Lining up my telescope

 

This story refers to my 3" refractor telescope  (you've seen it in previous blogs). I use it as my public astronomy telescope. It came into my hands (I bought it) in 1964, used, being available as a security by someone who had not repaid a loan. It came with a wooden, very stable tripod, which had nothing electric or automatic and it meant that any use of the telescope was done by hand. For public astronomy, that telescope and tripod are portable devices. I have no idea about how old the telescope is, but it has excellent optical performance. I have since bought a used EQ-3-like metal tripod which can be used with electrical and electronic components.

Whenever you are going to use a portable telescope to track stars and other objects in the sky, there is a first requirement if you want to track observing targets in the sky without having to do it yourself. For the purpose, the telescope's rotational axis needs to be exactly parallel to the Earth's rotational axis. This applies to equatorial telescope mounts used to adjust the pointing of the telescope's rotational axis to the north, or south point in the sky if you are somewhere in the Earth's southern hemisphere.

The North Pole in the Earth's northern hemisphere is found in the sky above the horizon  depending on the geographical latitude of your location. At the Earth's equator, the latitude is 0. Therefore the North Pole is 0 degrees above the horizon. Here, at the Trottier Observatory, we are close to 49.2 degrees latitude. So the North Pole is 49.2 degrees above level. At the North Pole, the latitude is 90 degrees, right above the top of your head, if you are standing there. the rotation axis would have to be set to 90 degrees. The stars move almost parallel to the horizon as would the Sun and Moon. The seasons determine position above or below the horizon.  This is an effect of the position of the Earth in its orbit around the Sun. 

For visual observing using our pole star (name: Polaris) as the north pole in the sky is adequate, it is very close to the correct point. 

That also means that the angle of the rotational axis with respect to level at the Earth North Pole is equal to the latitude at which the telescope is being used. When correctly positioned, there will be little need to manually correct deviation from tracking. The rotational axis is driven by a small electric motor at the speed the Earth turns. The Earth turns 15 degrees eastward in one hour. In order to stay pointed at a star, or other astronomical object in the sky, the rotational axis moves fast enough to achieve to achieve the same speed westward. Remember the Sun, the Moon, the stars rise in the East and set in the West. This is the result of the  Earth's rotation eastward.

The housing at right angle at the top of the rotational axis housing contains the axis which allows the telescope to move "up and down". The axis inside itself is set at a permanent 90 degrees angle to the rotational axis, but free to turn on itself. The lock is activated by the black lock handle to the upper right of the label "declination axis". This axis is the "declination axis" and the telescope is firmly attached to this declination axis, which is fixed to the rotation axis. In this picture my 3" telescope is clamped to the locked declination axis, and the rotational axis is turning westward. The telescope has been set onto an object up or down, the declination axis is clamped, and cannot move up or down again until unlocked. So, to repeat, the rotational axis turns westward at 15 degrees per hour, the telescope is fixed to the clamped declination axis, pointed at the chosen object, the telescope moves westward driven by the motion of the rotation axis which is driven by the motor. That means that the telescope moves along the same arc and speed of the clear sky and the object to which the telescope is pointed.

So far, I have written about the telescope. What point in the sky do you point the tracking mechanism to in order to achieve correct tracking? The point is that one in the sky which is exactly above the Earth's rotational axis, our precise North Pole on Earth. If you stood on the Earth's North Pole, the North Pole in the sky would be exactly above your head, the highest point in the sky. The rotational axis in the tripod is usually a hollow tube which may contain a small finder telescope. There are mechanical adjustment controls to move the whole mounting mechanism sideways and up and down. If you adjust the rotational axis to point exactly to the North Point, using the finder scope in the rotational axis, your tracking will be accurate. 

Nowadays, technology has evolved so that an equatorial mount is not needed. Computer programming allows an alt-azimuth mounted telescope to be controlled to follow the arc of a star or other object in the sky, as well as compensate for the field rotation which results from rising in the East and setting in the West. Objects launched into space from Earth are unlikely to be followed in this fashion. 

I have to repeat the setup routines for my 3" every time  we do a public astronomical evening for SFU, or other schools and locations. Obviously, I can't leave the observing equipment and the telescope behind after the end of the evening. Fun every time...