h1

Get $5 off your first purchase from OpticalInstruments.com

February 21, 2017

Hey, I was temporarily without a 10mm eyepiece (long story) and I have been sufficiently happy with the Bresser 20mm 70-degree that came with my AR102S Comet Edition that I plunked down thirty bucks for the 10mm version (sale price, down from $50). It was only my second-ever purchase from OpticalInstruments.com (after the Bresser Spektar spotting scope a couple of years ago), but they rewarded my ‘ongoing support’ with this deal. You can use this link and unique code:

https://opticalinstruments-com.myshopify.com/?redeem=58abdb56bb070f0018560f59

to get $5 off your first purchase, and if you do, I’ll get a $5 kickback. As far as I know, there is no limit to how often this can be used by people making their first purchase there. So if you’ve been tempted by something at that store, here’s your chance to save a little dough. Happy shopping!

h1

The 28mm RKE in action

February 20, 2017

Still cloudy here, but we got a gap earlier this evening, a persistent sucker hole right over Orion, and I got a whole 10 minutes of observing in. I was using the Bresser AR102S Comet Edition and for eyepieces the 20mm 70-degree that came with it, and my new 28mm RKE from Edmund.

Both eyepieces will just fit in the belt of Orion, with Alnitak and Mintaka in the last 5% or so of the field on either side. So the belt turns out to be a good test of edge characteristics. The 28mm RKE is way sharper at the edges, by the way. You might think that its 45-degree apparent field of view would feel positively claustrophobic after the 70-degree field of the Bresser eyepiece.

But it doesn’t, because of the magical floating stars effect. It’s real! It’s one of the most arresting things I have experienced in almost a decade of observing. As your eye gets closer to the eyepiece, you begin to be able to see the image. As you move in until you can see the entire field, the point where the eyepiece barrel disappears from view coincides exactly with the point where you are far enough to see the field stop of the eyepiece. If you hold up right there, you see the image created by the eyepiece floating in space, with a thin ring of unresolved darkness around it, which if you back out a bit will be the eyepiece barrel, and if you move in a bit will be the eyepiece field stop. In either case, the eye relief is great enough that you can still see the rest of the scope in your peripheral vision, past the thin ring of darkness at the edge of the field.

I have never, ever seen anything like this. It is exactly as cool and immersive as the legends have it. I can imagine building a whole observing kit consisting of this one eyepiece and a series of Barlows of various magnifications.

Anyway, if you have been on the fence about this eyepiece like I was, just get it. It’s amazing.

h1

Me and the ‘Stig

February 19, 2017

This story started a few nights ago. I had been monkeying around with the AR102S, both at its native aperture and stopped down, and I decided to see how it compared to the C80ED. In particular, I wanted to compare the rich-field views of both scopes (such as they are here – I was observing from the driveway after all), so I was looking at the belt and sword of Orion. The results of that comparo were not very surprising – with it’s wider aperture and shorter focal length, the AR102S goes significantly wider and brighter, but the longer focal ratio and low-dispersion glass of the C80ED produce a better-corrected image.

What was not only surprising, but actively alarming, was that at low power I was getting ugly star images in the C80ED. Even in the center of the field, stars were not focusing down to nice little round points, but to crosses and shapes like flying geese. I wondered if my diagonal might have gotten banged up, so I swapped diagonals. The problem persisted. The scope will not reach focus without a diagonal or extension tube, and I don’t have an extension tube, so I couldn’t try straight-through viewing. Still, it was exceptionally unlikely that both of my good diagonals got horked in the same way.

I didn’t know what to make of that. I figured maybe the scope had gotten out of collimation somehow, and I was pondering whether to mess with it. It’s always been optically excellent and mechanically solid (overbuilt, in fact), and I was loathe to take it apart (as opposed to the TravelScope 70 and SkyScanner 100, both of which were crying out for disassembly).

Then a few days later I ran across this thread on CN, in which a guy was having the same problem I had. It sounded like it was more likely astigmatism (aka the Stig) in the eyes than in the telescope. Apparently it’s worse at low powers where the exit pupil is large, which makes sense – astigmatism is caused by having corneas that are out of round (football-shaped rather than basket-ball shaped), but as the exit pupils get smaller, the less of the cornea is involved in vision, and the more likely it is that the ‘active’ portion will approximate a radially even curvature.

astigmatism-of-the-eye

One commenter recommended making a little diaphragm between thumb and forefinger to stop down the exit pupil. I tried that, but it was awfully difficult to hold my finger and my eye all steady and in alignment. Then I had the idea of using a collimation cap from one of my reflectors. That stopped down the exit pupil to a 1mm circle, which made the image d-i-m, but the star images cleaned right up. Then I took away the collimation cap and tried the view with and without glasses, and the glasses also cleaned up the star images.

