Analog Sky Drifter Prototype 1 Notes


So it's been a while since I updated this blog. That's because the Drifter prototype 1 was finished 8-12 months ahead of schedule, in time for Oregon Star Party (OSP) 2018. I was too busy building it to post any updates. Many thanks to Jerry Oltion for helping me out of three or four jambs, providing his workshop and vast experience. It wouldn't have happened without his help.

Starting at OSP, the Drifter has been used by about 50 people from noobs to god-level observers.

Here are my notes so far.

IPD Adjustment

Everyone has differently-spaced eyes. The distance between eyes is called the interpupillary distance, IPD.

In general, the biggest roadblock to a successful binoscope is an intuitive IPD adjustment that can hold image coalignment for long/multiple observing sessions through the full altitude range of a scope and use by several individuals. So far, this has been remarkably successful.

The Drifter's handheld binocular style IPD adjustment--just grab the eyepieces and pull them apart--has been intuitive for most people. Out of 50 people, one declined to pull the sides apart for fear of breaking something, and two stuck their hands in the ends where the tertiary adjustment knobs are and affected collimation/merging.

One particularly successful group consisted of an eight-year-old, an individual with high-powered eyeglasses, and an individual with a wide (70mm+) IPD. The three spent probably 20 minutes taking turns viewing several targets without image merging issues. Three different sizes of people, three very different things going on with their eyes, all three could sit down at the scope and have a great observation. The individual with the glasses said she'd never been able to get a sharp view through a monocular telescope before, but upon looking through the Drifter said "I think you've got a convert."

At the EAS (Eugene Astronomical Society) monthly star party, the scope was pointed at the moon for a couple hours with a few dozen people viewing the scope with the 17.5mm Morpheus eyepieces. At least 50% of these are general members of the public with low knowledge of the night sky and little experience at any telescope. Despite the fact that most of them had already observed the moon through other scopes that night, almost everyone exclaimed with a "Wow", "Incredible", etc. Aside from the typical star party mistake of bumping the scope and losing the target, the scope pretty much ran itself for this crowd. This outing felt particularly successful.

Exposed Secondary Mirrors

The Drifter has partial secondary rings. It's designed this way to minimize the weight of the UTA and to minimize the width of the whole scope. It also reduces the number of struts required, which makes precise alignment of the struts less critical.

These partial rings leave the secondaries very exposed. In a real dark site like OSP, I worried about bumping or accidentally grabbing the secondaries. Indeed, one time when reaching for a strut to move the scope, I did grab a secondary mount.

I'll add a lightweight plastic or aluminum shroud to the UTA to prevent this. Obviously it will also help with stray light and dew prevention as well.


The struts hold collimation from zenith to horizon. Not surprising since they're only 16 inches long, but still nice to confirm. They also hold collimation perfectly through repeated cycles of collapsing/extending. This concept of the mirror box being the container for the UTA and not having to (dis)assemble struts/trusses with screws every single viewing session is a grand slam. Grab-and-go on a scope that pulls in as much light as a 12". It's closer than ever before.

The struts are a little stiff to slide in and out. I decided to err on the side of too stiff, but playing with the diameter of the clamps will tune this.

The struts don't stay straight with the scope  collapsed. They would touch the primary mirror surface if there weren't a protective cover or if I collapsed it completely. I'm guessing this is a combo of strut attachment angle not being completely perfect, the slight imperfections in strut clamp alignment, the weight of the UTA on the bottom struts.

In the next version of the mirror box, I am going to try to telescope the struts into the tubes that form the frame so that they stay straight. They will be cut more precisely, mounted more precisely, and they will attach to a CNC aluminum plate rather than a plywood surface that can deform a bit.


The ergonomics of binoscope use are very different from a standard dobsonian scope. You sit or stand at the front of the scope with your head between the primaries. In the case of the Drifter, your head is also below the center of the primaries by about eight inches. It's an 8" scope that's already f/5, shorter than usual because of the addition of the tertiary mirror, and is shorter than usual because of the high tertiary angle. These factors combine to substantially lower the eyepiece height.

