Uncle Rod's Astro Blog

With apologies to William Tyler Olcott, whose In Starland with a Three-inch Telescope, first published in 1909, is one of the few antique observing guides that are both charming indoors and still fun to use in the field. but our topic for today, muchachos, is not Olcott’s book (though it deserves an article) or three-inch refractors, but instead four-inch refractors, and actually one specific four-inch that has been making waves of late, the Explore Scientific AR-102 f/6.5 achromat.

I’ve written about this telescope, which we’ve had here since 2013, several times, so why revisit it? Simply because it is one of the best deals in amateur astronomy at the moment. Explore Scientific is now selling this JOC made scope for a freaking amazing $299.00. I thought this refractor was a steal when it was $500.00, but now it is just crazy. If you have even an inkling you might have a use for a telescope like this, don’t hesitate. Order one. Now. Who knows how long this nuttiness will last?

Exactly what is “a telescope like this,” though? What sort of a telescope do we have here? It is a 102mm aperture achromatic refractor with a focal length of 663mm due to its f/6.5 focal ratio. That makes it fast, especially for an achro. And indeed its prime attraction is probably its richest-field-telescope nature. That’s what I used it for at first. With a 2-inch focuser, a fairly flat field, and its unobstructed nature, this little wonder put my old RFT, an Orion StarBlast mini-Dob, to shame. I don’t hesitate to say the view of the North American Nebula, NGC 7000, I had with the AR102 at the 2013 Deep South Regional Star Gaze was one of the two best looks at that nebula I’ve ever had.

So, it’s a good RFT. But for an RFT’s low-magnification/wide-field nature to shine, you really need dark skies. Compromised skies make the eyepiece background bright at low power and you don’t see too much. Despite being a great deal, is this telescope usable from the average suburban backyard? That’s one reason we are talking about it again, to get an answer to that question. I also wanted to go into the AR’s mechanical characteristics in a little more depth.

First, let’s address the general, beginning with what’s in the box. What is in there is an amazing amount of stuff. I was impressed even when the scope was at its old higher price. In addition to the OTA, there’s a decent Vixen compatible dovetail on a set of sturdy, hinged tube rings. In addition to the Vixen dovetail on the bottom, there’s a handle-like bracket on top that makes it easy to carry and mount the refractor. This handle is slotted, so you can mount things on it. Maybe a red dot finder, maybe a piggyback scope—whatever.

Digging down, you’ll also find a rather nice 8x50 finder. It isn’t much different in quality from what we’re seeing on many import scopes today, which is to say “pretty good and a darn-sight better than the 6x30s that used to come on inexpensive rigs.” The finder optics are reasonably sharp, even with my eyes. The finder rings are the two-screws-and-a-spring-mount type, and hold adjustment OK if not spectacularly well. The base-mounting shoe is of the “Meade” type where the ring mount goes over the base instead of sliding into it. Only criticism I have of the finder is that since it is not illuminated, the rather skinny crosshairs tend to disappear.

There’s also, almost unbelievably, a nice 2-inch compression-ring style star diagonal lurking in the box. It really is nice, with carbon fiber composition sides. I wouldn’t normally expect much of a diagonal thrown into the box with a scope, but testing reveals this 99% reflectivity dielectric job is as good as any of the other 2-inch diagonals I’ve got around here, which includes some pretty pricey ones.

Also included is an aperture cap (I like its molded-in handle), an end cap for the telescope’s focuser, a cap for the scope end of the diagonal, and a 2-inch/1.25-inch adapter (a good one) for said diagonal. And that is it. There are no eyepieces, but, hey, waddayawant for your 300 dollars? You don’t get a telescope case for this price either, but the cardboard (double) box the OTA and stuff ship in is sturdy and will likely last a while.