It wasn’t the scope, it was me. I have astigmatism, and it’s bad enough that stars look ugly at low power unless I wear glasses.

On one hand, that’s a big relief, because the C80ED scope has always been a rock-solid performer. Along with the Apex 127, it’s my reference standard for good optics. I was feeling a bit queasy at the thought that it might have gotten out of whack.

On the other hand, I now need to prioritize eye relief in my eyepiece collection. I have a bunch that are too tight to show the whole field when I’m wearing glasses. So I have some decisions to make.

That was the first major discovery of the night.

The second was that the AR102S can take 2″ eyepieces with the most minor tinkering. The 2″-to-1.25″ adapter at the top of the AR102S focuser drawtube screws right off. I had been worried that it might be permanently affixed, but when I tried turning it, it spun with remarkable ease. Once I had it off, I dropped in the 32mm Astro-Tech Titan, which is my only 2″ eyepiece, and the views were pretty darned good. Way wider than with any of my 1.25″ eyepieces, and pretty clean as well, although I need to a little more head-to-head testing on that score. Possibly the star images looked good because they were so small at only 14x.

bresser-ar102s-with-2-inch-ep

In any case, the 32mm Titan gives a significant boost in true field, from 3.6 degrees in the 32mm Plossl and 24mm ES68, to a whopping 4.88 degrees.

I don’t think there would be any advantage in going wider, at least in the AR102S. Astronomics seems to be out of Titans, but the equivalent 70-degree EPs are available through Bresser and Agena. The next step up would be a 35mm or 38mm, giving 13x and 12x, but those would push the exit pupil to 7.7mm and 8.5mm, and that’s just wasted light. At least in the AR102S – in the C80ED, longer 70-degree eyepieces would yield the following:

Focal length / magnification / exit pupil / true field

  • 35mm / 17.1x / 4.7mm / 4.1 degrees
  • 38mm / 15.8x / 5.1mm / 4.4 degrees

Either of those would be a good step up from the 3.7-degree max field that the 32mm Titan gives in the C80ED, without pushing the exit pupil uselessly wide.

Anyway, I’m just noodling now. The big news is that the C80ED is fine, I need to prioritize long eye relief in future EP purchases (and maybe thin the herd a bit?) so I can observe with glasses on, and the AR102S can take 2″ EPs after all.

h1

Unboxing the Edmund 28mm RKE

February 17, 2017

rke-unboxing-1

Look what came in the mail today.

rke-unboxing-2

Something small, in a gold box.

rke-unboxing-3

An eyepiece wrapped in paper, and a rubber eyeguard.

rke-unboxing-4

And here they are.

rke-unboxing-5

That is a big honkin’ eye lens. And that’s why I got this eyepiece. The 28mm RKE from Edmund is legendary for its “floating stars” effect where the big eye lens, the sharply raked barrel, and the long eye relief combine to create the impression that the eyepiece has disappeared and the image is simply floating in space. I’ve never experienced this, because I’ve never gotten to look through one of these before. But the reputation of this eyepiece, illustrated by several glowing threads on Cloudy Nights (like the ones that follow), was enough to convince me to take the plunge:

rke-unboxing-6

It didn’t come with a case, so I made my own out of an old prescription pill bottle. A little bubble wrap stuffed in the bottom and taped inside the lid, and I’ve got a nice padded case for free.

new-eyepiece-curse

And I need that case, because the new gear curse is in full effect. How does this eyepiece work in practice? No idea yet – with any luck, I might find out next Wednesday, when the clouds are finally supposed to part. I’ll keep you posted.

 

h1

From km/s to parsecs per million years

February 15, 2017

I’ve been following a fascinating thread on Cloudy Nights called “Classic Rich Field“, in which Danish observer Allan Dystrup is posting his observations and sketches of stellar associations, especially OB associations of big, hot, young stars. It’s a fascinating observing program, not least because he’s doing it all with a 55mm telescope. I’ll no doubt be talking more about OB associations in the future, as my interest in this area – both intellectual and aesthetic – has been steadily growing over the last few years.

