Review – Explore Scientific 127 ED Air-Spaced Triplet Apochromatic Refractor

Explore Scientific 127 ED

Photo by Brian Hancock of Memphis Astronomical Society

August 2013 — It’s that time of year again for me to start thinking about what I want to purchase for my annual “Astro Splurge”. The previous year, 2012, I acquired a large Explore Scientific AR152 6″ f/6.5 doublet achromatic refractor and a CG-5 mount. You can read all about that here. If you read that article and the following comments, you know I really like that big achromat.

Orion Atlas Pro AZ/EQ

So during my musings on the 2013 “Astro Splurge”, I had completely convinced myself I was going to purchase a new mount for the AR152. The mount I picked out was the newly released Orion Atlas Pro AZ/EQ. The mount sounded great in theory, but after a little more thinking, and being honest with myself, I came to the realization that once the novelty wore off, I probably wouldn’t use that big heavy mount very much. Sometimes, especially on weeknights, I can barely get motivated enough to carry out the CG-5. At this point I decided to go an entirely different route and started looking into 5″ class ED apochromatic refractors.

I kept hearing about how much better the apochromats (apos) perform than their achromat (achro) bretheren (it’s a never ending debate over on the Cloudy Nights refractor forum), so I thought I’d see what all the hub-bub was about first hand. The two main contenders came down to the Sky-Watcher Pro 120 ED and the Explore Scientific 127 ED Triplet (ED127). The Sky-Watcher is a 120mm/4.7″ f/7.5 doublet with one lens element being crown premium Schott Glass and the other lens element being FPL-53 O’hara extra-low dispersion (ED) glass. The Explore Scientific is a 127mm/5″ f/7.5 triplet with the center ED lens element being the slightly less desireable FPL-51 Hoya glass. The Sky-Watcher Pro 120ED is known to be a great performer with wonderful color correction, especially for a doublet, and comes with some nice accessories such as a 2″ diagonal, 9×50 RACI finder, and a metal case. However, I eventually decided on the Explore Scientific scope because it’s a triplet instead of a doublet, it has 7mm more aperture, Explore Scientific has an amazing lifetime warranty and great customer service.

In October I finally ordered the Explorer Scientific ED127 Essential Edition along with a new Celestron AVX mount (the successor to the venerable CG-5). We all know, of course, that a new scope deserves a new mount. Right? The “Essential Edition” of the ES ED127 is a no frills package — it comes with a white aluminum tube with sliding dew shield, cradle with carry handle, Vixen style dovtail bar, dual speed rack and pinion focuser, and two extensions for the focuser tube. It does not come with a finder scope, diagonal, or carry case like the carbon fiber version of the scope.

I received the scope and mount from AgenaAstro just a few days after placing my order. The scope arrived in a single box dropped off at the front door by the brown truck guy. When I first picked up the box I immediately heard something rattling around inside. Not a good sign! Luckily, it only turned out to be one of the thumb screws used to adjust the focuser tension. One thing I noticed immediately when lifting the scope out of its box is how front heavy the scope is compared to a doublet. That extra lens makes a big difference in the weight up front, so care must be taken when lifting the scope to prevent the nose from taking a dive and crashing into something. After unboxing the telescope, the first thing I tried was the focuser, a nice looking rack and pinion dual speed job. The focuser racked out easily enough; however, when I tried to rack it back in, it would hang in spots and the smaller fine focusing knob would spin but wouldn’t move the focuer tube in or out. My AR152 also arrived with a bad focuser, so this made for 2 bad focusers in a row from Explore Scientific! I called up Explore Scientific and explained the issue.  They put me through to Lance who immediately knew what the problem was and walked me through adjusting some set screws on the focuser housing which solved the issue. After the fix, Explore Scientific offered to replace the entire scope if I still wasn’t happy with it. They really stand behind their products.

Overall, the scope seems pretty solid mechanically (once I got the focuser issue worked out). The focuser is a rotatable 2″ dual speed rack and pinion focuser  with 2 thumb screws for controlling the tension and one to lock the focuser in place. There is also a thumb screw to lock and unlock the rotating mechanism. The focuser isn’t great, but it’s not junk either. The scope came with two focuser draw tube extensions to get more in or out focus when needed. I have to have at least one of the draw tube extensions attached while observing or my eyepeices will not come to focus. One of the great things about having the draw tube extenders is that you can remove both of them and have enough in-focus to use a binoviewer.

The ES ED127 also has a sliding dew shield which is nice. It makes the scope much easier to transport and store with the dew shield retracted.

Dew Shield Retracted

127 ED Dew Shield Extended

Dew Shield Extended

Like the AR152, the scope has a nice solid cradle with a carrying handle on top making it really easy to lift the scope for mounting.