Instructions? Well, there’s a warranty and a warranty card, but there are no instructions in the box. They really aren’t much needed. What is there to say about a rather simple OTA? The sole exception might be collimation directions, but most users will never need to collimate the scope. If you should need to collimate, or just want a set of instructions, the AR scopes are covered in Explore’s all-in-one instruction pamphlet, which can be downloaded from their website.

Let’s face it; the first thing you are going to notice when you have the AR102 out of that box is her ENORMOUS DEWSHIELD. It’s much like those JOC put on the refractors Meade shipped with their LXD55 and LXD75 telescopes some years ago. Why so large in diameter? Who knows? Aesthetically it’s not overly pleasing, but at least it doesn’t seem to impact the dew shield’s dew fighting abilities. My friend Jon had a hard time getting over the AR102’s looks despite my assertions on Cloudy Nights that “all scopes, like all cats, are gray in the dark,” but eventually the fantastic price suckered even him in. I suspect the same will be true of you. Me? I haven’t quite got to the pointing of thinking the scope looks pretty, but I at least now consider her “distinctive” rather than downright ugly.

Otherwise, the AR102 looks good, with a beautifully finished gleaming white tube set off by black trim and a black focuser, finder mount, and rings. It’s more than just good looking, however; the whole thing spells “quality.” This is a long ways from the minimalist short tube 80s, 90s, and 100s that graced the first years of the Chinese scope revolution.

Let’s examine the specifics of that quality starting at the objective end. Inside that humongous dew shield you’ll find the objective in its cell. It is well coated, and as is common these days the coating has a strong greenish tint. The lens is well coated enough to tend to disappear in normal light. The objective is held in a metal cell that is equipped with three sets of push-pull collimation screws. The objective is secured in this cell by a plastic retaining ring not unlike the corrector retaining ring on an SCT. Peering down the tube of our example doesn’t reveal any baffles, but the tube is decently blackened (similar to my C8) and apparently doesn’t need baffles since contrast is good even under my bright skies.

Swapping around to the other end, the focuser end, will leave you even more impressed. To put it simply, this is the best focuser I have ever seen on a refractor in this price range. It is a two-speed Crayford type, and reminds me a lot of the surprisingly excellent focuser on my 10-inch GSO Dobsonian, Zelda. It is smooth and even buttery in action, and I didn’t have to do any adjusting to enable it to work with my heavy TeleVue eyepieces without slipping. The knobs are aluminum, with the fine focus knob being incorporated into the right focuser knob. The only down-check is that the focuser is not rotatable, but you really only need that for imaging, and this being an achromat, only a masochist would do much picture taking with it. This focuser is a jewel.

About all that’s left to talk about on the OTA is the finder’s shoe mount. The only problem with it is its location. Due to the short length of the tube, the telescope’s tube ring assembly will prevent you from mounting the finder unless you slide the scope all the way back in the rings. That’s an annoyance, but not much of one, and won’t even be an annoyance if you leave the finder mounted all the time.

The AR102 package is cool, but it is just an OTA package. You will have to provide a mount. Luckily, the short tube and light weight (about 10-pounds) of this refractor mean you can get away with a fairly minimalist mounting. That doesn’t just save dollars; it turns the scope into a powerful grab ‘n go performer. The first time I used the AR102, it was on my VX, and it performed beautifully in that configuration, but I soon decided I wanted it on something lighter and simpler. Something easy to get into the backyard at a moment’s notice.

What sort of light and simple mounting should you seek? One of Celestron’s CG4 German equatorials would do. I’m not sure you need a German equatorial or motorized tracking, though. The wide-field nature of the AR102 means you can do without motors and polar alignments, and that whispers “point-and-shoot alt-az mount” to me. Sure, tracking is nice, but it is really only vital for high power lunar and planetary work, and this is not the scope for that anyway.

