Gemini Servo System Features

DC Servo motors

Servo motors have the advantage of a huge dynamic speed range. They track smoothly even at the lowest guiding speed consuming very low current but can ramp up to high speeds while delivering high torque.

Hand controller (4+1 keys)

16 Character dimmable red LED display

Separate Autoguider Connector

Mount Axes Encoders are supported but not required

Serial Port Communication

Real Time Clock

Battery backuped SRAM data

Feature Connector


Gemini supports focusing units having 9 to 12 volt DC motors (e.g. JMI's NGF series). In order to focus using the Gemini, you must have an adapter board accessory connected to the feature port.

The Gemini focusing function uses the Meade LX-200 serial line focusing commands. You can also change focus using the hand controller by putting it into focus mode. This is done by selecting "Focusing" from the Quick Menu. This menu item is only available if the adapter board was connected. When in Focus Mode, the RA buttons move in and out, and the dec buttons toggle between fast and slow focusing modes.

To disengage from the Focus Mode, press the Menu button.

Internal databases

User Defined Database

A user defined database with up to about 4000 objects can be uploaded using serial communication and are stored in the battery backuped SRAM memory. This feature can f.i. be used to store the observing program for a night.

Observing Log

Many user actions, f.i. startup and shutdown, object selections, GoTo operations are logged and stored into the SRAM memory together with timestamps and coordinates. This log can be uploaded and used to build the user's observing log.

Gemini Starts Up Aligned

Gemini can slew to the first alignment star if the mount is polar-aligned and the OTA is in a defined home position during startup. This feature also requires that Gemini have the correct latitude, longitude and UTC data.

Sophisticated Telescope Pointing Model

Simplified Sync Operation using TheSky

From Level 3, V1.01 up every location marked within the planetarium program TheSky can be used to synchronize Gemini's coordinate system without having to slew to the location before. This simplifies Sync and Image Link operations.

Polar Axis Correction

This feature assists you in accurately polar aligning your mount. Because Gemini's modeling feature calculates the mount's deviation from correct polar alignment, it can be used to guide you as you adjust the mounts altitude and elevation to achieve polar alignment.

After you have done several alignments on different objects, select an object from the Object Database that is near the celestial equator and the meridian. Goto the object and center it in the eyepiece using the HC buttons. Now, select "Align Telescope->Polar Axis Corr." from the menu. Gemini will move the mount to where the object should be if the mount were accurately polar aligned. You should adjust the polar alignment of the mount using the altitude and elevation knobs until the object is again centered in the eyepiece. Since the mount will approximate perfect polar alignment, all subsequent GoTo operations should result in target objects being near the center of the eyepiece. If not (for instance, because the mount's initial misalignment exceeded several degrees), you may repeat the correction process. Note that after using this function, the modeling parameters will be reset and you will need to rebuild the model by performing several alignments on different objects.

Permanent Setup


To use this feature, ensure that the mount is in the CWD position before applying power. After selecting "Cold Start", simply choose an object from the database and select "GoTo". For even more convenience, Gemini offers a "GoTo Bright Star" feature, listing the commonly known bright stars currently above the horizon. Just select one of them, or a database object, and Gemini will slew reasonably close to the selected object. Use the hand controller to center the object. Now all you need to do is select the "Align Telescope" menu item and select "Synchronize". You can then add additional alignment stars as before.

GPS Input

Gemini's Level 3 software allows the user to set Gemini's longitude, latitude, UTC date and UTC time from a GPS receiver connected to the serial port. Any GPS receiver can be used if it is capable of transmitting either NMEA 0183 protocol or Garmin Textout protocol at 4800bps or 9600bps.

Gemini attempts to query a GPS when it is powered on. You can also explicitly have it read the GPS data by using the "Setup->Geogr. Location-> Query GPS Rec" menu item at any time.

Sophisticated Tracking Modes

Sidereal tracking follows the sky at a constant rate.

Terrestrial Mode switches tracking off to allow for terrestrial use or observing geostationary satellites.