Canadian astronomer Glenn LeDrew has made some very substantial contributions to the thread as well. His two posts on runaway stars alone are worth copying, pasting, and saving (1, 2). Especially for this arresting fact that he related:

1 km/s is almost exactly 1 parsec/million years [1.023 pc/myr, in fact, according to this page – ed.]

What a stunning way to put the scale of a parsec into common terms – there are about as many kilometers in a parsec as there are seconds in a million years. Or, if you prefer the more familiar light years, about as many kilometers in a light year as there are seconds in 313,000 years. Suddenly the universe feels ungraspably, inhumanly big. Which of course it always has been – it’s just easy to forget that.

Who thinks on these scales, besides astronomers? Paleontologists. And the question that popped into my head immediately upon reading that near-equivalence was, “If an object had been traveling at 1 km/s since the Late Jurassic, how far would it have traveled?” By Late Jurassic I was thinking about 145 million years ago, when Apatosaurus, Stegosaurus, and Allosaurus roamed the American West. When I go dig each summer, it’s in sediments laid down during that time, preserving the bones of those animals. And if you travel at 1.023 pc/myr, after 145 million years you will have traveled 1.023 x 145 = 148.3 parsecs, or 485 light years.

milky-way-sketch-10-galaxy-diameter-and-thickness-with-earth-distance

A diagram I made for my RTMC talk last year, showing the size of the galaxy and our place in it.

That’s nothing. That’s barely farther than the distance to the Pleiades, one of the closest open star clusters to earth. It’s about a third of the way to the Orion Nebula. (I have these distances loaded in RAM for a reason.) It’s a little less than half of the 1,100-light-year thickness of the ‘thin disk‘ of the Milky Way, which holds about 85% of the stars in the disk of the Milky Way, including our sun.

That’s amazing. If you started out right on the centerline of the plane of the galaxy, halfway between the bottom and the top of the galactic disk, and you started flying directly up or down (galactic north or south) at 1 km/s (fast as a speeding bullet), you’d have to fly for more than 170 million years to reach the edge of thin disk. To get to the edge of the galaxy’s halo at 30,000 parsecs would take 29.3 billion years, or just over twice the age of the universe (and 3.5 times the age of the Milky Way itself).

Space is big, y’all.

h1

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

February 11, 2017

60mm-aperture-mask-6-comet-edition-close-up

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

3-inch-sub-aperture-mask

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.

60mm-aperture-mask-1-gallon-jar

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.

60mm-aperture-mask-2-marking

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.

60mm-aperture-mask-3-completed-mask

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.

60mm-aperture-mask-4-comet-edition-before

Here’s the scope before…

60mm-aperture-mask-5-comet-edition-after

…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.
skyscanner-aperture-mask-test-fit-jar-lid

h1

Hacking the SkyScanner 100: six easy pieces

February 7, 2017

skyscanner-100-before-hacking

Remember this thing? It’s a lovely little scope, but I got tired of crouching over it. And it’s made out of tractable materials – rolled steel and particle board, mostly – and costs next to nothing as these things go ($100 as of this writing), so it was basically crying out for customization. I made six fixes – two on the base, two on the outside of the tube, and two on the inside of the tube. They’re all cheap, fast, and easy, hence the title of the post (with apologies to Richard Feynman!).

skyscanner-hacks-1-eyepiece-rack-and-handle

My first two mods were to the base. (1) I had a leftover eyepiece rack, which I screwed to the base in the only place and at the only angle that it would fit. It works great. My very first scope, an Orion XT6, came with an eyepiece rack like this (which has since been dropped from the base model XT6), as did my XT10, and I’ve always found them to be very convenient. It took me a long time to realize that an eyepiece rack didn’t have to be horizontal to work, especially if the eyepieces are always capped so they can’t fall out.

(2) The second mod is the wire handle on the top of the base, which I scavenged from an earthquake stabilization kit for furniture. It’s just a small woven steel wire with an eyelet at either end which is screwed into the particle board that makes up the base. When I put it on, I thought I’d cut a piece of aquarium tube to slide over it as a cushion. I still might do that at some point, but so far I haven’t needed to. The whole scope and mount only weighs 6 lbs, maybe 7 with a full eyepiece rack, and I’m never carrying the scope that far. Basically from the garage to the driveway, or from the car to a picnic table. So the wire handle has not had the opportunity to get uncomfortable yet. This was the simplest mod but may be the one that has made the most difference in terms of overall convenience. Orion should just build ’em this way, even if it bumped up the price by five bucks.

