Televue Delos Eyepiece Review

History of the Delos Line
Tele Vue launched the Delos range in the Spring of 2011 with initially just two focal lengths, the 6mm and 10mm though their creation began over two years early in the Autumn of 2008.  At the time, the chief designer of the 100 degree apparent field Ethos range, Paul Dellechiaie, began investigating a new eyepiece design in the 70 degree range which would retain the key performance characteristics of the Ethos range (read my review of the 21mm Ethos eyepiece here).

Seen by many as the spiritual successor to the 60 degree apparent field Radian series which has been phased out over the last few years, the new Delos range launched with an apparent field of view of 72^o and a generous 20mm of eye-relief.  Named by Al Nagler, Delos is a Greek Island which is the mythical birthplace of Apollo, god of light and also gave recognition to Paul Dellechiaie, principal designer of both the Ethos and Delos range of eyepieces.

While launching with just two eyepieces, Tele Vue has now filled the short-to-mid focal length range from 3.5mm to 17.3mm, stopping just short of where the Panoptic range begins at 19mm.

Original Purchase
My original purchase of a Delos eyepiece came when I was putting together my airline portable astronomical equipment for a trip to Oman (read that observing report here) in 2012 which included some time in the desert hundreds of kilometres from any settlement of note.  My APM TMB 105 triplet refractor (review of the APM TMB 105 here) has a focal length of 650mm and I was looking for an eyepiece to fill the gap between the low power 3.4^o field of the 35mm Panoptic and the high magnification, narrower fields of Tele Vue 3-6mm Nagler Zoom.  Forgetting that I already owned a 10mm Tele Vue Radian which was buried in an eyepiece case (perhaps indicating I already own too many eyepieces!) I selected the recently released 10mm Delos which would produce a 65x magnification and a 1.1^o field allowing me take in the full extent of some the larger DSOs.  Forgetting that I owned that 10mm Radian, a very good eyepiece in its own right, might be one of the best astronomical mistakes I have ever made.

Initial impression
The first impression of the 10mm Delos was very favourable, as it usually is with all Tele Vue eyepieces but what really hits you is how tall and thin they are, especially in the shorter focal lengths.  It is a trait that reveals their heritage as decedents of the Ethos eyepiece line which are also tall and thin.  They certainly have a solid feel about them and for a 1.25” eyepiece they are quite heavy, with the 10mm Delos weighing almost double that of the 10mm Radian and standing approximately 4cm taller.  I should say though that the weight is not so extreme that I have ever experienced any balance issues that can be the bane of switching to and from heavyweight 2” eyepieces.

The build quality is excellent, with all the usual features one would expect to find on an eyepiece in this price range.  There is a bevelled undercut on the chrome barrel as a safety feature to prevent the eyepiece from slipping out of the focuser as well as the Tele Vue rubberised grip allowing a firm hold while wearing gloves.  Each barrel is threaded to accept 1.25” filters.  Like most Tele Vue eyepieces, the Delos uses the standard, soft fold down eye-guard.  However, perhaps the most interesting feature is the new, continuously adjustable height eye-guard that can be locked into any position.  This new addition is clearly an advancement over the older click-stop instadjust eye-guard found on the Radian and Nagler Type-4s, which has a finite series of positions.  All the Delos eyepieces have a fairly large 35mm diameter eye lens and this adjustable eye-guard prevents stray light from reflecting off the first surface as well as any external light from entering the observer’s eye resulting in an improved perception of contrast.  Adjusting the eye-guard is a very simple process.  Take hold of the eyepiece name ring and then twist the rubberised grip of the upper part of the eye-guard anti-clockwise to loosen it; then pull the eye-guard to the required height before simply turning barrel clockwise while holding the name ring to lock the position.  Indicator marks on the eyepiece body means it is very easy to dial in the required setting for your preferred position.

Just like the Ethos range, the coatings look exceptional and have a violet / dark green tint when viewed at an angle, appearing slightly darker than on some of my older eyepieces indicating greater light throughput including my 17mm Nagler Type 4 as shown in the accompanying photograph.

