Basic Telescope Mounts - An Introduction
Mounts for astronomical telescopes vary in type, physical size and weight. Before we look briefly at each of these aspects we need to stress an important point about mounts for telescopes. The advice we give to our customers regarding the suitability of a mount for a given telescope is based on decades of experience using all types of telescopes, suitability based on the customer's needs regardless of budget, or suitability based on budget. Essentially this means that if a customer has a fixed budget, we will recommend the ideal mount within this budget. If the required mount is outside the budget, we will not recommend another that is inside the budget. The reason is simple, there are a great many telescopes in use currently that are under-mounted, that is to say, the mount is incapable of supporting the bulk and weight distribution the telescope is placing on it, so that it cannot track with enough precision for astrophotography and video imaging. Some telescope mounts such as driven German equatorials and even fork mounts perform at their best when the workload asked of them is well within the capability of the motor torque and bearing strength/mechanical quality. Some mounts have inadequate bearing types, some have weak motors that struggle to drive correctly if there is a weight balance issue, some have design issues that surface under severe load. Even some manufacturers offer telescope products that include a mount unsuitable for carrying the load. It is a problem that has existed for many years, and although most manufacturers are now taking more seriously the issue with mount capability (with complete telescope products), there are still instances where us experienced astronomers disagree with the manufacturer's specification.
This may seem a somewhat gloomy way to open an introduction to mounts section, but in reality we are suggesting that the problems of under-mounting should not occur at all for our customers, as we simply will not recommend under-mounted telescopes. It is a false economy, and will frustrate rather than enhance the enjoyment of visual observation and imaging.
In order to clarify the correct way of thinking about telescope mounts, we need to simplify proceedings by removing any preconceived notions about them and focus on mount fundamentals. What are the important criteria that create a successful astronomical telescope mount?
a.) Steady and balanced support.
b.) Precision electronic and mechanical tracking. (We will cover these aspects when we discuss modern GOTO mounts in a separate section).
There are two ways of supporting a telescope for astronomy.
1. Alt-azimuth. A mount that has two axis of revolution, altitude and azimuth so that one axis is 90o or perpendicular to the other. Possible revolution movements of the telescope on an alt-azimuth mount are directly up and down in altitude, left to right or vice versaparallel to the flat horizon in azimuth, or any angle in between the two if both axis move at the same time. A manual alt-azimuth is the simplest mount to use manually as the telescope is moved manually up or down, left or right, till the desired object is in the field of the eyepiece. Keeping the object there requires constant movement in both axis, either by nudging the telescope "along a bit and up a bit", or using slow-motion manual controls if the mount has this. So, a robust manual alt-azimuth mount will support a telescope in a balanced way for manual "point and view" observing.
2. Equatorial. A mount whose two axis of revolution are inclined so that they are no longer altitude and azimuth axis. One axis (lets say the former azimuth axis), points at the North Celestial Pole (NCP) from any position north of the Earth's equator, or the South Celestrial Pole (SCP) from any position south of the Earth's equator. This means that the former plane of azimuth movement is also tipped perpendicular to its own axis. This new axis is termed the Right Ascension (RA) axis, and the plane, or new circle of revolution (projected against the night sky) is the plane of Right Ascension. Any movement around this circle by the telescope is termed Movement in Right Ascension. Perpendicular to this axis is the former altitude axis, which is now termed the Declination (Dec.) axis, and the new plane or circle of revolution around this axis, also projected against the night sky, is the plane of Declination. Any movement around this circle by the telescope is termed Movement in Declination.
Just as we can find any position on Earth by its latitude and longitude co-ordinates, we can find any object in the night sky by its celestial co-ordinates of RA and Dec. The RA circle of revolution is the Celestial Equator, and positional measurements around the RA circle are in increments of Hours, Minutes and Seconds of arc. There are sixty arc seconds in every arc minute, and sixty arc minutes in every hour. There are 24hrs in the complete RA circle. If we want to know how many degrees make an hour, we simply divide the total number of hours in a circle into the total number of degrees in a circle. 360/24 = 15. One hour equals 15o. As there are roughly 24 hours in a complete day, we can say that a star moves across the sky in RA at a rate of 15o per hour. The Dec. circle uses the more familiar angular measurements of degrees, minutes and seconds. Sixty seconds in one minute, sixty minutes in one degree, and three-hundred and sixty degrees in a complete circle.
