TRG – Telescope Review Guide

Yes, it’s only July but it’s not too early to start thinking about what to buy for that astronomer on your Christmas list! But what would be the best telescope to get? The choices are almost endless! Here then is a short list. You can’t go wrong with any of these:

Orion Skyquest XT4.5

Celestron CGEM 800

Orion Skyview Pro 127

Orion SpaceProbe 130

Orion SkyQuest XT8

Keep checking back as I’ll be continuing my research into great Christmas telescopes and publishing my findings here!

In my last article I discussed the various prism systems one could expect to find in binoculars. Now I’ll talk about lenses and lens coatings and how they affect the viewing experience.

Chromatic aberration is caused because light of different colors does not bend the same amount when passing between mediums of differing refractive indices such as glass and air.

This prism shows how different wavelengths of light is bent in differing amounts as it passes from air to glass and back to air again.

Blue light, for example, will not focus to the same plane as red light. The effect can create a ring of color around sources of light, and results in a general blurriness to the image. Chromatic aberration is minimised by using an achromatic doublet, or achromat, in which two materials, often crown and flint glass, with differing refractive indices are bonded together to form a single lens. While this reduces the amount of chromatic aberration over a certain range of wavelengths,  it does not produce perfect correction.
This problem can be reduced in several ways. One method is to apply a thin film to the eyepiece element that corrects. The more traditional approach is to eliminate the aberration by using multiple elements of different types of glass and curvature.
An apochromat is a lens or lens system which has even better correction of chromatic aberration, combined with improved correction of spherical aberration. Apochromatic lenses are designed to bring three wavelengths, typically red, green, and blue, into focus in the same plane. Apochromats are much more expensive than achromats.

Antireflection lens coatings reduce the amount of light reflecting off of the lens and allow more light to pass through. Without coatings, up to 50% of the light entering the binoculars can be lost to reflections because of the many glass surfaces within. The more expensive brands will have multiple coatings on all the lenses which will help to give the brightest and clearest images. The most used and least expensive coating is a single-layer of magnesium fluoride but there are also modern broadband multicoatings. Magnesium fluoride reduces reflections from 5% to 1%. Modern lens coatings , such as zinc sulphide or titanium dioxide, consist of complex multi-layers and reflect only 0.25% or less to yield an image with maximum brightness and natural colours. To save money, some optics manufacturers coat only some of the air-to-glass surfaces. Common antireflection coatings often look somewhat bluish, since they reflect slightly more blue light than other visible wavelengths, though green and pink tinged coatings are also used. Some binoculars ruby coatings intended to reduce glare in bright light and improve the contrast between brown and green objects. You should avoid any binocular that uses these coatings because it will perform poorly for astronomical use.

Coating symbols:

Coated (C) – One or more surfaces are coated.
Fully-Coated (FC) – All air-to-glass surfaces are coated but plastic lenses may not be.
Multi-Coated (MC) – One or more surfaces are coated.
Fully Multi-Coated (FMC) – All air-to-glass surfaces are coated.

Binoculars come with two types of focusing mechanisms. Most people opt for the center-focus model, which uses a centrally mounted wheel to adjust both eyepieces at once. There is also a separate adjustment for the right eyepiece, which helps to correct for any difference in near or farsightedness between your eyes.
The second focusing system uses individually focused eyepieces and has no centrally located focusing mechanism. Even though focusing is slower compared to the previous model, binoculars that use individual focus tend to be more rugged and less prone to moisture infiltrations.

Hermetically sealed binoculars filled with dry gas, often nitrogen, will not be susceptible to clouding due to condensation at low temperatures; this will also help to prevent mildew, although air may leak in over a period of years if the binoculars are not properly maintained.

Because binoculars are basically two small telescopes mounted side by side, an error in collimation (optical and mechanical alignment) can lead to numerous problems including eyestrain and double-images. For most binoculars collimation problems are not immediately obvious when you first pick the instrument up and view through it. If after using the binoculars for several minutes your eyes feel uncomfortable as they compensate for the barrel misalignment, most probably the binoculars are out of collimation, which means that the two barrels don’t point in the same direction. This is a serious problem, and you shouldn’t buy those binoculars.

Keep your binoculars in their protective carrying case to prevent dust and grit getting into the mechanism. This can clog up the lubricants and make the controls grind which could eventually seize up. Also avoid knocking them because prisms are often mounted lightly and a bump can misalign one, causing double vision. For the best view keep the front and rear lens surfaces clean with optical cleaning fluid and a soft lint free cloth.

Now that you know everything there is to know about choosing binoculars, I’ll write some articles about just what exactly you can expect to see with the new binoculars that you’ll want to buy with your income tax refund!

