I was recently asked to translate my review of the Ethos 13mm and the Explore Scientific 14mm. I found that a machine translation delivered inacceptable results. Instead I translated the text myself but as I am not a native English speaker I found it impossible to translate the text word by word. I decided to re-write the article in English. I did my best to use the correct technical terms and I am sure that I made a number of mistakes. But I think that the article is understandable and easier to read than a machine translation.
14mm Explore Scientific 100°
One hundred degrees apparent field of view is what I noticed first about the new 14mm Explore Scientific eyepiece. Only a few eyepieces offer such a large field. On the european (mainstream-) market of fall 2009 only Televue’s Ethos series featured one hundred degrees. Namely their 13mm Ethos is a direct competitor to the 14mm. Or is Televue’s first Ethos more than just a competitor? In fall 2009 I had the chance to compare them in detail.
Explore Scientific’s website offers an extensive amount of information about their 14mm. According to their site eye relief is 14.5 millimetres. A schematic view shows a 9 lens design with 5 groups. Lanthanum glas and at least 3 other glasses were used. The effective field stop diametre is 24.43mm. Of course the eyepiece offers an FMC grade coating, blackened edges and an internal baffle system. An unusual feature for an astronomical eyepiece is the waterproof built. According to the manufacturer each eyepiece is filled with nitrogen gas and spends some thirty minutes one metre underwater.
The 13mm Ethos however comes with combined 2” and 1.25” barrels. It’s negative element uses as much diametre as possible inside of the 28.5mm filter thread. The carefully blackened lens retainer is screwed into the filter thread and offers some 26 millimetres free diametre for the negative element. Of course Televue offers a lot of information as well. 22.3 millimetres for the effective field stop diametre, 15 millimetres of eye relief and ‘ultra low reflectance, high efficiency coatings tuned to the composition of each element’. What they don’t tell us however is the lens setup. In the past Televue’s eyepiece-comparison table gave us the lens setup for every Televue eyepiece on the market. For their Ethos series they left that field empty and maybe to keep some nosy eyes out of the internals of their newest eyepiece designs they left a small note in the package: ‘Warning: Do not unscrew any sections of the eyepiece, as the lenses may fall out, thereby voiding the warranty.’
Comparing both eyepieces I had the usual look at their physical dimensions. The Explore Scientific weighs an astonishing 877g (31 oz). So the Ethos appears lightweight with 571g (20 oz) but of course that’s still quite a load for most focusers. The Explore Scientific is about 155 millimetres (6.1”) long and 70 millimetres (2.75”) in diametre. With the lens caps on the Ethos reaches 150 millimetres length. The eyepiece could’ve been some 7 millimetres shorter if Televue hadn’t used the standard lensecap from their type 5 Naglers for the eyelens. These lense caps have a two diametre design and fit on the folded rubber eyeguard with one side and on a 2” barrel with the other side. However the combined 2” and 1.25” barrel design of the Ethos doesn’t allow a 2” cap on the bottom side of the eyepiece. The Ethos has a maximum diametre of 63.5 millimetres (2.5”). Important for binocular use.
Regardless of their origin workmanship is great for both eyepieces. I can only talk about small details. The Explore Scientific’s cap for the 2” barrel has a loose fit. The 1.25” filter thread of the Ethos is not blackened but very close to the (blackened) lens retainer and the lens itself. The coatings appear to be good on both eyepieces. The Explore Scientific has a larger rubber grip and handles slightly better than the Ethos with its relatively small rubber grip. Their packaging is very similar. Sturdy cardboard boxes with foam plastic mouldings. Positive detail for the Ethos: It comes with a page of instructions. Negative detail for the Explore Scientific: The eyepiece is not that much larger than the Ethos so it wasn’t at all necessary to put it into a box with almost twice the volume of that of the Ethos. And considering the size of both boxes I ask myself who will cary an eyepiece around in a box like that? Have You ever bought an eyepiece from Pentax? They use TwistPacks with well fitted foam plastic to protect their eyepieces. Meade used them too for their series 4000 eyepieces but without foam plastic. It doesn’t need much space and the eyepiece is well protected. Both eyepieces are expensive enough to expect something more than just two caps for their protection.
