Archive for the ‘Gear’ Category

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Anyone tried the AstroBox yet?

June 5, 2017

http://www.astrobox.rocks/

Just learned about this. Am seriously considering springing for the eclipse box. If I do, I’ll post a review when it comes in. If you or someone you know has gotten one of these, lemme know how it worked out.

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Young crescent moon, pleasant surprises, the Bresser gets a name

March 1, 2017

earthshine-feb-28-2017-450

Got out tonight for a few short burst of observing amidst other things. I set up the C80ED and caught the young crescent moon as it was going down. Above is my best shot. It is still wildly inferior to the one I have up in the banner image, to the right of the blog title. That one I shot with my XT6, which had about three times the light gathering ability and almost twice the angular resolution of the C80ED, and I got that shot one night earlier in the lunar cycle. That was back in the early days, when we were still living in Merced. From my driveway I had a straight shot almost to the horizon, so I could catch a 2-day old moon. Here I have lots of trees and buildings in the way, so I generally have to wait an extra night to get a shot at the moon from the driveway.

Then I was out again in the half hour before midnight to try some things with the Bresser Messier AR102S Comet Edition. First, I put it on the lightweight Manfrotto CXPRO4 tripod and DwarfStar alt-az mount that I have previously only used for much smaller scopes (example 1, example 2). Orion was going down over LA so it was pretty stinky, but I still had a long look at both the belt and the sword, and I powered up to split the Trapezium and Sigma Orionis. Then I swept up to hit M35 in Gemini, then back down to Meissa at the ‘head’ of Orion. I finished on Jupiter, using the 60mm aperture mask to knock down the CA.

bresser-on-dwarfstar-1

I was deliberately bouncing around the sky, looking at a variety of targets at a variety of magnifications, to see if the Manfrotto/DwarfStar combo would keep up. I’m a pretty forgiving observer – witness my near-pathological devotion to cheap scopes and stuff made out of junk – but one thing I just can’t handle is an undermounted scope. My first Mak was a 4″ which I hated and sold away before I realized that I hated it because I’d never put it on a solid mount. That experience left me traumatized when it comes to rickety mounts.

The Bresser/Manfrotto/DwarfStar rig doesn’t look like it should work. It looks like the definition of a spindly undermounted disaster. But it was fine. I never had any problem slewing, tracking, or focusing. It helps that the Bresser is lighter than it looks, and carbon fiber is a lot stronger than it looks.

(In the photo, I have the optional eyepiece rack attached to the DwarfStar – I don’t think I’ve ever shown a photo of the mount with it in place. It’s useful.)

I was also pleasantly surprised by the views I got of Jupiter. To get to a decent magnification I used the 8.8mm ES82, both natively (52x) and Barlowed (104x), and a Celestron 8-24mm zoom dialed down to 8 (57x). In both eyepieces I could see the North and South Equatorial Belts and stacks of minor belts marching away toward the poles. There was some CA, but I could minimize the effect by keeping Jupiter in the center of the field, and my eye centered over the eyepiece. The view was so good that I slipped out of gear testing mode and just stared for a few pleasant minutes. I was also happy to find that with the rubber eyeguard removed, I could see the entire field of the 8-24mm zoom at all magnifications while wearing glasses. Which I have to do now. In fact, the other night at the Salton Sea I made almost all of my observations with glasses on.

And lastly, the Bresser Messier AR102S Comet Edition – whew! – finally has a name. I posted on Cloudy Nights about the Messier survey I’m starting with it (thread here), and CN user ‘Glob’ wrote,

mwedel, I read and enjoy your blog, let me suggest nicknaming the 4″ “The Ferret” as King Louis XV called Messier.

I responded:

That is a lovely suggestion, and it put a huge smile on my face. One thing I haven’t blogged about yet is that basically by serendipity I managed to pick up an 80mm prototype of the Bresser ‘reflactor’. So now I have two, big and little, otherwise nearly identical. Ferrets are mustelids (weasel family), along with wolverines, badgers, skunks, fishers, martens, stoats, weasels, and otters. My late grandfather was an accomplished taxidermist and one of his stuffed badgers is sitting on top of a bookcase about four feet from me as I type. It’s just about the same size as the 4″ reflactor. So I’m going to take your charming suggestion, with one modification: the 80mm will be the Ferret, as I anticipate some effort to ferret out all the Messiers with it, and the 4″ is henceforth the Badger, because it can just knock them around with all that aperture. Thanks for helping me solve that long-standing and vexing problem!