In part two of my series I discussed digital binoculars and binocular optics and how field of view and magnification played into which binocular to buy. I’ll continue to discuss binocular optics by discussing the various prism systems employed by binoculars.

In binoculars prisms are used to provide correctly oriented images and to shorten the optical path. The typical binocular design can be porro-prism or roof prism. The size and design of the prisms will affect image sharpness, with quality optical glass delivering clarity from edge to edge of the field of view. Prisms are located inside binoculars are like mirrors. It is a reflective coating on glass that bends and refracts light to bring the objects you are looking at to your eyes. The BAK-4 prism is made of a high quality glass and produces sharp images and good edge to edge sharpness. Generally, higher quality binoculars will use BAK-4 prisms. Phase coated prisms have a coating process that enhances the resolution and contrast of images coming through the binocular and are generally applied only on more expensive binoculars.

The Full Moon as seen through binoculars

Roof Prism System

In roof prism binoculars the prisms overlap closely, allowing the objective lenses to line up directly with the eyepiece. The result is a slim, streamlined shape in which the lenses and prisms are in a straight line. An Abbe-Koenig prism is a type of reflecting prism used to invert an image (rotate it by 180°).  The prism is made from two glass prisms which are optically cemented together to form a symmetric, shallow V-shaped assembly.
A Schmidt-Pechan prism is a type of optical prism used to rotate an image by 180°. They are commonly used in binoculars as an image erecting system.
The prism consists of two glass prisms separated by an air-gap. Multiple total internal reflections of the light cause a vertical flipping of the image; a “roof” section of the second prism also flips the image laterally, together causing a 180° rotation of the image. The image’s handedness is not changed.
Compared to the double-Porro prism or Abbe-Koenig designs, the Schmidt-Pechan is much more compact. However, the large number of reflections and glass/air transitions create more light loss than the other designs. The multiple internal reflections also cause a polarisation-dependent phase-lag of the transmitted light, in a manner similar to a Fresnel rhomb. This must be suppressed by special phase-correction coatings to avoid unwanted interference effects on the image. Phase-corrected prism coating and dielectric prism coating are effective techniques for reducing reflections. Light reflected from one roof surface is 1/2 of a wavelength shifted from the light hitting the other roof surface, sometimes referred to as “out of phase” or “phase shift”. Although the light waves are subsequently forced back together when they reach the viewer’s eye, this phenomenon results in reduced contrast and image resolution. This effect does not occur in Porro prism designs.

Porro Prism System

In porro prism binoculars the objective or front lens is offset from the eyepiece. Porro prism binoculars provide greater depth perception and generally offer a wider field of view. A Porro prism binocular will inherently produce an intrinsically brighter image than a roof prism binocular of the same magnification, objective size, and optical quality, as less light is absorbed along the optical path. However the optical quality of the best roof-prism binoculars with up-to-date coating processes as used in Schmidt-Pechan models is comparable with the best Porro binoculars.

Of course, you will want to consider the following factors:
 Coatings: reduce glare, and protect against water and other potential damage
 Quality of construction: the grade of glass, the quality of the prisms and the material used in the barrel are just a few of the factors to be mindful of. BK-7 borosilicate flint glass are of lower quality; for optimal optics, make sure you have BaK-4 barium crown glass prisms
 Long eye relief is a worthwhile feature for eyeglass wearers.

I’ll wrap up what you need to know to buy an astronomical binocular in my next installment and then conclude the series with talking about just what can been seen in the night skies using binoculars.

In Part One, I pointed out that binoculars can be a viable alternative to telescopes for astronomical viewing. I touched on magnification and aperture as ways of distinguishing and choosing which binocular is right for you. In part two, I’ll talk about digital binoculars

Digital Binoculars

and go into more detail about how magnification and aperture plays into binocular selection.

Digital binoculars are gaining in populararity – these capture a digital image seen through the binoculars. Zoom binoculars have the ability to quickly and very efficiently zoom in on the object of interest, but the image quality is compromised in cheaper models. The extra workings and glass inside reduce the amount of light available, making them unsuitable for astronomy. Avoid binoculars that claim to be “focus-free”.  Also be beware of advertising jargon like “high-powered”. Increasing the magnification will decrease the brightness and field of view, which makes objects faint and fuzzy. A lower magnification will maximise the amount of light transferred.

The larger the aperture, the brighter the image will be; but the greater the size, the binoculars will weigh and cost more. For general astronomy use, choose binoculars with an aperture of 50mm. An observation binocular with a 60mm objective lens will still be fairly portable while an observation binocular with a 80 to 100mm + objective lens is far more suited for static use. The size of the objective lens and the power of magnification are the two major factors that determine the light transmission of the binocular. For example, 50mm (the diameter of the objective lens) divided by 10 (the power of magnification) gives a figure of 5, which is the diameter (mm) of the exit pupil and indicates the amount of light reaching the eye. In general the larger the exit pupil diameter the brighter the binocular will appear and the better the resolution will be, enhancing colour and contrast perception, especially in low light conditions. However, since the human eye pupil dilates on average from 2.5mm to 7mm depending on light conditions it follows that an exit pupil above 7 is not beneficial as the human eye cannot accommodate it.