Time to look through the eyepieces eventually. First I checked how good internal baffles and blackened edges work. Looking into each eyepiece from a distance and against a bright background offers dark interiors. The developers did a good job on both eyepieces. The inner walls of the Ethos’ look somewhat darker than those of the Explore Scientific. When I turn the Explore Scientific there’s an angle that produces a highlight on a part of the interior. The area around the first lens however is very dark in both eyepieces. Turning them around looking through the negative element reveals highlights in both eyepieces. This view reveals that the Ethos has a somewhat rough or bumpy surface inside and this might be the reason for its darker appearance I mentioned before. I have to mention however that it depends on the internal baffles if the slight disadvantages of the Explore Scientific will lead to visible drawbacks. If internal baffling prevents light from the scope to fall on the inner walls it should be hard or impossible to notice a difference.
After that I was really curious how both eyepieces would perform in practice. I challenged them with a nightmare for eyepieces: f/4. Of course a fast Newtonian will show a visible amount of coma in a field of 100 degrees and pinpoint stars to the edge can not be expected. That’s why I used several coma correctors as well. In detail I used my 200/800 Vixen R200SS with the original Vixen coma corrector, a Paracorr PCV-2000 and a special pre-production example of a planned coma corrector. Very secret ;). In addition to the 8” R200SS I also used my 300/1200 Orion UK Newtonian with the same coma correctors.
Without a coma corrector both eyepieces showed – of course – much coma towards the edge. Again to the same extend in both eyepieces. The Vixen corrector brought a great improvement but stars at the edge remained slightly bloated and showed a little bit of coma. Both the Paracorr and the prototype corrector delivered virtually perfect views up to the edge. I got those pinpoint stars that you phantasised about when You read Your first eyepiece advertisement. I got what I expected from some f/8 fluorite apo from an f/4 Newtonian. Great performance! And whenever I suddenly found some distorted stars at the edge I found that I was just obscuring a part of the exit pupil with my iris causing diffraction to deform the stars. Sorry – my fault! Moving my head a little to compensate for the movement of my eye fixed it immediately. Comparing the two correctors I found the Paracorr to add some 16% to the telescope’s focal length while the prototype corrector added only 6%. As a result the darker background with the Paracorr lead to a slight increase of the limiting magnitude at the cost of some true field of view. Concerning true field of view the difference between the two eyepieces was not very apparent even though switching eyepieces was done much faster than switching correctors. Important however was an adjustment of the distance between the coma corrector and the eyepiece which was necessary for an optimum performance at the edge of the field.
Observing with my Megrez 110 (110/655 doublet ED) again lead to very similar results. Stars close to the edge showed something like some coma oriented towards the center of the field. Visible in the outer 25% of the field the effect was easily tolerable even close to the edge of the field.
Summing up the review there are no major differences between both eyepieces. We are talking about details which is not what You expect compairing a Televue eyepiece with a Chinese one with similar specifications. Instead we found – maybe for the first time – that the Explore Scientific, be it a clone or not, could deliver virtually the same optical performance. The designers of the Explore Scientific did a good job and the producer took good care of workmanship. As a result the similar performance of both eyepieces makes details of the (mechanical) design more important. And considering this we see Televue’s old fashioned and impractical safety undercut and the incomplicated solution used (but not invented) by Explore Scientific. The smaller rubber grip of the Ethos is suddenly a notable difference. Of course Televue can offer important advantages like the dioptrix, the smaller diametre for binocular use and less weight. For a few observers even the possibility to use 1.25” focusers. But these advantages apply only to some buyers.
I don’t know it was just the attraction of the unknown or the consequence of experiences I already made. In the end I found myself in front of a door with an x-ray warning on it and the eyepieces behind that very door. It wasn’t easy to achive good shots. Especially the barrels of both eyepieces proved very opaque to X-ray and the film pieces were a little small for the big eyepieces. The results however solve the mystery and tell the story.
The picture of the Explore Scientific can be exactly matched with the schematic view on the Explore Scientific website. The two X-ray views of the Ethos show the best single shot and several shots with different exposure times combined. The negative element inside of the barrel remains almost completely invisible. But the rest of the setup was perfectly revealed. Televue could just put 9 lenses in 5 groups into their eyepiece specifications table. They don’t have to protect against somebody cloning their design. Because someone already did.