Telescopes

A Comparative Review of TeleVue Nagler, Delite and Delos short Focal-Length Eyepieces

Review by Steve Miles, in association with ‘365Astronomy’

18/01/2019

Preamble

Having owned and used most of the popular amateur telescope types, in varying f-ratios and apertures, I have ended up following a now well-trodden path back to using refractors (though not to the exclusion of the others, all of which of course have their uses). This is perhaps mainly driven by the recent availability of reasonably priced, good quality apochromatic refractors. CA has always been the Achilles heel of the achromat; using filters, or unwieldy long-focus telescopes to remove the effect of CA has its place, but having observed with inherently apochromatic reflectors (in all their different guises) left me wanting a refractor without CA. Looking at the available designs and prices I eventually opted for a TS Optics, 130mm, F7 triplet – a sort of ‘Goldilocks’ size (not too small, not too large…)

Checking through my eyepiece selection I noted that while I had several different types and grades of eyepiece ranging from 4mm to 42mm, I had little of any quality in the higher power sizes, and those that I did have were mainly standard Plossls. Having spent a fair sum on the OTA it seemed pointless not to furnish it with some good quality eyepieces.

While most of my existing collection would be fine for use with the new telescope for medium and wide field observing, I really wanted something for planets, moon and smaller DSOs. Working out the magnification range that I wanted suggested that I would need eyepieces in the range 3.5mm to 6mm. In company with many other observers I have found Plossls (or Super Plossls) to be good general-purpose eyepieces which, because of their relatively simple design, with only four or five lenses, usually have good light transmission, reasonable AFOV and flat field. To be sure, I could well have decided to follow this route, being that a smaller AFOV is fine for smaller objects, but, having used my Delite (62 Deg) and a Vixen 2 inch (65 Deg), I really wanted to be able to see these smaller objects in a wider-field context; planets alone are great to observe, but planets against a starry background provides better visual context.

Introduction

It is not my intention to repeat all the technical material regarding these eyepieces, it is all available from the manufacturer’s website. I did however, do some simple but revealing tests using optical test cards to confirm the basics (Appendix 2). If a supplier like TeleVue says it’s 82mm AFOV, that’s fine by me. Likewise, I expect the features, construction and finish to be as described. Otherwise, what’s the point in buying off the top shelf? Going through your local, respected dealer helps guarantee this. Researching the TeleVue website suggested that 5mm Delite (62 Deg), Delos (72 De) and Nagler 6 (82 Deg) eyepieces would be a good starting point, and this was confirmed by my local supplier, 365 Astronomy [link]. These three designs were chosen as the basis of this review. A 4mm Delite and a 3.5mm Nagler 6 were also included in order to test the telescope under higher powers.

It is worth mentioning here that the difference between focal lengths of, say, 28 mm and 30mm (2mm) is only some 7% in terms of magnification, the difference between a 3.5 mm and a 5.5mm (again, 2mm) is much greater at around 57%. Selection of the best focal length is therefore much more critical in the shorter range than in the longer. For example, with the telescope focal length of 910mm, a 3.5mm eyepiece gives a magnification of x260, while a 5.5mm eyepiece gives x165.

So here we go…

In the hand

The weight of all three types is reassuring, though not so heavy as to cause balance problems in most telescopes; the Delos being the heaviest of the three.  Indeed, at the focus end of a 5-inch triplet a little extra weight helps balance the tube. All are very well constructed and finished, with modified undercuts on the barrel to help prevent compression rings catching. Barrels are all 1.25 inch. Knurled bodies provide good grip even when wearing gloves. All have rubber eyecups, though the Delite and Delos also have an adjustable, and lockable, collar to help finding the right spot for eye placement, which the Nagler 6 lacks. There are also markings on the body to show the eyecup setting – which is useful for quick adjustment. All TV ep’s carry the trademark green print on the body showing the maker’s name and focal length. All lenses are beautifully coated and spotlessly clean. Looking through both types showed no obvious tint or colouration and no obvious features that might cause light scatter or obstruction.  Instructions for use, lens covers, and technical data are provided in the box; which incidentally feels rather on the cheap side – surely a decent storage case with foam inserts is not unreasonable at the price asked. TeleVue please note!

