An in-depth review of the Newise 200X - a modified Newtonian

by Ian Morison, Jodrell Bank Observatory

The CapeNewise 200X modified Newtonian

Mounted on the Losmandy GM8 Equatorial Mount - note the captive bolts to hold the dewshield

Image: Ian Morison

An Innovative Telescope

The Newise 200x is a modified Newtonian telescope whose innovative design, discussed in detail below, gives the prospect of having both the planetary imaging performance of a high quality Newtonian telescope (but in a much more compact form) and also the ability to form a wide flat field that is perfect for astro-imaging whilst giving impressive visual views of open star clusters such as the Pleiades. The telescope was tested in comparison to other high quality telescopes and the promised optical performance was confirmed. In addition the telescope was found to be a very pleasant telescope to use with its compact size and low weight making it easy to handle and with its Newtonian focus always enabling a comfortable viewing position to be found.

The Newise Design

The classic Newtonian uses a parabolic primary mirror whose reflected light falls on a secondary flat mirror (the flat) to direct the converging light cone to a point outside the telescope tube where the image is observed with an eyepiece. The secondary mirror is supported with a spider, usually in the form of a cross. This causes some diffraction (scattering) of the light and makes bright stars appear as a cross with central bright disc. The diffraction caused by the spider some diverts light from its proper place in the image and so reduces the image quality of extended objects, particularly planets. Though giving a theoretically perfect image at the centre of the field of view, the image quality falls off away from the centre due to an “aberration” (= optical problem) called “coma”. Images of stars away from the centre appear like little comets becoming more so the further away from the centre of the field. As a result, Newtonian telescopes do not naturally make good wide field instruments. A cost problem is that large parabolic mirrors are quite expensive to manufacture requiring hand “figuring” by an optical specialist.

The secondary mirror of a Newtonian also has an effect on the image, particularly when planetary imaging. The central obstruction pushes some of the light away from the central region of the Airy Disc into the surrounding rings reducing what the author calls the “micro contrast” of the image. (This is why planetary observers often use refractors which have no central obstruction.) With a central obstruction of less than 20 % there is little observable effect, from 20 to 25% the problem is not severe, but beyond 30% it does become a real issue and reduces the effective aperture of the telescope relating to its resolution. A typical Newtonian has a central obstruction of ~ 20-25%.

In 1933 Bernhard Schmidt invented the “Schmidt Camera”. This uses a spherical mirror and a full aperture “corrector plate” to provide a coma free very wide field image. However the field was anything but flat and the film was formed onto a spherical surface to compensate for the field curvature. It should be noted that the corrector plate was at the radius of curvature of the mirror (a distance twice that of the focus from the mirror).

This idea became the basis for the Schmidt-Cassegrain (SC) telescope. However the corrector plate is at a compromise position near to the focus of the mirror rather than its radius of curvature and so coma is not corrected over a wide field and the field is still not flat. Focal reducers can be bought to reduce the typical f 10 focal ratio to f 6.3 and these do help to flatten the field somewhat. The secondary mirror is supported by the corrector plate so that there are no diffraction spikes, but the secondary mirror has to be ~ 34% of the aperture of the primary mirror so pushing light out from the central disc into the surrounding rings and slightly reducing the image quality when observing planets. The effective resolution of a 200 mm aperture SC would be roughly equivalent to a 130mm refractor. The design does address the cost issue, as a spherical mirror can be made by machine and a cost effective way was invented to produce the corrector plate so that the combined cost could be less than a parabolic mirror.

With this introduction, where does Peter Wise’s new design fit in and what should be its theoretical strengths or weaknesses compared to the Newtonian or Schmidt Cassegrain?

The Newise design uses a spherical mirror as in a Schmidt-Cassegrain but rather than a single corrector plate uses two, much smaller achromatic doublets, to firstly correct for the spherical aberration of the spherical mirror but also – the clever bit – to provide a very wide flat field. There is some further subtlety in that the configuration – one diverging doublet prior to the secondary mirror and one converging doublet after - has the effect of reducing the required size of secondary mirror. The ratio is just 25%, comparable to most Newtonians and significantly less than that required in the SC design (~34%) so helping to give better planetary images.

The secondary mirror is supported by an optically flat glass plate so there are no diffraction spikes and the overall size of the telescope is much reduced compared to that of a 200mm f6 Newtonian. (See accompanying photograph.) The much shorter length compared to the equivalent Newtonian reduces its “polar moment of inertia”. Don’t worry what this is but the result is that any vibrations induced in the mount after moving the telescope will contain less energy and damp out more quickly. One should point out that a corrector plate or, in this case, the flat glass plate secondary support, will attract dew. A dew shield is provided with the telescope which proved very effective.