At OSP, I didn't have an extra box to set the scope on top of to raise eyepiece height for low targets. What should have been an epic journey from M8 to M11 was a frustrating series of

M8: sitting on the ground, a little hunched
M20: stretching up a bit beyond my full seated stature because it's just a bit higher but still too short for a chair.
M24: Ok now I definitely need a chair, go get the adjustable chair, but still it's kinda low so now I'm scrunching down and craning my neck back, which is really bad for neck health
M16: Now I'm finally getting into usable chair range where it's working more like a dob, though still in the low range where getting up from the chair is awkward

Everyone's seated height is different, so everyone experiences these things differently. My torso is particularly short for a 5'10" male so I experience it more like someone of typical proportions in the 5'2"-5'6" range.

Setting the scope on a box approximately 12" (30 cm) tall fixed this problem for low objects. M8 was still adjusting the chair to a low height, but everything else was very comfortable seated. However, being up on the box put high targets out of reach for anyone under 6' tall.

I'm thinking about how to make the rocker box adjustable so the altitude bearings can be raised for low targets and quickly/safely/easily lowered for high targets. Four positions: One for low targets, one for high targets, and one of each for the scope + equatorial table.

Unique Mirror Configuration

Standard 45 degree bino - red is human head outline, black circles are primaries, secondaries and tertiaries

Commercial bino


Many binoscopes either put the eyepieces between the primaries--pushing the primaries at least a foot apart--or the eyepieces come out at a 45 degree angle, which invites the primaries to scoot a little closer together. The Drifter pushes the primaries together as closely as they can be packed by making the eyepieces come out at a 62 degree nominal angle. I did this because I wanted a more compact scope, but there are two other significant effects:

a) Angular rotation of the cages for IPD is minimized compared to zero degree primaries. This aids in maintaining collimation of the secondaries relative to the primaries.

b) The head is kept at a comfortable distance from the equipment and far from the light path. This is especially important the closer one gets to zenith. The idea of hovering over the scope at zenith seemed very uncomfortable to me. The Drifter's 8" offset really cures this issue well, so you never feel like you're going to accidentally fall into the scope, or that your shoulders are going to obstruct the primaries. While this setup eats a little light transmission and lowers the eyepieces to a challenging spot for low targets, I wouldn't change it for eyepieces directly between the primaries.

Secondary Vane Transform Shimmer

I didn't experience this effect at all, and no one else reported it. In a rotating ring binoscope, the vanes are only aligned perfectly for one particular IPD. So while they were rarely/never aligned perfectly, no one reported adverse effects from this situation. (You are in an incredibly small club of maybe 20-30 humans on Earth if you even understand what this one is about.)

Secondary Mounts

The meh: I tried Gary Seronik's secondary mount concept of a pivot and rotation rather than the standard three- or four-screw push/pull mount. In one sense I really like it because it's intuitive. On the other hand--and maybe it's because I've 3D printed it, so my parts are plastic--there is a large degree of recoil when you're adjusting. Tightening the adjustments without changing collimation was incredibly finicky. Originally I did two thumb screws for tightening, but the pivot couldn't be held securely with a thumb nut. (To be fair to Gary, he recommends a jam nut here, not a thumb nut. When I replaced the thumb nut with a jam nut, it could be tightened very well with a crescent wrench.)

The good: When the adjustments were locked down, they really stayed put. I haven't had to adjust these except in the case where they have been bumped by my hand or something like that.

Given the fact that I can get plenty of co-alignment adjustment from the tertiary mounts, I am very tempted to simply remove collimation adjustment from the secondary mounts altogether. The 3D printing is incredibly accurate. The laser-cut secondary vanes are incredibly accurate. The vanes attach to accurately-printed secondary rings. The secondary rings are perfectly parallel to the primaries. The rest of the scope is CNC routed. We're talking about sub-millimeter accuracy on an f/5 scope, where you still get sharp images when the collimation laser is an inch off-center.

