Equinox Systems GEMINI Manuel d'utilisateur télécharger pdf (Page 37)

Gemini Users Manual 32

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 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.

3.3.3 RA Safety Limits

Whether you are imaging or observing visually, you do need to be concerned about the

east and west limits of the mount. The system will never make a complete turn in RA.

There are default limits set for how far the RA axis can be turned to the east and to the

west: 122 degrees to the west (telescope on the west side of mount) and 114 degrees to

the east (telescope on the east side of the mount). The limits for the Mountain

Instruments MI-250 are 92 degrees to the east, and 95 degrees to the west. These angles

are measured from the CWD position. During a GoTo operation, if the mount cannot

reach its target on the current side of the meridian, it will make a meridian flip.

Note: The “east” and “west” defaults given above are for the northern hemisphere. They

are swapped for the southern hemisphere.

To change the safety limits, use the Hand Controller to move the scope to where you

want the new limit to be set, and select the “Setup→Mount Parameter→Set Safety Limit”

menu item. You have to start up the system accurately in Startup Position and slew to the

position you want to set as the new limit. While this is no problem if you are narrowing

down the allowed range, going beyond the old limits is a bit tricky. About 10 degrees

before you reach the current limit, the system will softly ramp down the speed until the

motor stops near to the real limit. Carefully continue moving with centering speed or

slewing slowly until the system stops the motor and you hear the buzzer. You are now

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Equinox Systems GEMINI Manuel d'utilisateur Page 37