Frequently Asked Questions (FAQs) on telescopes
Astronomically, you can see the Moon, the Sun if correctly filtered, all of the planets except perhaps Pluto, some surface details on Mars, Jupiter and Saturn, multiple stars, globular and open clusters, bright nebulae, galaxies and nearby galaxy clusters. Terrestrially, there are wildlife, sports, etc., but remember that daytime viewing is often over areas which may radiate heat so that very distant subjects may shimmer.
Yes and no. Bright objects like the Moon, some planets and some star clusters will show colours and features just like photographs, but faint objects are more difficult. The eye is not sensitive enough to detect colour at low light levels so even bright nebulae appear as shades of gray in small telescopes. Colour films and digital images can be exposed long enough to collect light across the visible spectrum so photographs show colours than you don't see visually.
A telescope has three types of power and they are measured against the performance of a normal human eye. They are magnifying power, light gathering power and resolving power. All three are important but the most important is resolving power. The longer the focal length of a telescope, the more a particular eyepiece will magnify the image. However, there is a practical magnification limit of 2x per mm of telescope aperture. Using an eyepiece which gives a magnification beyond that limit is normally of little use. The amount of light that a telescope can gather depends on size of the aperture and the more light that can be gathered, the better the resolution. What you will see through your telescope will then depend on these three powers. For example, compared to the human eye, and using the 2x per mm rule, a 150mm aperture telescope will have a maximum practical magnifying power of 300x, a light gathering power of 600x and a resolving power of 0.8 arc-seconds.
The larger the aperture, the higher the practical magnification limit. Since more light is collected and brought to focus by a larger aperture telescope, fainter objects can be seen with it than with smaller apertures. Under good seeing conditions when air is not turbulent, a larger aperture objective gives higher resolution, letting you see finer details.
Depending on the type of telescope and whether or not it is used in combination with a star diagonal, the image you see may be either upside-down, backwards, rotated, or normally oriented.
For most astronomical observing, it makes little difference if an object is seen upside-down or at an otherwise odd angle (after all, there’s no "right side up" in space!). However, for terrestrial viewing you certainly don’t want to see everything upside-down. And when stargazing, it’s hard to compare what you’re seeing to your star chart if the image is inverted or flopped.
Different telescope produce different image orientations which diagonals may be able to modify.
Refractor and Cassegrain telescopes, when used without a diagonal (which isn’t usually the case), produce an inverted (upside-down) image. The view in Newtonian reflectors is also inverted, or rotated at an angle depending on the eyepiece angle with respect to vertical. Straight-through finder scopes also invert the field of view. If you’re using a star chart, all you have to do is turn it upside down to match the view through the eyepiece.
Refractor or Cassegrain telescopes used in combination with a standard 90 degree "star diagonal" will provide a right-side-up, but backwards (mirror-reversed), image. This will not work with a Newtonian reflector.
If you want your image right side up and not backwards you need an Erect Image diagonal - available in 45 degrees and 90 degrees. Note: They do not work with Newtonian reflectors.
If your instrument is for land use only, select the alt-azimuth mounting, but if it’s for astronomical or dual use, the equatorial mounting is the best choice. Make sure that the mounting you select is strong enough to carry the telescope you've chosen. Heavier or longer telescopes need stronger mounts to be stable at high magnifications. When in doubt, over-mount the instrument; choose the mount one size up if you want extra stability.
A motor drive is necessary for many types of astrophotography, but it is more than just a convenience for visual observation as well. At 200x magnification, the Earth's rotation will move an object out of your field of view in about twenty seconds. A Right Ascension motor drive will keep an object in the centre of the field where the image is the best without producing the objectionable vibrations experienced with manual tracking. Adding a Declination motor drive and a hand controller allows you to guide for astrophotography.
Colour filters, which usually thread into the eyepiece barrel, are almost a necessity for viewing planetary detail. By using an appropriate colour, you can highlight a specific planetary feature. This often allows you see two to three times as much detail as in an unfiltered view.