This is actually one of the themes of my testing so far, is that these manufacturing processes are accurate enough that including lots of adjustments is unnecessary. I want to see how many adjustments I can eliminate, starting with the secondary mount. A return to that 50's ethos of "Build the scope right the first time."

Tertiary Mounts

In place under the tertiary assembly

The Edmund Optics tertiary mounts performed beautifully. They provided a lot of movement for coalignment and merging of the images at a very convenient place for the user. Assuming the primaries are pretty accurately aligned to begin with, there is no reason to adjust collimation or coplanar position of the primaries as part of the coalignment process.

For one user with a lower than 58mm IPD, the tertiary adjustments even allowed image merging in the 34mm eyepieces by over-aligning in the horizontal plane.

I was worried about the springs being too light for the weight of the tertiaries, but I didn't see any collimation issues when changing altitude.

The screws require an incredibly light touch. I was concerned about this from the start. It would be nice if they weren't machined so perfectly and it took a little more force to turn the screws. They are very easy to bump out of alignment if you stick your fingers in the recesses.

I like having just two screws to adjust. You can't see them when you're adjusting them, so it's all by feel. Three might be too confusing. Having two adjustments though, it is less-than-intuitive sometimes which direction the adjustment is controlling, especially if you're already bumped up against the side of the housing and turning the wrong direction.

There needs to be some feedback when the mirror hits the housing so you don't keep cranking the screws. Perhaps simply adding stops to the screws in the form of threadlocked nuts is the best way. Or a plastic "C" I can superglue on so it can be installed any time in the assembly process.

These mounts are also pretty expensive at $60/piece, so I'm thinking about whether I should roll my own. They are very, very nice so hats off to Edmund Optics.

Primary Mounts

The primary mounts suggested by Gary Seronik work great. I was unable to use my linear adjustment bearings since the mirrors would not glue to the pads I'd created.

I have a new scheme for mounting heavy mirrors to 3D printed plastic, but I'm also wondering in this specific case whether I even need to have lateral adjustments for the primaries. CNC should be good enough. I left one primary adjustable in a very sloppy way, but am going to install a threaded insert to align it to the other mirror which is already mounted in this way.

Changing Eyepieces

Changing eyepieces was really easy. Having the 2"-1.25" adapters right there on top of the tertiary assemblies was very intuitive and handy.

Different eyepieces have different effective IPDs because they have different eye relief. Despite my IPD being about 63mm, I had to push the 34mm eyepieces together to the 58mm IPD setting to get them to merge correctly.

The 34mm, 17mm, and 11mm eyepieces can be used without adjusting coalignment. Shorter eyepieces seem to require some tweaking. I don't know why yet. Perhaps as I get the primary alignment dialed in, this issue will be resolved.

I'm loving the Morpheus eyepieces. Love the big eye lenses, the views are beautiful, 76 degrees feels as big as Naglers, parfocal is handy.

Filter Cartridges

I expected these to work well, and they do. One thing that had never occurred to me was how easy it would make changing eyepieces on the same target. The Veil Nebula was easy to switch from 34mm to 17.5mm down to 11mm to zoom on the detail. This would have taken 12 screwings/unscrewings of filters from eyepiece barrels without the filter cartridges. Screw that.

In the dark, it is a little hard to align the cartridges for insertion. A thin line of luminescent paint along the upper-edge of the cartridge port will fix this.


Focusers worked without issue. The one on the left is feeling a little loose. I need to check up on that. The glow-in-the-dark knobs were intuitive for the vast majority of users.

The fine focus knobs have been on the chopping block from the beginning. They are heavy and expensive. But to my eyes, I'm now extremely glad I have them. They really aid in getting each eye dialed in quickly.

Ground Board Size

I went with a 300mm ground board to keep the rocker box footprint small. I wanted to see how small it could go. The azimuth bearing works fine, but it is a little too tippy. This will get bumped up substantially in the next version to ensure a rock-solid experience.


In order to get the first prototype done for OSP, I had to knock together a mirror box and rocker box very quickly. There was not time to balance the scope properly or figure out how to make it lightweight. Counterweights were required. Altogether it weighs 72 pounds as follows:

6 lbs UTA

8 lbs primary mirrors

15 lbs counterweight

43 lbs plywood/hardware

Obviously the scope can get much lighter. My goal is 35 pounds (16 kilos) without a battery.