There are plenty of manual alt-az rigs on the market at the moment in every price range. Luckily I already had one I suspected would serve, SkyWatcher’s AZ-4. Slapped the scope on the mount, unscrewed and stowed the enormous pan handle that comes with the AZ-4 in the interests of good balance, and we were off to the races. My particular model of AZ-4, whose tripod is saddled with extruded aluminum legs, is not quite the Rock of Gibraltar with the AR102 onboard, but it is more than good enough and is a pleasure to carry out the door.

So you should get an AZ-4 to go with your AR102? That would be fine, but, unfortunately, SkyWatcher doesn’t seem to be selling that mount in the U.S. anymore. You can, however, get exactly the same (if more expensive) thing wearing an Orion badge in the form of their Versago II Orion used to sell an upscale configuration of the mount with tubular steel legs instead of extruded aluminum ones, but they don’t seem to offer that anymore. All in all, the Versago II is a perfectly reasonable choice.

I think there may be a better and cheaper alternative, however. Explore Scientific sells an alt-az mount for less than 200 dollars, the Twilight I, which not only has tubular steel legs on its tripod, but slow motions on the mount head, both things the Synta made AZ-4/Versago lacks. Its payload capacity is also rated slightly higher than that of the AZ-4, 18-pounds versus 15-pounds, though I’d guess they are probably nearly identical in performance. It gets better. If you buy the Twilight I as a package deal with the OTA it you’ll be paying a mere 150 for the mount.

Naturally, spending more for a top-of-the line alt-azimuth mounting will make the experience of using the 102 even better, but I believe either of these two inexpensive mounts are more than sufficient and are well-suited to the OTA. I’ve never been annoyed with the telescope on the AZ-4 and suspect I’d like the Twilight I even better.

So you’ve got the AR102 and a suitable mounting for it; what can you expect from this bargain rig of a refractor? As above, I was thrilled by its performance under dark skies. Not just looking at specific objects, but merely scanning along the summer Milky Way with a 35mm Panoptic in the diagonal. You can imagine. The question on my mind, however, given the hordes of you snapping up the AR102 at its reduced price, was how the scope would do for the average suburbanite in the backyard. While I did a little backyard sky-surfing with the telescope last year, I thought I’d best double check its efficacy under light polluted skies.

That's a big dew shield, Paw-Paw...

On our last go-round with the AR102, I’d already discovered some things about the refractor and the deep sky from my backyard. First of all, that a pedigree isn’t everything. The AR102 bested my expensive 80mm APO on DSOs, and particularly globular clusters. Just as you’d expect, the AR102’s extra inch of aperture was an insurmountable advantage. M13 and M92 looked better and showed more resolution in the 102 than in my Megrez II fluorite APO.

Perhaps more surprising? I expected my other 4-inch achromat, an f/9.8 C102 to do better on the deep sky than the AR102. Nope. Despite it throwing up an essentially perfect star test, the images of the summer deep sky objects were really no better at all in the C102 at comparable magnifications than in the AR102, which star tests well, if not as well as the longer scope.

The truth is that what makes the most difference—assuming your optics aren’t really punk—on the deep sky in the backyard is magnification. If you can get your power up enough to spread out the background sky-glow a bit and increase contrast, most DSOs will look better. So, I believed my goal this time would be to see how the scope took power—with the understanding that this time of the year with the jet stream roaring overhead there’s only so much you can expect from any instrument.

Before pumping up the power, I wanted to reacquaint myself with the facts of the matter concerning the AR102’s color. And what better way to do that than with Sirius? The star is nearly as punishing as Venus for achromatic refractors. I inserted a wide-field 16mm eyepiece and headed to the Dog Star.

How bad was it? Oh, there was no lack of the color purple, which formed a halo around the crazy-bright star, but you know what? It wasn’t as bad as I expected. In the house, looking at the specs of the scope and trying to imagine how much chromatic aberration it would show led me to exaggerate its failings in that regard. I’d be nuts to say the color around Sirius wasn’t prominent. It was, but what I saw in the eyepiece was not an ugly view at all. If the seeing had been better, it would actually have been a rather pretty view. And remember, poor seeing tends to make chromatic aberration look worse.

