Archive for the ‘reflactors’ Category


From sub-aperture mask to replacement dust cap

February 23, 2017


Here’s something dumb. The Bresser AR102S Comet Edition is optimized for two things: widefield, low-power scanning, and portability. At 20″ for the OTA it’s just within the bounds of airline carry-on-ability, but you can unscrew the dewshield and shave off another 4″, at which point the options for storage and transport expand wildly.

BUT the stock dust cap for the objective is dome-shaped, for no good or obvious reason, which means it sticks out about a full centimeter longer than necessary. When you’re thinking about flying with a scope, that is one centimeter more stupidity than you should have to put up with.

There’s another problem with the stock dust cap: when the scope gets cold, it gets loose and falls out easily. Nothing unique to this scope about that – I’ve had to shim the majority of my scopes’ dust caps for the same problem, including the C80ED and XT10. One cheap package of sticky-back green felt has kept me going since 2010. I think I’ve used almost a third of it.

Now, I already have a nice 60mm sub-aperture mask for this scope (construction details here). If I could plug the central hole securely, I’d have a replacement dust cap that would be shorter, would get tighter rather than looser if it shrunk in the cold, and would serve double-duty as both a dust cap and a sub-aperture mask. The problem was finding a plug the right size, with a good lip on it to keep dust out, that would grab the edges of the mask hole securely.


And it’s the dollar store to the rescue again, with this container of Tootsie Rolls that is intended to double as a coin bank. The hard plastic lid snaps down into the cardboard tube very securely, and the plug bit is just a shade over 60mm in diameter.


I used the Dremel and some sandpaper to enlarge the hole in the sub-aperture mask ever so slightly, and voila. There’s a small lip that runs around the top edge, and even a little recess in which to hook a finger and pull out the plug.


Here you can see the ridges on the plug. By sanding in short increments, I was able to fine-tune the hole diameter until the plug snapped in very securely, without stressing either piece. I need to put some tape or a little epoxy or something over the perforated slot, which is intended to be punched out so the candy container can become a coin bank. Or cut out the center and replace it with another, smaller plug, so I’d have a dust cap and two aperture masks in one package…


Boom. Now the scope is a centimeter shorter for travel, and I don’t have to keep the sub-aperture mask in my eyepiece case.

What I really want is for someone with even rudimentary 3D modeling skills to create a series of nested aperture masks, like Russian dolls, in 10 or 20mm increments, which could be 3D printed on demand in whatever combinations people needed. Most of them could be standard sizes, with only the outermost adapter for each telescope model needing to be custom. Then you could order the adapter for your scope and whatever set of nested masks you wanted, or maybe all of them to simplify, so your 100mm scope could also be an 80mm, a 60mm, a 40mm, and even a 20mm (the “Galileo model”) if you liked, just by taking out the relevant bits from the dust cap. Sure, it would be gross overkill for most people, but for those of us who like playing “what if” (“what if my C80ED was a C40ED?”) it would be a godsend. And with 3D printing no-one would be stuck with a bunch of useless stock when the idea inevitably bombed.

Anyway, if someone would to that, it would save me the trouble of building my own “Mask-ryoshka” dust cap out of junk from the dollar store. But if I’m being totally honest, avoiding building my own stuff out of junk from the dollar store was never the point of the exercise, was it?*

* With apologies to Adam Savage.

Why and how to make a sub-aperture mask for a refractor

February 11, 2017


Here’s the Bresser Messier AR102S Comet Edition with a homemade aperture mask. I just converted the scope from a 102mm f/4.5 to a 60mm f/7.7.

“WAT!? You took a refractor, the most aperture-challenged of the three basic telescope designs, and made it even smaller?”

Yup. For several reasons.

The first and most obvious is to control chromatic aberration (CA), also known as false color. Despite the name ‘achromat’, which literally means ‘no color’, doublet refractors without extra-low dispersion (ED) glass do show some false color, because their lenses do not bring all of the colors of light to the same focus point (they’re still a LOT better than scopes with a singlet objective lens, like those used by Galileo). For dim objects like galaxies, nebulae, and most field stars, the effect is not noticeable, even in large and optically fast scopes like the AR102S Comet Edition (nickname needed). But bright objects like the moon, planets, and first magnitude stars will be surrounded by purplish halos, and may have yellowish margins. In effect, the purple and yellow-orange parts of the spectrum are forming out-of-focus images that are superimposed on the main in-focus image.