Comet/User Defined Tracking

Comets and Minor Planets follow their own trajectories through the Solar System. Gemini can be commanded to follow their movement in a variety of ways.

Available on the hand controller is a Comet/User Defined submenu of the tracking speed selection. Once selected (after centering the object), a training phase can be started. Unlike training PEC, you don't have to follow the motions exactly. Just keep the object in the eyepiece for a sufficient amount of time. Finally, center it and press the menu button to end the training phase. Gemini calculates the necessary tracking rates in RA as well as in DEC and the mount will follow the object in both axes.

The amount of time needed for training will vary with the object and the type of observation. For an object with a relatively slow and constant motion, a few minutes of training should be sufficient for visual observing. For photography, you may want to consider training for up to half of the intended exposure time. For an object that changes speed fairly quickly (such as a Near Earth Asteroid), shorter training sessions just before the observation will work best. In general however, for an object with relatively constant speed, the longer the training (up to 45 minutes) and the more exactly the object is centered at the beginning and end of training, the better the tracking will be.

Alternatively, if you know the ephemerides of the object, you can input the actual position and the position in 1 to 255 hours together with this timespan. Gemini will set the tracking rates accordingly.

Additionally, new serial line commands have been added to allow the user to set his or her own RA/DEC tracking rates. This "User Defined Tracking" (to be used instead of a training phase) is useful for tracking minor planets and comets by setting the necessary rates using a computer program. For more information on how computer software can use these commands, see the Serial Line Protocol description.

Closed Loop Tracking

Gemini can be instructed to stay at, or slowly move to, given RA and Dec. coordinates regardless of how accurately the mount is polar aligned. The difference between the desired coordinates and the actual coordinates is checked about 22 times a second. If there is a difference, Gemini moves one step in the requested direction. This lets the telescope gently follow the coordinate at about 0.5x guiding speed.

There are several applications for this advanced tracking feature:

  1. Visual observing. There is no need to precisely polar align the telescope for visual observing. The internal pointing model is not only used for the GoTo operation, but with Closed Loop Tracking also for keeping the object centered.

  2. Tracking comets or minor planets. This requires a connected computer sending updated coordinates whenever the object coordinates change. The coordinates are to be transferred with the :Sr and Sd commands. No :MS# slew command is to be issued. If the use of the :MS# command cannot be suppressed inside the computers software, Gemini's "Move Lock" command :ML# can be employed to let Gemini ignore further :MS# commands.

Gemini allows you to center an object with the hand controller during closed loop tracking without moving back to the calculated position. The difference between the calculated and actual position is stored and added to the new coordinates within the correction cycles.

Adaptive King Rate Tracking

Gemini Level 2 includes the option of constant King Rate tracking for objects at lower elevation. Level 3 takes this a step further by varying the tracking speed depending on the distance of the object above the horizon.

You can enable the adaptive King rate using the "Setup->Mount Parameter->Tracking Speed->Adaptive King" menu item.

Variable Solar/Lunar Tracking Rates

Earth and Moon move in slightly elliptic orbits. According to Kepler's Law, the velocity changes with the distance from the gravitational center. Gemini calculates the actual tracking rate necessary to follow the object at the moment you select the tracking rate (considering the motion within the next hour). For the Moon, the parallax is taken into account, too.

Western Tracking Limit Warning

While tracking, an audible warning (a short beep about every 20 seconds) sounds some minutes before the western safety limit is reached. This gives the user an opportunity to close his camera shutter or take whatever other action is necessary.

Enhanced Search Mode Features

The Search Mode can now be accessed by remote control, a feature particularly useful in video astronomy. In addition, a new "Z" shaped search pattern, "Wobble Object," was added to assist in recognizing faint objects.

Permanent PEC with Advanced Functions

Level 3 provides a greatly enhanced set of PEC (Periodic Error Correction) features. Gemini L3 provides the following new PEC functions:

PEC Delay Correction - Advances the playback of PEC corrections to compensate for latency between the error and the correction during PEC recording.

PEC Drift Correction - Removes any systematic drift component from the recorded PEC data caused by differential refraction or polar misalignment.

