timer' s range, an RA timer value of 65535 (the maximum value) will be displayed and 
used instead. 
Coordinate Controlled Tracking 
Calculating the tracking rate using coordinates is useful if you want to track an object for 
which you have an ephemeris.  Knowing 2 different positions of the object at different 
times, Gemini can calculate the speed required to track the object.  You can use the 
  Setup
Mount Parameter
Tracking Speed
Comet/User Def
Calculate Rates   menu 
item to enter coordinates from an ephemeris for the object you want to observe.  Gemini 
asks for three pieces of data: two sets of object coordinates, and the amount of elapsed 
time (in hours) between them.   The coordinate difference is used to calculate the 
necessary tracking rates in both RA and Dec. Since only the difference is used, the 
coordinates need not reflect the exact location of the object at the current time.  If the RA 
value is outside the possible range, the input will be rejected. 
PC Controlled Tracking 
In addition to programming to track visible objects, serial line command capability has 
been provided 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 produce these commands, see Appendix 8.4   
Serial Line Protocol. 
Comet/User Defined tracking produces the same undesirable field rotation as Closed 
Loop tracking if the mount is not adequately polar aligned.  With the tracking rate set 
correctly for a moving object, that object will be imaged as a point, but other objects that 
are moving at a different speed will be trailed on the image. 
3.3.2  Meridian Flip 
One of the characteristics of a German Equatorial Mount is that it cannot continuously 
track an object from the eastern horizon to the western horizon.  At some point after the 
object has crossed the meridian, the telescope must be flipped to the other side of the 
mount to avoid tracking into the mount or tripod.  This is called a   meridian flip.    While 
this is not a great concern for visual observing, it must be taken into account when 
planning a long exposure photograph.  You' ll need to plan ahead so that you don' t face 
the need to do a meridian flip after 15 minutes of a planned one hour exposure.  The hour 
angle displayed (in minutes) when you select an object from the database can help you to 
plan your exposure.  For example, if the HA is minus 10 minutes, you know that the 
object will cross the meridian in 10 minutes.  It would be best to position the telescope on 
the east side of the mount (if safety limits allow), or wait to begin your exposure until the 
object has moved so that you 
can
 position the telescope on the east side. 
Gemini Users Manual 
2  
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