The telescope can be transported in 2 main parts--telescope tube and mount. Loosen the thumbscrews on the tube rings and remove the telescope tube from the mount. We suggest removing the accessories (finderscope and bracket, and the eyepiece) from the optical tube. Cover the telescope tube and the eyepiece with their caps. It is also convenient to remove the fine-adjustment control cables and counterweight rod/counterweights. The accessory tray should be removed in order to transport with the 3 tripod legs closed. The telescope can be transported in a vehicle without a problem. Padded insulation can prevent scratches on the tube but it is not necessary. The mirrors may go out of collimation after a bumpy ride but collimation may be required after transportation anyway, with or without padding.
It is unnecessary to separate the optical tube and the mount when storing the telescope. It can be stored in one unit in a clean, dry, and dust-free environment. If it has to be stored outdoors, cover it with a heavy-duty plastic cover to prevent it from getting wet. Make sure that the dust cap for the front of the telescope and the cover for the rear opening are on. Accessories should be stored separately in a box, with all their caps on. Some people do store the reflecting telescope in two parts, leaving the telescope tube upside down on the ground to prevent dust from settling down on the primary mirror. However, it is not proven that it really helps.
The sky is mapped out in a spherical coordinate system similar to the system of Latitude and Longitude on the surface of the Earth. On the imaginary celestial sphere, the coordinates are Declination, which is equivalent to Latitude and measured in degrees, and Right Ascension, which is equivalent to Longitude, but measured in hours. The celestial equator is a projection of the Earth's equator onto the celestial sphere. Because the positions of stars and other distant celestial objects, as plotted on this celestial sphere, change very slowly with time, their listed coordinates and star charts are only updated every fifty years. On the other hand, planets change position so rapidly that their coordinates must be obtained from current astronomy periodicals. The setting circles on your equatorial mount can be aligned with the celestial sphere to aid in finding astronomical objects.
Periodicals like 'SkyNews', 'Sky&Telescope' and 'Astronomy' will tell you where the moon and planets are, and the location of all other objects can be found in Star Charts. The quickest way to find objects is to learn the Constellations and use the finderscope, but if the object is too faint you may want to use setting circles.
For visual use, only a rudimentary alignment of the polar axis of your equatorial mount is required. First, the finderscope should already have been aligned to the telescope by centring a distant fixed object in the telescope's field and then adjusting the finderscope with its adjusting screws until the object is at the centre of the crosshairs. The angle of the polar axis should also be set equal to your Latitude. Now, with the mount approximately level, align the telescope parallel to the polar axis (i.e. set the Declination axis to 90 degrees). In the northern hemisphere, adjust the polar axis until the star Polaris (the 'North star') appears in the centre of the field of your finderscope. This alignment is good enough for visual observation. If you are trying astrophotography, a more accurate alignment is required, and you will need a polar scope if your mount is equipped for it. For longer exposure times, the mount is usually adjusted using a very accurate star-movement measurement technique called 'drift alignment.'
A polar scope is a specialised finderscope which is used to align an equatorial mount with the celestial pole. It is usually mounted in a tube which runs along the RA axis. For northern use, it will have a mark for Polaris, the pole star, which is slightly offset from the North Celestial Pole. It must be rotated so that the offset mark of Polaris is correctly aligned relative to the direction of epsilon-Cassiopeia on a line from it (through Polaris) to Alkaid, the end star on the Big Dipper's handle. For the southern hemisphere, four stars of the constellation Octans will be marked in the polar scope. These need to be placed over the real stars in the sky, by rotating the polar scope and adjusting the pointing of the polar axis.
Most telescopes can be adapted to act as lenses for single lens reflex (SLR) cameras. For the basic technique of 'prime focus' photography, all you generally need are a camera body, a T-ring specifically made for your camera body (allows it to connect to a T-thread) and in some cases a combination T-adapter designed for your telescope (supplies the T-thread). This configuration is fine for terrestrial use, or for the Moon or the correctly filtered Sun, but for fainter astronomical objects you will need to do time exposures using an equatorial mount with a Right Ascension motor to correct for the Earth's rotation. For exposures longer than a few seconds, you should use dual axis motor drives and a hand controller to guide the telescope.
