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Explore Scientific 25mm 100 degree eyepiece

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Explore Scientific 25mm 100 degree eyepiece

by Richard Lines



Richard Lines, Worcestershire, England

Back in those halcyon days before the financial crisis I was the proud possessor of a fine selection of Ethos eyepieces. I’d not originally planned on getting them, but on completion of a 14”f6 Newtonian with a 2” focuser, my collection of cheap Revelation plossls was serving only to highlight the deficiencies of taking the bargain route.  The sweet spot in the middle was surrounded by a doughnut of unfocussed mush. I guess it was at least useful as a built-in collimation check. The purchase of a paracorr coma corrector did a little to help, but was by no means the complete answer. The edge of the field with the 2” Plossls was grim to behold, I reckoned I was losing 2 magnitudes at the edge simply because stars weren’t in focus.  So I bit the bullet and shelled out megabucks for my first Ethos, the 13mm.  A big success, and the 17mm and 21mm followed shortly afterwards as when they became available. Don’t get the impression at this point that I’m a Televue addict; I’m not.  Back in the 80’s I bought some of the MK1 Naglers (which I still have), but since then I have eschewed the green-and-black in favour of more modest optical offerings which work fine at longer focal ratios.  The 11mm  Mk1 Nagler I still regard as one of the best eyepieces ever made, but the 7mm and especially the 4.8mm have been easily eclipsed in the intervening years. I also didn’t like the way the range kept changing; Televue seemed to update their products faster than the mobile phone manufacturers. As a point of principle I like to try various manufacturers products,  and a 40mm Pentax XW and a 28mm UWAN were added to bolster the low power/ wide field end. So, for low and medium powers, I had pretty much the dream collection. Many happy hours were spent looking at Makarian’s chain or Stephan’s quintet.  Those Ethoses really did the biz and it was impossible to quibble with the optical quality.

Then several things happened at once. The nation’s bankers lost the plot and belts had to be tightened. To the last hole in my case. I bought a new house with my girlfriend and life began to look distinctly expensive.  Priorities were chosen and sacrifices made; much of the expensive eyepiece collection went up for grabs on Astro B+S to pay the mortgage.  The 14” Newtonian would not fit into the new garden and was anything but portable, so that went as well.  I was left with my old faithful 5” achromat, but aperture fever’s a **** and it wasn’t long before I was at least planning the next mini-Palomar even if I couldn’t afford it. Not as big as before, it would have to be car-portable, but big enough. I had an ancient 12” f4 Orion Optics (UK) tube assembly that had seen very little use over the years because I wasn’t too convinced about the optical quality. I took the optics up to OO’s new factory in Newcastle-under-Lyme and got them to check out the mirror accuracy.  It proved to be dire. The mirror was one of the pre-zygo ones and clocked up a truly miserable Strehl value of 0.52 with a 0.63 lambda pv. There and then I ordered up a 1/10 wave ultra grade replacement on the flexible friend.  As this would take some weeks to arrive, I had chance to get the cell modified to accomodate its new, considerably fatter and more accurate occupant.

So with the refurbished telescope complete I was back to the original situation, with no widefield eyepieces equal to the task. Going from an f6 scope to an f4, the situation was even worse than that which the old 14” had presented. Whatever I picked would have to be very good with off-axis aberrations. I was going to allow myself one low-power eyepiece around the 25mm mark. Anything longer would have made the exit pupil too large. The current cost of an Ethos 21mm comes in at £720 which was more than I’d paid for the new 12” primary mirror, so I started to shop around. Most reviews rated Explore Scientific virtually as good as Televue, and they recently had a new toy out; the 25mm 100 degree eyepiece. This latest offering does not seem to have arrived this side of the Atlantic yet, but it was on offer in the USA for $599 from EyepiecesEtc (plus shipping and import duties).  Don Pensack runs this supplier, and he was very helpful. The deal was duly done, and,   a month later (after George Osborne had helped himself to a piece of the action)  unit number 187 was in my possession.  I awaited the first clear night. 

In the meantime I was left to admire the box, which is a work of art. It is covered with sections of star atlas which looks like it might be borrowed from Wil Tirion’s Sky Atlas 2000. Certainly a refreshing change to the TeleVue funereal black. The box is also easy to open.  Televue take note; your boxes can be notoriously difficult to open in the dark. Inside, the eyepiece comes well packed in a polyethylene foam cutout.



The eyepiece itself is about the same size as an Ethos 21mm and weighs much the same at just over 1 kg. The metalwork is beautifully executed and the coatings look perfect. The field lens virtually fills the inside of the 2” barrel; it is difficult to see how it could be made any bigger. The lettering is milled into the aluminium body so will never wear off.  The rubber parts are all of good quality and the barrel has the usual taper to prevent the owner letting it drop out of the focuser if the holding screw is loose. There were just a couple of minor quibbles:

  • The cover for the eye lens does not fit correctly over the rubber eye guard. The eye guard itself comes away from the eyepiece rather easily. Personally I don’t like them and I quickly learnt to fold it over.

  • Holding the eyepiece up to the light revealed a “ring of fire” around the field stop. I wasn’t too bothered about this since other premium eyepieces show the same effect.  This is just visible in one of the photos.