In the telescope

All three eyepiece types fitted perfectly as you would expect. Some observers seem to prefer a double- tapered barrel, or no undercut at all. My focuser, as is common these days, uses a compression ring, and, as with other eyepieces, I found that the undercut occasionally caught on the edge of the compression ring when trying to remove the eyepiece, despite the modified undercut.

A quick check of focus across all three showed that while TV claims all 1.25” eyepieces are parfocal (i.e. focus remains the same when swapping to a different focal length eyepiece), I found that some minor adjustment was still necessary when switching eyepieces. However, the adjustment was small – a tweak on the 10:1 focuser was all that was usually required.

Daylight Tests

Date & Time: 29/12/2018, 1430-1700hrs

Seeing: Full overcast, light grey which persisted throughout the tests. Tests discontinued when sky darkening was noticed.

Weather: Ground moist after rain, windspeed approx. 2mph, temp 8 Deg C.

Location: Urban area in northern Hove (nr Brighton, Sussex, England)

Viewing distance: 19.5 metres

Equipment: TS Optics Photoline 130mm F7 Triplet on manual equatorial mount, TS 2-inch Quartz Dielectric diagonal, TV Delos 4.5 mm, TV Delite 4mm and 5mm, and TV Nagler type 6; 3.5mm and 5mm. All optics were allowed to cool outside for one hour before the tests.

Optical test cards comprising an Amsler Grid, a contrast scale, resolution and separation charts were set up as far from my garden observing site as possible (about 20m). You can download all these graphics from the internet or make your own – see Appendix 2 for my own printable version. (Please observe Copyright on the Koren Lens Test Chart). Charts were all mounted on a sheet of very black card about A4 size. Each eyepiece was checked in turn for linearity, field flatness, contrast, its ability to separate very fine features, and CA. The tests were repeated over the course of two hours with short spaces between to allow the eye to relax.

Results of daytime tests

Delite 5mm (referenced against a generic 6mm Plossl)

I already own and use an 11mm version of this ep, so knew pretty much what to expect. Immediately obvious over a standard Plossl is the much better FOV, allowing the whole Amsler grid to be viewed, as opposed to just part of it. With the more complex lens design in the Delite I had expected a warmer and possibly slightly dimmer image, but noticed neither of these effects – even after extensive viewing. Images were very sharp, with very good contrast, the Delite came to focus rather easier than the Plossl. I noted very slight pincushion effects on the Amsler grid but only at the extreme edges; this is a tough test though, and at night the effect would probably be quite invisible. There was no CA attributable to the eyepiece and only the barest hint of flaring where the white areas of the charts met the black background (i.e. extreme contrast boundaries). There were no reflections or ghost images. With careful focussing the extreme end of the resolution chart was visible. The slightly unusual eyecup adjustment worked well enough and eye placement was fairly straightforward (though I do prefer the twist-up design found elsewhere, partly as it is much simpler to change the setting for another user while the eyepiece is still in the focuser). Magnification with this set-up was x182.

The overall impression that the eyepiece left me with was transparency – it was crystal clear, full of detail and allowed space for the eye to move around the image; a pleasure to use.

Nagler type 6,   5mm  (& 3.5mm)

Both the 5mm and 3.5mm ep’s gave near identical results in terms of definition, with little difference in brightness between the two, the 5mm having a very slight edge. Both were colour neutral, though the 3.5mm did just hint at being slightly bluer. This may just be that it was beginning to reveal some residual CA being generated in the objective lens – so no weight has been given to this). Having also found that the 3.5mm was somewhat over the limit of usable magnification in the 130mm triplet it was removed from the test.  

Moving on, the 5mm Nagler 6 had neither quite the brightness, contrast, or fine detail of the Delite 5mm. However, while the brightness and contrast differences were noticed after only a few minutes observing, it took quite while before the difference in fine detail was noticed. Little nuances in the test cards were spotted with the Delite; once seen, they became visible in the Nagler 6 – but not the other way around. Magnification was x182.