The Celestron Advanced VX mount has proven to be a very good match for the ED127 for both visual and planetary imaging. Only one 11 lb. counterweight is required to balance the scope in RA. The mount also carries the big AR152 better than the CG-5 even though the CG-5 is rated at a higher capacity. Everything about the AVX is more solid than the CG-5. A very impressive mount.

I have read a few complaints on the Cloudy Nights board about people receiving their ED127 and discovering an inordinate amount of dust and debris between the lens elements. Luckily I didn’t have that issue with mine. There was barely any dust on or between the lens elements. You can always expect a little dust on the lens with any new refractor, but the ED127 I received was unusually clean.

ED127 Lens Immediately After Unboxing

Optical Performance

Okay — enough of the mechanical stuff. Let’s get to where the rubber meets the road — optical performance!

First I’ll address what’s on everyone’s mind — chromatic aberration (CA).

As a quick review — chromatic abberation is an optical defect caused by different wavelengths of light not coming to focus at the same point. This happens with lenses because different wavelengths of light travel through glass at different speeds. This optical aberration manifests itself as a colorful halo around bright objects as well as reduced contrast caused by light not being concentrated at one focal point. Apochromatic lenses aim to dramatically or entirely eliminate this defect through the use of special extra-low dispersion (ED) glass. More economical apochromatic refracting telescopes such as the ED127 do a very good job elimating CA, but usually aren’t as good as the more premium branded telescopes. And it can vary from sample to sample with the cheaper scopes since the QC on them isn’t near as stringent.

The Explore Scientific ED127, or at least my sample, does indeed show a slight amount of CA on the brightest objects such as Venus, Jupiter, Sirius, and the limb of the Moon. If atmospheric conditions are good, it’s very difficult to detect the CA, but if the conditions are poor or the telescope isn’t thermally stable, it’s quite obvious. With that said, the CA is nothing compared to a standard doublet achromat.

I’m by no means an experienced star tester, but the ED127 star test looked very good. The collimation was spot on and the diffraction rings were, for the most part, the same on both sides of focus. My AR152 shows a fair amount of overcorrection when star tested, but I couldn’t detect much, if any, undercorrection or overcorrection in the ED127. Of course, Explore Scientific supposedly guarantees that all of their apochromatic refractors test at .25PV or better. I’m sure a more experienced star tester could find something I’m missing, but overall, the star test looks really good. A lot of other owners of the ES ED127 have reported similar results which tells me these scopes have fairly consistent optics.

Moon and Planets

I mainly got this telescope to use for lunar and planetary observation and imaging and it does not disappoint. The scope turned out to be a good deep sky performer too, but more on that later. The clarity the scope delivers is amazing! All it took was pointing it at the Moon once to make me a believer. Amazing contrast and no more purple crater shadows! Even at 1″ less aperture, the ED127 seems to resolve small details better than my 6″ achromat. Compared to the 6″ achro, finding perfect focus is much easier with the ED127 — sometimes I have to fiddle with the focuser a bit on the big achro to find perfect focus, but the ED127 snaps right into focus and there is no doubt when you are at best focus.

I’m still a newbie when it comes to planetary imaging, but I’ve managed to get some pretty good results (for a newbie) with the ED127 and a Celestron NexImage 5 camera. Pictures speak louder than words, so here’s a few of the images I’ve taken with the ED127.

ES 127 ED Jupiter

Jupiter at f/15

ES 127 ED Jupiter Double Shadow Transit

Jupiter Double Shadow Transit at f/15

ES 127 ED Jupiter

Jupiter at f/15

ES 127 ED Moon

Moon at f/7.5 2×2 binning

ES 127 ED Moon

Moon f/7.5 2×2 binning

ES 127 ED Moon Plato Crater

Theophilus, Cyrillus, and Catharina f/7.5

ES 127 ED Moon Plato Crater

Close Up of Plato Crater and Alpine Valley

ES 127 ED Saturn

Saturn at F/15

Deep Sky

A 5″ scope is typically not the scope you would turn to for deep sky observing; however, the ES ED127 has proven to be no slouch for a 5″ scope. I will throw out one caveat here — no matter how good the optics, this is still just a 5″ scope. If you like to chase down the faint fuzzies, you’d be better served by a larger scope — I would say this scope is on par with a 6″ reflecting type scope when it comes to the fuzzies.

Last autumn I set up the ED127 side by side with the AR152 for a deep sky shoot out and the results were not what I expected. I expected the big 6″ achro to provicde slightly better views on every object, but the reverse turned out to be true. Maybe one of these days I’ll post a detailed object by object recap of the shoot out, but that’s for another day.

ES 127 ED AR152

ED127 (foreground) and AR152 set up for a deep sky shoot out (Don’t worry, those blinds were closed before the shoot out.)

