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 

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