The 20mm of eye-relief across the range, which matches that of the older Radian series, makes the Delos a good choice for glasses wearing astronomers and the eye-guard can also be rotated for users of the astigmatism correcting Dioptrx units.

First Light
First light came in late spring 2012 at a Baker Street Irregular Astronomer meeting in Regents Park in London and one object seemed a perfect fit for the 10mm Delos, the Double Cluster in Perseus.  The 1.1^o field and 65x magnification perfectly frames that deep sky jewel and the view did not disappoint.  Paired with my APM TMB 105 triplet, the cluster was text book jewels on black velvet and exhibited full field sharpness.  I rarely changed eyepieces that first night, even when viewing Saturn.  That spellbinding planet, which I never tire of observing, presented a beautiful small disc and ring of pure colour with hints of detail not usually seen at such modest magnification.

Over the next few months I found myself often reaching for the 10mm Delos and after some of the most memorable stargazing of my life in the Omani desert using it, I decided to add a couple more Delos to fill in some of the focal length gaps in my eyepiece collection, purchasing the 4.5mm and 14mm.  After using all three eyepieces for almost two years, I have found each to perform identically which is why a review of several from the range can be written.

Continued Use
Quite simply, the planetary performance matches the very best dedicated planetary eyepieces I have ever used such as the TMB Super Monocentric.  Using the 4.5mm in my APM LZOS 180mm Triplet Apo reveals staggering levels of detail and low contrast surface features on all the planets.  At 280x, detail on Jupiter that does not reveal itself in other 4-5mm eyepieces are readily apparent in the Delos.  Mars, a planet which teases with low contrast surface features has presented the best views I have ever had of the red planet during the most recent 2014 opposition.  I only wish I had the APM 180mm and the 4.5mm Delos during the Martian perhelic-opposition of 2003.

When viewing the planets or the Moon, the colour rendition is very natural, clear and bright, possibly a touch more pure than even the Ethos which are renowned for their colour neutral images, and certainly more so than the typically warmer view seen Tele Vue Nagler eyepieces.  Having compared a 10mm Delos side by side with a 10mm Ethos, the difference was subtle, but the image seen in the Delos appeared slightly more neutral than the Ethos.  In that comparison, using 16” f/4.5 Dobsonian, the consensus opinion was that while the performance of both were truly excellent, the sharpness and overall ability to detect the lowest of low contrast features on the surface of planet was just edged by the Delos.

If you have ever allowed a planet to drift towards the field stop of an eyepiece, you may have noticed that the planetary disc stretches into an ellipse as it approaches the edge of the field of view.  Not so with the Delos.  These eyepieces have the best f-theta distortion mapping, known as angular magnification distortion, of any eyepiece I have ever seen.  The planetary disc retains its true shape even as it crosses the field stop.  This means the full 72^o apparent field of view is useable in the Delos, which is especially useful for those of us who use un-driven scopes.

With virtually perfect f-theta distortion mapping, rectilinear distortion (f-tan theta distortion) might be a problem as it is impossible to correct for both at the same time.  Rectilinear distortion is an off-axis aberration and is the result of change of image scale as a function of field height, distorting shapes.  Straight lines will appear curved away from the centre of the field.  Positive shifts will produce an effect known as pincushion distortion and negative shifts result in barrel distortion.  When panning the telescope round, I detected a very slight pincushion distortion with stars travelling in shallow arcs but the impact was minimal and far lower than seen in some other eyepiece designs.  Truth be told, it has never really been a distortion that troubles me.  Daytime use of the Delos EPs showed straight lines having slight curvature towards the field edge, but not encountering geometric shapes when used as an astronomical eyepiece, this is not really a problem.

