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A large Newtonian telescope is set up for astrophotography. This Losmandy G-11 mount has analog setting circles as well as digital setting circles. The scope also has a Telrad finder and piggyback refractor for guiding.

We have already discussed using star charts, star hopping and the geometric method in section 110, Finding Your Way Around the Night Sky. Here we will talk about some other ways of finding things that we want to shoot in the sky.

Finder Scope

Many telescopes come with a finder scope, usually a small compact 35mm or 50mm aperture refractor, attached. It can be a useful accessory in finding objects if we know exactly where to look. Some of these finders come with a straight-through view, which can yield an image that is reversed and inverted, so this must be accounted for when moving the scope, and can sometimes get confusing. The other problem with finders is that often you can't see a faint nebula or galaxy, or a small faint planetary nebula at all in them. They are helpful with long focal length telescopes, at least in getting close.

If you have a finder and want to use it, be sure to align it accurately with the main scope when you set up your equipment.

Setting Circles

Setting circles are engraved rings with markings for right ascension and declination on their respective axes. The declination circle is fixed. The right ascension circle is not fixed, and it can be "driven" and made to turn with the right ascension axis, or not.

To use setting circles, the right ascension and declination of the object is looked up and written down or memorized. The right ascension of a bright star nearby is also needed. The scope is pointed to the star and the right ascension circle is turned until it reads correctly for the star. The declination should be correct for the star at this point, but it is a good idea to check it the first time the circles are used. If the declination reads incorrectly, the declination setting circle can be unlocked and corrected and then locked down again. The scope should now be pointing at the star, and the right ascension and declination circles should correctly read its position.

At this time, the right ascension and declination axes are unlocked, and the scope is moved until the right ascension and declination of the target object are dialed in on the right ascension and declination circles. The object should then be in the field of view of the scope or camera.

Many factors can conspire to make setting circles less than accurate however, such as inaccurate polar alignment, the optical axis not being perpendicular to the declination axis, and the right ascension and declination axes not being perpendicular to each other.

If the right ascension circle is not driven, then it should be set to the right ascension of the current object in the field of view just before it is moved to the next object.

Digital setting circles

Digital setting circles employ an encoder on each of the mount's axes that tells its position precisely. These encoders send signals to an electronic digital readout that displays the scope's position. Once the digital setting circles are initialized and aligned on the location of a known object or set of stars, the scope is moved until the right ascension and declination of the target object are displayed.

Go To

Many new mounts come with computerized Go To control. The observing location's latitude and longitude, date, time, and other parameters are input, and the mount is aligned on several reference stars. Then the scope is pointed with a hand controller that interfaces with the mount. An object can be looked up in a database in the controller, or its right ascension and declination can be directly input, and the scope will then slew to the target object.

Computer Planetarium Programs

Many Go To mounts will also interface with a separate computer running a planetarium program. After the program is synced with the mount's controller, you can go to an object simply by clicking on it on a map display in the planetarium program.

Finding Your Target - The Bottom Line

Finder scopes, setting circles and Go To telescope mounts can all be used to find your target object. A basic knowledge of the night sky is also required.

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