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.
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.
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.
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.
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.
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!