There is a lot of optical incremental encoders on the market, but UsDigital in the USA can supply good ones at a low price. These are the same used by most encoder mounting hardware manufacturers. An optical incremental encoder is made up of 2 major parts, the disk and the sensor. The disk of an optical incremental encoder is patterned with a single track of lines near the outside edge of the disk. The disk count is defined as the number of dark/light linepairs that occur per revolution. The sensor, made by Hewlett Packard, sends two signals which give the information to determine the direction of the rotation.

Four series of optical incremental encoders are suitable for telescope applications. These are E2, E3, S1 and S2 series, all from UsDigital. The first two series (E) are non contacting rotary to digital position feedback devices designed to easily mount to an existing shaft. The last ones (S) are shaft optical incremental encoders and are often used on commercial telescopes. All of these optical encoders are referred as quadrature optical encoders. Quadrature is defined as the phase lag or lead between the two channels. On an optical incremental encoder, you may read numbers such as 1000. These numbers refer to the number of Cycles Per Revolution (CPR) and each cycle can be decoded into 1 or 4 codes, referred to as X1 or X4 resolution multiplication. The Ouranos interface always works in X4 resolution multiplication. For a 1000 CPR optical incremental encoder, this will result in 4000 codes per revolution.

The E2 and S1 series can offer up to 4096 codes per revolution, are small and their current consumption is less than 20mA. The E3 and S2 are bigger with a much higher current consumption (up to 40mA) and offers up to 8192 codes per revolution.

New from UsDigital !!! The T2 inclinometer with up to 7200 codes/rev
This is great for alt-azimuth mount. Internally, an optical encoder disk is suspended between two micro-ball bearings. A weight placed on one side of the wheel assembly causes it to follow the source of gravity. The inclinometer can be installed anywhere on the altitude axis so you can leave it on the telescope tube and you won't have to be worry about the altitude encoder anymore. Very easy to install and no more encoder handling...

If you're not sure about making the encoder mounting hardware yourself, try asking for a free encoder installation instruction from JMI. This will give you some clues on how encoders can be mounted on your own telescope.

For more information about optical encoder: UsDigital

How to install encoders on a telescope

German Mount

A Losmandy GM8 mount (or GM11); close look at the RA axis and similar setup for the DEC axis
Encoders are both S1 4096 codes/rev. The gear are 2 sets of Boston Gear's #Y3296 and #Y3248. The gear ratio is 2/1, giving a total resolution of 8192 codes/rev. This setup was very easy to do with little machining.

An Optic-Craft like german mount with the use of friction wheels; close look at the RA axis and similar setup for the DEC axis
Encoders are both S2 8192 codes/rev and total resolution of the setup is 8284 codes/rev.

Another Optic-Craft like setup for RA axis and for DEC axis
A 8192 codes/rev E3 was chosen for RA axis. A small 1/4" shaft was added to the RA to receive the optical disk. This is a direct driven encoder setup.
The Dec setup is different and uses a small 0.75" shaft mounted between two small ball bearings and linked to the Dec shaft by way of a flat belt. Another 8192 codes/rev E3 mounted on the 0.75" gives over 20 000 codes / rev.

Great pages on the use of encoders on a Vixen GP astronomical mount.

Alt-Azimuth Mount

A standard 10" dobson mount; the azimuth setup and the altitude setup
For the azimuth setup, two pulleys and a timing belt were used, giving a ratio of 4:1. With the use of a S1 2000 codes/rev, this gives 8000 codes/rev of axis. In altitude, a S2 8192 is directly attached into a small 0.75" shaft. Since the new H2 encoders from UsDigital are now on the market, one of these will soon replace the actual setup.

Suppliers

UsDigital Corp
11100 NE 34th Circle | Vancouver, WA
98682 USA
Tel.  (360) 260-2468 | Fax (360) 260-2469

Jim's Mobile Incorporated
810 Quail Street, Unit E | Lakewood, CO
80215 USA
Tel. : (303) 233-5353 | Fax.: (303) 233-5359