Unboxing the Bresser Messier AR102S Comet Edition

January 31, 2017


I’ve been interested in this scope since late 2014. The Celestron TravelScope 70 turned me on to the joys of refractors back in 2012, which led to the C102, which led to the C80ED, which got me firmly hooked on low-power, widefield scanning, which led to this.

This is the Bresser Messier AR102S Comet Edition, which I believe is a record for the longest name of any telescope I’ve owned. And you actually do need all of it, because there is another Bresser Messier AR102S that is a completely different scope. That other AR102S is a standard f/6 achromat. The AR102S Comet Edition is an f/4.5 rich-field scope. And as you can see from the photo, it’s built funny. Instead of having the focuser at the back end of the tube, the focuser is mounted on the side of the tube, as in a reflector, and a reflector-style secondary mirror* bounces the light from the objective lens to the eyepiece. This makes for a very short, compact scope, and theoretically for easy collimation via that secondary mirror (I haven’t tried that yet). Scopes like this are sometimes called “reflactors” because they combine an objective lens with a secondary mirror. I’ve seen ATM builds using this design, but I’ve never seen another one marketed commercially.

* Existential telescope question: is it still a secondary mirror if there’s no primary?


As far as I know, this scope has only ever been sold as part of a travel kit that includes the OTA, an eyepiece, an alt-az mount, 7×50 binoculars, and a backpack to carry it all. That package has a list price of $349, but the list price has been creeping downward. Explore Scientific’s online store and OpticalInstruments.com both carry the AR102S Comet Edition (man does this scope need a nickname) package for $299, but B&H Photo-Video has it for $249 with free shipping. Amazon used to have it for $249 as well, but I seem to have gotten the last of those – as of this writing, the price is hovering in the $340s.

I’ll have a first light report along soon, this one is mostly photos of the unboxing and the scope.


Outer box…


…contains the middle box…


…contains the inner box. That’s right, three boxes before you get to anything other than packing material and the instructions.


Inside box number three are the backpack and two more boxes.


Inside the backpack is the OTA in a plastic bag, and on the right you can see the eyepiece peeking out of the side pocket.


Oh, also in the backpack are the 7×50 binos. Everything bagged.


And inside the bags, the telescope OTA with wrapping paper, the binocular case with the binos in yet another plastic bag inside that, and the eyepiece bolt case with the eyepiece in yet another plastic bag inside that. Oh, and a couple of hex wrenches inside the bag with the bolt case, for collimating the OTA.


And here’s everything finally outside of the various bags, bolts, and cases.


The OTA is 20″ long and 4″ in diameter, with a 4 1/4″ diameter dew shield. In the shots before this one, you can see the dovetail on the right side of the OTA, and here you can see the shoe for a finder (not included) on the left side of the OTA. Having the dovetail on one side and the finder shoe on the other is convenient, because it means the OTA can’t roll over and bang the focuser if you set it down on a flat surface.


Here’s the lens cap. If you’re thinking it looks like a Meade, you’re not wrong.


And way down inside the dewshield, 4 1/4″ in, is the objective lens, with its dark green anti-reflection coatings. The achromatic doublet is fully multicoated. The dewshield has an outside diameter of 4 1/4″, and an inside diameter of 4 1/8″. Past the objective lens you can also see the single baffle inside the OTA, which is otherwise just painted flat black inside.


Remember those other two boxes? The long one has the tripod and the short one has the alt-az head.


The alt-az head, which is metal, and the eyepiece tray, which is plastic.


The mount assembled. The alt-az head looks like my SkyWatcher AZ-4/Orion VersaGo II, but it lacks the adjustable tension knobs.


Here’s a feature that I really like: the eyepiece tray goes solidly onto the spreader bars with no tools. It threads over a central bolt, and then rotates to snap into position. This is super-handy at the end of the night, because I can unlock and rotate the eyepiece tray without taking it off, and fold the tripod legs in just enough to get through the door.


The whole rig set up. The tube looks not quite square here, but that’s just field distortion from the iPhone camera, which we’ve seen before here.

The OTA weighs 6.2 lbs, the mount weighs 6.8, so the whole rig clocks in at 13 lbs even. That’s pretty portable, although certainly at least flirting with being undermounted. More on that in the first light report.


User end. The eyepiece is a Bresser 20mm 70-degree model, which is currently on sale for $40, down from $60, at OpticalInstruments.com. If you’re thinking that $60 seems like not much money for a new fully-multicoated 70-degree eyepiece, I agree, and I am likewise suspicious. I assume it’s some kind of Erfle, but I haven’t taken it apart to confirm. The size, form factor, and even barrel detailing are very similar to Orion Sirius Plossls, but the eye lens is just slightly too big for Sirius dust caps to fit (which is a shame, since it gets in the way of me stuffing this thing in my pocket while I swap Plossls and Expanses around). It gives 23x and a 3-degree true field of view.


