On this page you will find answers to most of the basic questions.


What is the Sky Commander? – The Sky Commander is a small self contained computer designed for the specific purpose of locating and identifying celestial objects with a telescope.

How Does it Work? – The computer ‘reads’ the position of the telescope with the use of two optical encoder modules. The encoders are attached mechanically to the telescope axes in such a way that the encoder shaft turns as the telescope is turned. At the beginning of an observing session, the user initializes the computer by pointing the telescope at two different stars. By sensing the change in position between the two stars, the computer is able to fix on the orientation of the both telescope and sky.

How do I point the telescope to an object? – A six button keypad and LCD display form the easy operator interface. First you select a catalog, e.g. Messier, NGC or Planet. Then you enter the catalog number (four digits). Arrow buttons are used for this purpose. Pressing the ENTER button makes the object ‘current’. The user moves the telescope while watching the LCD. As the scope moves closer to the target object, the display digits roll down to zero in AZ and EL. Arrows are displayed to indicate direction.

What does the display show me? – The Liquid Crystal Display (LCD for short) shows you the position of the telescope in RA and declination, or the relative position of the scope to the target object (‘zero in’ display). It also shows you the current catalog and object number (used for object selection) and some information such as object type, magnitude, and constellation.

How do I get it set up on my telescope? – The Sky Commander works on both Alt-Azimuth (e.g. Dobsonian) and equatorial type scopes. Sky Engineering supplies a general purpose encoder mounting kit (pn ALT-AZ-1000) for Dobsonian type scopes. Since it is a general purpose kit, some modifications may be required to make it work depending on what scope you have. Dobsonian type scopes are by far the easiest to fit encoders because there is generally plenty of room to work with. For other type scopes, the Sky Commander will work with most commercially available encoder setups. A standard encoder cable connector (modular RJ45 also used for computer network cables) is used.

What’s the deal with encoder resolutions? – Angular resolution of the system is determined by the encoders themselves. The Sky Commander can handle a wide range of resolutions. Typical encoders are 4000 step per revolution, but other resolutions are also used, (2160, 4096, 8192, etc). The important thing is that you enter the encoder information into the Sky Commander so it knows how many counts per degree of scope rotation. This information only needs to be entered one time, but can be changed whenever needed.

Do higher resolution encoders mean higher accuracy? – Possibly, but be aware that the encoders themselves have an inherent ‘runout’ error which may exceed the step size, especially for the higher resolution units.

What is the RS232 connector for? – RS232 is a standard for serial communications between computers. In the Sky Commander it is used to tell a PC the position of the telescope so it can be displayed on the screen. In this way the user can tap into the huge resources provided by many Astronomy programs. On the XP4 RS232 is used as a ‘Flash Port’ when updating the software or internal database.

How is the Sky Commander powered? – The Sky Commander is powered from an internal 9 volt battery which will last a very long time (about 40 hours) under normal use. There is also an optional 2.1mm (5.5mm OD) power jack for connection to 12 volt DC.

What do I need to buy? – If your scope already has optical encoders installed with a standard encoder cable, you need just the Sky Commander XP4 itself, CMP-XP-4000. If you have a Dobsonian scope you will need the SYS-XP-4000 (includes cable and two 4000 step encoders) and Dob kit, ALT-AZ-1000.