The Gemini HowTo
How do I start?
Before your Gemini system arrives you should check the mechanics of your
Check if the axes are lubricated, the bearings' needles are rolling
freely in their cages and the plastic coupling discs and the surfaces they
are gliding on are clean and dry.
Check the worm gears (worms and wheels) if they are clean and slightly
Dismount the stepper motors (keep the plastic cardan coupling disc)
and adjust the worms so that they can be turned easily with your fingers
- at every position of the wheel.
Download the Gemini manual from the Losmandy
HGM website, print and read it.
What else do I need?
At least a good power supply or battery. Although the servos need
very little current while tracking, they must get the energy to overcome
the friction and to accelerate the OTA, counterweights, ... to slewing
A "stiff" (having a low internal resistance) power supply allowing
for at least 3 amps current is recommended.
12 volts or something around 13.8 volts are fine.
If you want to connect the Gemini to a PC, you'll need a serial cable
with a 4pin telephone type RJ-11 modular plug and a connector for the serial
port of your PC. The layout is given in the Gemini manual. Standard cables
can be purchased from Losmandy HGM.
You may also use a modified RJ11 to DB9-adapter. The modification needed
is the replacement of the
6pin-plug by a 4pin-plug which has to be mirrored (turned around 180
degrees before crimping on).
The motor cables are too short
The motor cables are flat straight 8-wire cables that can easily be self-made
with the required length using a 8/8 modular crimp tool.
I'm not sure if my power chord has the correct polarity
You can try to use it. The Gemini is protected by a diode against wrong
polarity power supply.
Navigate through the menu
After selecting the Menu Mode by pressing the Menu Key, you are always
in the main menu, and "M E N U" is displayed. Pressing any HC key brings
the actual main menu point into the menu, normally that is "Align Telescope"
after switching on or the last menu point you've used. The handcontroller
DEC keys scroll trough the possible selections, the RA+ key selects it,
RA- goes back to the previous selection (menu point, number,...), but has
no effect while you are in the main menu. There is a picture showing the
HC with the names of the keys in the manual.
Check what Setup Values are stored
All settings can be done using the Setup menu point. Alternatively, you
can use the Gemini Control Program or partially a planetarium program. With
Level 3, V1.01 up UTC date and time, mount type and location are displayed
and can be input directly at a Cold Start.
If you want to check the current UTC time because you are not sure about
the correct timezone or Gemini's UTC date and time format, click
here to get
To use Setup, select menu mode, scroll with
DEC keys until you get "Setup" displayed, select it with RA+, then scroll
through with DEC keys to the item you want to check. Let's say you want
to check which longitude is stored, so
press the Menu key to get "M E N U" displayed,
scroll thru the main menu with the DEC Keys until "Setup" is displayed
press RA+ to enter Setup mode: "Hand Controller" is displayed
press DEC- (or scroll thru the complete menu with DEC+) until "Geogr. Location"
press RA+ to get "Longitude" displayed.
press RA+ to get "Longit. W118°20'" displayed. This is the longitude of
Scott's shop. The "W" is blinking.
If you only wanted to have a look press RA-. Nothing will be changed, you
can select the next item to be displayed in the menu tree.
Change Setup Values
Begin as described in above. Once you get the value displayed that you
want to change, you can adjust the blinking character with the DEC keys.
If it is correct, the next input character should blink, special characters
(degree/minute/second signs and similiar) will be jumped over. If you've
made a mistake, you can go backwards to the wrong input character with
RA- again. Your input will be stored once you leave the input area with
RA+ (to the right side) and you'll get an acknowledging message.
The Setup->Set Time menu point is an exception insofar that it allows
to change f.i. only the date and to go back to the right, accepting the
new date and leaving the time value unchanged. In addition to the usual
characters and numbers displayed you may see an arrow character -> displayed.
It serves as "Enter", if you select it and press RA+ the displayed value
(without the arrow of course) will be stored.
What Setup input is needed for a G-11?
- The coordinates of your observing site and the timezone as described
above with Setup->Geogr. Location->... For southern hemisphere users: the
minus sign of the latitude reverses the tracking direction.
- The UTC/GMT date and time with Setup->Set Timer.
But if I have a GM-8 or HGM-200?
First of all, select Setup->Mount Parameter->Mount Type and choose you
type of mount. Having done this, set your location, Date and Time as described
above. Set the Safety Limits for RA to the values you need.
Mmh ... isn't there an easier way?
Do you use TheSky planetarium software? If yes, connect your Gemini to
the PC (cabling is shown in the manual), activate the link, select the
option "Initialize" in the Telescope Menu, begin with "Set GMT", after
this use the other setting options (Location, Time, Date).
