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Before we go any further I must apologize for there being no thread in the MASU on series last week. Personal commitments dramatically reduced the time available and there was insufficient time to write and appropriate post.
In the last article What When & Where we looked at how the sky is mapped and how the position of objects is described and transposed onto that map.
Let's quickly review what we have covered so far:
- Equatorial Plane: GREEN circle and plane passes through the centre of the earth and is perpendicular to the Earth's axis of rotation.
- Declination δ: PURPLE segment that describes the angle an object is north or south of the Equatorial plane expressed in degrees
- Right Ascension RA: GREEN segment that describes how far an object is around the Equatorial Plane from a fixed point and is expressed as a time where a full circle is represented by 24 hours.
- First Point of Aries A: RED arrow is the reference point from which the RA is measured.
- Ecliptic: GREY circle is the path the sun takes across the sky as the earth orbits around it over a full year.
- Celestial Sphere: BLACK circle that denotes an imaginary sphere onto which the position of any celestial object is projected.
So how do we relate all this to where we are on the surface of the Earth? What we need to do is look at two planes that directly impact how the celestial coordinated relate to us standing on the surface of the Earth.
The first plane is the Equatorial Plane (GREEN circle and plane in the image above) and the second is the Ecliptic (GREY circle in the image above). We have already looked at the Equatorial Plane fairly extensively and described how it is perpendicular to the Earth's axis of rotation.
The second plane is the Ecliptic (GREY circle) which corresponds to the plane of the Earth's orbit around the Sun. As the Earth's axis of rotation is inclined to this plane so is the Ecliptic and the two planes intersect along the GREY line in the image above. The Ecliptic also corresponds with the path the sun takes across the sky as we orbit around it over a year.
The most important part that needs to be looked at here is what is referred to as the First Point of Aries (RED arrow in the diagram above). This point is located on our imaginary celestial sphere along the line of intersection between the Ecliptic and the Equatorial plane and corresponds with the position of the Sun on the Vernal equinox.
Before we go any further equinox events are often associated with the season in which they occur, however, the seasons in the northern and southern hemispheres are reversed, therefore the spring equinox in the northern hemisphere is the autumn equinox in the southern hemisphere. However, you will often see equinox events described as vernal and autumnal and while these names derived from Latin and mean spring and autumn by convention they are associated with the seasons in the northern hemisphere.
As we are ultimately measuring everything by their relationship with two planes that the earth is moving on and rotating with respect to, the coordinate system will appear to rotate over a period of a day and year. This is where things start to get somewhat complex as in order to locate anything you need to relate the celestial coordinates to the real world. This requires a method of allowing for the movement of the Earth as it rotates on its axis and orbits the Sun as well as the location of the observer on the surface of the Earth.
The mathematics behind relating the celestial coordinate system are too complex to cover here and have to a certain extent become unnecessary for reasons we will discuss later.
Originally when telescopes were first pointed at the night sky their performance was not that great but as the technology improved being able to accurately point and track a telescope became more and more critical. Fortunately for us, the massive increase in computing power along with the reduction in cost of electronics and electromechanical systems has made it not only possible but affordable to construct computerized telescope mounts. Once these are correctly aligned all the operator needs to do is select what they wish to do and instruct the telescope to Go To that object whereupon the telescope will automatically slew to where that object is in the sky and then track it as the Earth rotates. Telescope mounts like this are often referred to as GO TO mounts and the power and sophistication of them is steadily increasing over time.
To do this you need to have a system that can rotate the telescope around two axes that are aligned at right angles to each other. The alignment of these axes varies but the two most common methods are as follows:
Altitude Azimuth Mount (Alt-Az): In this format the Azimuth (Az) axis is aligned vertically while the Altitude (Alt) axis is horizontal. The major advantages and disadvantages of this type of mount are:
- Simplicity of Alignment:. It is fairly easy to align the axes so they are horizontal and vertical making them simpler to set up and operate.
- Conversion Calculation: The axes do not correspond directly with the astronomical coordinates. Consequently the system needs to be capable of doing the arithmetic computations required to convert the astronomical coordinates to the Az and Alt coordinates used by the mount. This has become somewhat less of a problem with the increase in power and of micro controllers making them capable of carrying out the arithmetic calculations required to convert between the different axial alignments.
