup: The Argo Navis comes with a very good user manual and very clear instructions for setting up encoder resolution, direction, time, location, etc. Setup is very simple and straightforward and only minimally more complicated than setting up the Sky Commander.
Nightly Alignment and Setup: The Argo Navis uses two star alignment (same as the Sky Commander). It has one additional step, Mode fix alt ref where you point your Starmaster straight up at 90 degrees to the rocker box. This defines the altitude encoder reference point. After setting up mode fix alt and doing two star alignment, you are set for the entire evening.
Pointing accuracy: It is difficult for me to say whether the Argo Navis points any more precisely than Sky Commanders I have used in the past. I never had difficulty finding objects with the Sky Commander which was very accurate. After alignment, the Argo Navis consistently placed selected catalog objects in a circle outlined by a Televue 12mm Nagler 4 with Paracorr (24 arcsec).
Ergonomics: The rotating menu dial with enter and exit buttons is very simple and intuitive to use. The controls are easy to use when you wear gloves.
Touring function: This is one of the most useful functions to me. You can tour objects in a constellation or region of the sky after defining the angle of the field of the tour. The user has tremendous latitude in setting parameters for the tour. In the middle of or at the end of an observing session when your getting a bit punchy from fatique, it's fun to go into area of sky you usually don't look at and punch in parameters (e.g., non-stellar, magnitude 14 or greater) and just tour around that region of the sky to find unfamiliar and surprising pearls of the heavens.
Data display readout: Data for objects includes common names, size and magnitude and features of morphology. These are very helpful for reference when you are scrolling through objects in your tour choices.

After using the Argo Navis for a few nights, it is difficult for me to imagine ever going back to using the Sky Commander. The magnitude of computer performance compared to its price make the Argo Navis a good buy for digital setting circle/computer users.

Equipment index
posted 31 October 2005, revised 01 November 2005