There are a few methods for locating deep sky objects. We'll introduce below two 'manual' methods:
By using azimuthal circles and sky coordinates.
Due to the perspective, the sky looks spherical. It is then possible to establish an system of coordinates anolog to the earth's meridians and parallels coordinates. In this case, we are speaking of ascension and declination.
Right ascension: it is the equivalent of to the geographic longitude. Due to the fact that the sky apparently rotates around the Earth in 24 hours, those coordinates are divided in 24 hours and equivalent to 360 degrees.
Declination: it is the equivalent of the latitude. For objects located on the celestial equator, its value is 0. It is equal to +90 for the celestial North Pole and -90 for the South Pole.
Telescopes with an equatorial mount uses both azimuth circle axes. They are then a must for find deep sky objects. The procedure is as follows:
First, put the telescope in station, which means to align it so that its right ascension axis is perfectly parallel to the axis of rotation of the Earth. This step is necessary to reduce future coordinate errors.
Next, look for a reference star with precisely known coordinates. Generally this is a bright star which is located close to the object to locate. Center this star in the telescope and to write down the values of the right ascension and declination showned on the azimuthal circles.
Subtract the object right ascension from the reference star ones (both should be from similar catalogues). The difference gives the path lenght from the star to the object.
Move the telescope around its axes carefully and watch the declination circles so that the right ascension and declination increase equal those previously calculated.
This way it is possible to center the object in the eyepiece. The best station, the better will be the resulting centering of object in the field of the eyepiece. It is advised to follow this process with an eyeglass with a larger field of view. It is then easier to look if the object is visible.
With sky maps and star hopping.
Deep sky objects position are indicated on stellar maps.
Although it is not necessary to have an equatorial mount, it is advisable as the location is easier. The reason is very simple: the maps are based on the right ascension and declination. If the mount is equatorial, the mount control move the telescope in those directions. However, if the telescope mount is azimuthal, the movements are much less intuitive.
It is necessary to know what is the finderscope apparent field of view, as well as the eyepiece of lower power that is used to observe. This way it is possible to relate the scale of the map and the separation between two stars in the finder. Is it possible to determine this with two shining stars that can be easily located. Once the scale is known, it is possible to even cut a piece of colored transparency plastic with the size of the finder's field.
It is preferable for the finderscope to be without a zenithal prism, since this accessory inverts the image and the printed maps are made for a direct vision through the telescope. If the prism of the finderscope cannot be disassembled and you only have printed maps, the best thing is to scan them and to mirror the images with a drawing program and then to print them.
It is necessary to have the finderscope parallel to the telescope. Since the magnification is small, it is not necessary to have a perfect collimation.
Locating the object:
First, look for a relatively bright star located near the object to observe.
Next, find the north. If you have an equatorial mount and the telescope is in station (although in an approximate way), slightly move the declination control to bring the telescope towards the polestar; the north is the direction from which new stars enter.
The map can now be oriented so that the north is in the same direction as the one being observed in the finderscope. Observe the stars that appear in the finderscope and locate them on the map.
To get from the reference star to the object, move the controls of the right ascension and declination by taking stars as successive references and use groups of stars such as triangles. The triangles have the advantage that they give an approximate orientation to where to move the telescope.
Once the object is located, it can be a reference to star hopping to new objects.
Some amateur astronomers prefer to learn the sky as precesly as possible, using maps to find their way between the stars. Other like to use the automated GOTO technology. Place your telescope in station and enter the name of the deep sky object in the system. A few seconds later, the telescope points toward the object.
The main advantage for amateur astronomer is that it reduced the time spent to look for the object and gives them more time to study the object.
How to observe deep sky objects?