Setting the RA Safety Limits
Switch the Gemini off, move the counterweight shaft to its downmost position
and switch the system on again. Slew to the (eastern, western) position
where you want the limit to be set. The slew may be breaked before you
old limit if you wants to extend it, slew slowly (by intermittend keypresses)
to the old limit. The slew will be stopped and the buzzer sounds. Slew
to the position where you want the new limit to be set. Press the menu
key and select Setup->Mount Parameter->Set Safety Limit.
I want to use my mount's axis encoders.
Do you have a G11 and the Losmandy encoders? Just take the Y-cable and
plug it into the Gemini's encoder socket before switching it on. The encoders
are autodetected: Once the first encoder tick is signalled, they are activated.
For other mounts and encoders you'll have to define the directions and
resolutions in Setup->Encoder. There is also a Test Encoder submenu for
testing. After one revolution in DEC there must be the same value shown
in the display. If not, the DEC encoder resolution value is wrong.
Make sure that there is no play in the encoder gears.
BUT: For the first tests of your Gemini system, start without axis encoders.
They add some possibilities for errors.
Does the mount has to be polar aligned?
For photography, yes.
For visual observing you can use the modelling feature. Usually I align
my mount by moving in until I can see Polaris thru the hollow RA axis,
without the polar finder.
What does menu point "Align Telescope" mean?
With "Align Telescope" you tell the Gemini where your telescope actually
is pointing to, allowing it to adjust its internal coordinate system. The
"Initial Align" does that for the first time, resetting all the modelling
values and telling the system at which side of the mount your telescope
is (not where it is pointing to, that's not the same). The East side is
the side where you can point at the whole western hemisphere, from the
West to the meridian, without having to do a meridian flip. Vice versa
for the West side.
How does the modelling work?
The Gemini uses an internal model how to correct RA and DEC values for
polar misalignment, axes nonperpendicularities and play. The parameters
are calculated from object positions you teach in, a best fit is
calculated and the model is synchronized to the latest position given.
In general, the better, the stable, the repeatable the mechanics and
optics work, the better are the results.
How do I activate the Modelling feature?
By selecting different objects (that you know and you can identify!) from
the internal databases, making a GoTo to them, centering them in your eyepiece,
pressing the menu key and doing an "Align Telescope->Additional Align".
Especially for the first Additional Aligns it is important, that the objects
do have a different hour angle (distance to the meridian) the time you
do it, if there is not much time between the alignments, that means a different
RA. Doing an Initial Align at Capella and an Additional Align at Rigel
or Bellatrix is not a good idea, the same for an Initial Align at Procyon
and an Additional Align at Castor or Pollux thereafter - there are only
tiny differences in RA.
And what's happening after a Meridian Flip?
After a meridian flip you'll experiencing a constant offset caused by gear
play. Now the opposite side of the worm and teeth are in contact. Additionally,
all parts with a certain amount of freedom to move (f.i. the main mirror of an
SCT) will have flopped to the other side. Once you make the first Additional
Alignment after the meridian flip, the Gemini will know how big this play and
flop is and will take it into account. Please do this at an object in the middle
of the hemisphere, that the gear is in good contact. Maybe refocussing helps
to move the mirror into a stable position. Objects near the zenith are no good idea.
How is the modelling used?
Whenever you do a GoTo, the modelled deviations are added to the coordinates
of the object and the GoTo is made to the corrected position.
If you want to check the effects of the modelling, select GuideTo (Guide->)
after the GoTo to see the difference caused by the modelling. GuideTo always
shows the position of the object in relation to the Initial Alignment, as if no
Additional Aligns were done.
Am I informed about the parameters calculated?
Two parameters, the Azimuth and Elevation values of polar
misalignment are displayed after each additional alignment. The other values,
RA/DEC axes Nonperpendicularity at the Pole NP and
Nonperpendicularity at the Equator NE, the gear play and
mirror flop values FR and FD as well as the Index Errors
IH and ID, showing the deviation of the alignment objects from
the real position are stored into the Information Buffer and can be examined
using the Show Information menu point.
This values give you valueable information about your alignment, your mount
and your OTA. If you use the modelling correctly, avoiding systematical errors
(f.i. rotating the diagonal), they should converge quickly to typical values.
The signs may change, f.i. if you reverse the attachment of the OTA to the
I'm having problems with the serial link to my PC
- Please check if your cable has the required layout.
- Check if you are using the correct COM port of your PC.
- Check if the port is set to 9600bit/s, 8 data bits, no parity.
- Activate the FIFOs for the port, full buffering works best.
- If everything seems to be O.K., startup the Gemini in "Debug Mode"
by pressing any key of the hand controller while switching on.
- Start your planetarium software and try to establish the link to the
Gemini. If it works, fine. If not, display the contents of the information
buffer of the Gemini. In Debug Mode, the command characters (without the
numerical values) are stored there. If you can see something like :GR#:GD#
the Gemini receives the characters transmitted from the PC properly.