Thoughts about Newtonians
As this article will show, Newtonians are the most versatile telescope type but, first, some background about their “contrast”. The overall contrast of a Newtonian is bound to be somewhat less than a refractor as the two mirrors each scatter rather more light than does a doublet lens. In addition, the secondary mirror of a Newtonian “diffracts” some light away from the Airy disk into the surrounding rings of the Airy Pattern. This reduces the contrast of faint details in the image and can reduce the effective resolution of the viewed image. Table 1 shows the amount by which the light is transferred from the disk to the rings. A 20% obstruction has little effect and is the aim for Newtonians designed to observe the planets. For wide field imaging, a larger secondary mirror is needed to illuminate typical camera sensors but then the small reduction in resolution and contrast is not a real problem. So, depending on the proposed usage a Newtonian will have a range of secondary mirror sizes whose percentage obstruction ranges from ~45% for a wide field imaging Newtonian down to ~22% for a planetary Newtonian.
Short focal length Newtonians for astroimaging
It should be pointed out that short focal length Newtonians suffer greatly from coma and a coma corrector will need to be budgeted for. The telescope tubes can have either be made from metal or Carbon Fibre. The latter are more expensive but will not reduce in length as the night cools down so the focus is not affected.
These are usually of 6 or 8 inch aperture and have a focal ratio of f/4 – so good for astroimaging. They range widely in price from around £489 as for the Sky-Watcher Quattro 150P f/4 Newtonian (Figure-1) – which includes a Modified Aplanatic Super Coma (this must be a good buy) to around £4,500 for an Orion Optics (UK) AG8, f/3.8, Newtonian with an included 3 inch Wynne coma corrector (this itself costs ~£1,000) – so the telescope is easily capable of covering a full frame sensor.
My telescope in this range is a mid-price, TS Photon 6-inch f/4, Carbon Tube, Newtonian at £999 with a 4 element aplanatic GSU coma corrector costing £248 and is perfect for use with up to APS-C sensor cameras.
Many Newtonians of around f/6 in focal ratio are sold on Dobsonian mounts. These give one the best possible viewing experience for their price. A skywatcher 6-inch Dobsonian costs just £298 and has a 2 inch Rack and Pinion focuser whilst an Ursa Major 8-inch Dobsonian, Figure-2, which costs £369 is designed and manufactured to a very high standard by Guan Sheng Optical (GSO) in Taiwan. This employs a mount that includes roller bearings for the azimuth motion. Their StellarLyra, 10-inch, Dobsonian, which also has a roller bearing on the azimuth axis, costs £619. There is also a 12-inch Dobsonian for £869. [Incidentally, the roller bearing assembly can be bought to upgrade other Dobsonians for £39.] The 8-inch aperture Dobsonians are generally recommended for beginners.
Newtonians can also be bought as tube assemblies to mount on separate mounts. But be warned, with their weight at either end of the optical tube, they have a high “Moment of Inertia” as compared to a Schmidt-Cassegrain and a solid mount is required to mount them.
Giant Dobsonian Newtonians
The Dobsonian mount is how very large Newtonians are mounted. Often with an open truss construction, they range in apertures from 16 to 24 inches or more. A StellaLyra 16 inch Dobsonian, costs £1,899 and has a high quality GSO primary mirror and a roller bearing azimuth movement. The SkyWatcher Stargate 500P, “GoTo”, 20 inch telescope, Figure-3, costs £5,499 and has optical encoders on each axis and provides motorised operation in both Alt and Azimuth axes, and can slew to and track over 42,900 night sky objects from the Synscan handset’s extensive computerised database.
Newtonians can be optimised for planetary viewing and imaging. The objective is to have as small a secondary mirror as possible so minimizing its diffraction effects. This can be achieved in three ways. The first by having a long focal length – as the secondary mirror will then be further away from the primary mirror it can be smaller whilst still intercepting the light beam. A focal ratio of f/8 is often used. Secondly, by making the focal plane close to the tube side. This reduces the distance of the secondary to the focal plane so, again, it can be smaller and thirdly by allowing some vignetting (darkening of the image towards the field edges) as, of course, a planet will be viewed in the centre of the field of view.
First Light Optics sell two 6 inch, f/8, planetary Newtonians. One made by Bresser is £258 and has a 2 inch rack and pinion focuser. The second is their own Ursa Major brand at £269, Figure-4. It has the roller bearing azimuth assembly, excellent GSO mirrors, but only a 1.25 inch Crayford focuser. One might think that having a 2 inch focuser is better (and it is for most telescopes) but, in this case, 1.25 inch is better and gives no problem for planetary viewing. The reason is that the secondary only has to illuminate a smaller maximum field of view as determined by the maximum field stop size of a 1.25 inch eyepiece rather that the larger area that could be provided by a 2 inch eyepiece. To cover the maximum field of a 2 inch eyepiece, the secondary obstruction produced by the secondary mirror of the Bresser has to be 33% whilst that of the Ursa major is just 22% – which is very close to being negligible. So, the Ursa Major scope is better. I bought one for my grandson and the first light view of Jupiter was one of the best I have ever seen. Io was very close to the limb of Jupiter but easily seen as a pinpoint of light.
A derivative Newtonian type – a Maksutov-Newtonian
These use a spherical primary mirror but have a thick “Maksutov Type” meniscus corrector plate. An excellent example is the SkyWatcher 190 DN Pro at £1,329, Figure-5. Maksutov-Newtonans have relatively small secondary mirrors, 26% in this case, so are excellent for planetary observing. I have a Russian Intes-Micro Altair MN 56 with just a 22% secondary obstruction. To achieve this, it has a very low profile focuser so the eyepiece is only just outside the telescope tube minimising the distance between the secondary mirror and focal plane. Their MN 58 f/8, 127 mm aperture, Maksutov-Newtonian costs £1,100 and is a superb planetary scope, with just a 20% secondary obstruction.