So, it’s official now: from now on, the Bresser AR102S is the Badger, and the 80mm will be the Ferret. More info on the Ferret one of these days. I’m going out with this family photo of the two – Badger’s up front, Ferret looms behind:

bresser-ar102s-comet-edition-and-80mm-prototype-1

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From sub-aperture mask to replacement dust cap

February 23, 2017

aperture-mask-2-4-length-comparison

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.

aperture-mask-2-1-tootsie-roll-can

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.

aperture-mask-2-2-external

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.

aperture-mask-2-3-internal

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…

aperture-mask-2-5-dust-cap-replacement

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.
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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.

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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.

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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.

 

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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

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Upgrading secondary collimation bolts on a reflector

February 5, 2017

dscn1615

Here’s a fast, cheap, and easy hack that I do to every reflector that passes through my hands. I hate messing around with hex wrenches while collimating my reflectors, so I replace the Allen bolts with standard hex-cap bolts that can be turned by hand and lightly tightened with a socket wrench or pliers.

I’ve done the mod to all three of the StarBlast 4.5s that the PVAA has placed with the Claremont Public Library – which I am responsible for servicing every couple of months – as well as to my XT10, my SkyScanner 100, London’s XT4.5, and the 5″ f/5 SkyWatcher Newt I had a few years ago. You’ll notice that so far, all of the scopes I’ve done this to have been Synta-made and Orion or SkyWatcher branded. All of the smaller ones have taken identical hardware, but I did the XT10 so long ago I don’t remember – I think it took longer and possibly larger-diameter bolts, but I could be talking crazy.

If this is something you’re interested in doing, you need to take two measurements, make a run to the hardware store, and do about five minutes of work when you get back home. Or you can get a set of Bob’s Knobs, which are much nicer and designed for no-tool use. But making your own with hex-cap bolts costs less than five bucks and gives passable results, and doesn’t stop you from picking up Bob’s Knobs later if you like.

The first thing you want to know, that you can only find out from your assembled spider/secondary mirror mount, is the length of bolt that you’ll need. The secondary holder has two parts, the hub that the spider attaches to, and the 45-degree-angled mirror holder that is usually attached to the back of the secondary mirror itself with double-sided tape. The collimation bolts engage with threads in the hub, and bear against the flat back surface of the mirror holder. The Allen bolts that the scopes ship with are much shorter than the distance from the mirror holder to the front of the hub. So collimation requires sticking a hex wrench down the hole blindly and fumbling a bit to get it seated in the socket (at least for me – if that doesn’t bother you, this post will probably not be of much use).

secondary-mirror-diagram

If you’re going to replace those little shorty Allen bolts with regular bolts, you need to know the distance from the mirror holder to the front of the hub – it’s the dimension between the dotted lines in this diagram, labeled “min. length for bolts”. Your replacement bolts need to have shafts at least this long, or their caps are going to run into the hub before they engage with the mirror holder. It doesn’t really matter how much longer they are, as long as it’s not ridiculous – you don’t want them sticking so far out of the front of the scope that they’ll catch on things or scatter light into the tube.

The second thing you need to know is the type of collimation bolt your scope has – its diameter and thread pitch. If you don’t know that, and you probably won’t the first time out, just back one (and only one!) of your Allen bolts all the way out, and take it with you to the hardware store.

metric-bolt-gauge

At the hardware store you’ll find a bolt gauge like this one. Actually you’ll probably find two, one for English hardware and one for metric. If you have a scope made in China, it probably uses metric hardware, so start there.

testing-original-bolt

Here’s a close-up of me testing one of the collimation bolts from the SkyScanner in the metric bolt gauge. As you can see, it fit the 4mm socket.

replacement-bolt-bag

I already knew from measuring the scope’s secondary that I needed bolts longer than 20mm. And here’s my part: a 4mm x 25mm (diameter x length) bolt, part #81494 at Orchard Supply and Hardware. I bought six – three for my SkyScanner 100, and three for London’s XT4.5, which I hadn’t done yet.

testing-replacement-bolt

My motto is “trust but verify”, especially before buying hardware. If unbagging a part to test it in the store makes you queasy, you can just push the end of one bolt through the bag, enough to try it on the bolt gauge. This won’t destroy the packaging should you need to put it back – buy it or leave it, you can poke the bolt back into the package and only leave a tiny hole (in this case, 4mm!).

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Here are the old bolts ready to go into the bag, which has all of the original Allen bolts from half a dozen reflectors now. I don’t know why I save them. I ‘m kind of an astro-hoarder. If anyone out there wants these, let me know and I’ll send them to you gratis.

