Definition of terms:
Adaptive King Tracking Speed:
In the absence of the Earth’s atmosphere, sidereal
rate tracking would be all that is
needed to accurately track objects with fixed RA and Dec. coordinates
(any object outside the solar system). However, because the Earth’s atmosphere
refracts (bends) light, it complicates the situation. The light from an object close to
the horizon must pass through more atmosphere than an object higher in the sky. It is
therefore subject to more refraction, causing the object to appear to be slightly higher in the
sky than it actually is. This is called refraction. As the object climbs higher in the sky, it is
subject to less and less refraction, and catches up with its refracted image. This means
that objects appear to move at a rate slightly slower than sidereal rate when they are close to
the horizon. Several methods have been devised to compensate for this effect called
The King method (published 1931 by Edward Skinner King) helps to avoid
star trailing in photographs caused by differential refraction. Differential
refraction may especially affect wide field photographs with long exposures.
King proposed two measures:
The values necessary to accomplish this depend on the geographical
coordinates of the
observing site and the declination of the object being photographed.
Gemini's Adaptive King rate tracking varies the tracking speed to match
the required value for the selected object’s declination. The user must offset the
elevation of the polar axis himself.
- Offsetting the polar axis slightly towards the zenith, and
- Adjusting the tracking rate.
Alignment: Used to build a model. If no model is built, the
first use of Alignment will do an Initial alignment. Additional use of
the Alignment instruction will add stars to the model. Note: that
a model has to be built on both sides of the meridian. To switch
sides a meridian flip must be done. When
switching sides, (a meridian flip) the first Alignment will start
building a model on the other side, if no model has been built on that
Models can be built on either side of the meridian. The gemini-2
will automatically switch between them according to which side of the
meridian it is on. (Pointing to the west from the east side in not
considered switching sides, and the reverse is also true. If a meridian
flip has not been accomplished, you have not switched sides.) This
allows you to include some western stars in your eastern model, and some
eastern stars in your western model. How doing this effects the
model has not been tested yet. When the results of doing this is
known, I will change this paragraph. If you are using the built in
model builder, which has an East and West button, be advised that these
two buttons only select stars on the East or West. They do not
select which model is being used. To switch to the other model the
mount must go through the pole. An easy way to do this is to
select a star that has a HA greater than +/-4.6 This almost always
assures that the mount will flip sides.
Ask if Possible:
This selection will allow the telescope display the "Quick Start,
Cold Start, Warm Start, or
Warm Restart menu", if something has
not been detected where a Cold Start is mandatory. If the
SRam has been corrupted, or the log file corrupted, then the
Gemini-2 is more than likely going to force a cold start
bypassing the above menu selection. Also a low battery in the main unit
could cause the SRAM to lose it contents, and force a cold start.
All Speed Mode: This mode is for centering, slewing, and
You get to this mode by Menu-->HC-->Mode
and select All-Speed button.
Again only the buttons on the back of the hand controller can change speed by pressing the opposite button.
In this mode, all speeds are available, from Guiding
Speed to Slewing Speed, by using the opposite-button trick. You can use this mode, for example, while
drift aligning, when you need very fine centering of a star at the crosshairs and a fast
move from the meridian to the eastern or western horizon. However, for normal observing,
you'll probably select either the Visual or Photo Mode.
Guiding only works in "All Speed mode" or "Photo_mode"
This menu is a HC menu see
Menu-->Mount-->Balance on the hand controller. It indicates in
PWM (Pulse Width Modulation) and offset, how much power is being
supplied to the motor and, how far the motor is from the desired
Civil Time or Local Time - The time that you normally put on
your clocks/watches. If you are observing daylight saving
time, that time would include the daylight saving time. When
observing daylight saving time, normally the time zone offset will
be changed by +1.