The problem is that CA gets bad fast as refractors get bigger. There are a couple of standards that are commonly used to describe the focal ratio necessary to minimize CA to acceptable levels, the Conrady standard and the Sidgwick standard. By the Conrady standard, the focal ratio must be 5 times the aperture in inches; by the less stringent Sidgwick standard, 3 times the aperture in inches is good enough. Note that the standards describe focal ratios, not focal lengths, so they go up fast with increasing aperture. Here are some apertures, focal ratios, and focal lengths required to meet the Sidgwick standard:

  • 50mm (2″) : f/6 : 300mm
  • 76mm (3″) : f/9 : 684mm
  • 102mm (4″) : f/12 : 1224mm
  • 127mm (5″) : f/15 : 1905mm
  • 152mm (6″) : f/18 : 2736mm

This, along with mounting considerations, explains why reflectors and catadioptric scopes are progressively more common past 4″ in aperture. A 6″, f/8 Newtonian will be free of false color (as are all reflectors) and has such a gently converging light cone that it is easy to collimate and to focus – it’s easy for such scopes to achieve ‘planet-killer’ status if the mirror is good. A 6″, f/8 achromat will be a beast to mount and it will show lurid false color on bright objects.

But people still make, buy, and use such scopes! Why? Horses for courses: big, fast achromats can be superb deep-sky scopes, where chromatic aberration is typically not a problem. With the fixed sizes of standard eyepieces, achieving wide true fields requires short focal lengths (not just short focal ratios), and bright images require aperture, which drives the development of large but optically fast scopes like the AR102S Comet Edition. At f/4.5, it is well into ghastly CA territory on bright targets. The other night I stayed up late to catch Jupiter, and in the AR102S the planet wouldn’t even come to a clean focus. It was just a bright ball of light inside a sea of purple. I switched over to London’s 60mm f/11 Meade refractor and Jupiter snapped into a sharp and essentially color-free focus. There was a moon emerging from behind the limb of planet, already one moon-diameter out into black space, that was completely invisible in the CA-smudged view of the AR102S.

I’m okay with that – as I noted in a previous post, observing bright solar system targets with the AR102S is deliberate misuse of the scope. When I want good planetary views, I have a 5″ Mak and a 10″ Dob that can both be pushed to 500x (assuming the atmosphere is steady enough). But their max fields of view are pathetic compared to the AR102S – about 1.1 degrees for the Mak, and a shade over 2 degrees for the Dob, versus 3.6 degrees for the refractor, which is enough to take in all of Orion’s sword at once, with space left over on either side.

Still, I’m not going to take all of my scopes out with me every time I go observing, and chances are good that at some point I’ll want to look at something bright even if my main goal for the evening was low-power sweeping with the AR102S. Under those circumstances, it’s easier to have an aperture mask shoved in my eyepiece case than to pack a second scope. Hence this project and this post.

But I’m getting ahead of myself. There are other reasons to stop down a scope besides reducing CA:

  • To reduce glare from bright objects. Mostly applies to the moon when it’s full or very gibbous.
  • To give a more aesthetically pleasing image when the seeing is bad. Opinions differ on this point. Some folks prefer to look through a larger aperture despite the increased susceptibility to bad seeing, on the grounds that in the moments when the atmosphere does settle down a bit, you’ll see more detail. I suppose it depends on whether one is in exploration mode or aesthetic observation mode.
  • To make it easier to focus. F/4.5 is a steep light cone, and it’s easy to overshoot the point of best focus. Stopping down the scope makes a shallower light cone, so it’s easier to watch the image transition from out of focus, to near focus, to in focus. I’m going to test this method of finding best focus on some close double stars.

I had done some calculations in advance to figure out what sizes of aperture masks I’d want to try out. Given that the AR102S has a fixed focal length of 459mm, here are the focal ratios at full aperture and at 10mm decrements:

  • 102mm gives 459/102 = f/4.5
  • 90mm gives 459/90 = f/5.1
  • 80mm gives 459/80 = f/5.7
  • 70mm gives 459/70 = f/6.5
  • 60mm gives 459/60 = f/7.7
  • 50mm gives 459/50 = f/9.2
  • 40mm gives 459/40 = f/11.5


I didn’t want to trade away too much resolving power, so I tested the scope on the moon using cardboard masks of 76mm and 60mm, made from the light cardboard spacers from a box of wet cat food. The 76mm is shown above. Perhaps unsurprisingly, at this aperture and focal ratio (f/6) the view was still unappealingly soft. But 60mm looked good, with minimal CA. This makes sense – the working focal ratio of f/7.7 is a healthy step beyond the f/7.2 that the Sidgwick standard suggests for a 60mm aperture. Going any smaller would be trading away valuable resolution, without significantly improving the image.


The light cardboard aperture masks were fast and easy to make, but they weren’t very sturdy. To make a more permanent mask, I needed plastic, heavier cardboard, or foam-core board. So I unscrewed the dewshield from the scope and walked down to the dollar store, where I looked for food packages and storage containers that might fit. Finally on the last aisle I found this 1-gallon plastic jar. The lid slip-fit over the dewshield with just a bit of extra room, which I knew I could shim out with some sticky-back felt.