There is one aspect of the design of any telescope that uses a secondary mirror which has to be a compromise; the diameter of the secondary mirror. (To be more precise, its minor axis, as the secondary flat has an elliptical shape.) If you put your eye at the centre of the field of view in the image plane you should see the whole of the mirror. If you now move your eye to one side of the centre you will find that there comes a point where you will no longer see the whole of the mirror. This implies that when used as a telescope the image near the edge of the field (beyond this point) will receive less light and star images will be less bright. This is called vignetting. To reduce the vignetting a larger secondary mirror must be used, but this will increase the secondary obstruction and so reduce the performance when observing planets. So a telescope can be optimised for planetary viewing by using the smallest possible secondary or for wide field astro-imaging by using a much larger mirror to eliminate (or greatly reduce) the vignetting. For wide field viewing the large central obstruction really does not matter. The size of the secondary mirror on the Newise, as on many scopes, is a very reasonable compromise. There is no vignetting over the central ~1cm of the field of view with an acceptable loss of light at the edge of the field.

Theoretically, the Newise design should provide slightly better planetary images that either a Newtonian (no spider diffraction) and a Schmidt-Cassegrain (smaller central obstruction). Its design gives a substantially wider aberration free field than either and the design gives a flat field that neither of the other designs can give without additional correcting elements. We will see how, in practice, it performs later.

I would make one final comment. Though Peter Wise has never used the term, I see no reason why this telescope should not be called a Wise-Newtonian ranking as an optical design alongside the Schmidt-Newtonian and Maksutov-Newtonian.

The Construction

During its short life the telescope has gone through several cosmetic makeovers. The standard 200 mm version now comes with a nice black powder-finished drawn aluminium tube with all the other parts made of anodised aluminium. However I have seen a pillar box red telescope that was on show at Astrofest – quite dramatic – and there were tubes in lime green and pink should one fancy something a bit different – my wife quite liked them! It is only 620mm long and, with a weight of 8 kg quite easy to carry and mount – - in contrast to the steel tubed 200mm f4 Meade Schmidt-Newtonian which is longer and weighs considerably more. I found it reassuringly solid. For those who would like even less weight a 6 kg carbon fibre tube version is available at extra cost. (The larger Newise telescopes come with carbon fibre tubes as standard.) It comes with a very nice 2” Crayford style focuser with an adaptor for 1.25 inch eyepieces. The Finder gives an erect image with a right angled eyepiece. It was very good optically and nice to use, so I may well buy one for my 235mm Celestron. One very nice feature was the way in which the provided dew shield was mounted. There were three captive circular knurled “nuts” that screwed down to lock the dew shield in place.

What was provided

With the telescope and finder were the following items:

30mm 80 degree apparent field 2” eyepiece

11mm 80 degree apparent field 1.25” eyepiece

Adaptor for mounting an SLR camera.

Custom Dew shield

Allen Key for collimation.

Comparison testing: the CapeNewise 200X and Celestron 9.25" Schmidt-Cassegrain

200x on Meade LXD 75 mount with Celestron 9.25" on Losmandy mount - you can see evidence of the dew (close to freezing) on the Newise dewshield. It did not dew up whilst the Celestron did.

Image: Ian Morison

Optical testing

This was done in three ways: by comparison observations with telescopes of known quality, star testing with stars and a Geoptic Artificial star and by the use of a “Ronchi” Grating.

Star testing

The Newise was used to observe both a "Geoptic" artificial star and a stars at high elevation on nights of good seeing. Star testing is a fairly vicious test and virtually no telescopes survive unscathed. The Newise held up very well indeed. No obvious astigmatism and just a little under correction for spherical abberation. None of my reflectors (including an excellent Newtonian better than 1/6 wave) gives a significantly better star test. I do have one apochromat refractor that does - but so it should considering its cost! It is just what one would expect from a good amateur telescope.

Ronchi Test

Observing an out of focus star through a finely lined "Ronchi" grating is known as the Ronchi Test. It is not as sensitive as the star test - perhaps a good thing - but can show if a lens or mirror is significantly over or under corrected and - where it works quite well - if there are regions of the lens or mirror, called zones, that do not conform to the overall shape. If all is perfect, one should see a set of parallel straight lines. If they are bowed one way or the other the optic will be either over or under corrected and, if they wiggle, the surface is not of a uniform shape and has zonal errors. The lines were smooth (which indicate smoothly polished lens surfaces), and there were no obvious wiggles in the parallel lines (so no significant spherical abberation or obvious zonal errors). Good.

Mounting

The telescope's tube rings are fitted to a high quality standard "Vixen Type" dovetail so I could immediately mount it onto a Meade LDX75 computerised mount which proved quite adequate for both visual and photographic use. It does, perhaps, deserve an even better mount, so an adaptor was made so that it could be fitted onto a Losmandy G8 mount controlled with an Argo Navis computer. Images above show it on both the Meade and Losmandy equatorial mounts. I have to say that the Losmandy combination really looks rather nice and received some complimentary comments at a "star party".