Like I said, the ED127 proved to be a little bit better than the AR152 on every object, except for diffuse objects which were about equal. Of particular note are objects that are stellar in nature such as open and globular star clusters. Stars focused tighter and had a lot more “punch” than in the achromat. A fringe killer filter does help tighten things up a bit in the achro, but it still doesn’t match the clarity of the apo. I feel like the apo is more capable of reaching its theoretical limiting magnitude than the big achro. I was quite surprised to see M13, the Great Globular Cluster in Hercules, slightly better resolved in the ED127. Common wisdom dictates that aperture always wins on globular star clusters.

I also compared a couple of galaxies, M31, The Andromeda Galaxy, and M33, The Triangulum Galaxy. I really didn’t see much difference between the apo and the achro when observing galaxies. M33 seemed a little bit easier to pick out in the apo because of the improved contrast, but the difference was very slight. M110, a darf satellite galaxy of Andromeda, was slightly brighter in the AR152 than the ED127. M110 fit in the same field of view as M31, in both scopes using a 38mm Q70 eyepiece.


I must say that I am very happy with the Explore Scientific ED127 and there is indeed some truth to the hype around ED and apochromatic refractors. Does the difference in performance justify the huge increase in price from an achro? It’s really hard to say, but for me, I would say yes. I think I’ve discovered my personal perfect telescope in the 5″ apo. For me it’s a perfect balance of portability, ease of maintenance, and performance. Have I stopped using my 6″ achromat? Nope. I find the AR152 to be the perfect outreach scope. I’ve also been using the AR152 during the spring to avoid pollen on the lens of the more expensive apo. Also, if you check out my review of the AR152, I’ve taken some pretty decent images with it. The image of Tycho in the blog header was taken with the AR152. Bottom line, if you don’t want to drop the coin for one of the premium apo refractors, but you want a little taste of that apo performance, the budget ED apos like the Explore Scientific ED127 or the Sky-watcher Pro 120 ED are a great choice. Clear skies!

This entry was posted in Telescope Reviews and tagged , , , , , , , , , , . Bookmark the permalink.

14 Responses to Review – Explore Scientific 127 ED Air-Spaced Triplet Apochromatic Refractor

  1. M42 says:

    Excellent review of the 127 ED and superb photos! I agree 100% with you about the more affordable APO’s. Personally, I would love to have a TEC 140, but given the cost I’m happy with the ES 127ED or Skywatcher 120 APO, both of which provide clean sharp views.

    • Michael Wood says:

      Thanks! Like you, I would also love to have a TEC 140, but the cost benefit analysis fails here for me. The 127 ED can be justified, but the premium options are in a completely different league. Maybe some day. 🙂

  2. Mark says:

    I just wanted to say great review, I enjoyed reading it but I do have a question for you.
    I own 2 of the doublets refractors the ES152mm and the ES102mm.
    I have considered trading them both back to ES for the 127ED, because of all the talk of how there is such a big difference in CA with the lenses.
    My 102 does show allot of CA when pointed at brighter targets, but not so much with the 152mm it actually doesn’t show much if any unless you pick a very bright star like Vega.
    Even then its just a very small amount, I think I got a very good example with my152.
    In your opinion would it be worth the extra money to trade these back for the 127ED.
    I’m looking at about 1000.00 difference in price, is the performance of the 127ED over the 152mm worth the jump?
    Just curious = )


    • Michael Wood says:

      Thanks for reading Mark! With the 127ED you will get a little more contrast and stars will focus down tighter since more energy is being focused in the Airy disk. Whereas with the doublet, some of the energy is being lost to unfocused wavelengths of light. With that said, with the exception of planets and double stars with strong color contrasts, the 152 is just as good as the 127ED. You will be hard pressed to see any difference between the 152 and the 127ED on 90% of the deep sky objects you look at. If you have a good sample of the 152, mine has quite a bit of visible color, and you are happy with it, I would stick with the 152 and 102 combo. The only reason I would consider trading those two in for the 127ED is if I wanted to get into astrophotography. Truth be told, my 152 still gets just as much use as the 127ED. The only time I really use the 127ED these days is if I’m imaging. It’s also fun to take the 152 to outreach events because it always impresses people and everyone wants to look through it. So to sum up, yes, the 127ED does offer up a “prettier” view; however, if you’re happy with the 152 and you’re not planning on imaging, I’d stick with the 152. BTW, if you’re spending a lot of time in the refractor forum on cloudynights, give it a break for a couple of weeks and you’ll find you’re much happier just using the gear you have. The forum over there always makes me feel like my gear is all just garbage, but when I stop reading the forum and just enjoy what I have, I don’t even think about it.