Remember also that as we age our eye pupil does not dilate so much, so a large exit pupil of 7mm is not so important for a 50 year old person compared with a 25 year old. So a 4mm exit pupil on a 25×100 observation binocular will be more than satisfactory for most users in most conditions, whereas a 40×100 will only give 2.5mm exit pupil, drastically reducing the amount of light reaching the eye. A standard 7×50 pair will have a 7mm exit pupil, the average human eye pupil size at night.

Most binoculars are not suitable for use with eyeglasses. You have to put your eye close to the eyepiece, but the glasses prevent you from getting close enough. You can of course take your glasses off to use the binoculars, but this can be a nuisance. It is possible to buy special binoculars which can be used with glasses. Standard binoculars have eye relief ranging from only a few millimetres to 15 millimetres. Long eye relief (15 to 25 millimetres or more) is necessary for eyeglass wearers. A poorly designed optical system can force the observer to press his or her eye close to the eyepiece in order to see an unvignetted image, or alternatively may have an exit pupil larger than the observer’s pupil at a comfortable viewing position, resulting in loss of light and a dimmer image.

The eyepieces of binoculars are usually permanently mounted in the binoculars, causing them to have a pre-determined magnification and field of view. Usually binocular eyepieces have 3-4 elements with marginal correction for colour and edge sharpness. The correction on Siebert 6 element eyepieces are comparable to a Japanese made Meade 26mm Super Plossl. The eyepieces do not add colour correction or false colour.
Wide-angle binoculars have a field of view that is wider than average (60 or higher).

The Field of View is the size of an area that can be viewed using the binoculars.

Binoculars are often advertised with their field of view specified in one of two ways: angular field of view, and linear field of view. Angular field of view is typically specified in degrees, while linear field of view is a ratio of lengths. For example, a pair of binoculars with a 5.8 degree (angular) field of view might be advertised as having a (linear) field of view of 305 feet per 1000 yards or 102mm per meter. As long as the field of view (FOV) is less than about 10 degrees or so, the following approximation formulas allow one to convert between linear and angular field of view. Let A be the angular field of view in degrees. Let L be the linear field of view in feet per 1000 yards. Let M be the linear field of view in millimetres per meter. Then:

A = 0.0191 \times L
A = 0.0573 \times M
L = 52.4 \times A
M = 17.5 \times A

Generally, higher powered binoculars give you a smaller field-of-view and the opposite is true for lower powered binoculars. For astronomy, a wide field of view is desirable because if offers a more pleasant viewing experience, and you can see more of the sky at a better edge performance compared to a narrower field.

Come back for part three when I talk about the various prism systems employed by binoculars.

Telescopes aren’t the only viewing tool at the amateur astonomer’s disposal. In part one of a series, I’ll discuss how binoculars can be very useful for several reasons: they are relatively inexpensive, have a large field of view and show images right side up (which makes finding things in the sky easier), are easily portable and require little to no setup.

So, how do you choose a good pair of binoculars for amateur astronomy?

There are different ways of categorising binoculars. Usually, they are distinguished by their magnification and the size of the aperture. A combination of a small magnification and large lens produces a brighter view. A 7×50 pair, for example, gives a brighter image than a 10 x 50, but the image magnification is smaller. A standard 7 x 50 pair is considered one of the best all-round binoculars for practicality, performance and price. For high-powered observation, a magnification of 25x with a 100mm objective lens is recommended.

Observation binocular

Observation binoculars come with either straight or angled eyepieces. The benefit of the angled design, usually 45º, is the ease of use and comfort of viewing. The angled eyepiece model is much more user-friendly, allowing more flexibility for people of different heights to use the binoculars without the need to continually adjust the tripod. It is also of benefit when the binocular is trained on the night sky. The straight eyepiece design model will require a higher adjustment of the tripod for each user and this may make the tripod less stable.

If you are forced to go out to the country where you cannot fix your binoculars to something, then consider image stabilised binoculars. These are far more expensive but have other applications as well.
Stabilisation may be enabled or disabled by the user as required. Stabilisation will allow binoculars up to 20× to be hand-held. Major brands making image stabilised binoculars include are Canon, Nikon, and Bushnell.

When I continue with Part 2 of my Astronomical Binocular Review Guide, I’ll talk about various features to look for in a binocular, including digital binoculars.

