The first part of this article gave an introduction of German Equatorial Mounts (GEMs) and discussed polar aligment and how it is been done. This second part of the article talks about Declination, Right Ascension, setting circles, balancing the mount and load capacity.
The celestial equator is a projection of the earth’s equator in the sky. Declination of an object describes the angle to the celestial equator. Similar to the latitude scale of the earth, Declination is measured in degrees. Values are positive (0 to 90 deg) for objects north of the celestial equator, and negative south of it (0 to -90 deg). The advantages of an equatorial type mount are clear: because we have aligned the mount exactly with the polar axis, we have calibrated the mount’s celestial equator as well. The celestial equator is always orthogonal (right angle) to the polar axis. With that, the Declination of an object is completely independent of location or observing time.
It may be a bit confusing for beginners to imagine the celestial equator and the related Declination. A really simple way to picture this is to replace the sphere of the earth with a flat, circular platform on which the telescope stands. The platform has the size of the earth’s equator and the axis is the polar axis. Even when moving the telescope to any place on this disc, the
Declination of the object will be always the same. The famous Andromeda Galaxy (M31) can for example always be found at DEC 41° 16’, this is independent if we observe it from New York, Los Angeles or Munich.
The last degree of movement at a GEM is called Right Ascension (R.A.). After proper alignment, the R.A. axis points exactly at the NCP / SCP and with that, any R.A. rotation describes a circle as do the stars in the sky. Motors (or hand controls) can follow the apparent movement of a celestial object perfectly. Right Ascension is commonly used in units of time (hours, minutes and seconds).
- The star Omega Pisces in the constellation Pisces is very close to the zero point of Right Ascension and serves astronomers as easy to find reference.
Greenwich has been arbitrarily selected as start point for the earth’s longitude scale (0°), similarly the celestial zero point for Right Ascension (00h 00min 00sec) has been chosen arbitrarily to be the vernal equinox. A useful reference for astronomers is located in the constellation Pisces. With R.A. of 23h 59m 19s, the star Omega Pisces is currently very close to the zero point. Right Ascension of all other objects are designated by how long they lag behind this coordinate after it passes overhead moving toward the west.
German Equatorial Mounts are equipped with setting circles for R.A. and DEC and slow motion controls (or motors) that move the telescope in these directions. Setting circles are scaled discs attached to the R.A. and DEC axes.
- Even faint celestial objects, like nebulae and galaxies, can be found with setting circles.
They are marked with a 90 – 0 – 90 degree scale for DEC and a 0h to 24h scale for R.A. Setting circles are ideal to point the telescope at a particular object just by setting its coordinates. Most GEMs have movable setting circles. Their use is quite easy; astronomers point the telescope to a star with known R.A. and DEC coordinates close to the wanted object. They adjust the setting circles to the exact coordinates of the known star and move the telescope until the scales show exactly the wanted coordinates. It is surprising how easy it is finding faint objects with this method.
Balancing the Mount
All German Equatorial Mounts need one or more counterweights to balance the telescope. The position of counterweights is variable for balancing any load. Balancing is of utmost importance because it minimizes the burden on the gear and bearings. Clutches that hold the telescope in place are also much less stressed with a balanced setup. Conversely, a highly unbalanced set-up will most certainly damage the gearbox (and probably the motor). Furthermore, it is much more convenient to work with a balanced telescope that stays put when released instead of shifting and slipping at its own.
It is often forgotten that the focuser is extracted when the telescope is in use. Due to the weight of diagonal, eyepiece and sometimes even a camera, this extraction shifts the center of gravity significantly. This has to be considered during the balancing procedure.
Most manufacturers are quite “generous” when describing load capacity of their GEMs. Specifications should be taken with care, particularly for entry level mounts. Load capacity includes, with the exception of the balance weights, everything that is put on the mount: telescope, finder scope, diagonal, eyepiece, accessories, additional mounting hardware, and when shooting astrophotography, camera and addition accessories.
Do not overload the mount, it will make it unstable and results in unwanted mount wiggling and mount shake. This effect is multiplied by the magnification power of the telescope. Weak, wiggly mounts can make observing indeed quite unpleasant. Stable solid mounts are absolutely paramount for any kind of astrophotography. Astrophotographers tend to load their mounts only with 50% of the actual load capacity for best stability. There is much truth in the astrophotographers proverb:
“A good mount with a mediocre telescope will provide much better Images than a good telescope on a weak mount.”