Here’s the left side of the back end, showing the finder shoe, the collimation bolts for the secondary, and another look at the focuser. The focuser is an all-metal rack-and-pinion job. Oddly enough, the focuser drawtube is 2″ in diameter but the 1.25″ adapter at the top is permanently mounted. So it’s a 2″ focuser that only accepts 1.25″ eyepieces. I think there’s a reason for this – the focal plane is 6-7 inches (150-175 mm) away from the center of the OTA, which means at least a third of the 459-mm light path occurs after the light hits the secondary. I think a 1.25″ drawtube would cut into the light path and stop down the scope.

The finder shoe is not one I’m familiar with. Almost all of my experience is with gear made by Synta (Orion/Celestron/SkyWatcher), which uses the same mostly-but-not-quite industry standard dovetail shoe for finders. This is a weird square rig that is outside my experience. I probably won’t use a magnifying finder – I can get by okay just dead-reckoning, and when I feel like cheating I can lay a laser pointer along the dovetail shoe or the square edge of the focuser and get on target very fast. But I might put a counterweight there, to get the balance point a little farther back so the eyepiece height would change less going from horizon to zenith. Or here’s an interesting thought: I bet I could gin up an eyepiece rack that would attach to the finder shoe. That would be cool, convenient, and a counterweight.

Here’s the focuser again, with the axis drawn in blue. This is to make a point. I’ve seen one or two folks on Cloudy Nights alleging that this is a “leftover scope” – that Bresser/Explore Scientific had some leftover tubes, leftover secondaries, and leftover focusers, so they cobbled it all together into this Frankenscope. But that doesn’t hold up. The focuser is a single-piece aluminum casting with two features of note. First, it wraps tight to the 4″ diameter tube, which if it was leftover from a reflector would have housed a smaller-than-4″ mirror. There are 3″ reflectors out there, like Orion’s SpaceProbe 3, but no-one puts 2″ focusers on them. Second, and more importantly, the focuser knobs point across the tube on this scope – that’s what the blue line shows in the image above – as opposed to down the length of the tube as in all mass-produced reflectors. Again, the focuser is a single chunk of aluminum – the 2″ tube can’t be separated from the base, or rotated relative to it. So I’m confident that this focuser was purpose-built for this scope.

The “leftover scope” idea was pretty dumb anyway. The most expensive part of any refractor is the objective lens, which has to be figured to a tolerance of a millionth of an inch. The rest is just steel, aluminum, plastic, and fasteners, which cost peanuts by comparison. As far as I’ve been able to tell, neither Bresser/Explore Scientific nor their parent/partner Jinghua ever sold a 4″ f/4.5 scope before. It doesn’t make any sense to figure a bunch of bespoke objectives – the expensive part, especially after full multi-coating – just to sell the cheap hardware.

So, somebody decided that a fast, 4″ reflactor was a good idea. Were they right? Tune in next time and find out.


  1. Matt – thanks for getting this! I’ve always thought this was an odd duck of a scope, and now I get to hear all about it.

    In pondering this scope, I’ve never understood the whys and wherefores of it. Why build a scope like this, instead of just making it a normal refractor and using a diagonal – like EVERY OTHER REFRACTOR in the world?!?

    Plus, what the heck Is going in the last two inches between the end of the scope and where the mirror is – is that taken up entirely by the, ahem, “secondary” mirror? (An interesting conundrum you pose there.) What happens if you dared unscrew that brass-looking screw on the back in the middle? (Dare, dare!)

    And collimation? For a refractor? Seems to me that’s defeating the purpose. The secondary would be doing the equivalent job as a diagonal, so that it wouldn’t need to be collimated – it should just be permanently mounted in place and aligned with the focuser. But your photo definitely shows some collimation screws at the back.

    And yeah, I don’t have any first-hand – or even second-hand – experience, but I would bet that that $40 70 degree EP is just an Erfle – and is going to look positively awful in that very fast scope – I’ll go out on a limb and say that the outer 25% of the FOV will not be sharp. A regular old 32mm Plossl will give you 3.4 degrees and be sharper than the included EP. But that Erfle would look good in an SCT or Mak.

    I’m eager to see your first light report.

  2. Thanks for the thorough unpack. Can’t wait to see results thru it.

  3. Thanks for the article Matt. What an interesting scope. Existentially speaking, we should just call the “secondary” the “diagonal”. No primary mirror so no secondary either, but just like any other refractor it uses a star diagonal, just that this one is built in. And its collimatable, a great bonus to get the best from such a short tube. My favorite star diagonal is also collimatable, so nothing really unusual in that.
    Can’t wait to see a first light report – please compare to the C80ED.
    Thanks again. Always enjoy your writing.

