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Most portable Newt?


ollypenrice

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Now be warned, I have a trick up my sleeve. What would you say would be the smallest possible size for a diffraction limted Newtonian giving 25x magnification?

I intend to spring a surprize on this one but go on, hazard a guess. We're all friends here....

Ho hum.

Olly

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Well......

A 150mm f4 with a 25mm eyepiece would give a maginification of x 24 but to see the Airy diffraction disk you'd need a bit more magnification ( the human eye resolution is around 2 min arc)

Ken

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Diffraction limited at x25? You're going to need something like 1" of aperture .... a bit smaller than Isaac Newton's original demonstration model. (Which very probably wasn't diffraction limited, and wouldn't have stayed that way anyway as speculum metal mirrors needed frequent re-polishing.)

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I don't even know what diffraction limited means!:D

I just asked my wife what the size of an Airy Disc is but she was not impressed. :)

I'm serious! well about my first point anyway. I'd be interested to hear how these things are calculated - in the manner of a maths teacher - 'please show your workings' :D

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I'd be interested to hear how these things are calculated - in the manner of a maths teacher - 'please show your workings'

You get destructive interference when the light arriving from two point sources is N+0.5 wavelengths different. The diameter of the Airy disc is limited by the first ring of destructive interference. For two slits the calculation is easy but for a circular aperture you need a bit of calculus to do the sums: integrating over all pairs of points in the aperture, it turns out that the diameter of the first dark ring is 1.22 * wavelength / diameter radians; setting wavelength to 550 nm (the wavelength to which the eye is most sensitive) and converting units, this works out as 5.45/D arc seconds where D is in inches.

So a 1" objective has an Airy disk diameter of 5.45 arc secs; in practice the resolution is slightly better than this as most of the light is concentrated towards the disk centre. Dawes' empirical resolution limit - based on being able to seperate double stars of equal brightness - is 4.56/D arc secs.

A magnification of x25 increases the apparent seperation as seen by the eye to 4.56x25 = 114 arc secs or 1.9 arc minutes - just barely enough for the eye to resolve.

(Incidentally it is this ecact same calculation which proves that magnifications much exceeding x25 per inch are unnecessary!)

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Newton's first version had a mirror diameter of 1.3 inches and a focal ratio of f/5.[12] He found that the telescope did work without color distortion and that he could see the four Galilean moons of Jupiter and the crescent phase of the planet Venus with it.

Newtonian telescope - Wikipedia, the free encyclopedia

Portable, but not much use for seeing the dust-lane in M31.

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Right, you will not believe this but it is absolutely true; a dutch optician, Ralf Ottow, is staying here at the moment and he has a 15.5 MILLIMETRE Newtonian with him.

It has a glass front plate because a spider would have been unworkable and it uses a tiny microscope eyepiece. Not only does this mini-scope work, it works well. You can resolve car number plates easily when they are nowhere near legible to the eye. The eye relief is excellent and the views sharp. Ralf has seen banding on Jupiter with it!

He has also got with him a home made 5 inch SCT and an incredible 12.5 inch watercooled Newtonian. I can detect absolutely no coma in the Newt using Naglers. Ralf says it does have coma but to me it is sensibly perfect. It leaves on Saturday. Damn!

Olly

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Prolly can't find the darned thing ... at least it's hard to lose a 16" Dob amongst general household clutter :D

Heh heh! I will get the pics up but give me a sec, it's all night and all day here at the moment!!

Olly

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