Motorized GEMs come with one or two motors to control either only R.A. or both, R.A. and DEC. Most manual mounts can be retrofitted with small motors. For stargazing it is often more convenient to have only a R.A. motor drive and correct possible DEC deviations manually.
Completely different types are computerized GoTo mounts. These mounts have not only coordinates of thousands of celestial objects stored in their memory; they have also electronic versions of setting circles built in; they are called rotary sensors. A computer calculates the position, counts the necessary sensor impulses and drives the motors to the right point. Before the computer can find any object, it needs calibration on reference points, which are usually three known stars (three-star-alignment). Some stargazers conclude falsely that this alignment substitutes for proper polar alignment of the mount – this is not the case. After a successful three-star-alignment, the computer can point the telescope automatically at any wanted celestial object.
German Equatorial Mounts offer many advantages that are desired for observing, but are definitely required for astrophotography. Portable GEMs are heavier and more difficult to set up than Alt-Az mounts and they always need to be propperly aligned before they can be used in the intended way. The biggest advantage of GEMs is that they are suited to eliminate apparent star movement and with that, they are clearly the mount of choice for astrophotographers.
German Equatorial Mount – Part 1 @ Astronomy Source
The March Equinox @ timeanddate.com
- Refractor 70mm on alt-azimuth mount
This article is for people who are in the market for their first telescope – but have only a very limited budget (say $100) at their disposal. Some amateur astronomers will say spending $100 on a telescope is wasted money, simply because there is no great telescope for that amount. Good optics is expensive, but not everyone has hundreds of dollars to spend and there are indeed some decent telescopes that are fun and do not break the bank. A tight budget should never prevent anyone to start a fascinating hobby.
Telescopes are available in three basic types: refractors, reflectors and catadioptrics and they come with different mounts. An Astronomy Source article “Introduction to Telescopes” in coming soon. The link from souledout.org provides also a nice overview on telescope types.
What can you see with a beginners telescope?
Quite a lot. I heard people complaining that the view through their basic telescope did not reveal bright and colorful images. Well, do not expect Hubble quality images with a $100 telescope. Suitable objects for these telescopes are the moon, the bright planets Jupiter and Saturn, some of the brighter deep space objects, and stars. Observe Andromeda Galaxy (M31) or Orion Nebula (M42), you can also see the famous Hercules Cluster (M13). Stars shine in different colors due to their different temperatures and consistence, and there are many exciting double or ever multiple star systems to watch. Did you know that Polaris, the North Star, is actually a 3-star system?
When looking for a budget telescope, keep in mind:
- Do not consider department store telescopes that come with a ton of accessories, parts and pieces, and promise magnifications of whopping 500x or even more. These items are usually toys. There is nothing wrong with toys, but really, don’t expect clear images or lasting fun and excitement with those. What you can expect is inferior optics a lot of plastic parts that will break soon.
- Glass lenses. Plastic lenses simply do not deliver the view quality of well polished and adjusted glass lenses.
New versus used
Telescopes can last very long if they are treated well. Used scopes are often available for half the price of a new one and many astronomers choose to buy used, simply because they want more: for the same money they get a much better (used) telescope, offering better images.
- Be very careful buying a telescope at a yard sale. You may get lucky and find the deal of the century. More likely is however that the item is in a bad shape, because it has been neglected over years.
- However, used equipment sold by serious amateur astronomers is usually in pretty good condition. True astronomers care about their equipment. A good source buying used telescopes and accessories is www.cloudynights.com. Check their classifieds and see what it offered here. Sellers describe their items in detail and provide usually very reliable information about the condition of their goods. Look for other dedicated astronomy sites that offer classifieds.
- If you decide to buy used, you should wait – and be quick. This sounds like a paradoxon, but you should watch the classifieds for a while to get a feeling about a fair market price of the the item you have in mind. When you see the right one, be quick to get it. Good items are often sold within a couple of hours.
Further items for amateur astronomers:
- Planisphere. Easy to operate map like device, that shows you where stars, constellations and planets are – all year long.