I wanted to make sure the lid would fit before I did the hard work of cutting, so I put the felt on first. This was very familiar – it seems like every other scope I get has a loose dust cover that has to be shimmed to fit correctly. I’ve been slowly chipping away at the same package of sticky-back felt since 2010. I didn’t have a compass handy, so I used a small paper ruler to make a ring of marks around concentric 60mm circle inside the lid. Then found a lid to a jar of vitamins that was exactly 60mm in diameter and used that to trace the circle neatly.


I was going to cut out the aperture using hobby knife, but the plastic was too tough. So I moved up to a box knife, and then a linoleum knife. Then I said heck with it and got the Dremel. The hole I cut wasn’t perfectly circular and had rough edges to boot, so I wrapped some sandpaper around a pill bottle to make a tool for rounding out the aperture.


Here’s the scope before…


…and after.

Even with the aperture mask, the AR102S is not a champion scope on solar system targets. The C80ED blows it away, which makes sense – it has a 33% resolution advantage over the stopped-down AR102S, and frankly just better glass. But at least the view now is clean and not appallingly degraded. A dramatic way to see the difference is to get a good tight focus on the moon with the mask on, then quickly take it off without removing one’s eye from the eyepiece, and watch the view get a lot brighter and a lot softer at the same time.

I have a few more things I want to do. The 60mm aperture mask fits over the end of the scope so securely that it could work as a dust cover, if only I can find or make something to plug the central hole. Also, I think I am going to play with making aperture masks in other sizes, just to see what happens.

And finally, I have another 4″ scope that will be fun to make an aperture mask for. But that will be a subject for another post.


Misusing a fast reflactor: moon-gazing with the Bresser AR102S Comet Edition

February 1, 2017


I was going to hold off before I posted anything on the performance of the Bresser Messier AR102S Comet Edition (which I swear I am going to start calling something else, just as soon as I think of a good nickname). That’s because I haven’t had a chance to try it out under the conditions where it is likely to do well. This is an optically fast scope, optimized for low-power, widefield scanning, ideally under dark skies where those faint stars can really pop. A great time and place to test-drive this scope would have been last Saturday night at the Salton Sea, right after the new moon, when Terry Nakazono and I stayed up observing almost to dawn, (expect that observing report in the not-too-distant future).

Unfortunately, the scope arrived Sunday afternoon, a day late for our Salton trip. Mount Baldy is covered in ice and snow, and I haven’t had time on these school nights to get up there anyway, much less to get to anyplace farther away. And the skies down here in Claremont have been crappy this week. Never clouded out, but never truly clear either – there has been a thin, high-altitude haze that is really good at both obscuring dim stuff and reflecting light pollution. So the only objects that have looked good are the bright, small things – the moon, Venus, Jupiter, and double stars. In other words, exactly the things that the AR102S Comet Edition is not specialized for.

So I don’t feel like I can give the scope a fair review yet, because all I’ve been able to do with it are the things it’s not built for. It’s like taking a Lamborghini Huracán on a Moab jeep trail – it’s not going to work out well, in ways that are completely predictable, and you’re not going to learn very much by doing it.

But when has that ever stopped me? I may not be able to review the scope, but I can still play around with it.


Here’s a raw shot of the moon, taken with a Nikon Coolpix 4500 shooting afocally (and handheld) through the supplied 20mm 70-degree eyepiece.


Same shot tweaked with Unsharp Mask and Curves in GIMP.


And then converted to black and white.

I didn’t learn much. Yes, there is chromatic aberration. In other news, water is wet and the Pope is Catholic. On the plus side, the Trapezium in Orion is split into four members at 23x. Haven’t tried it on the Double Double or any other double stars, really.

On the to-do list are (1) to get this scope out someplace dark and clear and really put it through its paces, at a variety of magnifications – and using a variety of eyepiece designs – on a variety of targets, (2) to do some actual testing on close double stars and doubles with significant magnitude differences, and (3) to experiment with sub-aperture masks to knock down the CA on bright stuff. “Why not just use a smaller or better-corrected scope?” you may wonder. Well, this is sold as a travel kit, and if by using a sub-aperture mask I can make it into a passable solar system scope, I’ve just made it a better all-rounder when and if I take it on the road.

Given the waxing moon and the continuing lousy forecast for the coming week, I’ll probably have to tackle that to-do list in reverse order. Stay tuned.


Parting shot. I got this Earthshine pic by doing a half-second exposure with the Coolpix. It’s not nearly as good as the one in the banner at the top of the page. I need to try again next cycle when the moon is younger.


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 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 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.