The next clear night I installed the eyepiece my 70mm F6 ED refractor. The expected magnification was *17 which gives a true field of view approaching 6 degrees. In the UK it never gets truly dark this time of year but I went for a tour down the Milky Way through Cygnus.

The good news; the eye relief is comfortable. Like any other 100 degree eyepiece you have to relax and stop trying to take in the entire field at once. You can let your eye roll round the field without being very conscious of the field edge. There is still not enough eye relief for spectacle wearers to see more than about 50 degrees worth, but this is about par for the course with all widefield eyepieces I have tried.

The bad news; Assessing telescopic equipment usually entails several evenings of careful observation and comparison to make sure you’re seeing the best the kit can offer.  Occasionally however, there have been a few moments in my astronomy career when a ten second look tells you all you need to know. This was one such moment.  In short, I had bought a problem.  Still with the 70mm apo, the centre of the field of view focused up quite well, but the edge showed a lot of field curvature from somewhere.  Around halfway out the image starts to go horribly wrong. The focuser had to be racked in some distance to get best focus at the edge of the FOV. Furthermore, best focus was an ugly blob rather than a nice tight point of light.  Worse, there was a lot of gaudy chromatic aberration towards the edge.  The field stop was almost in focus, but not quite. All things considered, this eyepiece was quickly proving to be a big disappointment.  By this point the clouds were rolling over so I shut up shop for the night, hoping that some of the issues were down to field curvature in the refractor. The following night looked more promising, and I took out the new 12” Newtonian hoping that its flatter focal plane would suit the eyepiece better. I verified the collimation first. The magnification would be *48 and the real field 2 degrees. The secondary mirror is big enough to fully illuminate a 2” diameter circle in the focal plane.

Another issue reared its ugly head at this point. The 12” was rebuilt with the 21mm and 13mm Ethos in mind, and has limited travel on the focuser. The ES eyepiece needs to be racked in quite a long way compared to previous eyepieces I have owned, and it came to focus with about 3mm to spare. This does not leave enough to fit the paracorr, so coma was always going to be very much in evidence. The basic setup at least got rid of the field curvature problem; the same focus now applied all over the field of view. Unfortunately the edge distortion was still unacceptably prominent. The shape was different in that stars were now stretched out into radial lines towards the edge.  Coma was certainly there but that was really the least of the optical distortions. The light is spread out in a spectrum down the line with the blue-violet end innermost . I lined up the instrument on Vega, admittedly a harsh test for CA, but this is supposed to be a premium eyepiece designed to rival the best. The spectral line at the edge extended for 2-3 degrees of the 100 degree FOV with the unfocussed blue light very obvious. The red end is more compact and controlled, but still nowhere near satisfactory. Adding a nebula filter was interesting, the filter response was easily traced out in the residual CA! With fainter stars the CA is less obvious but it never fails to mar the image.

Very fast scopes are always a tough test so I fitted a Revelation ED Barlow lens doubling both the magnification and the focal ratio. This had the additional benefit of offering a direct comparison with my remaining 13mm Ethos. Operating at F8 should be easy for any half-decent eyepiece. The result; the distortions remained virtually unchanged! The contrast with the E13 could hardly have been more stark; the Ethos maintained perfect focus except for a very minor amount of coma at the edge. No trace of CA and a nice snappy focus.

Having paid for this eyepiece with my own money one has to be careful to maintain objectivity in writing this review. Before committing pen to paper I read David Knisley’s comments on fair and unbiased composure. On the other hand, I believe that same fact gives me a special entitlement to tell it exactly as it is, fully conscious of the fact that a bad review will ruin my own chances of selling it on! I have not had the opportunity to test the shorter ES products but I would willingly trust the majority opinion of the many who have; the general consensus is that these products are excellent and are the equal of the equivalent TV product. It would seem that where ES can copy TV, they do a very good job. However this eyepiece is a step into a focal length that TV have deliberately left alone thus there is no prototype.  The result seems to be a very compromised optical design probably rushed out to fill the niche in the market. Every time I use this eyepiece I slightly regret not saving up for a replacement Ethos 21mm. The usual ES mechanical excellence is all there, but that and a fancy box served up with argon purging are no substitute for poor optics. For half the money a new UWAN/ Nirvana 28mm provides better correction across an 82 degree field, or the usual Televue Nagler range comes in at this price bracket secondhand.

Two possible explanations are that the eyepiece is damaged or incorrectly assembled.  It arrived in seemingly perfect condition with no signs of abuse in transit. It’s possible that this one was put together incorrectly but each eyepiece is supposed to be inspected in the USA before release to the customer. It seems to have the correct focal length. Perhaps this one just slipped through.

A websearch reveals very little in the way of reviews but there are a couple of references that mention in a subtle way that this eyepiece might not measure up. ES are currently advertising a 30mm 100 degree eyepiece with a 3” barrel. On my experience with the 25mm it would be a brave man who forks out the huge asking price on this one…

As stated earlier, I’m not a TV addict but it’s impossible not to admire the innovation they create and their ability to get their product right first time. Others can do it, notably  Pentax , Takahashi.

Maybe it’s time to put this one down to experience.  Astronomers local to me in the UK are welcome to try this eyepiece for themselves and make up their own minds. I would appreciate a second opinion myself.

The old cliché remains true; “Buy Televue and you only cry once”.

Richard Lines

Worcestershire, UK

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