- Field of View Rotation: Because the axes of the telescope mount is not aligned with the axis that the Earth is rotating the net result is a field of view that rotates around the line of sight. This isn't a problem if you are just looking at the object but becomes problematic if you intend to do any astrophotography that involves taking exposures over time. There are special camera adaptors that will correct for this rotation but they are fairly complex and uncommon which makes them expensive.
Equatorial Mount: In this format the primary axis is aligned with the Earth's axis of rotation. This type of mount is also sometimes referred to as a polar alignment as the RA axis is pointed directly at the celestial pole. The pros and cons of this method are:
- Axes Commonality: Since the primary axis is aligned with the Earth's axis of rotation the celestial coordinates RA and δ relate directly with the axes of the mount making calculations considerably simpler.
- Tracking: Tracking is simpler as once an object is centered it will remain centred provided the RA axis is driven at the same rate as the earth is rotating.
- Field of View Rotation: There is no rotation of the field of view as the telescope tracks an object making it suitable for astrophotography where long exposures are required.
- Alignment: It is considerably more difficult to align the axes of the mount so they correspond with the axes of the celestial coordinates There are some tricks that can be utilized to make it easier which I will cover shortly but it is still fairly difficult to do properly.
- Mount Stability: The closer to the equator the further away from vertical the RA axis gets. Consequently you can have some serious problems with keeping the whole thing stable and it is possible to end up with telescope and mount inverting itself by toppling over. This can be somewhat embarrassing but it can also be costly and cause serious damage to your very expensive telescope.
There are considerably more differences but these are the major ones and there is one other point that warrants discussing.
The type of mount that you use to a great extent depends on what you wish to do with your telescope. If you do not intend to do any astrophotography then an Alt-Az mount is probably the way to go. If you do intend to carry out any astrophotography then the Equatorial mount is probably the way to go. You can, however, get mounts that are capable of being aligned as an Alt-Az and RA δ mount. Not all mounts are capable of being aligned according to both systems, but they are becoming more common and the reputable manufacturers produce such mounts. It is something to keep in mind when selecting a telescope and mount and worth asking the people you are purchasing the telescope from.
Some Helpful Tricks: As I promised earlier there are a couple of tricks that may be useful when aligning your telescope. If you are going to use an Equatorial alignment there are several things you will need to calculate in order to align the RA axis of your mount.
- True North: This will give you the horizontal component of the RA axis and varies from magnetic north depending on where you are on Earth but there are several ways to calculate and mark it out.
- Polaris or the North Star can be very useful if you are in the northern hemisphere as it is very close to the North Celestial pole. If it is visible at your location then it makes the whole process of aligning an equatorial mount reasonably simple. Polaris is not exactly at the north celestial pole but the small error is fairly easy to accommodate during the final alignment.
- Magnetic Deviation & Declination will be indicated on most quality maps that cover the local area. You can use this value and a compass to locate true north but it is difficult to ascertain due north to more than about 1° without the use of expensive and fragile equipment.
- Solar Transit is probably the easiest way to do it but it only works when the sun is not too close to being directly overhead at noon. The first part of this technique involves finding out what time the Sun Transits at your location. In this case the term Transits refers to when the Sun is half way between the point it rises and sets. Consequently any shadow it casts will be aligned with the celestial poles and you can then use anything that is vertical like a plumb bob, to cast a shadow that is aligned with the celestial poles. One point, however, is calculating the Transit time. You can't just utilize the local time as this is often adjusted according to time zone specifications and local conditions. Many planetarium programs will give you the Transit time of the object but you can calculate it by adding a correction to UTC. To calculate the value multiply the longitude of your location in degrees by 4 minutes per degree then either adding it if you are east or subtracting if you are west to UTC.
- Latitude: This is important as it will give you the angle the RA axis needs to be inclined at in order for it to be parallel to the Earth's axis of rotation.
- Polaris is definitely the easiest if it is visible for the same reasons listed above and the difference can easily be corrected.
- Cartography is the other major option and provided you know your latitude accurately enough you can utilize it to set the angle of the RA axis from horizontal.
There are also numerous other ways of fine tuning the alignment of your telescope and mount that we will discus in later threads when we look at subjects like astrophotography and tracking.
As usual you can read more by following these links.
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