TRG – Telescope Review Guide

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!

Sporting the classic long-tube Newtonian design, the Orion SpaceProbe 130 mm provides great all-around viewing performance. One minute you can be studying lunar crater formations at high power; the next, the ethereal glow of a nebular star factory. A few minutes later, the ringed visage of Saturn. All served in crisp detail by way of the Orion SpaceProbe’s light-gathering 130mm (5.1″) primary mirror of 900mm focal length (f/6.9).

The Orion SpaceProbe 130 ’s mirror lies at the base of the rolled-steel optical tube, which is outfitted with a smooth rack-and-pinion focuser. The Orion SpaceProbe 130 accepts 1.25″ eyepieces, including the two fully coated Explorer II eyepieces that come standard with the SpaceProbe 130mm: a 25mm for low-power viewing at 36x and a 10mm for higher-power looks at 90x. Also standard is a 6×30 achromatic finder scope to help home in on celestial targets.

The Orion SpaceProbe 130 comes on the sturdy EQ-2 equatorial model, with adjustable aluminum legs and accessory tray. Slow-motion controls allow fine manual pointing of the telescope and easy star tracking. Addition of an optional EQ-2M AstroTrack electronic drive enables the telescope to track objects automatically.

 The Orion SpaceProbe 130ST

The Orion SpaceProbe 130ST is a step up from the SpaceProbe 130 for the more serious beginning or intermediate stargazer who wants additional performance, particularly for targeting deep-sky objects.

The Orion SpaceProbe 130ST is indeed more compact than the standard Orion Space Probe model. The Orion Space Probe 130 ST’s optical tube measures 24″ long compared to 33″ on the standard Orion Space Probe 130. The focal length of the Orion Space Probe ST’s 130mm (5.1″) primary mirror is 650mm (f/5), yielding a wider field of view and brighter images for a given eyepiece focal length.

The Orion SpaceProbe 130ST has a primary diffraction-limited parabolic mirror, the same type used on much larger reflectors costing many times as much. On a short-focal-length design like this one, a parabolic mirror is a must for focusing incoming light to a point and delivering sharp, detailed images. Moreover, the secondary mirror of the Orion SpaceProbe 130ST is held in an advanced holder with thin 0.5mm metal vanes, to reduce diffraction spikes and light loss. These features combined with the included 25mm (26x) and 10mm (65x) Sirius Plössl eyepieces and quick-release 6×30 achromatic finder scope make this one sweet optical system for astronomy! It also includes a collimation cap and center-marked primary mirror for easy optical alignment.

The Orion SpaceProbe 130ST comes on an EQ-2 equatorial mount with dual setting circles and slow-motion hand controls. An optional EQ-2M or AstroTrack electronic drive can be added for automatic tracking. The adjustable aluminum tripod is strong yet lightweight and includes an accessory tray.

Whether you’re looking for a better-quality first telescope or a portable instrument to complement your big Dob, you’ll love the Orion SpaceProbe 130ST.

Buying a new telescope can be confusing, especially for the first timer. Many are left wondering which telescope to get. Understanding telescopes and their various accessories can go a long way in choosing the best one for you. Whether you are a beginner or old pro, in this first of a six part series, I will help you better identify the different types of telescopes and help you find the best telescope for you.

Aperture of a telescope

A telescope’s singular most important attribute is its aperture. Aperture is a measure of a telescope’s light gathering ability and determines the brightness and sharpness of everything viewed through your scope. Aperture is the diameter of the main lens or mirror and as the aperture increases so does the details of the image you see. A good 10″ aperture scope shows sharper images than even a well-made 6″ aperture telescope.

Since aperture is so important, many astronomers are misled into believing that getting the biggest aperture you can afford is the simplest way to choosing the best telescope for you. This isn’t always the case. If you make your decision based solely on the size of aperture, you might end up owning a big telescope that sits unused.

Where will you use your new telescope? If  nearby in the backyard then having a large telescope will be of advantage to you. If you need to carry the telescope to to an observing site away from city lights, you will definitely need something compact, yet powerful. Don’t forget that you may have to assemble and set up your telescope in the dark. A scope that is too big to carry outdoors and too difficult to set up just won’t get used.

Telescope Power

Power and design, like aperture, are other important considerations when deciding which telescope to purchase. Power is a critical factor since it determines how close a view you can get of space objects such as planets but it is not a decision making factor. You can control the  power by using different eyepieces.

An eyepiece is the small removable lens assembly you look into. Most telescopes come with several eyepieces and you can buy more separately. Remember to match the power of your eyepiece lens with the aperture of your scope for clear images. The lowest powers are the easiest to use, especially for beginners, and provide the most pleasant views.

Telescope Designs

Even among telescopes with the same aperture, some designs are more portable, others give sharper images while still others are more economical. There are three basic kinds of telescope to choose from depending on your specific requirements.

ﾷ Reflecting telescope,

ﾷ Refracting telescope, and

ﾷ Catadioptric telescope.
Despite their differences in size these 3 telescope types have the same light-gathering properties. They also have a similar purpose, to collect light and bring it to a point so it can be magnified and examined with an eyepiece, but each does it differently. Therefore, each type of telescope has its pros and cons, which you can match with your observing needs.

Come back soon for part 2 of this article when I will discuss refracting telescopes!