Given that the key difference in specification for the Nagler 6 was AFOV, attention was also paid to this. At the same magnification as the Delite, the Nagler 6 showed a marked increase in FOV. Whereas the Delite barely reached the four sides of the Amsler chart in its FOV, the Nagler easily encompassed the whole grid to its corners and gave a very satisfying wide view you could explore widely with your eye. However, this extra FOV comes at some cost, as the grid had now taken on a marked pincushion shape, with non-linearity becoming noticeable from as little as 40% out from centre. In the four corners of the grid distortion was marked, with a trace of CA and Coma beginning to appear right at the extremes of the grid. As noted before, this is a very tough test for any eyepiece and it remained for the night-time tests to see whether these effects were going to visible then, and to what extent they would affect the view.

A simple, fold-down, rubber eyecup is supplied, but in use I found it had an edge to it that was a little uncomfortable to delicate skin around the eyes, though once the correct eye-placement had been found this proved to be less of a problem. There is no adjustment to assist placement which is a pity, given that Nagler 6 eyepiece costs nearly half as much again as the Delite.

Overall though, the eyepiece offered compact size, low weight and significantly better performance than more basic ep’s on daytime subjects.

Delos 4.5mm

As with the other ep’s in this test the Delos is supplied in a simple, black, cardboard box, though this time with shaped foam inserts to properly support and protect the product. A label is fitted to the barrel of the eyepiece warning the user not to unscrew the barrel as the lenses will fall out and the warranty become void. Be warned.

The Delos 4.5mm eyepiece is substantially larger than the Nagler 6 or Delite and somewhat heavier too. A little extra care was therefore taken when fitting to ensure that the compression screws would hold securely with the eyepiece in any orientation; expensive eyepieces and concrete do not mix. The finish, as with the others in the test, was excellent. The large (and beautiful) eye lens and adjustable eye-relief feature immediately suggested that the eyepiece would be comfortable to use, though as will be seen in the night time tests, this was not entirely the case. As noted before, focus was not completely parfocal, but quite close and easily found. Magnification was x202.

As with the other ep’s in the test colour rendition was neutral, if with a very slight hint of warmth about it. Image brightness was on a par with the Delite 5mm, despite the greater magnification. The Amsler grid also showed very similar (but small) amounts of pincushion to the Delite, which was unobtrusive – bear in mind the difference in AFOV (62 Deg/72 Deg) and the Delos was the better of the two in this respect. As with the Delite, the resolution chart was easily read, right to the end of the scale and contrast was extremely good. There was little visible flaring across high contrast boundaries, nor were there any reflections or ghosting.  The Delos also had that ability to locate fine detail that would often go unnoticed in lesser designs. While studying the charts I noticed the texture of the black card I had mounted the charts on, and some very small print errors that my printer had produced (tiny streaks etc).

Overall, the Delos 4.5mm left me with the sense of using a quality eyepiece. The images, like the Delite, had a solidity, clarity, and detail that is just not there in basic and mid-range eyepieces. Add to this the better, and still quite linear, FOV and this eyepiece is one I would very much like in my collection.   

Night tests

Date & Time: 017/01/2019. 1730 – 1900hrs

Seeing: Dusk, skies falling dark, Mag 3 naked eye limit. Seeing: steady to good/occasional tremors

Weather: Windspeed approx. 4mph, temp 2 Deg C. Dew forming readily and temperatures falling quickly

Location: Urban area in northern Hove (nr Brighton, Sussex, England)

Equipment: TS Optics Photoline 130mm F7 Triplet on manual equatorial mount, TS 2-inch Quartz Dielectric diagonal, TeleVue Delite 5mm, Nagler 6, 5mm, and a Delos 4.5mm. All optics allowed to cool for one hour outside before use. Dew heater at 10 Deg.

Nagler type 6,  5mm

Taking advantage of the early evening skies, and before the moon swamped the night sky (waxing gibbous) the Nagler 6 was first assessed on the moon. Field of view was close 2 moon widths (ie just under 1 degree). At 180x magnification the sky around the moon darkened to near black, providing excellent contrast. There were no reflections or ghosting even with such a bright image. There was a trace of glare at the moon and sky boundary, though this was very slight. There was no CA evident in the image, right up to the field stop. Images were very slightly on the warm side of neutral and faint colours on the surface of the moon could be just made out. Focus was easy and despite there being no eye adjustment, finding the right place to view the image was straightforward and easily maintained. Detail was very good, not just around the terminator but also on valley and crater floors with subtle shading easily made out. The 82 Deg AFOV allowed the eye to roam around the image comfortably and find detail right at the edges of the view.

