EbolaBooze’s Astro Corner: Eye of Newt(onian) and Banks of Fog

Seeing is crap and the weather’s a dog.

Half a point to whoever gets the reference.

So! Still cloudy and windy, and seeing in the range of 4-5 arc seconds in the clear breaks. Here’s a (somewhat) recent synoptic chart for those interested.

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It’s summer now, for the record.

I did promise a breakdown of what Newtonian telescopes (Newtonians, Newts) were good for, and here it is!

 

Newtonian Telescopes

Named after their inventor; Sir Issac Newton, mathemator of the letter x, lord of apples, the deadliest son-of-a-bitch in space, the Newtonian is a versatile design, and one of my favourite types of telescope.

A classical Newtonian is a purely reflective system, composed of a parabolic (or spherical, or hyperbolic) concave primary mirror, and a flat elliptical secondary mirror angled at 45° to the light path.

16in Newt raytrace

Newtonians in their purely reflective form suffer from coma, which is the tendency for point sources of light (like stars) that are not in the centre of the field to be distorted, appearing to have a tail (coma) like a comet. In Newtonians, this is due to the fact that though a parabolic mirror will focus light rays to a common focus; this is true only for parallel, on-axis rays. Light rays from objects that are not on the optical axis i.e. the light rays strike the primary mirror at an angle, are not focused to the same point. This is only really a big problem on Newtonians with a focal ratio of f/6 or faster (lower), as the curve of the mirror increases with decreasing focal ratio.

The second major drawback of Newtonians is the fact that they have a central obstruction, caused by the secondary mirror required to reflect the light rays into the eyepiece/camera. The decreases the effective light-gathering area and the contrast obtainable by a scope of any given size, and is dependent on how large the central obstruction is. Again, the faster the focal ratio, the larger the secondary obstruction as the light cone has steeper angles and the mirror needs to be larger to intersect the whole of it.

Newtonians are also rotationally asymmetric, what with the focuser hanging off the side of the tube. If you’re mounting one on a German equatorial mount, more often than not the eyepiece will end up in an awkward position. For astrophotography though this doesn’t matter as much since the tube can be rotated to face the camera towards the mount axis.

That being said, the Newtonian is a design just as versatile as a refractor, and cheaper to boot. They can be had in apertures from 152mm (6”) all the way to massive monstrosities in the metre-class.

Aperture for aperture, a Newtonian is cheaper than almost any other type of telescope, and is the only option for amateur astronomers past around 50cm (20”) aperture.

The Dobsonian variant is one of the more popular types of telescope – in essence a Newtonian telescope placed on a simple alt-azimuth mount. Motorised tracking and computerised versions are available for ease of observation as well. These are primarily visual telescopes, able to offer impressive views of most objects in the sky for reasonable prices, up to apertures of half a metre.

DobCollapsible14Z

For astrophotography purposes, a Newtonian of focal ratio f/4 or faster on a good German equatorial mount is recommended. However, for decent images a coma corrector is a mandatory purchase – this will add around $300 to the price of the telescope, if a good quality one is purchased.

Focal lengths can range from 800mm for a fast 200mm aperture f/4 telescope, with the upper limit really determinant on the mirror size. A good upper limit for general astrophotography would be around 1600mm, achievable with a 406mm (16”) f/4 mirror, or a 306mm (12”) f/5 mirror. Various tube designs also exist, from solid metal tubes to open carbon-fibre truss designs.

254 174

Most Newtonians tend to be in the f/3 to f/6 range, and depending on aperture are suitable for imaging most nebulae and some deep space objects. A caveat is that faster Newtonians tend to be significantly more expensive, as the mirror geometry becomes increasingly complex to grind.

My current primary imaging rig is a 254mm (10”) f/4.7 customised Newtonian. The 1200mm focal length is suitable for most nebulae, star clusters and the closest few galaxies to us. Larger nebulae need a focal reducer for an increased field of view and further-away galaxies need a Barlow lens or a tele-extender. The medium focal length doesn’t lend itself well to planetary imaging. Ritchey-Chrétiens and other Cassegrain telescopes, with their much longer focal lengths are much more suited for planetary imaging.

More on those next week.

 

Clear skies!

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