  4. First, thanks to everyone for the kind words. It’s very satisfying for me to do posts like this, and I’m glad other people find them useful.

    Why build a scope like this, instead of just making it a normal refractor and using a diagonal – like EVERY OTHER REFRACTOR in the world?!?

    I can think of three reasons. One, it makes for a very short, compact scope. With the dewshield unscrewed, the scope is only 16″ long. That’s small enough to stuff in just about any backpack or carry-on case for airline travel. To make a standard refractor that short, you’d need a long-ish focuser drawtube, which would vignette the image and effectively stop down the scope.

    Two, vignetting again – normal refractors are forced to put the drawtube in front of the diagonal, where it has more potential to cut into the light path. Putting the diagonal in front of the drawtube is one way to potentially that problem, although it requires a much larger mirror.

    And three, the ability to collimate the scope is a feature, not a bug. I got significantly better performance out of my TravelScope 70 after I’d touched up collimation, and I think Terry Nakazono collimates all of his refractors. People don’t talk about it as much as Newtonian collimation, but it’s a factor, especially in very fast scopes.

    (A fourth, minor point is that now the user doesn’t have to pack a diagonal. That’s one less thing to mess with in a travel scope, which this is clearly built to be. Also, diagonals can get out of collimation and this keeps the views from being screwed up by a lousy diagonal.)

    Plus, what the heck Is going in the last two inches between the end of the scope and where the mirror is – is that taken up entirely by the, ahem, “secondary” mirror?

    Nah, I’m pretty sure there is a cylindrical mirror-holder just like in a Newtonian, for the collimation bolts to screw through.

    What happens if you dared unscrew that brass-looking screw on the back in the middle? (Dare, dare!)

    😀 I’m sure that will happen at some point. Another post I need to write deals with all the hacks I did to my SkyScanner, which necessitated it’s nearly complete disassembly. Taking apart a scope down to its bones is tremendously liberating.

    Good point, Steve, about the secondary actually being the diagonal.

  5. Matt,

    The finder mount on this scope is the same as that on my ES AR102, and it’s designed to fit only ES finders, all of which sell for $150 or more. I use double edge tape to secure a standard Vixen dovetail finder mount to holda an Orion green point lase mount as a finder for my 102, and with a 32mm or even an ES 24mm/82 FOV, it makes for a surprisingly precise system.

    Like other posters here, I have long wondered about this Bresser hybrid. I like the compactness of the ota and have to say that it makes for a handsome little unit sitting there on its tubular steel leg tripod.

    Now, of course the only question that matters: Does it work?

    Eager to read your First Light report.


  6. […] Stargazing for people who think they don't have time for stargazing. « Unboxing the Bresser Messier AR102S Comet Edition […]

  7. Matt, thanks for this unboxing report. Can’t wait to read your observing report on this, under dark skies.

  8. Doug, thanks for the info on the finder shoe. I found an adapter – at ScopeStuff, naturally (link) – that will mate a Vixen/Celestron/Orion-type shoe into the ES/Bresser shoe. It’s cheap enough that I’ll probably pick it up at some point. Although the scope more or less functions as its own finder, especially if I use the laser pointer to get in the right area, there are certainly times that a RACI finder mounted alongside would be nice.

    I have one other piece of news. In a fairly immense stroke of serendipity, I struck up a conversation on Cloudy Nights with the person who designed the original prototypes of the AR102S Comet Edition. That person is going to check with ES and see how much of the history of the scope can safely be made public. But I can tell you that my suspicions about the drivers for the design were correct. The designer cited the same three factors I mentioned in the comment above: (1) a reflactor can be slightly more compact than a standard refractor of the same focal length, and every inch counts for airline travel (the AR102S Comed Edition fits comfortably inside airline carry-on size limits, especially if the dewshield is removed); (2) putting the mirror in front of the drawtube avoids vignetting; and (3) it’s easy to make the scope collimatable, which is desirable at short focal lengths.

    Hopefully I’ll be able to say more about that in the not-too-distant future. In the meantime, we’re clouded here until at least Tuesday. So I am catching up on reading and organizing my notes. I still need to post an observing report from last weekend’s Salton run. Stay tuned!

  9. […] was rolling pretty light. I wanted to test the Bresser reflactor/bino set as a package, so I used the AR102S on the came-with mount and tripod. I essentially always have […]

  10. Wonderful, fabulous, marvelous scope. Easy to use and beautiful views. One of my favorite scopes to use after 56 years of astronomy and over 20 scopes. I changed the eyepiece ring to a 2″ one and one of my favorite eyepieces to use in it is a Nagler 20 mm type 2 I’ve owned for a long time for a great, well corrected, wide field, 23 power view.

  11. […] the awesome RFT with the strange design and incredibly long name is still around. Amazon is selling the package for $399 (link), BUT as of right now, Newegg.com has […]

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