The eyepiece was then checked on Rigel and Betelgeuse, both of which showed pin sharp stars when in focus and, as with the other Televue eyepieces in the test, matching discs when defocussed. Colours were exactly as expected and the star images remained almost completely free of distortion even close to the field stop (though not at it). Sweeping the telescope across nearby star fields in the Milky Way the pincushion effect noted during the daytime tests was just evident but much less obvious and did not detract at all from some impressive views. It took some time before I was ready to move on to the next eyepiece in the Review.

Delite 5mm

The Delite eyepiece with its 62 Deg AFOV gave about 1.6 moon widths FOV (approx. 0.8 Degree)  at a magnification of 180x. The image was a shade brighter than the Nagler 6 – swapping back to the Nagler confirmed this. The same sky darkening effect was also noticed, though any glare coming from the moon was less than that from the Nagler. Despite the brighter image there was no trace of ghosting or reflections, whether with the moon centred in the FOV or off to one side. Images were absolutely crystal clear with excellent detail around the terminator. The limb of the moon came to the sharpest focus I have seen in any eyepiece I have yet used (except perhaps a Vixen lanthanum 42mm). In focus there was no trace at all of CA anywhere in the image. Slight colour shadings were noted on the surface of the moon and the whole effect made the eyepiece disappear – the word ‘clinical’ comes to mind.

As with the Nagler, the Delite was lined up on Rigel and Betelgeuse, both stars showing there colours very clearly. At fine focus these images appeared just the tiniest bit tighter than the Nagler except at centre where the Nagler was very good indeed. Set against the darkening sky these stars were a wonderful sight. Sweeping the same star fields as before, hardly any pincushion distortion was visible – just as in the daylight tests – if you hadn’t seen the effect you would almost certainly never notice it at night.

Delos 4.5mm

The Delos 4.5mm eyepiece is designed to give an AFOV of 72 Degrees. At a magnification of 202x the view was about 1.5 moon widths (around 0.75 Deg). Images of the moon were about the same brightness as the Nagler 6 which is pretty good given the higher magnification. Images were very slightly warmer than the other two on test, and though the difference was very small indeed it had the effect of making the bright lunar surface just a little less glaring and more comfortable to view. Possibly as a consequence of this, glare between the moon’s limb and the dark sky behind was not a problem. Contrast and detail were very good and, as with the Delite, subtle shadings were easy to find and follow. The Delos’ AFOV is greater than the Delite, which as you might expect, allowed the eye more room to move around the image, though this was only achieved by very precise placement of the eye and careful setting of the eye adjuster. With the eye well placed, images were very good indeed, ‘classy’ in fact. However, with the eye misplaced it was easy to lose parts of the image and CA became evident close to the field stop.

Star tests were the same as for the other two eyepieces, colours were beautiful and exactly what was expected (if not more so). Focus was easy to find although not quite with the same snap as with the Delite. That said, you knew when focus was spot on.  Moving on to the same patch of stars as before and sweeping across them some slight edge distortion was noticed, but only right up close to the field stop. Just the slightest hint of coma and colour separation was visible in the last few percent of the FOV. This did nothing to detract from what was a fantastic view of stars spread out across a lovely wide space.

Discussion

At the start of the review and before testing began, I quite expected the Delos to come out top dog (observer bias!) Its size, weight and the beautiful polish on the lenses (which they all had, to be fair) made it stand out as something you would just want to own and have in your collection. Pride of ownership is part of it of course. There is no question that all of these three eyepieces are well worth the investment: the Delite is over £200, and the Delos and Nagler 6, over £300. All three provide a measure of optical excellence that is a real step up from middle range products – i.e. the first eyepieces you buy to upgrade from those that are supplied with a new telescope. However, each of these eyepieces is of a different design, not least of which is the AFOV, which ranges from 62 Degrees up to 82 Degrees, so it should be no surprise they each have their own characteristics.

