EbolaBooze’s Astro Corner: Telescopes! Huh, yeah, what are they good for?

Absolutely no-

Let’s stop there.

Since I bought some telescope parts a few weeks back, it’s been nothing but storms, hail, cloud and rain. Well, at least I’ve kept my 100% hit rate for telescope purchases to inclement weather!

So instead, let’s have a pictorial guide on the different types of telescopes, and what they’re good for – strengths and weaknesses!

Let’s start with a classic – the refractor!

 

Refractor Telescopes

A refractor telescope at its most basic consists of two lenses – an objective (the light-gathering element) and an eyepiece. Refractors suffer primarily from chromatic (failure to focus all colours) and spherical aberration (failure to focus all light rays at a single point). Modern refractors can use up to six glass elements of varying optical dispersion and design to correct for these aberrations.

For astronomy purposes, two refractor designs are prominent: achromatic refractors and apochromatic refractors, with accompanying sub-designs.

Achromatic refractors are the cheaper of the two types, correcting for some spherical aberration and bringing only two wavelengths of light to a sharp focus. These are good beginner visual astronomy telescopes, as apertures of up to 152mm (6”) can be cheaply bought (~$700), and due to the lack of additional glass elements are lighter than apochromats.

starwave_152mm_ota5_m

Apochromatic refractors have two or more glass elements, with an extra-low dispersion (ED) element to bring three wavelengths of light to a sharp focus, and remove virtually all spherical aberration. Obviously, using more elements and more expensive glass raises the cost and weight of these telescopes significantly. 80mm aperture apochromats are $600-$800, with price increasing significantly once apertures are larger than 120mm.

esprit120

As a rule of thumb, most affordable refractor telescopes tend to be of short focal length (<800mm), and small aperture (<152mm). Refractors tend to be the most versatile telescope design, useable for visual, as well as planetary, wide-field and deep-space imaging. For a given aperture, their views are more contrasty and brighter than pretty much any reflecting or Cassegrain telescope type, due to the lack of central obstruction. The larger apertures are prohibitively expensive though!

For planetary views, good colour correction and a long focal length is needed – 1200mm is about the lowest you want to go. You can get this focal length by adding a Barlow lens or a tele-extender to your visual train. This will allow you to see the bands of Jupiter and the rings of Saturn with ease on a good night. For deep-space views, aperture is king – a 152mm achromat will serve well.

 

The list of refractors that are good for imaging are as long as my arm, but what is important is that the majority of mid-price apochromatic refractors project what is called a curved focal plane. This has no bearing on visual astronomy, but is important to note if you want to use one for imaging. The effects of a curved focal plane means that the stars in the centre of your image will be in sharp focus, with the stars further out being blurry and distorted – the amount dependent on the degree of curvature. Below are two examples, at the high and low-end of imaging refractors to demonstrate what some extra glass can do.

My imaging refractor is a SkyWatcher ED80, a two-element ED semi-apochromat at f7.5, with a focal length of 600mm, so a pretty slow scope. It needs a field flattener to produce decent images.

The Takahashi FSQ-106ED (on my Lotto wishlist, at a cool $5300), is a four-element apochromatic Petzval design refractor, and it’s corrected for astigmatism, coma, chromatic aberration, spherical aberration and field curvature. No correctors are needed, as the correction is part of the optical design.0038147_takahashi-toa-150nfb-double-ed-triplet-ortho-apochromat-refractor

Hopefully that was informative; I’ll be talking about Newtonian telescopes next time!

 

Clear skies!

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