Smooth PEC Data - Smoothes recorded PEC data to eliminate spurious corrections caused by seeing, etc.

Average PEC Data - Allows you to make multiple PEC training runs and average the results.

Restore Saved Data - Restores PEC data that has been moved to a backup buffer by a previous PEC command or an additional training run.


Up to 10 bookmarks can now be placed at any coordinate in the sky, and Gemini's GoTo feature can slew to it. To set the bookmark, select the "RA/Dec Coordnt.->Set Bookmark" menu item. To have Gemini slew to the bookmark, select the "RA/Dec Coordnt. ->GoTo Bookmark" menu item.

You can overwrite old bookmarks using the Set Bookmark feature or add a new one as long as the maximum of 10 (0..9) is not reached. Bookmarks are stored permanently and can only be erased by a Reset Defaults operation.

Identify function


A convenient and fast place to (1) change Gemini's Centering Speed, (2) toggle between Visual and Photographic Modes, (3) Park the OTA, and (4) Focus via an auxiliary adapter board. To access these functions, simply hold the menu button down until the options start appearing on the display. When you see the option you want to select, release the menu button.

Appendix - Level 3 Menu Tree

Align Telescope

GoTo Bright Star
Reset Alignment
Additional Align
Polar Al. Assist
Polar Axis Corr. (available only if there was a pointing model established)

Object Database

Messier NGC
Solar System
Bright Stars
User Defined (available only if the user downloaded catalog objects)

Show Information

Goto <object>

Guide to <object>

RA/DEC Coordnt.

RA/DEC Display
Enter RA/DEC
Set Bookmark #x (0 <=x <=9, number of bookmark )
GoTo Bookmark#x


M/NGC/IC/Sh2 Cat
Messier Objects
NGC Objects
IC Objects
Sharpless 2 Obj.

Arbitrary Obj. submenu for all kinds of objects
Stellar Objects
NonStellar Obj.

All Magnitudes> submenu for all kinds of objects
Magnitude < 15m0
Magnitude < 14m0
Magnitude < 13m0
Magnitude < 12m0
Magnitude < 11m0
Magnitude < 10m0

Object Search

FOV 2 degrees
FOV 1.5 degrees
FOV 1 degree
FOV 50 arcmin
FOV 40 arcmin
FOV 30 arcmin
FOV 20 arcmin
FOV 10 arcmin
FOV 5 arcmin
Wobble object

Show Date/Time

UTC Date/Time
Julian Date
Civil Time
Local Sidereal


Hand Controller

Visual Mode: C-S
Photo Mode: G-C
All Speeds:G-C-S

Mount Parameter

Tracking Speed

Adaptive King
Closed Loop
Comet/User Def.

Train Tracking
Calculate Rates

Moving Speeds


TVC Value:


Train PEC
Clear PEC data
PEC Off (if On) OR PEC On (if Off)
Delay Correction
Drift Correction
Smooth PEC data
Average PEC data
Restore saved

Mount Type

Losmandy GM-8
Losmandy G-11
HGM 200/MI-250
Celestron CI700
Losmandy Titan
Losmandy Titan50

Set Safety Limit

Set Home Posit.


EncRes RA
EncRes DEC
Test Encoder
Ignore Encoder
Use Encoder

Set Alarm Timer

Alarm On
Alarm Off
Set Alarm Time

Set Date/UTC

Credit Info

Restore Defaults

> Losmandy
>Mountain Instr.

Dim Display

100% Brightness
53% Brightness
40% Brightness
27% Brightness
20% Brightness
13% Brightness
6.6% Brightness

Geogr. Location

Query GPS Rec.

Coordinate Epoch

Equinox of Date
Epoch J2000.0


Focusing (only available if focuser board connected)

Focus In, Focus Out with RA buttons, Focus Slow, Focus Fast with DEC buttons

2x Centering
8x Centering
16x Centering
32x Centering
64x Centering
128x Centering
-> Visual Mode
-> Photo Mode
Park mount!