Last of a series

Your telescope

No matter what telescope you select, choose one that will meet your precise needs and interests. The planets, the Moon, and close stars require high power, good contrast, and sharp resolution, and if these are the objects of your attention, a refractor or reflector would be a good choice. Because very faint objects like galaxies and nebulae need a large aperture, you should invest in a big reflector telescope to view these. An all-purpose midrange telescope should serve best, for example a 6″ to 8″ reflector or an 8″ Schmidt-Cassegrain.

Keep checking back. Future articles will delve into filters used to cope with light polluted skies, astrophotography, astronomy book and software reviews, astronomical binoculars, and reviews of specific telescope models.

Next to last in my 6 part series on choosing your first telescope.

Telescope mountings

In the first four parts, I’ve discuse aperture, power and the different types of telescopes. Now I’d like to talk about an often overlooked but very important aspect of using a telescope – the mounting. A shaky view is all it takes to a fanatastic viewing session! A good mount can enhance your views. There are two basic telescope mountings:
ﾷ The equatorial and
ﾷ The altazimuth.
An equatorial mount is designed so you can easily track the motion of the sky as the Earth turns and its motions indicate celestial directions in the eyepiece. This helps when you’re trying to find your way among the stars with a map. The altazimuth mounts are simpler and just swing up, down, left and right. You have to move the scope along every so often to follow the stars, moons and planets. An altazimuth mount is both cheaper and lighter for the same degree of stability.

Come back for my concluding article when I’ll wrap everything up.

Catadioptric telescope

Catadioptric telescopes use a combination of mirrors and lenses to fold the optics and form an image.  There are two popular designs, the Schmidt-Cassegrain and the Maksutov-Cassegrain.

In the Schmidt-Cassegrain, light enters through a thin aspheric Schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube to be intercepted by a small secondary mirror. The mirror then reflects the light out the back of the instrument where the image is formed at the eyepiece.

Advantages

ﾷ Most versatile type of telescope
ﾷ Best near focus capability of any type telescope
ﾷ First-rate for deep sky observing or astrophotography with fast film cameras or CCD’s
ﾷ Excellent for lunar, planetary and binary star observing plus terrestrial viewing and photography
ﾷ Closed tube design reduces image degrading air currents
ﾷ Compact and durable

Disadvantages

ﾷ More expensive than reflectors of equal aperture
ﾷ Slight light loss due to secondary mirror obstruction compared to refractors
The Maksutov-Cassegrain telescope design has basically the same advantages and disadvantages as the Schmidt. It uses a thick meniscus-correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt’s giving it slightly better resolution for planetary observing.

However, the Maksutov is heavier than the Schmidt and because of the thick correcting lens, it takes a long time to reach thermal stability at night in larger apertures. The Maksutov optical design typically is easier to make but requires more material for the corrector lens than the Schmidt Cassegrain.

Welcome back to part 3 of my series on how to buy a telescope.

The reflecting telescope

Reflecting telescopes use a parabolic mirror at the back of the telescope to gather and focus the light to a flat secondary mirror that the image out of an opening at the side of the main tube. You observe through an eyepiece on the side of the tube up near the top.

ﾷ Easy to use
ﾷ Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters because their larger apertures collects more light
ﾷ Low in optical irregularities and deliver very bright images
ﾷ Reasonably compact and portable
ﾷ A reflector costs the least per inch of aperture compared to refractors and catadioptrics

Disadvantages

ﾷ Not suited for terrestrial applications
ﾷ Slight light loss due to secondary obstruction when compared with refractors
ﾷ The tube is open to the air, which means dust on the optics even if the tube is kept under wraps

Check back often for the continuation of my series on how to buy a telescope. Part 4 will discuss catadioptric telescopes.

Part two in my series on buying a telescope

In part one, I discussed why aperture is important in choosing a good telescope and also about telescope power. As I continue my series, I will discuss the refracting telescope. Be sure to check back for part 3 on reflecting telescopes!

The refracting telescope

Refracting telescope are the most common form of the telescope – a long tube where light passes directly from the front objective lens directly to the eyepiece at the opposite end of the tube.

Advantages
ﾷ Easy to use  due to the simplicity of design.
ﾷ Good for distant terrestrial viewing
ﾷ Excellent for lunar, planetary and binary stargazing especially with larger apertures
ﾷ Sealed tube protects optics and reduces image degrading air currents
ﾷ Rugged, need little or no maintenance

Disadvantages
ﾷ Generally have small apertures, typically 3 to 5 inches
ﾷ Less suited for viewing small and faint deep sky objects such as distant galaxies and nebulae
ﾷ Heavier, longer and bulkier than equivalent aperture reflectors and catadioptrics
ﾷ Limited practical usefulness
ﾷ Good-quality refractors cost more per inch of aperture than any other kind of telescope.