The next version will use an all-aluminum frame, aluminum altitude bearings and rocker box, and a molded plastic enclosure for the mirror box.


There has been a lot of smoke in the Oregon sky over the last month. Some nights at OSP were marginal and there have only been a couple observing opportunities since. I've spent most of my time looking through 34mm/65 deg Sieberts (29x), 17.5/76 Morpheus (57x) and 11/82 Explore Scientifics (91x). My main scope of comparison is a 10" f/4.7 truss scope I've been using for about a year.

Friday night at OSP I was obsessing over whether the thing was working correctly for the audience. Saturday night I had the scope more to myself and I forgot about/shut out all the questions in my head and just used the thing to look at outer space.

My favorite object so far is M24, Small Sagittarius Cluster, Delle Caustiche. I've typically found this object too complex to really get to know, but when binoviewing the eyes linger. I realized I'd never noticed the open cluster NGC 6603 before. It was quite pronounced, looking as bright as M4 or M5 would in a 10" scope. The graceful arcs of stars never looked so good. The 34mm Sieberts gave a view of the full object framed against the background, whereas the 17.5mm Morpheus with its wider field gave an incredibly immersive view maybe 2x2 fields.

I am really impressed with M8 and the dark lane through the middle. It hangs there before you, tons of nebulosity, has a very lagoony quality. Switching to the 34mm, I could capture M8, M20, and M21 in one sharp frame with enough light gathering to clearly see both sides of the Trifid. Nice view. At 34mm, the nebulosity has that "handheld binocular look" you'd expect from a pair of 10x50's, where at 17.5mm it has switched to "telescope look" if that makes any sense. Often we switch eyepieces simply to change magnification, but with these wide + bright views, the Drifter is really providing aesthetically distinct experiences.

M17 looks huge at 17.5mm through two eyes. I'd have had to look through a 7mm in the 10" monoscope to get the same feeling of size. Extended objects in this size range look a lot bigger through two eyes.

M11 is clearly resolved through the 11mm and felt much brighter. Those neat lines of stars have such a surreal presence, one can't help imagine for a moment that some immense ancient power lined them up intentionally.

M31 looked good with multiple lanes, though it was mostly impressive due to the fact that it clearly continued beyond the 2.2 degree view of the 34mm Sieberts.

M51 had clear spiral structure but felt a little fuzzy. I was a little disappointed by this view but on reflection I was comparing it to my most recent view through an 18" on a pristine night. I'll have to view this through a 10 or 12" again to recalibrate my expectations.

The Veil was a delight as mentioned before due to the ability to quickly swap eyepieces with the NPB filters in the filter cartridges. Several people have really enjoyed the Veil. Finding the Veil is really easy when you can spot it at 30x and then switch to higher magnification without messing with filters.

The double cluster NGC 869 wedding ring/cowboy hat was very distinct at just 30x. Clearly resolving these stars with one eye with the 34mm is sometimes difficult for me in the 10". With two eyes--even on a scope with 10% shorter focal length--they popped. Another "binoscope magnification factor" data point. With the 17.5mm Morpheus the immersion factor felt like seeing it for the first time.

Borrowing a pair of 7mm Naglers allowed a nice 160x view of Saturn with Cassini Division visible. I need to spend more time in this magnification range and test it much more thoroughly.

I need more data before giving an opinion on the 4.5mm Morpheus (220x). I've seen some primary mirror misalignment issues at this magnification. The views have been soft but it may be seeing. I've only used them to look at planets low-ish in the sky on nights when smoke was present. Is it the primaries, is it the eyepieces, is it the 0.9mm exit pupil, is it the seeing, no way to know yet.

Speaking of eyepieces, I love the Sieberts though sadly I still experience significant spikes from my astigmatism due to the 6.8mm exit pupil. I'll definitely keep them forever, but I'm curious to see how the APM 30mm/72 performs in this regard with its 6mm exit pupil.