Astronomers must be patient; you must optimise your observing site and times, as well as your equipment. When you observe the Moon and the planets, and they appear as though water is running over them, you probably have bad 'seeing' because you are observing through turbulent air. Always observe objects as high in the sky as possible. Don't observe immediately after sunset and avoid viewing across heat-radiating ground objects such as buildings and parking lots. Let your telescope come to temperature with the surrounding air; sometimes the shimmering is due to 'tube currents' within the telescope tube. Try to enhance planetary detail by using colour filters. Optimise all that you can then be patient because good seeing comes and goes.
An eyepiece is a magnifier, much like a high power magnifying glass. When placed at the real image made by the lens or mirror of a telescope, the eyepiece projects a virtual image into your eye, enabling you to see the target.
A Barlow lens has a negative focal length which increases the effective focal length (E.F.L) of the objective lens or mirror of the telescope. It is always placed between the objective and the eyepiece and results in increased magnification and decreased field of view.
Not for visual purposes, as the eye cannot process the real image made by the objective. The telescope may be used without an eyepiece for cameras and other instruments.
Every telescope is different, but a rough rule of thumb is 30-50X per inch diameter of the objective. A good refractor may, however, use 100X/inch on bright objects, so this is not a hard rule. You can always increase the magnification above these limits, but it is pointless if you're not seeing more. This rule breaks down for larger instruments, as the distortion of the atmosphere limits practical magnification to 300X.
Apparent Field (A.F.) is the angle viewed by the eye when looking into the eyepiece. An eye by itself has an A.F. of about 100 degrees, so any well corrected design up to this value would be a benefit.
Eye relief, also known as exit pupil distance, is maximum distance between the eye and the eye lens of the eyepiece to see the eyepiece's field stop. (The field stop is the baffle at the image plane that produces the field edge.) Adequate eye relief is a very important factor for comfortable viewing. Eye relief generally decreases as power increases. Low eye relief (less than 10mm) requires you to get very close to the eyepieces, while higher eye relief (greater than 15mm) allows more distance. Eyeglass wearers need a higher amount of eye relief to allow room for their eyeglasses, however- many eyeglass wearers are surprised to find that they don't need their glasses when viewing through telescopes.
There's likely nothing wrong with the eyepiece: you have probably exceeded the resolving power of your telescope. A television set looks clear 10 metres away, but up close you can see the imperfections.
This often-asked question is quite irrelevant, as different design's performance varies with different telescopes. Different eyepiece designs have various characteristics. For example, an expensive wide field design is not required for planetary viewing, where the only important thing is maximum contrast. A Plossl or Orthoscopic would probably be best, but almost all designs are good performers on-axis for any f/ratio. Telescopes with f/ratios>10 are quite tolerant of simple low element eyepieces up to 55 deg. A.F., but telescopes <6 are a different matter. Off-axis performance requires powerful correction to properly image the highly convergent beam. Each eyepiece and telescope performs as a system, and their image can only be evaluated as much.
Exit Pupil is the size of the light beam the eyepiece projects into your eye. Exit pupil can be calculated as follows:
Most night-adapted eyes open to 5-7mm, so it's not a good idea to use eyepieces which give an exit pupil much larger than this, as the beam won't fit into entrance pupil of your eye.
Only for some objects, although under magnification is often a problem, even for experienced observers. The penalty for increased magnification is reduced field of view and brightness; faint objects grow fainter as the magnification is increased This is why larger aperture telescopes are so effective on faint objects; they provide enough light to stimulate the eye at high magnifications.
For example, a 4-inch telescope will only view a globular cluster effectively at 80X, and it will appear as a blob. A 6-inch will resolve the outer stars at 130X, an 8-inch will resolve further in at 200X. 10 and 12.5-inch telescopes will make them glitter to the core at 300 and 400X.
If brightness is not a factor, choose the eyepiece that will encompass the object, and then allow for a suitable backdrop. If you want to know the actual field of view the eyepiece will give (True Field), this can be calculated as:
When you pay more for an eyepiece you are usually paying for:- Field of view: Eyepieces that have many lenses to correct for the five major aberrations (these aberrations give increasingly worse, the lower the focal ratio of the telescope) have obviously higher costs in lenses and coatings.