Anyway, so far, so good. You get home, back out the Allen bolts, and replace them with the hex-cap bolts. Now, this is important: for your sanity, replace the bolts one at a time. If you screw all of the original Allen bolts out before putting in any of the replacements, your secondary is going to be flopping around uselessly. It may well rotate in place and end up not even facing the focuser drawtube. Take it from an idiot who has done this! But if you replace the bolts one at a time and get all of the replacements finger-tight, the mirror will maintain its radial orientation and may even stay in pretty good collimation through the procedure, although of course you’ll want to recheck and touch up the collimation when you’re finished.

There are loads of good sources on Newtonian collimation online so I’m not going to reinvent that particular wheel. I’ll just add a couple of tips that have made my life a lot easier. The first is to try to balance the push and pull on the three collimation bolts. In other words, if you want to screw in one bolt, back off another one first. If you only ever collimate by screwing in, you’re going to either run out of travel, jack up your mirror holder, or force it farther down the tube, depending on what the deal is with the mounting bolt (some are spring-loaded, some aren’t). When I sit down to collimate the secondary, I quickly go around to each bolt and turn it both ways, backing out first and then screwing in, to get a sense for what each bolt does.

The second tip is specific to these replacement hex-cap bolts on the Orion/Synta scopes that I own and service. Once I get the secondary collimation where I want it by tightening the bolts with my fingers, I go back around and give each one a small additional twist, maybe a sixth of a turn, with the little pliers I keep in my eyepiece box (see here). If I do this evenly to all three bolts, it doesn’t affect the collimation, and the extra bit of tightness helps the scope stay in collimation longer. That might no be needed or even helpful depending on how the mounting bolt engages the mirror holder. Play around with it and see what works for you.

Replacing these bolts was just one of half a dozen hacks I made to the SkyScanner 100. The rest will be covered in another post very soon. Until then, clear skies!

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Marking up sky atlases

February 4, 2017

I’m a book lover. Any space I’m in for long will have books on every available surface and piles of extras on the floor. Because of this love of books, for a long time I wouldn’t mark them up. This hands-off reverence extended to my sky atlases. But eventually I realized that sky atlases are tools, not heirloom pieces, and anything that makes them more useful when I’m observing is justified.

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Here’s a representative page from my working copy of the Pocket Sky Atlas (I also have a second copy, autographed by John Dobson, that actually is an heirloom piece now). The circles and polygons flag objects from various Astronomical League observing projects. Triangles are double and multiple stars, rectangles are Herschel 400 objects, big circles are for the Binocular Deep Sky objects, and an open letter C designates Caldwell objects. I also drew in the position of Almach, which is just off the edge of this chart, wrote in the number for the multiple star 57 Persei, and wrote down the magnitudes of Algol and some of the useful reference stars, including Almach. Arrows in the margins are left over from my Caldwell tour.

I’ve finished all of those projects except the Herschel 400. You’ll see that some of the little rectangles have a diagonal slash across one corner – that’s how I flag which ones I’ve already observed. I’ve actually seen all of the H400s on this chart, I just got lazy about marking them off in the atlas. But I did write ‘CLEAR’ in the corner of the page so I know not to waste my time looking for unobserved H400s here. Other pages have the numbers of the H400s I still need written in the margins, for quick sorting and bookkeeping at the eyepiece.

These marks are very helpful while I am working on a project, because I have an instant visual reminder of what’s available to see in any given stretch of sky. And once I’m done with a particular project, the marks still point me to a lot of ‘best in class’ objects that I might otherwise overlook or forget.

Oh, I also sketch in the positions of comets from time to time, with the dates of observation.

This method has worked so well for me that I have thought about picking up extra copies of the PSA (for $13!) just so I could mark them up with objects from other observing projects. I’ve done that with a couple of my other atlases. My copy of the Cambridge Double Star Atlas has all of the AL Binocular Double Star targets marked, and I use my Jumbo PSA (which is ridiculously useful) to keep track of targets from the last several years of Sky & Telescope’s Binocular Highlight column, to help me avoid repeats. Of course I have other lists for all of these things, both physical and digital, but it’s nice to have an easy reminder when I am out observing or doing desk research.

Do you mark up your atlases? If so, what system do you use? Let me know in the comments.

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Unboxing the Bresser Messier AR102S Comet Edition

January 31, 2017

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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?

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

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

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Outer box…

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…contains the middle box…

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

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

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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.

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

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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.

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

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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.

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

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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.

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

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

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

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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.

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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.

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

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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.
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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.