      • Mark says:

        Hi Michael,

        Just wanted to say thank you for the reply, that makes allot of sense.
        We go to the forums for information, and it can definitely get to you after a while and make you doubt yourself.

        I am very happy with the AR152, and I bought the AR102 thinking it would be a great scope to eventually get into wide field AP with.
        But like I said there is CA on the 102 on bright objects for sure.

        With a trade in I was looking to split the difference and cost to get the 127ED seamed like enough aperture to make me feel like I didn’t lose to much in the change from the AR152.

        Maybe it would make more sense to just upgrade the AR102 to the 102ED and the benefit to that is less weight and an easier place to practice and learn AP.

        Either way I go the cost is about the same, but if I did the later I would get to keep a very nice 152mm scope.

        Thanks again for the great advice

      • Michael Wood says:

        That’s actually a hard choice. If it was me personally laying out the cash, and I was definitely going to do one or the other versus just keeping what I had, I would trade in the 152 and 102 achros for the 127ED. While a short focal length 102mm apo is nice to have in one’s lineup, the 127ED will be much more versatile and will not leave you wanting. You will in general not notice the aperture difference in the 127ED and the 152. So again, if I was determined to upgrade instead of just keep what I had, I would go with the 127ED. I will throw a couple of caveats though — there are times when I wish the 127 had a wider field of view like the shorter focal length 102, but overall I think the 127 is more versatile. Also, the 127ED takes just as much effort to set up as the 152 because of the mount requirements, so if you want a grab and go, the 102ED is probably the right choice.

    • Johnny says:

      Hi Mark,
      I bought a AR152 about a year ago and mine like yours shows very little CA. I love it and the deep space views are excellent. I just bought a ES102APO before Christmas during the ES sale, couldn’t pass it up for their sale price! It is just excellent, no CA at all on any objects that I’ve viewed and very sharp views. If our weather ever clears up on my days off I want to get them side by side when the moons up and see if there is really any difference between them other than the slight CA of the AR152.

      • Michael Wood says:

        Thanks for the comment! Glad to hear you’re enjoying your scopes. I’d be interested in hearing what you think if you do that comparison.

  3. Mark says:

    Hi Michael,
    I hate to bother you again but out of curiosity, how do you feel about the weight of the scope on the AVX when imaging?
    Has it caused you any problems being it two thirds the mount capacity?
    After getting your feed back about the 127ED I did decide to send them back for a credit, and purchase a triplet.
    My concern is that for imaging there seems to be a 50% load rule for mounts, so that in itself has caused me to wonder if I am doing the right thing.
    I have the AVX mount, I have used it Visually for well over a year now with a C9.25, the AR152 and the AR102, for visual it has always been great no problems.
    Now I am at the point where I want to try AP I purchased 2 ZWO cameras ASI120mm’s and their guide scope.
    So the concern for me is side effects, the 127ED weighs 22 pounds plus finder and cameras might make it 25 total, the mount is rated for 30lbs.
    So this is way past the 50% recommended load, so since you have the experience with this scope/mount how do you feel about it at this weight for AP?
    I have no doubt its fine for visual, but I haven’t pulled the trigger yet on the scope I am still tossing around the idea of the 102ED just because of the weight.
    And of course you know I want to have something that satisfies both Visual and AP lol
    Maybe someday I will go to a CGEM and I would really hate to spend the money twice on the scope in the long run.
    So my concern is not so much the scope as it is the AVX handling the scope for AP.

    Thanks again

    • Michael Wood says:

      I haven’t had any problems imaging on the AVX, but I’m only doing solar system imaging, so I’m not adding a lot of weight to the system. I think you’ll be fine though until you decide to upgrade later. The AVX is a pretty solid platform — I know of a guy that images with a DSLR and an 8″ newt on an AVX.

  4. IP Frealey says:

    Have you checked collimation on the achro? I wouldn’t think a well collimated 6″ achro should be beat that much by a 5″ apo. But then again, what do I know. I’m not an optics person. Appreciate the review and look forward to more from you regarding these two scopes.

    • Michael Wood says:

      I have indeed checked the collimation on the 6″ and it seems to be spot on. Honestly, on deep sky objects it’s pretty much a wash between the 2 scopes under light polluted skies. I’ve never done a direct comparison under dark skies, so the 6″ my actually do a little better under those conditions.

  5. IP Frealey says:

    Thanks for the reply. I would be interested in seeing a shootout between these two scopes under dark skies.

    I’m on the fence right now between an APM 152 ED or getting one of these AR152’s and having the lens respaced. At times I’m even drawn to getting a big SCT. Anyway, thanks again for replying and taking time to do these reviews.

  6. Reginaldo da Silva says:

    If you still have Explore Scientific’s two achromatic and apochromatic refracting telescopes, you could make comparisons between them. As cited in the last post of your blog.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s