Some eyepieces, such as the older Radian are known to show some lateral chromatic aberration.  This radial shift of image points is the result of changing magnification with wavelength and appears as asymmetric colour flaring that causes stars to stretch into little rainbows and planets to show some chromatic aberration.  It is an off-axis aberration that increases with distance from zero at the centre of the eyepiece to the edge.  I have seen no evidence of this while using the Delos.  The only colour fringing I have seen was on Jupiter and Venus when each planet was already 2/3 of the way across the field stop when the trailing edge showed a slight blue/purple arc.  This lateral colour performance further reinforces my opinion that the full 72^o field is useable.

When viewing the Moon during its various phases, I have not detected any ghosting or internal reflections, demonstrating that internal baffling and blackened lens edges are doing an excellent job.  The suppression of stray light is so good in fact that the Moon can be just outside the field stop and the sky is very dark on the opposite side of the eyepiece such that low brightness stars are visible.

Without wishing to sound like I am heaping praise upon the Delos range, there is also an indescribable quality which makes them a very easy and relaxing eyepiece to use.  Some eyepieces can be a little fatiguing with their limited eye-relief and small eye lens, and this can mean that after a while it becomes increasingly difficult to see all the detail that is being shown.  However, the Delos eyepieces are just effortless to use.  I am not sure exactly what gives that feeling, but perhaps the long eye-relief, large eye lens and lack image distortion all work together to contribute to a very easy viewing experience.  This is very important, especially for planetary viewing where patience is key, waiting for that brief moment of tranquillity when the planet suddenly presents a wealth of detail.

While I have written extensively about the exceptional planetary performance of the Delos eyepieces, they are certainly no slouches in the deep sky stakes either.  The 72 degree apparent field is expansive and all the inherent qualities that make them such good planetary performers translate very well to viewing DSOs also.  The stunningly flat field and lack of optical aberrations really allow clusters to shine and galaxies and nebulae are resplendent.  Unless I lack the required field and magnification for a particular DSO among my Delos EPs, it is rare I use any other eyepiece for deep sky observing anymore.

If I have one criticism of the Delos range it is that eye placement is key.  Some might be inclined to misdiagnose the effect as spherical aberration of the exit pupil which can result in the phenomena known as “Kidney Beaning” but here it is because there is 20mm eye-relief which can result in an observer placing their eye too close to the eye lens.  By doing this, it is not possible to take in the entire exit pupil in one go resulting in some dark areas appearing in the field of view.  However, this problem is easily remedied by utilising the adjustable eye-guard which provides a natural guide for placement of the eye.  Once I have dialled in the correct eye-guard position, the problem goes away.

Conclusion
During extended use, in a variety of scopes, I have been amazed by the unparalleled on-axis and off-axis performance which allows the full 72^o apparent field to be used.  I have become very confident that if there is a detail that the sky conditions and scope will show, the Delos is the eyepiece above all others that will reveal it to me.  After two years of ownership, the Tele Vue Delos have become my favourite eyepieces and are always the first I reach for during any observing session.

Conventional wisdom suggests in the search for planetary observing perfection, the fewer lenses and air-to-glass surfaces the better but the Delos range is sharper, has superior contrast and reveals more fine detail than any other eyepiece I have ever used.  They also lack the uncomfortable characteristics of tight eye-relief and narrow fields-of-view typically seen with “dedicated planetary” eyepieces like Orthoscopics or Super Monocentric designs.  With the Delos, I really feel that they provide the best of both of worlds, dedicated planetary-type performance and fields of view that do not leave me feeling claustrophobic, without making any sacrifice to either characteristic.

There is also an intangible quality about the Delos eyepieces that makes them a joy to use.  I always feel very relaxed when using them, the eye-relief means the observer is not pressing their eye to the eye lens, and in multi-hour observation sessions, where fatigue can rob you of the ability to see all the fine detail, this quality cannot be underestimated in its importance.

Optical perfection does not come cheap, but at around £250 per eyepiece, they are more reasonably priced they many high end, wider-field designs, and unless you must have ultra-wide apparent fields, for me the Delos surpasses all others and are recommended without reservation.  The God of Light was born on the Island of Delos, and now the Delos reveals the light with more purity than any eyepiece that has come before it.