So far, it's been immensely enjoyable for me in the few nights I've sat with it. It's great to finally see things sharp and without most of the problems associated with the extreme astigmatism of both my eyes.

Update 2018/8/27: 

Front yard testing. (Two street lights and a hill blocking southern and eastern views.) Easily split both sides of the double-double early in the evening with the 11mm (91x). This is not to say that I tried the 17's and couldn't split it, so next time I'll try that.

Atmosphere was terrible for planetary, but since my eyes weren't straining I still enjoyed wavy Jupiter at 91x. Saturn is such a jewel at 30x drifting among the stars. If you're a hardcore planetary observer that may sound crazy but even on a night of poor seeing it's still a magnificent view through both eyes.

This was all with my glasses on, first observation with glasses. Usually I hate observing with my glasses, but it's less frustrating with both eyes open. Merging worked ok. Getting the field stop to merge into one neat sharp circle was not happening, but I wasn't really trying either. This could be a primary alignment issue since I still haven't remounted the left primary. Or it could be about getting my glasses in the right spot to make this happen. Glasses and high-AFOV eyepieces don't mix well. Will have to experiment with this more to come to grips with it.

Update 2018/09/02:

Observing party: Excellent seeing for a few hours outside of Cottage Grove. Several 10" scopes of different focal length from f/2.7 to f/5, a 20" f/4.8, a 25" f/2.5, and the Drifter.

Two views stood out for the group: First, the Lagoon Nebula with NPB filters through the 17.5mm. Extensive nebulosity with a prominent backwards "C" shape extending beyond Herschel 36. In reviewing sketches of the Lagoon online, the view was consistent with monoscopes of 16-18" aperture. (The bottom sketch here is the closest thing I can find to the Drifter view.) Everyone was very excited about this view.

The Veil Nebula through the 34mm (30x) using Astronomik/Lumicon OIII filters was another unique sight. At 2.2 degrees FOV, the entire object does not fit into a single field, but it is only 1.5 fields wide. (Honestly, having to scan a bit adds to the fun. I feel the same way about scanning M24 in the 17.5mm.) Filament could be seen all the way from the East Veil to the West Veil with very few breaks. Pickering's Triangle stood out much brighter than anything I've seen in 8-12" monoscopes. Next to the Drifter, the 25" was also pointed at the Veil and the image looked better than any terrestrial-based astrophoto I've seen. More detail and without the noise of all the background stars to distract. Despite the absolutely stunning views through the 25" of filaments stacked on filaments stacked on filaments, observers still found the view through the Drifter very worthwhile because of the massive field of view and relative brightness.

M24 showed enough detail in Barnard 92 and 93 to linger on them for a while.

People found the Double Cluster quite pleasing through both eyes as well, enough to stop what they were doing and come peek.

IFN (Integrated Flux Nebula) was easily visible west of M13 using the Siebert Optics 34mm, which operates on the Drifter at an exit pupil of 6.8mm. Here the Drifter kept up with the 25" better than any other scope due to the wide field and full exit pupil x 2. Hunting an IFN near M51 was interesting. While spiral structure within M51 looked less-defined than in the 10" scopes, the IFN was much more visible in the Drifter.

The view of the Blue Racquetball was somewhat disappointing, probably due to outsized expectations. I'd hoped two eyes would show more vivid color. Turns out the 20" shows much more vivid color. The Blue Racquetball is of course relatively small. This is another data point about object size and binocular effect. I looked at 90x and did not try 150x, so this is something to try for a future viewing.

By comparison, the Cat's Eye showed an unexpected amount of green color for a smaller aperture scope. 20x30 arcseconds vs. 15x20 arcseconds for the Blue Racquetball. Surface brightness 5.87 vs. 7.83.

Scope Issues

When I arrived, the right-hand secondary ring was a little loose. I'd adjusted this earlier in the day. There was moderate washboard up the last 1/4 mile to the viewing site, but I don't feel it should have been sufficient to cause a misadjustment. Unlike mirror mounts where we're just trying to tighten as far as possible, with the secondary rings we need to maintain a specific friction that allows the bearings to move.