- Red Flashlight. Eyes adapt to the dark, this is called night vision. Night vision takes a while, about 15-30 minutes, but allows to see much more and much fainter objects in the dark. Exposure to bright light, with the exception of red, erases light vision immediately and it takes another 15-30 minutes.
- A good astronomy book about telescopes and star watching gives helpful hints for the beginner and advanced astronomer.
Beginning astronomy is the start of a great, very exciting and fun hobby. A simple 70mm or 80mm refractor on alt-azimuth mount is probably the best choice as starter telescope for around $100. These telescopes are very easy to set up and simple to operate – just point and see the universe.
- German Equatorial Mount – Shown Celestron CG4
This article explains how German Equatorial Mounts work. It consists of 2 parts: The first part provides a general overview about these mounts. GEMs require polar alignment; it will be discussed what this is and how it is done. Part 2 talks about more specific subjects like Declination, Right Ascension, setting circles, mount balance and load capacity.
Most people are familiar with camera tripods that offer vertical movement and head rotation. Telescope mounts based on this principle are called Altitude-Azimuth mounts, or Alt-Az mounts for short. German Equatorial Mounts (GEMs) are more complex, but they are preferred by astrophotographers and observers who like to view particular objects for long periods. Furthermore, GEMs are ideal for finding faint celestial objects just by coordinates.
German Equatorial Mounts are designed to compensate for the earth’s rotation. They can keep a telescope steady at any object in the sky. For this, GEMs have special rotating axes and additionally a gearbox. Astronomers can either turn a slow motion gear knob manually to keep the object in view, or they can attach a small motor to move the telescope for them. If the mount is properly aligned and the motor is running (or the slow motion knob is turned), an object will stay in the field of view (FOV) as long as the observer desires.
The earth rotates, and so do observers who are standing on it. This gives the appearance as if stars circle in the sky. If observed with bare eyes, stars move very slowly (one revolution in 24h), but the telescope magnification increases this speed proportionally. A telescope with a power of 100, let stars “move” 100 times faster. If an observer uses for example an eyepiece with apparent FOV of 50 deg, and a telescope with magnification of 100, it takes only about 2 minutes for a star to wander from one side of the eyepiece to the opposite site. What might be slightly annoying for stargazing is completely unacceptable for astrophotography. GEMs offer a solution
- German Equatorial Mounts need to be polar aligned. Startrail photo credit: RClements
Exact compensation for the earth’s rotation is only possible if both, the polar axis of the mount and the polar axis of the earth are in perfect alignment. Other than Alt-Azimuth mounts, which offer only 2 axes of movement, German Equatorial Mounts have no less than 4 axes. These mounts use Altitude and Azimuth settings solely for aligning the polar axis of the mount. Azimuth adjustment turns the mount so that its polar axis points north. Altitude variations set the angle to point exactly at the north or south celestial pole (NCP or SCP). Astronomers in the northern hemisphere use the Pole Star as reference. Polaris is conveniently located within 0.8 degree of the actual NCP. Observers in the southern hemisphere refer to the constellation Octans for aligning the mount to the SCP. All GEMs are equipped with a tight tube like opening for aiming at the NCP or SCP. At most GEMs this opening is actually a bore in its polar axis.
Astrophotography demands very precise polar alignment because imaging faint galaxies or nebulae requires very long exposure times. Polar scopes improve alignment accuracy dramatically. These are small telescopes to be inserted in the aiming tube of the mount. These scopes have a small magnification but more importantly, they come with etchings on a lens. Etchings show constellations and a marker for Polaris. Using these markers provides a much more accurate polar alignment. More information about precision polar alignment methods will be provided in future AstronomySource articles.
Once the German Equatorial Mount has been properly aligned, latitude and azimuth screws are secured; they should not be touched during the remaining observing session. At first glance it seems rather strange that these axes are secured and left alone; are these not exactly the degrees of movements that are used with Alt-Az mounts to point at the objects we want to see? Yes – however GEMs offers two more degrees of movement to set Declination and Right Ascension, or DEC and R.A. for short.
Continue reading Part 2 of this German Equatorial Mount article on Declination, Right Ascension, Setting Circles, Mount Balancing, and Load Capacity.