Closed Loop Tracking Speed: The preceding discussion of tracking rates assumes
that the mount is accurately polar aligned, allowing tracking to be
accomplished using only the RA motor. However, for visual observing
sessions, there is no need for extremely accurate polar alignment. By
selecting Closed Loop tracking, Gemini can accurately track an object
even if polar alignment is off several degrees in any direction. Gemini
does this by comparing the coordinates of the object being tracked with
the actual coordinates of the telescope. If the coordinates do not
match, Gemini makes slow corrections in both RA and Dec. axes to point
the telescope back at the object. This process of compare and correct is
done about 22 times per second. Although Closed Loop tracking will keep
an object centered for a long time, it is not a substitute for a good
polar alignment – especially if you intend to photograph extended
objects. A misaligned polar axis will lead to field rotation, an effect
where objects in the field of view will rotate around the center.
However, after having done a good polar alignment, Closed Loop tracking
can be used to photograph slow moving objects, such as minor planets or
comets. This can work even if the object moves at a varying speed, by
repeatedly sending actual coordinates to the Gemini using a computer.
Because Gemini uses modeling parameters to calculate coordinates, Closed
Loop tracking is only available after a pointing model has been
Cold Start: This assumes
that no model has been build, or the mount has been physically moved, If
the mount is moved In Latitude or Longitude, or even a change of it's
position even slightly to the pole, or in vertical or
horizontal position at all (anything that will change Polar alignment), you have
to do a cold start. You start at CWD position with a cold start. It also
wipes out your current models, on both sides of the meridian. This is
the mode you use if you are mounted on a tripod. If the tripod
gets moved at all or bumped you will have to redo the cold start and
your model. The processor uses the CWD position as an approximate
starting point. When you do a synchronize or alignment on a bright star, the
processor now has a known good point to start it's calculations from, and also resets the limits (using the numbers that has already been set) to there correct position.
If this is the first time you have done a cold start, then after the
Synchronize/alignment on a bright star, go set your limits. This is
important, especially if you have a large scope mounted on your mount.
Warm Start and Warm Restart.
Comet/User Tracking Speed: This rate is very complex and a
separate web page will be generated for it.
Crossing the Pole: this is defined as when the Dec continue to
climb until it reaches 90 degrees, then when it starts back down on the
other side you have crossed the pole. Normally this is
accomplished along with a meridian flip, but it does not have to be.
CWD, Counter weights and shaft in a
vertical Down position with Dec pointed to True North, or True
South in the Southern Hemisphere.
Home Position, Home position is a user defined position. It
can be used to park the telescope at any position. It is
normally used in Observatories that have roll-of-roofs to park the
mount in an horizontal position so that a roof can be closed over
the telescope. You define it by moving the telescope to
the desired position and then use Menu-->Park-->Set home position.
When coming out of this position you must use a Warm Restart.
Lunar and Solar Tracking Speed: Neither the lunar nor solar tracking rates is
constant. Since the orbits of Earth and Moon are slightly elliptical, the apparent speed of Moon and Sun will change.
When you select the lunar or solar tracking rates, the tracking speed will be calculated
based on the actual position and the position one hour later.
Because the Moon orbits the Earth, its RA and Dec. change during the
course of the night. The lunar tracking rate compensates for the RA component of that
change; there is no compensation for the changes in Dec. While sidereal rate can
certainly be used for visual lunar observing, the lunar rate will do a better job keeping the
Moon centered in the telescope’s field of view. Since the Moon is relatively near the
Earth, even the location on Earth where you are observing must be taken into account.
The lunar tracking rate therefore takes the parallax into account by calculating
top-centric instead of geocentric positions.
Gemini computes the tracking rates by calculating the current position
of the Sun or Moon and the position one hour later. While the tracking rate for the
(distant) Sun will not change much during an observing session, the tracking rate for the
Moon might vary significantly as its parallax changes moving from the horizon to higher
elevations and vice-versa. You can reselect the lunar tracking rate periodically
throughout the observing session to allow Gemini to recalculate the correct rate as needed.
Because the Earth is orbiting the Sun, the Sun’s RA and Dec. do change
during the course of the day, but much more slowly than the Moon's. The solar
tracking rate again compensates for the RA component of that change, but not for the Dec.
Since the difference between solar and sidereal rate is only about 4 minutes a
day, you will see little difference between the two over a short period of time.
