In a previous blog, I described, in considerable detail, my enthusiasm for the light–weight but powerful 130mm f/5 Newtonian reflector. This modest instrument beat the codpiece off a way overpriced 90mm ED apochromat which retailed for about £1,000 (and is now discontinued) on every test object. This little Newtonian has an excellent SkyWatcher parabolic mirror and the secondary was upgraded to an Orion Optics UK sourced unit, with a semi–major diameter of 35mm, thus delivering a moderate 27 per cent linear obstruction (so considerably smaller than a similar–sized SCT or Maksutov). Both mirrors were also treated with Orion UK’s proprietary HiLux coatings, providing an overall reflectivity of 94 per cent. Further testing showed that it provided images that were effectively equal in brightness to a fine 127mm f/12 refractor (which has now been retired). The interior of the tube was lined with cork for improved insulation and then covered with standard flocking paper, maximising contrast and reducing stray light to near zero.
Such an instrument has provided excellent views of the heavens, from 20x in a 2.5 degree true field, to over 400x on very tricky double stars down to 1.0″ separation. During last year’s apparition of Jupiter, the 130mm f/5 Newtonian also proved its worth as a very capable planetary telescope. The instrument was also fitted with easy–to–use Bob’s Knobs on both the primary and secondary for ultra–precise collimation that takes just seconds to execute.
It may come as a surprise to some readers to learn that this author spends as much time looking through telescopes by day as by night, and over the past few months, he has been using the 130 as a ‘super’ spotting ‘scope, where it has dlivered crisp, colour–true images of a range of terrestrial targets, including, birds, trees and various man–made landmarks. This has led him to openly question the dominance of more conventional spotting ‘scopes (usually small refractors and catadioptrics). The only reservation I had while using the Newtonian for nature studies was that it delivered an image which was upside down and right–left reversed. And while this ought not to bother a determined telescopist, some have dismissed the small Newtonian as a daytime spotting ‘scope precisely because of the orientation of its images. Yet, there are ways to produce correct orientation views through Newtonians and it doesn’t require expensive prism diagonals and the like; enter the Newtonian erecting eyepiece.
I was able to get hold of a 10mm focal length ocular marketed by SkyWatcher for less than £30. It has basic but effective anti–reflection coatings and appears to consist of a simple Kellner design with an extra lens inserted so as to invert the image at the focal plane and, unlike conventional refractor diagonals, it flips the image so that left and right are correctly presented.
The eyepiece delivers a power of 65x, with a near 2mm exit pupil, which closely matches the diameter of my pupil during bright daylight. Optically, the unit delivers good, sharp images, with a well–defined field stop, although critical tests did reveal a small amount of lateral colour. Contrast is good but not quite in the same league as a dedicated astronomical eyepiece endowed with fully multi–coated optics. Yet, it is more than adequate for casual nature studies.
The instrument can be focused on objects as near as 25 yards without adjusting the position of the ocular, but by moving the eyepiece further up the barrel, or using the supplied extension tube (pictured below),objects even closer to the telescope can be brought to a good focus.
Doing an internet search, I have also become aware of a 20mm model (supplied by Celestron with some of their smaller Newtonians), which would produce a power of 33x or so with the 130. This would be a useful addition for those wishing to extend the range of magnifications achievable with the telescope during daylight hours, and I will report back on this if I am lucky enough to find one.
So to summarise, there is absolutely no reason why observers possessing small Newtonian reflectors cannot use them in a variety of terrestrial applications. Their increased light grasp and resolution over conventional spotting ‘scopes and small refractors will both surprise and delight their users. The range of terrestrial eyepiece options available are quite limited at the moment (as far as I can tell) though, but I would warmly welcome the introduction of more models offering a greater range of magnifications, as well as improving the optical quality of these designs. But one thing is clear; using erecting eyepieces increases the verstaility of what is, already, a great, all–round ‘scope.
Neil English is the author of Grab ‘n’ Go Astronomy.