Eye relief: Using larger, more expensive elements in eyepieces allows for a greater distance between the eye and eyepiece.
Coatings: 2-layer multicoatings on both faces of all lenses will typically add 25% to the cost of an eyepiece, but this is absolutely necessary to preserve the contrast of the image when the light has to go through 7-9 lenses.
All commercial eyepieces are made with spherical elements, as these are the only types that are easily mass produced. These naturally produce aberrations, which become much worse in highly convergent light beams. There is no way to avoid all aberrations when using spherical elements. Clever eyepiece designers can, however, minimise the objectionable ones and cause others to manifest themselves in an acceptable form.
A low powered eyepiece in a reflector produces a large exit pupil with a large image of the secondary mirror obstruction. During the day, when the pupil of the eye is small, if the size of the secondary obstruction image approaches the size of the pupil, it will appear as a darkened region in the centre of the field. At night, when the pupil of the eye is large, the darkened region is not noticed.
The 'Kidney Bean Effect' is not the same phenomenon as the before mentioned 'black spot'. In some long f.l or wide angle eyepieces, it is sometimes necessary to move the eye closer to the eyepiece in order to see the edge of the field. Sometimes, when this occurs, parts of the field between the centre and the edge are cut off, as part of the quickly converging beam misses the eye's pupil. This appears to the observer as a giant kidney bean shaped dark region that meanders around the field as head moves.
The only time the eyepiece alone may perform as well, is on-axis, in a high-contrast application, as the extra optics of the Barlow may cause a slight depreciation. Optically, for all other uses, the eyepice+barlow outperform the eyepiece working alone. The reason? Most of the aberrations caused by positive spherical lenses (Coma, Astigmatism, Curvature of Field and Spherical Aberration) can be reduced and sometimes almost eliminated by introducing a negative system (Barlow) which has the same aberrations in negative quantities! Spherical aberration of the system is reduced as the positive spherical aberration of the eyepiece is cancelled by the negative spherical aberration of the Barlow. The other aberrations cancel in a similar way!
This is one of the eyepiece designer's most powerful weapons, and it is used in most of the shorter focal length ultra-wide designs. Another great benefit of this idea is that the longer eye relief of the longer f.l. eyepiece used with the Barlow is retained.
Parfocal eyepiece sets reduce the amount of refocusing when changing powers, but it is rare when no refocusing is required. Parfocallizing of eyepiece sets is a non-performance factor when choosing oculars.
Yes. Conventional thought seems to be that all the light not reflected is transmitted through to the next medium. This is critical to the performance of high-element wide angle designs with many refractive surfaces.
The ghost image, and its evil twin, the out-of-focus ghost is caused by internal reflections inside the eyepiece. The only way to eliminate these is to eliminate air-spaces in the eyepieces, as the ghost is caused by a double bounce between two lenses in close proximity. While the ghost is an annoyance, the out of focus ghost is more of an enemy, as it reduces overall contrast of the image, which determines how much detail you'll be able to see. The treatment, if not eh cure, is di-electric multicoating of the lens-facing surfaces inside the eyepiece.
Eyepieces are the most critical factor concerning the performance of your telescope, excepting a dark sky. Eyepieces create the image your eye will see, and the right ones will give you the experience that makes amateur astronomy so rewarding. Even the best instrument will never perform to its potential visually with poor oculars. Since most manufacturers sell their telescopes with inexpensive ones, and since most people selling a telescope keep their good eyepieces, the aftermarket is your best source. Borrow as many as you can and try them out; for every object there will be an eyepiece that works best with your particular telescope. You'll probably be satisfied with 5-8 good eyepieces; and you'll use your telescope much more often with good ones.
Your optical tube is probably covered with a metal back plate. The 3 Phillip's-head screws are there to hold the metal plate in place. Loosen them and remove the metal plate. You should be able to see the back of the primary mirror and 2 sets of screws around it.