The Nagler 6, 5mm is a compact and well-engineered eyepiece with sharp, bright and contrasty images. It has a wide FOV and its on-axis performance is very good indeed. If you can live with some slight peculiarities right up close to the field stop then this would make an excellent high-power eyepiece, especially for planetary and small DSO use. Of the three eyepieces tested, this was the only one that may have benefited from the use of a field flattener with this particular telescope. Even without TeleVue’s new sliding eyecup adjustment it was easy to use. I would have preferred to have a twist-up eyecup or at least a softer rubber eyecup though. Overall, I was very impressed with the ‘can-do’ attitude of this eyepiece.

The Delos 6mm is a somewhat larger and heavier eyepiece than the other two. From the moment you open the box it is clearly a lovely product. Mated with a 130mm triplet apo it gave lovely wide views and allowed the eye to relax while observing. Personally, I found eye placement a little difficult, and poor placement led to some odd effects. However, provided you get your eye orthogonal to the eye lens and at the right distance, then you will undoubtedly enjoy this very classy eyepiece. After the review was over it was this eyepiece I went back to, for an hour or so enjoying the night sky.

The Delite 5mm is one of the more recent designs from Televue. Although physically longer than the Nagler 6 it has about the same body width and is therefore still quite compact. It shares the same sliding eye cup adjustment as the Delos: a twist and lock arrangement (though I prefer a twist-up mechanism as it is quicker and can be done with one hand).  The Delite has a 62 Deg AFOV, narrower than the other two but still a good step up from a standard Plossl design (usually around 50 Deg). The upside to this is that the eyepiece optics are not being ‘stretched’ to give a wider FOV and in consequence the image remained almost completely linear across the whole field – right up to the field stop in fact. There was virtually no coma, colour separation or pincushion even on the stringent daytime tests. What sets this eyepiece apart though is its transparency. Images are crystal clear, bright, highly detailed and showed the full performance of the triplet apo it was being tested on. The same level of detail was there over the whole FOV. I used the word ‘clinical’ before and I think this sums up this excellent eyepiece well, you just see what is there without being aware of the eyepiece itself. I would be hard pushed to chose between the very classy Delos (at £300+) and the Delite at around £100 less.

So, should you buy one of these TeleVue eyepieces?

Well, the point is: do you have a telescope that will make good use of them? If you have a beginners’ telescope then probably not. Better to spend the money on upgrading the telescope when you are ready and build up your eyepiece collection later. For telescopes in the mid-range the answer is less clear. To be sure, the TeleVue eyepieces will improve things, but if the telescope is not of matching quality you will not get the best from them.

However, if you have a telescope with really good optics (not necessarily and expensive one) then the situation is quite different, for you will not realise the full potential of the telescope without high quality eyepieces. All three of the eyepieces tested here are certainly that, will complement a decent telescope, and bring many years of observing pleasure.

Data Collection Grid (Night Tests) – Appendix 1

Ratings: 1 = Poor, 2 = Usable, 3 = Fair, 4 = Good, 5 = Very Good, 6 = Excellent

Pattern AFOV Eyepiece Focal length   Edge Distortion (pincushion) Field Curv’ Colouration   Transmission Resolution Contrast CA Comments
  PlÖssl   50 Deg   6mm       4 5 Slight pink   5 5 at centre 5 4 Ideal for public displays, inexpensive. Good value for money
  Delite   62 Deg   5mm 5 6 Neutral   6 6 6 6 Excellent images free of almost any artefact. Very easy to position eye. Eyepiece almost unnoticeable in use.
  Nagler 6   82 Deg   5mm 4 6 Neutral   5 6 at centre, 5 nearer field stop 5 5 Compact, lightweight and solid performer. Quality eyepiece ideal for planetary. Excellent detail in centre of FOV, less so near field stop.
Delos   72 Deg 4.5mm 5 6 Neutral/warm    6 6 5 5 A very classy eyepiece requiring careful eye positioning for best image.

Appendix 2

Images are public domain except for the Koren Lens Test Chart, please observe copyright as indicated. Print in black only.

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Posted on February 2nd, 2019.