Onward to crescent Luna. It would have been an even more telling test if the Moon had been a little closer to first quarter and brighter, but it was bright enough. What did I see? A sharp terminator with plenty of detail at 150x. It was not quite as sharp at 200x, but I believe that was more due to the seeing than the optics. I’ve used this telescope at a few public outreach events, and there a considerably fatter and brighter Moon still looked surprisingly good.

How much color, though? I’ll be straight-up about it:  the color looks more prominent in this 4-inch f/6.5 in my opinion—somewhat anyway—than it does in my 6-inch f/8 achromat. That is not to say it is debilitating or ruins the Moon. It doesn’t. There’s purple along the limb, and as the sky darkens, the shadows along the terminator become purple-hued, but it is still pretty and detailed. Keeping your eye positioned on the optical axis reduces the purple as much as possible.

So, the Moon looks good enough. Certainly the scope is sufficient for casual lunar exploration, particularly at lower powers. I will tell you right here and now that Luna at least looks better—worlds better—in this telescope than it ever did in my old Short Tube 80mm f/5 achromat, and I had plenty of fun taking plenty of grab ‘n go peeps at the Moon with that humble instrument.

Now it was time to push the magnification. The target? First off, Rigel. While a double star is not necessarily the best test of a telescope’s optical quality, an unequal one like Beta Orionis will show how your scope takes magnification. Under good seeing, Rigel’s magnitude 6.7 companion is doable with a 3-inch. The seeing wasn’t exactly good, however, so I wasn’t quite sure I’d spot that little spark, which is some 500 times dimmer than the primary. There it was, though, easy at 150x. I raised the power to 200x just to see how the image looked—it still looked good—but I really needn’t have.

OK, now for something slightly tougher, but not crazy tough given the wavering atmosphere. How about Polaris? While Polaris is dimmer and not as overwhelming as Rigel, its companion, Polaris B, is also dimmer, about 9^th magnitude. I find it tough with a 3-inch more often than not and occasionally hard with a 4-inch. Not tonight. I had to squint a little at first, but there it was at 150x in the gloaming, easy-peasy. If I haven’t mentioned it, in times of good seeing, stars looked like perfect little airy disks surrounded by rings (concentric rings; our AR102’s collimation is dead-on).

iPhone Moon with AR102

How about a little DSO action? My experiences with the deep sky on this night were similar to what I had in the summer. Objects looked just like I’d expect them to look in a 4-inch refractor. In the backyard, the sky was bright enough to prevent the scope from doing its RFT thing to good effect—the whole sword of Orion was visible at lower power, but looked awful washed out. Zooming in on M42 at 150x and 200x made the Great Nebula look easily as good as it does in my C102, and, given the light pollution, not much worse than it does in my 5-inch ED refractor. The seeing was not good enough to reveal any “extra” stars in the Trapezium, but the nebula looked fine, and I noted that M43 was almost threatening to assume its comma shape as the constellation climbed higher.

Just time enough for one more. M35 showed off its pretty cloud of stars on this less than perfect night. And not only that. By concentrating and using a 4.7 mm wide-field eyepiece, I was able to pick up at least a trace of its much dimmer and more distant companion cluster, NGC 2158 (magnitude 8.6).  I won’t say it was as easy to see as with the 5-inch or 6-inch refractors, but nevertheless it was there, if barely.

Are you on the fence? Again, if you think there’s even a remote chance of you having a use for the AR102, get off that fence and get one. It is an incredible buy and the legitimate heir to the much-loved Short Tube 80. I won’t hesitate to say this may be all the telescope some people ever need. It’s a powerful instrument for exploring the deep sky under dark skies. It’s OK for casual observation of the Moon and planets. Best of all, perhaps, it can surprise on galaxies, clusters, and nebulae from the suburban backyard. A true classic.