Wide field visual observations

The Newise comes with one of the 30mm, 80 degree wide field of view, eyepieces now available for the order of Ł60 or so. It has a field stop diameter of 45mm corresponding to an actual field of view of 2.0 degrees and gives a magnification of x 40. Such 80 degree apparent field eyepieces are called ultra-wide. I was really quite impressed with this eyepiece – it had better contrast (blacks were blacker) when viewing the Orion nebula than a similar and well rated eyepiece of mine with a similar specification, whilst the star images at the centre of the field of view were really excellent. The image quality did, as expected, fall off near the edge of the field of view. This is nothing to do with the telescope but the eyepiece. The only 30mm ultra wide angle eyepieces that can give pin-sharp stellar images to the edge of an 80 degree field are the Televue Nagler’s or equivalent costing upwards of Ł400, and you are not going to get one of those bundled with your telescope!!
I do, however, have two eyepieces that can give pin-sharp star images to the edge of the largest field that a 2” eyepiece can give. The first is a ~70mm eyepiece derived from a Meade Super Plossl that I acquired with one lens missing (I have no idea why!). With an apparent field of view of ~ 30 degrees it is rather like looking down a tunnel BUT it does provide edge to edge pin-sharp images and it did show a beautiful star field when observing the Pleiades Cluster with the Newise. At Astrofest 2007 I was able to purchase a new design of wide-field eyepiece having a 69 degree apparent field of view. It is a 40mm TMB Paragon sold by Burgess Optical. This gave an absolutely stunning view of the Pleiades indicating that the Newise does, indeed, have a wide well corrected field of view. Perhaps surprisingly, such a test cannot prove that the field is flat. As virtually all telescopes have curved field of views, optical designers often, as in the case of Thomas Beck who designed the Paragon, assume that there will be some field curvature and so include some correction. Bill Burgess of Burgess Optical who market the design was very interested to know how it would perform with a telescope having an inherently flat field. No problems!

Wide Field Photographic Observations

The Newise came with an adapter to enable a camera (given a suitable T-mount costing ~ Ł10) to be mounted with its film or CCD frame in the image plane. As – certainly in the case of the CCD, but not necessarily so in the case of a film camera – the detector surface is flat so the best test of a flat field is to produce star fields that have nice circular, pin point, images across the whole field. Sadly, I do not have a full frame digital camera which has a diagonal of 42mm (or 2 degrees on the sky with the Newise) but I do have a 10 Mpixel Nikon B80 with an APS-C sensor having a diagonal of 27mm (or 1.3 degrees of the sky). I used this to image both the Pleiades Cluster and the Sword of Orion. I quickly found that from my back garden in a town I could take no longer than a 30 second exposure without the light pollution fogging the image. I am thus not particularly proud of the two images shown below but here they are anyway. The stars of the trapezium show up quite well and are almost resolved in what is effectivly a very low power magnification image.

The Pleiades Cluster, M45

Nikon D80 on Newise 200X - Magnitude limited by light pollution!

Image: Ian Morison

The Orion Nebula region

Nikon D80 on Newise 200X - light pollution limited

Image: Ian Morison

Medium Field Observations

The Newise came supplied with an 11mm 80 degree apparent field of view eyepiece. This is a “generic” Chinese made optic which can be bought for the order of Ł50 from a number of suppliers and, with the Newise, gives a magnification of 109 and an actual field of view of 0.71 degrees. I have no direct eyepiece to compare it with and the only close competitor would be a Televue 11mm Nagler costing several hundred pounds. It actually performed very well. The performance falls off beyond ~ 85% of the field which is entirely as expected when used with an f6 focal ratio telescope. The “trapezium” area of the Orion nebula looked very nice and comparable, but with a much wider field, to my 12.5mm Celestron Ultima and Vixen Lanthanum 10mm eyepieces (both excellent) which straddle its focal length.

There is a good reason for using wider field eyepieces than the standard Plossl designs apart from the so called “spacewalk” effect. For a given field size, such as that to encompass a globular cluster, a shorter focal length can be used which results in a higher magnification. The higher magnification reduces the apparent sky background without affecting the brightness of the stars, so fainter stars can be seen.

There were two issues with the 11mm eyepiece, both easily rectified. The first is that the eye relief is quite small and, as a result, the pupil of the eye has to be quite close to the eye-lens of the eyepiece. The (rather too hard) rubber eye cap makes it impossible to get the eye close enough. The remedy is simple – just remove it. The eyepiece then fits snugly into the eye recess and the full field can be seen at once. The interior of the barrel that fits into the focuser is very nicely ribbed and blackened (as it should be) but there is a metal coupling section which attaches the barrel into the body of the eyepiece. This has a shiny surface on its interior. This would reduce the contrast of the image when observing the Moon but not on most other objects. Again the solution is simple. It is very easy to remove this by first unscrewing the barrel from the eyepiece and then unscrewing the coupler. One can then paint the interior with a good matt black paint – I used “blackboard” paint. With a generous application one can even give the paint a fine “ribbed” surface. Problem solved.