Meridian Flip: The process of causing the mount to switch sides of
In the Gemini 2 you can do this when selecting a target by checking the
flip box during a Go To command. If the mount can flip to get to
the target it will flip. If it cannot see the target with a flip,
then it will not flip even if the flip box is selected. See the
limits page for drawing what
is meant by East and West sides of the mount. Note the new Align menu
selection on the hand controller does not offer this box for flip.
Only using the Go To button (located on the right side of the Menu
button) will you get a the flip option box. It will be at the
bottom of the page of the object you selected. If you do a Go To -->
Coordinate Input --> check the flip box, and then Go To at the bottom of
the page, the Gemini-2 will do a meridian Flip to the current
coordinates, but only if it can safely do so, and can reach the current
coordinates from the other side of the mount. NOTE: The mount will never
do a meridian Flip while tracking at any of the defined tracking rates.
It only does meridian flips during slews or GoTo's. There are some programs
that can control the mount and make it seem to do a meridian flip during
tracking such as ACP and others.
If you are using the Model builder
to build a model and want to do a flip to the other model, you have to
pick a star that is between (+/- 4.6 and 6 HA) using the opposite side
(East/West buttons) IE. If you are on the east side, hit the West
button till a star comes up that has a HA larger than 4.6, and this will
almost always cause a meridian flip. This also holds true for the
west side and want to goto the east side with a meridian flip.
Just hitting the other side button and picking a star, will not insure a
flip to the other model. It is this way so that you can add stars
that is close to the meridian's other side to the existing model.
This helps in transitioning from east to west and back. Also to
ensure good meridian flips, while building a model use an alignment star
that is close to zenith that can be added to the model on both sides of
the meridian. This will help with better GOTO's and better
meridian flips. This
site shows pictures of G11 and what goes on during a meridian flip.
Model: This is the mathematical sum of terms that is built as one
builds a model. It is used to insure precise GOTO's.
Without a model, the mount really does not know the precise mechanical
to physical star locations. It takes into many mechanical
attributes of the mount, such as polar misalignment,
non-perpendicularities, and then flexure calculated as more stars is
added to the model. Since the Gemini-2
controller can control so many different types of mounts, with many
different types of telescopes attached, there is no practical way to
have a pre-defined model. Also a pre-defined model would not be
possible without perfect polar alignment. Building models normally
use the Bright Star catalog group of stars. These provide the most
accuracy. You can however use any object in the sky except the
moon and sun to build reliable models, as long as you can center on
them. There are two difference methods of providing a model in the
Gemini-2. One is more like what was done in the Gemini-1, while
the other is a graphics hand controller guide you through building a
model. Please see
There are actually two models built in the Gemini-2. One for the
East side, and one for the West side. It is recommended that you put
8-10 stars into each model, with at least 1 star from the other side of
the Meridian in each model. Try to make sure the first two stars
of each model are separated by at least 4
hours or more away from each other in RA, and then add more objects in between
them. So far the Hand Controller checks only for HA and elevation, not
for distances between the objects, but the hour angles are displayed for
each object, so you can select it manually. These models are stored in
the battery backed up SDam inside the main Processor. You do have
the option to store them into the micro-SDcard in the main unit, and
also retreave them from this memory. They are never worked on from the
micro-SDcard, only the battery backed up SDram in the processor.
Please note that the stars that are used to build a model is not
retained. Only the last star used in mormally known, so it can be
removed from the model if necessary.
Model Parameters: These are calculations in
Arc-Seconds of the different pointing and alignment errors
from doing a star alignment.
Object Search This menu allows you to search around where the mount is pointing. The field of view is in arc seconds,
and the search cycles is how many times the mount will make a circle at the arc seconds of deviation from center.
Hand controller commands are "Menu-->Search." Also see tutorial
- Alignment count - tells you how many points are in each
model. Note there is an East and West model built separately.