When the scope was pointed in the right direction, Mars refracted through the thickness of the secondary mirrors, creating significant glare--M13 almost entirely washed out. This is the first effect demonstrating clearly that a UTA shroud is needed despite the precise baffle of the tertiary housing.

I dropped my first filter cartridge on the ground when I forgot I'd already unscrewed it. Filter was fine. In general, you feel so much more comfortable handling the cartridges in the dark than the bare filters. This is a huge usability upgrade.

The question has been posed to me, why not just rotate one secondary ring to adjust for IPD. Lock down the other one, use it as a reference. After doing more observation in the 150x-220x range, it's becoming clear that the problem with this idea would be misalignment of the small exit pupil. When the two sides of the Drifter are not evenly extended/retracted, it can lead to exit pupils having very different vertical alignments. It is not immediately obvious to the observer that they need to rotate their head. Rather, it just looks like the images are misaligned. So the impulse is to mess with the merging. Now someone else adjusts the IPD and the merging is off again--not because the collimation has shifted but because the relative position of the eyepieces has shifted.

This is hardly noticeable at low power but critical at high power. Same goes for eyepiece collimation. You can move the image around quite a bit if the eyepiece isn't really tight. At f/5, we can maybe treat this as a feature rather than a bug. It prompts me to think about the following:

  1. Adding a mechanism for even movement of the secondary rings, like handheld binos or Jerry Oltion's binoscope. Just as a limiter not as the adjustment mechanism itself. I still want people to yank and push
  2. Different way of securing eyepieces besides small thumbscrews
  3. Will this issue limit max magnification before exit pupil?

Snack Issues

Jelly beans get hard when they get cold. Need to start working on a jelly bean heater.


People are really enjoying this scope, whether they have been observing for five minutes or five decades.

The impact of using both eyes is lost on absolutely no one.

The noobs don't need instructions beyond "Look through both eyepieces, push them together or pull them apart, the glowing knobs focus." A busy star party didn't destroy image co-alignment or reveal major issues of any kind.

Experienced observers were not offended by the GSO primaries.

When the Drifter is 35 pounds and sets up in 60 seconds without the need to collimate or coalign, it will redefine grab-and-go. A refractor will have no ergonomic advantage. The difference between 16 square inches of aperture and 114 square inches of aperture will be gloriously obvious. The difference between one eye and two eyes with 2" eyepieces will be gloriously obvious.

Mel Bartels put me on the Telescope Walkabout at OSP and posted his report. Many thanks to Mel for all the support and feedback.

On to Prototype 2!


  1. Lawsen Lew on December 2, 2018 at 6:32 pm

    Thanks for your review and abstract post. I would to build a 10” version with affordable GSO brand/make 10” f/5 mirrors. I have thought about 12”, but those only are f/4. Is PLA plastics strong enough and long lasting enough.

    • Analog Sky on December 11, 2018 at 6:54 am

      PLA unfortunately melts in the sun–so this makes it problematic for a lot of the desert-like places we like to take our telescopes. The Drifter uses carbon-filled nylon for maximum durability. Another good choice for your project would be copolymer (PET-G).

  2. Lawsen Lew on December 2, 2018 at 6:50 pm

    The GSO 12” mirror does comes in f/5, thus it is possible to make a 12” version binocular Newtonian telescope. The weight is getting more with increasing aperture. I have thought about a 16” f/4.5. 16” aperture will have the height eyepieces focuses too high up and will needs a ladder, not comfortably.

    • Analog Sky on December 11, 2018 at 6:58 am

      16″ does start to get pretty tall, though in a six-mirror binoscope like the Drifter, the scope ends up significantly shorter than a 16″ f/4.5 Dobsonian monoscope would be. One could also argue that on a bigger scope like this, you are mostly interested in the center of the field so you may get away with focal ratios closer to f/4 without coma correction.

  3. Travis Young on August 31, 2019 at 9:39 pm

    Hi, do you plan to ever share the STL’s or source-files, or to open-source this?

Leave a Comment