The piece of tape on the tube is covering the holes intended for mounting the dot finder. I never used it, and now the holes are in an inconvenient place. I’ll come up with a more permanent and better-looking solution than the tape, but at least it keeps dust out of the tube for now.

skyscanner-hacks-2-focuser-position-and-laser-trough

This photo shows the two mods to the outside of the tube. (3) Originally the focuser pointed straight up, with the focus knobs on the opposite side of the base arm. I wanted the focuser to face up at a comfortable angle, so I wouldn’t have to lean so far over the scope while using it. And I wanted the knobs on the same side as the base arm, so the eyepiece rack would face the user. Achieving both of those goals meant moving the scope’s dovetail bar about 135 degrees around the tube. To do that, I had to drill new holes in the tube. I used a paper wrap to get the new holes lined up with the old ones and with each other, made pilot dents using a thumbtack, then drilled them out with a cordless electric drill. It’s not a good idea to have metal filings flying around precision optics, so I removed both mirrors before drilling the holes. It’s fun to take a telescope all the way apart and put it back together, especially if it works better after you’ve done so. Everyone should try it.

(4) Once I had the dovetail moved over to the new holes, I had a couple of perfectly good holes in the tube in a convenient place, and at a convenient angle from the dovetail and the focuser. So I built a laser trough to go there. It’s an idea I got from Ken Crowder, a former PVAA member. Back in 2010 on one of my first trips to the Salton Sea, Ken had his 8-inch SCT set up with a video camera for taking integrated shots of deep sky objects. To help get on target, he had wooden bracket pretty much like the one you see above, into which he would lay his laser pointer. Lots of companies make special rings for adapting handheld laser pointers into telescope finders. But like me, Ken wanted to be able to do other stuff with his laser at a moment’s notice, like trace out constellations for newcomers, or point out especially nice things in the sky without moving his telescope. The wooden bracket lets you drop in the laser and get on target quickly, and then lift it out and use it for other things.

To fine-tune the aim, Ken would shim his with little scraps of wood and paper, and I intend to do the same if necessary. But with the SkyScanner’s 400mm focal length, a 32mm Plossl yields 12.5x and 4-degree true field of view, and even a 25mm gives 16x and a 3-degree field, so even without shimming the laser is usually good enough.

skyscanner-hacks-3-laser-trough-close-up

Here’s a close-up of the laser trough. I built it out of wood scraps and glue. The hardware store didn’t have hex-cap metric screws in the size I needed so I got machine screws and washers. I used a spade bit to cut little indentations for the hardware. The two square stringers on the bottom are to help keep the whole rig aligned with the long axis of the tube.

skyscanner-hacks-4-primary-center-spot-and-secondary-bolts

Finally, the two inside-the-tube mods. (5) I center-spotted the primary to aid in collimation. The best thing to use for this is a notebook reinforcing ring. I have a whole package of those somewhere, but I can’t find it. But I did find a package of the little round stickers of the kind you use to make price tags at garage sales, and made it into a ring with a handheld hole punch. It works great. I have doubts about its longevity, but if and when it falls off, I’ll just make another. It seriously takes less than five minutes. Most mass-produced reflectors these days ship with their primary mirrors already center-spotted, and it really helps with collimation.

(6) As explained in the last post (link), I swapped the stock Allen bolts for secondary collimation with standard hex-cap bolts that I can turn by hand and lightly tighten with a small pair of pliers.

skyscanner-hacks-5-all-ready-to-go

So how does the reborn SkyScanner work? Pretty darned well! It was already an extremely convenient and easy-to-use scope, and now it’s even moreso.

I’m not done hacking on it. As shipped, the primary mirror can’t be collimated. I read on CN about lengthening the bolt holes in the OTA that the mirror cell is screwed into, so that the mirror cell can be tilted to achieve primary collimation. I tried this and didn’t like the results. It’s very hard for me to get the mirror cell mounting bolts tightened down enough to keep the mirror cell from shifting. Especially because it’s natural to grab the back of the scope to help aim it, and in doing so I almost always shift the mirror cell relative to the OTA and subtly throw off collimation. Or not subtly – at f/4, every last arc-second of collimation matters. So I’m going to build a fully-collimatable mirror cell.

And I’m going to figure out a better way to cover those holes in the tube for the finder. And flock the inside of the tube, and make a long dewshield to keep stray light from hitting the secondary and the focuser drawtube. And probably do some other stuff I haven’t thought of yet. I’m basically going to treat this scope as a testbed for every hack I can think of. Should keep me busy for a while.