The two perpendicular great circles meet at two points. At these two points the Dec. is 0o. If we head north around the Dec. circle from this point to the NCP, the Dec. position of the NCP is +90o. If we head south around the circle to the SCP, its Dec. position is -90o. So the Dec. circle is divided into two sections, + for north of the celestial equator, and - for south of the celestial equator.
Another great circle is the Ecliptic. This is the circular path the Sun travels across the sky. The ecliptic is also measured in degrees of arc. At a rather unremarkable position in the constellation of Pisces, these three great circles meet, and at this point are the co-ordinate positions RA 0h, Dec. 0o and Ecliptic 0o.
On traditional commercial equatorial mounts (German equatorials), there are two Setting circles, one around the RA axis and the other around the Dec. axis. Both are marked in their respective increments so that the telescope can be swung round to point to the known co-ordinates of a star or deep-sky object. If we look up and face the NCP, stars appear to move in an anti-clockwise motion orbiting the NCP. Those stars that complete an orbit around the NCP without dipping below the horizon are the circumpolar stars for our latitude position. Of course if we lived at the north pole or south pole all visible stars are circumpolar. On the Earth's equator, there are no circumpolar stars. If we face south from a northern latitude, we have our back to the north, and so stars appear to move in a clockwise direction, each star tracing out its own great circle. Imagine another great circle that rises up from our northern horizon, passes through the NCP, sets at our southern horizon, and carries on through the SCP. We call this circle the Meridian, and it divides our personal hemisphere into two halves, east and west. From a position In the northern hemisphere, astronomical objects that rise and set appear to rise east of the meridian and set west of the meridian. So we can say that from a northern latitude stars rise in the east, are then considered eastern stars from the position of an equatorial mount, move west toward the meridian, cross the meridian and from then are considered western stars from the position of an equatorial mount.
An equatorial mount needs to have its RA axis pointing directly at the NCP or SCP depending where on Earth it is. In order to facilitate this, these mounts have a north or south latitude scale at its base and a means to manually tilt the axis so that the correct latitude is set on the scale. There is also facility for small east/west azimuth adjustment. The mount must be able to follow the motion of the stars at the correct rate, otherwise stars appear to quickly move out of the field of view of an eyepiece. A small motor driving the mount in RA at the correct rate, keeps the star in the field of view of the eyepiece. If the mount and tripod are not level east to west, the star can still drift in the eyepiece in Dec. Adjustment for this can be made by levelling the mount and tripod east/west correctly and by more precise polar alignment, (pointing the RA axis more accurately at the NCP or SCP).
The reason we need mounts with tilting axis and motors that run at Siderial rate, (the rate the stars appear to move), has nothing to do with the stars themselves. We are simply compensating for the rate of spin of the Earth, and the tilt of its North/South axis.
So, an equatorial mount supports a telescope so that its two rotational movements permit a balanced pointing to a particular celestial co-ordinate by compensating for the Earth's tilt, and supporting the telescope so it can track the stars by motorised tracking. This is not only true of German equatorials, but also for the fork mounted Schmidt Cassegrains popularised by Meade and Celestron. All that happens here is that a strong metal wedge is placed under the fork base of the telescope, so that polar alignment is achieved when the fork arms point at the NCP or SCP. The SCT tube assembly is supported between the two forks, the fork base sits on the angle of the wedge, and the wedge sits on top of the tripod.
At the beginning of this mount section we mentioned physical size and weight of a mount, and continued on to remark about the issue of under-mounting. The issue is one of compatibility. A heavy telescope tube assembly requires balancing in RA. This means that counterweights of a total weight close to the weight of the telescope are required. The mount then has to support nearly double the weight of the tube, plus any accessories such as heavy eyepieces, cameras, guidescope, dewshield, all of which must also be counterweighted. A shift in weight bias occurs at different points around the RA axis when the telescope is moved in RA and Dec. to point at different objects. This is even more of an issue with long tube assemblies such as medium aperture Newtonians and refractors. What is required is a mount large enough and heavy enough to easily handle the load. Many mounts have stated mounting weight limits recommended by the manufacturer. A sensible astronomer should accept many of these weight statements as being "slightly optimistic". Lightweight mounts commonly have inadequate tripods and small bearing surface diameters, neither of which are suited to heavy loads. What is recommended is a sturdy heavy mount for heavier instruments, one that won't wobble and shake in the slightest breeze, or every time the focuser is touched. If the image in the eyepiece moves with the slightest provocation, it is because several important stress points in the mount and tripod are too weak.
For observing and particularly imaging in astronomy, a robust mount is as important as the quality of the optics in the telescope.