However they can only be saved or restored together. You
cannot separate them. See Web interface
- Polar Axis Misalignment - can be calculated with 3 to 5
stars. The amount that your telescope is missaligned from
the polar axis. This is also called Polar Axis
- Orthogonally Error - This is calculated with the 6 through 9
stars. Orthogonally error is were the telescopes is not
perfectly aligned Perpendicular to the RA Axis. This can be
caused by the Telescope DEC axis Saddle plate not being
Perpendicular to the RA Axis, or the telescope not setting in
the DEC saddle plate Parallel to the saddle plate. Other
mechanical items can cause this but this is the most common.
This is also called Non-Perpendicularity on the hand controller
model error pages. There are two values given. The first non-perpendicularity measure is at
the pole, and the second at the celestial equator.
- Flexure - this is calculated with 10+ stars. This is
how much the counter weight shaft is flexing. It takes an
alignment of 10+ stars on both sides of the meridian to
- Model Parameters: These are calculations in
Arc-Seconds of the different pointing and alignment errors
from doing a star alignment.
- Refraction is also adjusted for if a model is built. The
setting in ASCOM driver allows you to turn this off for
coordinates that are sent from your PC, since some software
already applies this correction to the transmitted coordinates.
Park and Un-Park:
Park is the act of moving the mount to a fixed defined position, and stopping all tracking movement.
Un-Park is releasing the mount from a fixed park position, but does
not necessarily star up tracking again.
Parking Modes - Parking modes
defines how the mount and what rules the mount follows to perform an Un-Park. There are 3
different modes. The rules are in firmware after July 27, 2013
- Every Move or Mode 0 - Every command that moves the mount wakes the
mount up (current state). This includes pressing the
directional buttons, changing the tracking rate, using any
of the Un-Park button. If a computer is tied to the
controller, then any command from it can also wake up the
mount and it start tracking in RA. I highly recommend
using parking mode 2. It is less convenient, but a
much safer parking mode. This is the default mode, and is the
only mode in firmware before July 27, 2013
- Goto Command or Mode 1 - HC directional buttons, classical HC and
AG inputs are ignored, but "GoTo" commands and un-park
commands wake it up.
- Explicit Wakeup or Mode 2 - Only a Un-Park command :hW# will wake the mount up.
This mode is there to provide compatibility with the ASCOM
standard rules. The Un-Park buttons in the hand
controller, Web interface, and the Un-Park command in the
ASCOM driver send this string to the mount. Also
checking the Tracking box in the ASCOM Driver will send that
command.* The ASCOM
driver now has a check box under Configuring Park, that will
select this mode of Operation. Please note that
parking or un-parking does not change from one mode to the
other. Only the Un-Park command in the ASCOM driver
and HC will start tracking again. *Except as noted above.
- The parking modes can now be changed in the Web
interface on the Functions tab.
Photo mode: This speed is used for Centering and guiding.
get to this mode by Menu-->HC-->Mode
Guiding Speed is the principal speed, so pressing a Hand Controller
directional button moves the telescope at the selected Guiding Speed; Slewing Speed is
not available. The following changes in speed are only available on
the buttons on the back of the Gemini hand controller, or a
Standard Hand controller if plugged in.
Acceleration to Centering Speed is available in four stages (to allow easy
centering of an object in the field of view or on a CCD chip) by pressing the opposite
button while Gemini is guiding:
Pressing the opposite button once changes to 1/8 of Centering Speed to allow fine
centering of the target. Then, after about 2 seconds, the speed will increase to ¼ Centering Speed, after another 2 seconds to ½ Centering Speed, and finally to full
Pressing the opposite button twice changes to full Centering Speed immediately.
If you want to guide a photograph manually, you may do so by plugging a Standard Hand
Controller into Gemini’s autoguider port. This way you are sure that
you cannot accidentally move the scope at centering speed while
guiding. A Standard Hand Controller is available from Losmandy
under part number HC for $70.00 (price subject to Losmandy's current pricing
- Polar Alignment: when the mount physical position (not the telescope) is aligned in AZ and Alt to true
north, or true south. After any type of polar alignment, where the mount
horizontal axis or vertical axis is moved, you have to start at CWD
again and build a new model. Moving the mount in horizontal axis or vertical axis
invalidates the mathematics used to build a model. In this regard,
any model in the mount has to be disregarded, and rebuilt from the
start. Also don't expect to be able to see Polaris in your
telescope at CWD after doing a polar alignment. This is mainly do to
Polaris not being at true north, but a bout 3/4 degree off of true
- Polar Align Assist: This is a routine that will help
get the telescope into 1 degree alignment of the pole. It is a pretty
simple routine. You have to do it after a cold start, and with no model
built. The routine will ask you to pick two stars. You
should pick the first about +/-6 hours from the meridian, and the second
on as close to the meridian as possible. For the complete
procedure see this page.