I also used two excellent eyepieces to give somewhat higher powers; a Burgess optical 9mm “planetary” eyepiece and a Takahashi 7.5mm LE. The supplied 11 mm eyepiece was also used with an x2 Barlow lens to give the effect of 5.5mm eyepiece.

Saturn

One of the images above shows the Newise set up alongside a 9.25" Celestron Schmidt Cassegrain. It has a different optical design to most other such telescopes and is a very good performer. On a cold and dewy night, but with good seeing, I compared the images of Saturn as observed with the two scopes. There was very little in it. As a result of the larger aperture one of Saturn's fainter moons was more easily visible in the Celestron than in the Newise, but otherwise the images were very comparable. The Newise has a slightly "warmer" colour rendition than the Celestron which one would only notice by switching quickly from on the the other. I would like to point out to amateurs moving up from a smaller to a larger telescope, that, quite often, the view through the larger may appear worse than in the smaller. This is due to the scale size of the irregularities in the atmosphere and smaller scopes are less limited by the seeing than larger ones. Often a smaller scope will give a better image of a planet like Saturn, but under nights of good seeing the larger telescope will come into its own and give greater resolution. A colleague recently moved up from a 5" to an 8" telescope and, for a while, was somewhat dissapointed. Following a couple of nights of good seeing he isn't any longer!

Digital Imaging of the Moon

I am much happier with these images of the Moon! If using a digital SLR with the ~24mm x ~15mm sized CCD sensor, then the focal length is just right to sit the Moon nicely in the frame with a bit to spare. These images were taken in February, March and April 2007 - and included the night of the total eclipse of the Moon!

Moon 25th February 2007

Nikon D80 on Newise 200X

Image: Ian Morison

Moon 26th February 2007

Nikon D80 on Newise 200X

Image: Ian Morison

Full Moon on the night of the Total Eclipse

Nikon D80 on Newise 200X

Image: Ian Morison

The Moon in Eclipse

Nikon D80 on Newise 200X

Image: Ian Morison

Value for Money

As the Newise X200 is unique, one cannot really compare it to anything else. However to give a feel as to how its cost compares, lets consider a Meade LX200R tube assembly. This is an f10 modified Schmidt-Cassegrain design. The basic tube costs Ł1099. This is less than the Newwise, but the tube assembly is totally “bare”. For Visual use it would certainly need a 2” star diagonal costing ~ Ł100 and a finder scope ~ Ł80 making the costs comparable. For many nights observing, a dewshield would be needed as well. In addition the Newise comes with two very acceptable eyepieces that are together worth about Ł100. Not bad value for money in my judgement.

Conclusion

As I hope the observations and images have shown, the telescope put up a very creditable performance indeed. I have telescopes that can provide very high quality planetary imaging - but do not have a wide flat field. I also have an 8” scope with an even wider field. But this is not as flat or well corrected as the Newise and the scope does not perform quite as well on planets. In contrast, the Newise is able to carry out a very wide range of observations in a highly competent way and would be an ideal telescope to take out and about to reach darker skies.

I had expected the Newise to perform well - it has had two excellent reviews in the British astronomical press - and I was not disappointed, but what endeared me most to the Newise was the ease and convenience of its use. I liked the Newtonian focus that can be positioned easily for comfortable viewing (and which enable a camara to be easily mounted ). I liked the fact that it was not too heavy and, due to its short length, easy to handle. I liked the Crayford type focuser, the right angle finder and the fact that one could collimate it whilst viewing the effects of any adjustment in the eyepiece. I liked the way the custom dew-shield was locked into position. To be honest, there wasn’t really anything that I didn’t like!

I think that this about says it all. A telescope that is easy to use is one that will be used often!

Reviewer's statement

The reviewer has no formal links with Cape Newise or its distributers but was loaned this telesope in order to evaluate it and write this review. The reviewer is Telescope Adviser for the UK Society for Popular Astronomy and co-author of two astronomy books for the Amateur. He owns and uses the following telescopes:

80mm Astronomica ED-80 Refractor

102mm Takahashi Apochromatic Refractor.

127mm Alter 500 Maksutov-Cassegrain.

150mm Helios Achromatic refractor

150mm Meade Schmidt-Newtonian

150mm Intes Micro MN-66 Maksutov-Newtonian

200mm Orion Optics, UK, Newtonian Reflector

235mm Celestron Schmidt-Cassegrain.