[ngcles]

Hi Afro & All,

Interesting thread. If you intend on building a Newtonian telescope (irrespective of how it will be mounted) , then I'd strongly suggest you get a hold of an older publication called "Newtonian Notes" by Kenneth Novak which discusses a lot of optical theory in layman's terms and provide quite a bit of guidance on the implications of f/ratio, secondary sizes, coma, off-axis astigmatism and the size of the illuminated field etc etc.

There is no real right or wrong on f/ ratio for a visual-use Newtonian, but there are consequences that have to be lived with either way you go. You simply need to decide what you can and cannot live with or live without.

Just dealing with photography/imaging, a short f/ ratio will provide a wider field on the chip/film need shorter exposures etc. A long f/ ratio will generally provide narrower fields with larger image scale but also need longer exposures meaning a better (more expensive) mount is needed and is more demanding on the setup (and the operator).

Visually however, in terms of "image brightness" the f/ratio has no effect at all provided the telescope is used at the same magnification. In other words, for example a 10" f/9 used at x150 will have an identically "bright" image to a 10" f/5 used at x150. The longer focal length telescope will however use a longer focal length eyepiece. It will also, provided the seeing conditions are almost perfect, produce a slightly more contrasty image because the diffraction effects of the tiny secondary mirror will be negligible compared to the f/5 with a larger secondary.

The optical defect called "coma" affects all telescopes containing a paraboloidal surface. This includes all Newtonians and classical cassegrainians and some others.

The shorter the f/ ratio, the "stronger" (ie deeper) the parabola, the more coma intrudes and a larger portion of the outer areas of the field are seen to be affected with imperfect star-images. In slow telescopes, coma is hardly noticeable. In "slow" small telescopes (say an 8" f/10 Newtonian) the difference between a spheroid and a paraboloid is so small it does not bear considering and with so many "slow" commercial 'scopes of this sort, the manufacturer doesn’t bother to parabolise the mirror at all. No problem with that – it just isn’t really necessary on say a 4½” f/9 Newtonian. Slow optics are also more forgiving on slight mis-collimation and use/need very small secondary mirrors (ie less than 15% by diameter compared to the primary) so that diffraction caused by the secondary has no real practical effect on visual image quality and aesthetics.

If you intend the telescope to be a pretty much dedicated planetary Newtonian, a long f/ ratio is preferable for these reasons. It also means you can achieve high magnifications with longer focal length eyepieces that are more comfortable on the eye in use. Simple eyepieces of older design also perform much better with slow f/ratios because the light-cone does not converge so steeply. Long f/ ratios also have many practical cons for visual use once you reach 8” +.

The tube will be a monster and need a very substantial, heavy mount making portability a serious problem. Long focal length telescopes find it hard to achieve wide fields for things like Eta Carinae (NGC 3372) M42 etc etc as well. Many years ago I had a friend with an 8” f/11 Newtonian which was a very nice and sharp ‘scope that produced lovely images, but was a pain in the you-know-what to use.

“Faster” f/ ratios necessitate much more care when fabricating the mirror and coma will become an increasingly serious issue once you go faster than f/5. The simple solution is to use premium quality advanced design eyepieces that cope much better with short f/ratios and to use a Paracorr that will all but eliminate it. In the US a whole generation of new super-fast Newtonians are being born – like say 28” f/3.6. But they need a very, very experienced and dedicated optician to figure the optic correctly and are extremely demanding on collimation. They also need very large secondary mirrors (in the order of 30% obstruction by diameter) which I personally do not like for visual use, but they do allow you to own a 28” ‘scope and only need 1 step up to observe at zenith – a decided advantage. They are also therefore lighter and therefore need less “engineering” to be rigid.

In my opinion, the sensible thing of course is to compromise and pick a middling f/stop (depending on the diameter of the aperture) that will not be too expensive or too hard to make, be very usable for most or all visual purposes, keep the secondary mirror under 20% by diameter and have a 100% illuminated field big enough to fully illuminate an eyepiece field at say the 2.5mm exit-pupil size while maintaining an at least 75% illuminated field at the edges of your lowest power eyepiece. This is why personally I chose f/4.9 for my 18”. For me it was the best compromise and it meets all the above criterion.

On that, a 13.5” (let’s not split hairs) at about the f/5 mark, I think would be a very good compromise if it uses a 66mm secondary. Faster than f/4.5 and you’ll need a parracor, slower than f/6.5 will be a very long telescope that will be much less “portable”-friendly.

Of course if you are going to image with a Newtonian, there are all sorts of other considerations that need to be taken into account when choosing f/ stop and secondary mirror size.

All telescopes are a compromise of one sort or another. We trade money (ie cost per inch af aperture) against aperture against portability against image quality against width of field against aberration control.

I’m not sure where you would get “Newtonian Notes” nowadays. The last time I saw it was at BINTEL. Others may still carry it. It used to be only about $20-odd, about 150 A4 pages, cheap type-face, hand-drawn illustrations but had a wealth of good, practical advice and wisdom on how to design a Newtonian. The other book mentioned on building dobsonians is also mandatory reading but tends to concentrate more on building the thing and engineering it.


Best,

Les D