- Polar Axis Correction: This routine will be available after a model
is built. Asking for it will move the mount to a star.
You then use your horizontal and vertical mount controls (not the
buttons on the Gemini controller) to center the star. After
this is done, you will have to build a new model, as the PAC routine
wipes out the current model.
- Pulse Guiding: - This is when the guiding commands are
sent as serial commands to the Gemini-1 or Gemini-2, and do not use
the Guider ST4 input jack. These commands can be sent into the
Serial input, USB input or by using the ASCOM Gemini.net driver the
Ethernet input of the Gemini-2. The Gemini-1 only has a Serial
input. Note that the ASCOM Gemini.net driver can use
any of the serial, USB or Ethernet inputs to provide pulse guiding,
but is not required if the software doing the guiding has native
Gemini driver of it's own. But the ASCOM Gemini.net using
Ethernet is the suggested mode, as it provides the fastest
communications speeds, and requires less PC overhead. All the modes below are a form of
- Non-Precision Guiding - non-precision pulse guiding uses the
PC to tell Gemini when to start guiding movement and when to end
it. This mode is used with the Gemini-1.
- Precision Pulse Guiding - telling Gemini on how long the
pulse should be. The PC then gets out of the way, letting Gemini
control the length of the pulse. Gemini-2 can use this mode,
where Gemini-1 had some problems doing this. This setting
should be turned off for Gemini-1.
- Asynchronous Pulse guide - Asynchronous Pulse guide means
being able to send the pulses asynchronously and not having to
wait for the Gemini to complete the correction. This means that
both, RA and DEC can be guided simultaneously. Both, Gemini-1
and Gemini-2 can use this mode and this should be left turned on
for best guiding results.
- Synchronous Pulse guide - When a pulse is sent, the
driver/PC needs to wait until that pulse correction is completed
by Gemini before continuing. This mode may be used with some
older software that does not allow both axis to be guided
- Quick Start: This menu offers the select where it leads you through
entering all of of your location data, such as time zone, mount type, and location
These are messages telling you that the tracking error in RA or DEC has exceeded the set
as seen at the bottom of the Servo Page on the Web interface. They are set to 30 as a default, but you
can change them to anything you want. Do not change the RA or DEC
Slewing Power Percentage. You can change the RA/DEC Tracking Lag
or the RA/DEC Tracking Power Percentage. The higher values will
lessen the messages seen.
- Reset Model: clears the model, for the side the telescope is on. To
clear the model for both sides of the telescope, you must do a meridian
flip to get to the other side, then Reset the model on that side also.
- Sidereal Speed: Sidereal rate is simply the rate that compensates for the Earth’s motion relative to the
celestial sphere. It keeps the mount pointed at a constant RA and Dec. coordinate in the
sky and will probably be the rate that you use the vast majority of the time. With sidereal
tracking selected, the telescope makes one revolution per sidereal day (86164.0905
seconds). Note: The sidereal rate assumes that your mount is accurately polar aligned.
- Sidereal Time Sidereal time is
measured according to the positions of the stars in the sky. A sidereal
day is the time it takes for a particular star to travel around and
reach same position in the sky. A sidereal day is slightly shorter than
a solar day (24 hours), lasting 23 hours, 56 minutes, and 4.1 seconds. A sidereal day
is divided into 24 sidereal hours, which are each divided into 60
sidereal minutes, and so on.
You can use this site to calculate your local Sidereal time from your
or for a much more detailed explanation see:
- Solar System Catalogs: These are not really catalogs at
all but are calculated by the Gemini-2 internally. Any that
are highlighted, brighter (looks like they are already pressed), are
not visible objects.